Forecasting Fog for Aviation: Kenya Case Study
Reviewers: Thank you for reviewing the script. As you read through it, please be as critical as possible so we find
any errors or inconsistences. If something is wrong, add the correct information - what we should say instead. Also
add any information that you think is missing or would make things clearer. We plan to publish the module in
September so we need your feedback as soon as possible. If you could send your comments back before 23
August, that would be very helpful! Thanks!
Vesa: Should we say Aeronautical or Aviation?
KMD:


When do morning flights begin?
What is Mr. Mwathi's first name?
1.0.0 INTRODUCTION
1.1.0 Defining the Problem
If you're an aviation forecaster, you've undoubtedly dealt with fog and low clouds and know the impacts that the
associated low ceilings and visibility can have on aviation operations - from causing flight delays and passenger
discomfort to even closing airports. Diverting aircraft to other airports requires additional fuel.
Forecasting fog and low clouds is challenging. These conditions often form on the mesoscale, and numerical
models have a hard time predicting these smaller-scale features. Both low clouds and fog are relatively easy to
identify on visible satellite imagery. But differentiating between the two is difficult because the satellite views the
cloud layer from the top. Also, when these conditions occur at night, visible imagery is not available (unless the
satellite has special visible nighttime observing technology) and the features are very difficult to discern from the
ground surface, which often has a similar temperature to the cloud tops. This makes it critical for forecasters to
understand the processes involved in the development of fog and low clouds, and follow a forecast process that
makes optimal use of all available data.
1.2.0 About the Lesson
This lesson aims to improve aviation [Vesa/KMD: Should this say aviation or aeronautical?] forecasts of fog in the
African airspace by teaching forecasters to make more accurate forecasts using satellite imagery, numerical
weather prediction, and other available data. We will present a process for diagnosing and forecasting fog, and
apply it to a case over the Nairobi, Kenya region on 20 July 2012. You will assume the role of aviation forecaster
on the night shift at Nairobi's Jomo Kenyatta International Airport (JKIA). You will analyze various products to
determine whether the current Terminal Aerodome Forecast (TAF) is valid or needs to be amended and then
follow conditions to see what happened.
The lesson is intended for aviation forecasters, general weather forecasters interested in aviation meteorology,
pilots, and meteorological forecasting instructors and students (WMO). By the end of the lesson, learners should
be able to:
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
Describe the impacts of fog and reduced visibliities on aviation planning and operations
Diagnose the development, presence, and evolution of radiative fog as required in aviation forecast and
warning preparation using satellite imagery and other tools
Develop skills in writing and amending a TAF for the development of radiation fog that meet customers needs
and follow ICAO standards
The following prerequisites will enable learners to get the most out of the lesson.
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Basic Instruction Package in Meteorology (BIP-M)
Basic ability to interpret satellite images, RGB products, and NWP products for forecasting fog
Basic understanding of fog development
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Ability to interpret METAR and TAF codes and tephigrams
Understanding of the steps in the TAF writing process
Some of this information is found in the following COMET lessons, which learners are encouraged to take before
starting the case study:
 Writing TAFs for Ceilings and Visibilities, Africa Edition lesson (mww: add url), which outlines the TAFwriting process and applies it to a low cloud/fog case over Cape Town, South Africa
 Fog: Its Processes and Impacts on Aviation and Aviation Forecasting
(https://www.meted.ucar.edu/training_module.php?id=795), which provides extensive background
information on fog formation and its impacts on aviation forecasting
This lesson is one of three aviation weather case studies developed by the ASMET project to improve aviation
forecasting in Africa. The other two lessons focus on convection and clear air turbulence, and are accessible via
the ASMET website (http://www.meted.ucar.edu/communities/asmet/). The ASMET case studies also support
COMET's Review of Aeronautical Meteorology – Africa online learning curriculum, which provides training that
supports the WMO/ICAO competencies for Aeronautical Meteorological Forecasters.
1.3.0 About Fog
Fog is the suspension of very small water droplets in the air that reduces surface-based visibility. Some of the
more common types are radiation fog, advection fog, upslope/terrain-induced fog, rain/post-frontal fog, blocked
flow fog/stratus, and valley fog. This lesson focuses on radiation fog, which affects many tropical regions, including
Eastern Africa.
Radiation fog is caused by nocturnal infrared cooling at and/or near the ground surface. It forms and completes its
life cycle in situ (although it can be advected under the right conditions). The key low-level ingredients required to
generate radiation fog are moisture, rapid cooling, and calm or light winds (generally 5 kt or less).
Additional characteristics of radiation fog include:
Duration
Generally short duration (< 24 hr), often dissipating by afternoon
Intensity
