NATS 101
Lecture 2
Basic weather symbols and
fronts
We describe weather in terms of:
temperature
humidity
pressure
wind
clouds
precipitation
patterns
visibility
sunlight/UV
IfWe
we “feel”
observe
weather
theseas
weather
the heat
elements
we feel,atthe
an“weight”
instanceofinthe
time,
air
we breathe.
then
we obtain a measure of the weather
If we measure these weather elements over many years,
then we obtain the “climate” of the region.
Therefore:
-
climate represents an average of daily weather over a
long period of time
-
Weather is the instantaneous condition of the
atmosphere.
“Climate is what we expect”
“Weather is what we get”
Pressure and Wind
Atmospheric pressure impacts every aspect of weather
although we do not easily recognize differences in pressure.
- Air moves from high pressure to low pressure → wind
- Air tends to rise in regions of low pressure and sink in
regions of high pressure
Pressure Units: mb, hPa, inches Hg, mm Hg
Wind Units: m/s, mph, km/h, kts
Temperature