Considerable variation, especially over open areas or near water sources where
fog tends to be denser; dense areas may be isolated but can present an aviation
hazard
Coverage
Typically remains in one place, and is patchy and localized
Depth
Varies with the depth of the radiation inversion
Time of Day
Tends to form late at night or in early morning hours; can also form after
precipitation that clears near or after sunset
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Fog typically dissipates within a few hours with increasing wind, cloud cover, advection of drier air, and/or solar
radiation.
For more information on the various types of fog, access the COMET lesson Fog: Its Processes and Impacts on
Aviation and Aviation Forecasting, Additional Fog Types at
http://www.meted.ucar.edu/fog_int/navmenu.php?tab=1&page=4.0.0.
2.0.0 Case Study: 00 UTC
2.1.0 Case Setup
Assume that you are an aviation forecaster working the 6 PM to 8 AM (1500 to 0500 UTC) (KMD: Is a 14-hr shift
normal or too long?) shift at Nairobi’s JKIA on 19 to 20 July 2012.
The region is usually covered by stratified clouds in June, July, and August. That’s due to temperature inversions
resulting from the cold Mascarene high pressure cell in the south. Late afternoons, evenings, and nights are
usually cloud-free, while mornings and daytime are typically cloudy.
2.2.0 TAF
You are working with another aviation forecaster, who wrote the 00 UTC 20 July TAF for JKIA valid to 06 UTC 21
July. You are responsible for monitoring the TAF forecast [OR: for monitoring conditions??] between midnight and
08 AM local time and writing the 06 UTC (9 AM local time) TAF. Note that Kenya is at +3000 UTC.
Review the 00 UTC 20 July TAF that was prepared at 22 UTC 19 July, then answer the question below. Note that
you can roll over each part of the TAF for a definition. Other resources on writing and interpreting TAFs are
available via the Aviation Resources link in the navigation menu on the left.
TAF HKJK 192200Z 2000/2106 06005KT 9999 SCT016
BECMG 2002/2006 VRB05KT 8000 RADZ BKN015 BKN080
BECMG 2008/2011 07010KT 9999 SCT023
TEMPO 2013/2019 BKN025
BECMG 2021/2100 SCTO20
TEMPO 2100/2206 VRB05KT 8000 –DZ SCTO12 =
How will this forecast impact aviation customers? Select all that apply.
 The conditions will lower to MVFR at 02 UTC so pilots flying VFR [KMD: Is this correct?] will have to plan
alternate destinations and take on additional fuel**
 Conditions are forecast to drop below IFR conditions and will ground any flights landing at the airport
 Pilots flying VFR should be able to land in Nairobi between 8 UTC and 11 UTC when conditions return to
VFR**
 VFR conditions are expected throughout the forecast period so aviation operations should not be affected
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FEEDBACK: The correct answers are A and C. The forecast calls for conditions to fall to MVFR at 02 UTC and
remain there until 06 UTC. This means that pilots expecting to fly under VFR conditions will need to plan an
alternate destination and take on more fuel. Conditions are forecast to improve by 08 UTC. In this forecast period,
IFR conditions are not forecast, making B incorrect. Since MFVR conditions are forecast, D is wrong as well.
Now we’ll compare the 00 UTC TAF to the 00 UTC METAR, sounding, and surface chart.
2.3.0 METAR
METAR HKJK 200000Z 00000KT 9999 BKN015 15/14 Q1023 NOSIG=
TAF HKJK 192200Z 2000/2106 06005KT 9999 SCT016
BECMG 2002/2006 VRB05KT 8000 RADZ BKN015 BKN080
BECMG 2008/2011 07010KT 9999 SCT023
TEMPO 2013/2019 BKN025
BECMG 2021/2100 SCT020
TEMPO 2100/2206 VRB05KT 8000 –DZ SCT012 =
Here’s the METAR for 00 UTC 20 July. How are current conditions different from those described in the TAF?
 The 1500 ft ceiling developed a little early **
 The drizzle predicted near 02 UTC has already started with the predicted decrease in visibility
 The observed wind speed is higher than that predicted at 00 UTC
 The visibility was predicted to be obscured by rain at 00 UTC but the rain has not yet begun
Feedback: The correct answer is A. The ceiling, which was predicted to move in between 02 and 06 UTC, has
come in early. B is incorrect because drizzle is not observed at 00 UTC. The observed wind at 00 UTC is calm, so
C is incorrect, and D is not correct because rain was not recorded at 00 UTC.
2.4.0 Sounding
Several depictions of the sounding are available for Nairobi (Dagoretti Corner) and the surrounding 150-km square
area at 00 UTC 20 July. Review them, then answer the questions below.
tephi_Nairobi_00utc20july2012.jpg 44461
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tephi_Nairobi_00utc20july2012_zooom.jpg
44462
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Question 1: What are conditions in the upper troposphere between 700 and 300 hPa?
 Winds shift around 400 hPa from northeast at lower levels to southwest in the higher levels
 Clouds are probably present near 400 hPa because the atmosphere is near saturation
 Wind speed is reduced compared to values below 700 hPa
 Drier air is indicated as the temperature and dew point lines become farther spaced apart**
Feedback: The correct answer is D. Conditions above 700 hPa are considerably drier than in the lower
atmosphere. A wind shift occurs around 400 hPa but it goes from generally northwest to generally southeast
above that level, making A incorrect. B is wrong because the dew point depression increases considerably
above 700 hPa, indicating that clouds are not present above that level. Winds in the upper levels are stronger
than seen below 700 hPa, making C incorrect.
Question 2: Are there any indications of cloud below 300 hPa? If so, choose the right level.


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
No clouds are present below 300 hPa
750 to 700 hPa **
650 to 600 hPa
550 to 500 hPa
500 to 450 hPa
Feedback: The correct answer is B. The temperature and dew point lines are close enough to indicate clouds
in the layer between 750 and 700 hPa.
Question 3: How would you characterize the lower atmosphere? Select all that apply.
 The stable layer between 720 hPa to 690 hPa is capping the tops of the stratus clouds **
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
The surface-based stable layer indicates that the surface is cooling less rapidly than the air above it
The difference between the temperature and dew point values in the layer below 720 hPa shows
significant moisture **
The winds are stronger near the surface than above the inversion at 720 hPa
Feedback: The correct answers are A and C. The stable layer between 720 hPa and 690 hPa is acting to
decouple the lower levels of the atmosphere from the levels above. Air below is closer to saturation and the
saturated layer corresponds to the level of the observed stratus clouds. A surface-based inversion is
developing because the surface is cooling more rapidly that the air above it, making B incorrect. The winds in
the stable layer are lighter than those seen above, making D wrong as well.
Question 4: How would you expect the conditions to affect the stratus that formed at 00 UTC? Select all that apply.
 Strong winds in the surface-based stable layer will promote mixing of the air and dissipation of the stratus
layer
 Unstable conditions near the surface will result in stratus dissipation
 The presence of the stable layer below 690 hPa will suppress mixing of air in the cloud layer, so the
stratus will persist**
 The dry air above the stratus layer will promote radiational cooling at the cloud top**
Feedback: The correct answers are C and D. The stability at low levels, calm winds, and dry air above the
stratus will all work together to maintain and perhaps deepen the cloud layer overnight. Since the winds are
relatively weak in the layer near the surface, which is stable overall, A and B are incorrect.
Radiative heat loss is maximized when the layer immediately above the fog or low clouds is relatively dry, the
winds are weak, and there are no cloud layers aloft. On a clear night, the rate of fog-top radiational heat loss is
much more rapid than that in the lowest few meters of the atmosphere, which can result in thickening of the
cloud layer.
2.5.0 SYNOPS
Vesa/KMD: We need a station legend explaining the numbers. We need to extract ceiling, visibility
and whether fog is present. Also, what's the unit for vv?
synop_20july2012_00utc.jpg 44463
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Here's the surface synoptic chart for 00 UTC 20 July. What conditions are indicated? Select all that apply.
 Light and onshore winds **
 Convergence southeast of Nairobi **
 Visibility above 10 km **
 General anticyclonic flow**
Feedback: All of the answers are correct. The 00 UTC synoptic chart shows weak winds and a general anticyclonic
pattern associated with high pressure. Weak convergence can be seen with onshore flow to the north and east of
Nairobi and southwesterly flow to the southeast. Visibilities are greater than 10 km (actually 20 to 30 km)
[VESA/KMD: IS THAT RIGHT? IT'S HARD FOR US TO READ THE CHART]
2.6.0 Weather Story
Now that you have reviewed the TAF and some observational data, you have a basic idea of the state of the
atmosphere, which we refer to as your “mental model.” From this, we can build the “weather story,” which
describes the current conditions and how they are expected to evolve over the forecast period. We'll start by
viewing satellite images and products.
Keep in mind what we've seen so far.
 Stable conditions with weak onshore flow and some low-level convergence
 The presence of stratus at 1500 ft, which has brought the flight category down to MVFR
 Dry air in the middle and upper levels of the troposphere, which will promote radiational cooling and should
help maintain the stratus layer
One question that we need to answer is whether the stratus layer will reduce radiational cooling at the surface and
suppress fog formation. The 00 UTC METAR indicated that the surface dew point was just 1oC lower than the
temperature, so very little additional cooling is necessary for fog to form.
Next we'll examine the satellite imagery to determine the areal extent of any clouds/fog.
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2.7.0 Satellite Images and Products
Fog and low stratus can be inferred from single channel imagery, RGBs, and derived products. The following
tables describe the satellite images and products that are useful for identifying fog and low-level clouds. Review
the information, then we will look at some of the key products for the case.
SINGLE CHANNELS AND
DIFFERENCES
USEFUL
PERIOD
USE/INTERPRETATION
Water vapour 7.3 microns
24 hrs

Good for detecting low-level to mid-level moisture

Provides information about the medium to lower
levels of the atmosphere when the atmosphere is dry
(700 to 650 hPa) and for 650 hPa and above when
the atmosphere is moist

Dark grey areas indicate a dry atmosphere (areas of
minimum RH) and the presence of radiative cooling

Useful for checking for areas of fog or low-level
clouds

At night, dark areas indicate fog or low cloud

During the day, very bright white areas indicate fog or
low cloud

At dawn, fog is difficult to distinguish from the
surrounding areas, although animating the images
can be helpful [Vesa/KMD: Is this right?]
Ch 4 minus 9 (MSG IR 3.9
µm minus IR 10.8 µm)
temperature difference,
known as the “fog product"
24 hrs
EUMETSAT's [VESA: Is it EUMETSAT and/or other organizations?] Meteorological Product Extraction Facilities
(MPEF) produce a range of meteorological products derived from satellite data. One of the most useful products
for fog detection is the Cloud Analysis Indices [Vesa: Does the "I" in CLAI stand for indices or image?] or CLAI.
The CLAI product is produced every three hours (corresponding to the ECMWF analysis/forecast periods) and
helps distinguish the different layers of clouds.
PRODUCT
COVERAGE
USE/INTERPRETATION
MSG Cloud Analysis Image
(MPEF CLAI)
24 hours

Distinguishes different types of clouds
based on temperature and height. Lighter
colours mean lower levels.

VESA: VERIFY THE FOLLOWING WITH
MARIANNE K:

In this product, fog is pink; low-level clouds
are brown; deep convection with cirrus at
the top is blue/cyan; and sandy surfaces are
light tan
[Vesa: Can you verify that it can
help distinguish low clouds and
fog?]
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The following RGBs also provide useful information about fog and low cloud. [Vesa: Do any of these [or other]
products help distinguish fog from low clouds?]
RGB PRODUCT
COVERAGE
USE/INTERPRETATION
24-hour microphysics RGB
24 hours,
particularly useful
at night

[Vesa: Is it ok to add this?] Helps confirm the
presence of fog [OR LOW CLOUD?] and lets us
monitor its development - if it's increasing or
decreasing/dissipating

Very useful because it is available at night and
throughout Africa via EUMETCAST

Fog [or low cloud???] is light green [Vesa: Add
more about what the other colors mean]
Natural color RGB (3,2,1)
Daytime only

Fog is milky white
HRV fog RGB (12,12,9)
Daytime only

Fog is grey against the blue background
Modified airmass RGB for
fog (5-6,9-4,5-0)
24 hours

Colors vary by time of day

During daytime, fog is purple

At night, fog is cyan
2.8.0 Reviewing the Products
Click the tabs to review the water vapour, microphysics, and fog [and CLAI??] products for 00 UTC 20 July 2012.
Then answer the questions below about conditions in the Nairobi region.
msg_wv_20july2012_kenya_00utc.jpg 44480
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msg_microphys_20july2012_kenya_00utc .jpg 44481
msg_fog_20july2012_kenya_00utc.jpg 44482
should we show this? if so, in a tab or after the questions? spec! mark Nairobi better
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Derived products can help in differentiating between cloud layers, particularly the MPEF CLAI.
Using the drawing tool, outline the areas in the Nairobi region that have the lowest clouds. Recall
that in this product, fog is pink; low-level clouds are brown; deep convection with cirrus at the top
is blue/cyan; and sandy surfaces are light tan [mw: update based on MK's info].
Feedback: In the CLAI product, the pink areas are the lowest cloud layers. But we cannot
determine whether those clouds are actually fog or low stratus. Vesa: is this true? What about the
brown interspersed with the pink northeast of Nairobi? Also, put labels that identify features on
image with the feedback
Question 1: The water vapour image shows:
a) Moisture advecting in from the convection to the west
b) Dry midlevels**
c) Dry slot associated with sinking air
Question 2: The microphysical RGB shows: Vesa: WHICH IS RIGHT?
a) Widespread area of fog and/or low clouds over Nairobi
b) An area of cirrus to the southwest of Nairobi
c) Deep convection to the south of Nairobi
Question 3: The IR fog product shows: (Select all that apply.)
a) A more narrow region fog or low cloud near Nairobi**
b) Widespread fog/low clouds over Lake Victoria**
c) Clear skies over Nairobi
Feedback for questions 1-3: The satellite products support what was seen in the soundings - clear skies above the
stratus deck. The dark grey colour in the water vapor imagery indicates that the middle and upper troposphere are
relatively dry. These conditions promote radiative cooling and deepening of the stratus layer, and support the
development of radiation fog. Note that fog itself cannot be detected on water vapour images. Stratus is evident
over the Nairobi region in the light green area [Vesa: Confirm the colors] in the microphysical RGB, and in the dark
areas in the fog product. The pink areas in the CLAI product are the lowest cloud layers but we cannot determine
whether the clouds are actually fog or low stratus.
Question 4: In general, can you differentiate low clouds from fog in these satellite products?
a) No, because cloudy areas cannot be differentiated from cloud-free areas
b) No, because the products only show cloud tops **
c) Yes, because IR sensors can see through clouds
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Feedback: The correct answer is B. Satellite imagery shows data from cloud-top level, making it difficult to
differentiate between fog and stratus. We can, however, sometimes identify fog when it follows terrain
features, such as mountains, or if we animate multiple images. [Vesa: do we need to say something about the
products that should help differentiate between fog and stratus and what their strengths/limitations are?]
Question 4: Based on the current weather situation, how might conditions evolve? [liz's comment: no stations were
reporting fog at 00Z]
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Strong winds will cause the stratus to dissipate and prevent fog from forming
Unstable conditions near the surface and cloud level will mix dry air with the cloud and the cloud will
disperse
The sun will rise soon and the stratus will block radiation from reaching the surface, which will lead to fog
formation
Additional radiational cooling at night could aid in the lowering of cloud ceilings and formation of fog and
reduction of visibility**
Feedback: The correct answer is D. Conditions need to be closely monitored to see if fog forms at the airport
with additional nighttime cooling. Winds were calm at 00 UTC so A is incorrect. We’ve seen that the
atmosphere is stable up to 700 hPa, so B is incorrect. With sunrise, we would anticipate that the fog and low
clouds might be more likely to dissipate, so C is incorrect.
2.9.0 Building the Weather Story
Recall our findings from the METAR, tephigram, and SYNOPS: stability at low levels, calm winds, drier conditions
aloft and stratified low-level clouds.
Based on the satellite imagery, how would you change your mental model about the current conditions? Select all
that apply.
 Low clouds are present over Nairobi **
 The low clouds are not isolated over the airport so conditions forcing the low ceilings are more
widespread **
 Drier air in the midlevels will promote radiational cooling **
 Ceilings are likely to persist and prevent fog from forming
Feedback: The correct answers are A, B, and C. The areal coverage of the low clouds shows that they are
relatively widespread. It seems that the low clouds will persist at least overnight and may reduce radiational
cooling at the surface and thus the chance of formation of fog. However, we have not yet looked at numerical
weather prediction (NWP) data to see how the wind field is expected to change. In addition, the dew point
depression at the airport is only one degree, so very little cooling is necessary for fog to initiate.
Next, we will look at some of the NWP data available for 00 UTC and evaluate how well the model depicts the
weather situation.
2.10.0 Charts to Check
As we continue to develop our weather story, we take the mental model developed while examining the
observational data and start to think about how the situation will change over time. We will focus on the 00 UTC to
09 UTC time period and see how evolving conditions will affect aviation operations. But first we will look at the 00
UTC analysis and see how it compares to the observations. If they compare favourably, we can have greater
confidence in the forecasts.
Here’s are the ECMWF charts that we will check. Review the information, then we will examine each one.
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Chart
Wind vertical profile: sfc to 700 hPa
Humidity – sfc to 700
Surface pressure
What to look for
Wind speed values and how they change
with height. Are winds sufficiently weak to
promote formation of radiation fog?
Boundary layer humidity values. Is the
atmosphere sufficiently moist to become
saturated with additional cooling?
Areas of relatively high pressure and how
the pattern changes with time.
2.11.0 Checking the 00 UTC Charts
Click the tabs to review the 00 UTC analyses of wind speed, humidity, and MSLP, then answer the questions
below. Note that the Nairobi station is at 823 hPa.
Tabbed interface of stills mw: remove div chart from msdb
ecmwf_windverticalprofile_00utc_20july2012 [get valid at...]
Humidity SFC to 700 hPa Vertical Cross Section
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ecmwf_vert_xsection_humidity_0utc_20july2012.jpg 44470
Mean Sea Level Pressure (solid blue contours; hPa) 20 July 2012 00 UTC. Redraw ridge in different
colour [Vesa: Can you send the product from 00 to 12 UTC?]
Question: How well do the 00 UTC NWP analyses agree with the observational data?




The model does not capture the stable conditions in the low levels
The intial conditions from the model support the conditions depicted in the observations**
The low-level moisture field does not support the presence of the low clouds observed
Strong winds in the low levels do not reflect conditions in the sounding
Feedback: The correct answer is B. The initial conditions, represented by the 00 UTC forecast, capture the stable
conditions in the surface pressure chart, the moisture conditions in the cross-section, and the low-level calm winds
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in the wind profile. Since the analysis does a good job of representing conditions at 00 UTC, we can have good
confidence in the model forecasts.
2.12.0 Checking the 00 to 09 UTC Charts
Now we'll review the 00 to 09 UTC forecasts, paying particular attention to conditions that could impact winds,
relative humidity, and the stability of the lower levels of the atmosphere. Click each tab to view the forecasts, then
answer the questions below.
Vertical Profile of Wind Speed
ecmwf_windverticalprofile_00utc_20july2012
Humidity SFC to 700 hPa Vertical Cross Section
ecmwf_vert_xsection_humidity_0utc_20july2012.jpg 44470
Mean Sea Level Pressure (solid blue contours; hPa) 20 July 2012 00 UTC. Redraw ridge in different
colour [Vesa: Can you send the product from 00 to 12 UTC?]
Question 1: Based on the model forecasts, how do you expect the low-level winds to change?



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Winds will increase significantly in the upper levels, indicating an approaching jet streak
Winds will steadily increase overnight and become much stronger after sunrise (03:37 UTC)
Winds will remain light near the surface and not change much over time**
Winds will become completely calm over the forecast period
Feedback: The correct answer is C. Low-level winds in the stable layer do not change significantly over the
forecast period. They are light but not calm, making D incorrect. Winds in the upper levels do not show a
significant increase so A is incorrect. More generally, winds are not expected to increase significantly over time
so B is also wrong.
Question 2: What does the relative humidity cross section indicate about the atmosphere around Nairobi?
a) The lower atmosphere is dry with moist air above
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b) The lower atmosphere is dry with dry air above
c) The lower atmosphere is moist with moist air above
d) The lower atmosphere is moist with dry air above **
Feedback: The correct answer is D. The air near the airport is relatively moist up to 700 hPa, with values of
70% or above. The air above 700 hPa is much drier, with values falling below 30%.
Question 3: What seasonal weather system is indicated by the mean sea level pressure configuration over the
region. [Vesa/KMD: Can this be marked on the chart???]




East African ridge **
Meridional ridge
Arabian ridge
East African low-level jet
Feedback. The correct answer is A. The main meteorological feature during this season is generally the East
African ridge, which is over the continent at this time. The ridge rises from the Mascarene high pressure cell in
the south. The East African low-level jet is present but further to the east over the Indian Ocean. The jet brings
cold air from the south, which creates a temperature inversion that leads to stable conditions at lower levels
and unstable conditions aloft. This is a conducive environment for the formation of low clouds.
Question 4: Based on the NWP data, how will the forecast conditions affect the stratus deck and the potential for
the development of fog? Select all that apply.




Dissipating high pressure at the surface will destablize the lower levels and cause the stratus clouds to
dissipate
Very moist conditions in the low levels will help maintain the stratus deck and could support fog
development later**
Light winds mean less mixing of the lower levels, which can support the development of fog**
Dry conditions above the stratus clouds will promote radiational cooling, which will help maintain the
clouds and promote the development of fog **
Feedback: The correct answers are B, C, and D. Moist conditions in the lower levels and the dry upper
troposphere will enhance radiative cooling. This will promote the development of fog and support the
persistance of stratus clouds.
2.13.0 Comparing Observational Data with NWP
Here's what we've seen so far. The observational data show:
 Stability and moisture at low levels, with calm winds and stratified low-level clouds over Nairobi and the vicinity
 Drier conditions in the middle and upper levels of the troposphere
The NWP charts show:
 Moisture below 700 hPa will continue so existing clouds can be expected to persist and the development of
fog is possible.
 The vertical wind profile does not change significantly. There are calm winds at low levels, which are
conducive to the development of low cloud and fog
 The dry upper troposphere will enhance radiative cooling and support the maintenance of stratus clouds and
the development of fog; the air near the airport is relatively moist up to 700 hPa, and very dry above
 High pressure associated with the East African ridge is over the continent and expected to persist until 09
UTC; this creates an environment conducive for the formation of low clouds and fog
Question 1: How are conditions expected to change between 00 UTC and 09 UTC? [confirm after see animations[

Low-level winds are expected to increase with time and help dissipate the clouds
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


Dry conditions will advect in from the west and destabilize the low levels
High pressure will weaken with the approach of storms systems
The moist stable conditions with calm winds are expected to persist**
Feedback: The correct answer is D. The model forecasts show that the current conditions will persist.
Question 2: Based on the forecast, what aviation-related features or conditions might be of concern and need to be
monitored over the next few hours? Select all that apply.




With continued cooling, fog may form, reducing visibilty and lowering the flight category **
Drier conditions will prevent fog from forming, and conditions should not significantly change
The stratus is expected to persist and ceilings may lower, affecting traffic landing at the airport **
With winds expected to increase, clouds may dissipate and greatly improve conditions
Feedback: The correct answers are A and C. Ceilings and visibility are the main concerns over the next few
hours. Conditions need to be closely monitored, especially for the development of fog. If it forms, it will cause
poor visibility, which can delay departures and arrivals. It can also increase operational costs through
additional fuel consumption and possibly result in aircraft rerouting to alternate aerodromes. Passenger
comfort may also be compromised.
3.0.0 Case Study: 04 UTC
3.1.0 METARs
It’s now 04 UTC (7AM local time) and the sun has risen. The 01 to 04 UTC METAR observations show that fog
has formed while the wind remains weak and variable.
00 UTC: METAR HKJK 200000Z 00000KT 9999 BKN015 15/14 Q1023 NOSIG=
01 UTC: METAR HKJK 200100Z 02003KT 9999 BKN015 15/14 Q1023 NOSIG=
02 UTC: METAR HKJK 200200Z VRB03KT 9999 BKN015 14/13 Q1023 NOSIG=
03 UTC: METAR HKJK 200300Z 22003KT 9999 BKN015 12/12 Q1022 TEMPO 0300 6000 BKN002
04 UTC: METAR HKJK 200400Z VRB01KT 1000 BCGF SCT003 OVC015 12/12 Q1023 BECMG FM0430
0600 FG BKNOO1=
3.2.0 Satellite Products
tabs for 3 animations:



msg_microphys_20july2012_kenya_00to04ani.jpg 44483
msg_fog_20july2012_kenya_00to04ani.jpg 44486
msg_wv_20july2012_kenya_00to04ani.jpg 44485
Click the tabs to view animations of the microphysical, fog, and water vapor satellite products from 00 to 04 UTC.
What has happened to the low cloud layer over Nairobi during this time?




It has gradually extended **
It has started to dissolve
It has dissipated
It has not changed
Feedback: The correct answer is A. The cloud layer near and east of Nairobi is slowly spreading outward.
3.3.0 Decision
Based on the data through 04 UTC, is there any reason to amend the 00 UTC TAF? If so, click the part of the TAF
that no longer is valid. If the TAF is fine and does not need be amended, click “No Change Required.”
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No Change Required
TAF HKJK 192200Z 2000/2106 06005KT 9999 SCT016
BECMG 2002/2006 VRB05KT 8000 RADZ BKN015 BKN080**
BECMG 2008/2011 07010KT 9999 SCT023**
TEMPO 2013/2019 BKN025
BECMG 2021/2100 SCTO20
TEMPO 2100/2206 VRB05KT 8000 –DZ SCTO12 =
Feedback: The first change group (in cyan) needs to be amended because patchy fog has formed at the airport
and dropped the visibility to 1000 m. The second change group (in green) indicates that the stratus should
disperse in the morning after sunrise. Since that's what we expected, it needs no amendment. The third change
group (in grey) does not need to be amended and, in any case, will be updated in the next TAF.
3.4.0 Amending the TAF
In writing the amendment, you may want to check the Aviation Resources (linked in navigational bar) for detailed
guidance on how TAFs are formulated.
Here's the previous TAF valid 00 UTC on the 20th to 06 UTC on 21st.
TAF HKJK 192200Z 2000/2106 06005KT 9999 SCT016
BECMG 2002/2006 VRB05KT 8000 RADZ BKN015 BKN080
Changed temp group to becmg so that this time period works at 00Z we will write the
amendment when the visibility drops] Sunrise is 6:37 am (3:37 UTC)
BECMG 2008/2011 07010KT 9999 SCT023
TEMPO 2013/2019 BKN025
BECMG 2021/2100 SCTO20
TEMPO 2100/2206 VRB05KT 8000 –DZ SCTO12 =
Question 1: Re-write the cyan change group by selecting the correct term for each group.

Wind: 00000KT**/VRB05KT/09008KT

VIS: 9999/8000/2000/0800**

WX: DZ / FG** / MIFG (shallow fog) / BR (mist) / CAVOK
Feedback: Since we anticipate that the fog and reduced visibility will persist, the TAF needs to be amended.
Calming winds and horizontal visilibity of less than 1000 m need to be included.
Question 2: How does this portion of the amendment affect the forecast flight category?

The flight categories are not forecast to change

The amended change group drops the flight catgory from MVFR to VLIFR

As the visibility drops to 800 m, the flight category moves from the current observed value of LIFR to
VLIFR **

Flight categories will be changed from IFR to VFR in the change group
Feedback: Vesa/KMD: Do you want to add anything else? The correct answers are B and C. An amendment to
the TAF is necessary because of the significant drop in visibility, what that means for the flight categories, and the
potential impact on aeronautical operations.
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3.5.0 Amended TAF and Its Impacts
Here is the amended TAF:
TAF AMD[amended] HKJK 200430Z [time of issue] 2004[changed from 0000 to 0400] 2106 [validity period]
00000KT [calm, wind amended] 0800 [800 metres, visibility amended] FG [Fog, wx amended] OVC015
BECMG 2008/2011 07010KT 9999 SCT023
TEMPO 2013/2019 BKN025
BECMG 2021/2100 SCTO20
TEMPO 2100/2206 VRB05KT 8000 –DZ SCTO12 =
TAF AMD HKJK 200430Z 2004/2106 00000KT 0800 FG OVC015
BECMG 2008/2011 07010KT 9999 SCT023
TEMPO 2013/2019 BKN025
BECMG 2021/2100 SCTO20
TEMPO 2100/2206 VRB05KT 8000 –DZ SCTO12 =
How might the conditions impact aviation users? Select all that apply.





Flights may need to be rerouted to an alternative aerodrome **
Flights may need to remain in a holding pattern until the weather clears **
Departures may be delayed **
Passengers may experience discomfort during landing or takeoff **
Additional fuel may be needed, increasing operational costs **
Feedback: All of these are possible when fog is present and visibilities are so low
3.6.0 What Happened
Click the tabs to see what happened from 00 to 09 UTC 20 July. Then answer the question below.
4 TABS FOR:
Vesa: Can you get visible imagery for this period - 04 to 09 UTC??



Microphysics RGB
msg_microphys_20july2012_kenya_00to09ani 44471
Fog Product
msg_fog_20july2012_kenya_00to09ani.jpg 44472
METARs from 02 to 08 UTC
METAR HKJK 200300Z 22003KT 9999 BKN015 12/12 Q1022 TEMPO 0300 6000 BKN002=
METAR HKJK 200400Z VRB01KT 1000 BCGF SCT003 OVC015 12/12 Q1023 BECMG FM0430 0600 FG
BKNOOI=
METAR HKJK 200500Z 19004KT 0800 FG SCT001 0VC015 11/11 Q1024 TEMPO FM0600 3000
BKNOO1 OVC015=
METAR HKJK 200600Z 21006KT 4000 BKN003 BKN016 13/12 Q1025 TEMPO AT 0700 9999 SCT009
BKN018=
METAR HKJK 200700Z 20004KT 9999 SCT008 BKN017 14/12 Q1025 NOSIG=
METAR HKJK 200800Z 20006KT 9999 SCT010 BKN018 16/13 Q1025 NOSIG=
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METAR HKJK 200900Z 19007KT 9999 SCT020 18/13 Q1024 NOSIG=

06 UTC Synop
How did the weather situation evolve?




Fog formed and visibility was reduced to 800 m **
Drizzle and fog were observed
The low cloud layer dissolved completely in the Nairobi region by 06 UTC
The ceiling lowered to 100 m
Feedback: The correct answer is A. According to the METAR observations, visibility was at its lowest at 05 UTC,
1.5 hours after local sunrise. No drizzle was observed, only patches of fog and fog, which lifted later. According to
the satellite products, the low cloud layer remained over the Nairobi region the whole morning and broke up shortly
after the time forecasted in the TAF. The 06 UTC SYNOP indicates foggy weather (in green) over the region with
reduced visibility (as low as 400 m).
4.0.0 Summary
4.1.0 Summary
Fog and low stratus can have a significant impact on aviation operations through reduced ceiling and visibilities.
Satellite imagery and products are very effective tools for detecting the presence of low clouds and fog,
determining their aeral coverage, and monitoring their evolution. This is especially true of radiation fog, where the
fog area tends to dissipate from the edges inward with daytime heating.
Since satellites look down on clouds, it is hard to distinguish areas of low cloud from fog (which starts at ground
level). Therefore, it is vital to use all of your weather data and integrate satellite information with surface and upper
air observations to determine where fog is actually occurring.
Satellite and other observational data are vital tools for evaluating NWP guidance. Based on how the models
intialize, you may need to adjust the forecast output before writing the details of your aviation forecast.
Aviation Resources (link from navigational bar)
Guidelines for using "BECMG" and "TEMPO" groups:
 Indicate significant changes in the weather elements using the “BECMG” or “TEMPO”
groups showing the time duration expected. The beginning and end times are included in
whole hours UTC.
 TEMPO indicator and its time group are used for expected fluctuations in meteorological
conditions that may cross threshold values and last for less than an hour over less than
half the forecast period. Longer fluctuations are represented by the “BECMG” group.
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


“FM” abbreviation is used when a set of forecast conditions are expected to change
significantly. It is followed by a four-figure time group in whole hours and minutes
indicating when change is expected to occur.
“PROB” abbreviation is used to indicate the possibility of alternative values for a forecast
element. Include the probability in tens of percent and the time period during which the
alternatives are expected after the forecast element followed by the alternative value.
PROB can be used with TEMPO to indicate the probability of temporary fluctuations in
conditions. Probabilities less than 30% should not be indicated using TEMPO, and
probabilities greater than 50% should be represented using BECMG or TEMPO groups,
or contained in a separate FM group.
Recommendation: The number of change and probability groups should be kept to a
minimum and should not normally exceed five groups.
Criteria for Using Conditional Groups based on Phenomena:
Wind




Visibility

Wind direction shift by 60° or more when speed is 20 km/h (10
kt) or more
Mean surface wind speed is forecast to change by 20 km/h (10
kt) or more
Wind gusts of 20 km (10 kt) or more above mean wind speed of
30 km/h (15kt)
Be aware of your own criteria and change groups or amend the
TAF if wind changes cross those operational thresholds. For
example, a wind direction shift that would require a runway
change. Include tailwind and crosswind thresholds.
Change (improve or deteriorate) and cross the following values:
150, 350, 600, 800, 1500 or 3000 m; or 5000 m at airports using
primarily visual flight rules

Weather
At the beginning or end, or intensity change of these
weather phenomena:
o
o
o
o
o

freezing precipitation
moderate or heavy precipitation
dust storm
sandstorm
or other phenomena expected to
significantly impact visibility
At the beginning or end of these weather phenomena:
o
o
o
o
o
o
o
Ceiling


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ice crystals
freezing fog
low drifting dust, sand or snow
blowing dust, sand or snow
thunderstorm (with or without precipitation)
squall
funnel cloud (tornado or waterspout)
When the ceiling of BKN or OVC skies will change (lift or fall)
and cross the following values: 30, 60, 150 or 300 m (100, 200,
500 or 1000 ft); or 450 m (1500 ft) at airports using primarily
visual flight rules
When the amount of coverage of cloud below 450 m (1 500 ft) is
forecast to change: from SKC, FEW or SCT to BKN or OVC; or
from BKN or OVC to SKC, FEW or SCT;
Vertical
Visibility
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
When the vertical visibility will change (improve or deteriorate)
and cross the following values: 30, 60, 150 or 300 m (100, 200,
500 or 1 000 ft);
 Or any other criteria based on local aerodrome operating minima
Note: Other criteria based on local aerodrome operating minima are to
be considered in parallel with similar criteria for the issuance of SPECI
developed in response to Appendix 3, 2.3.2 l).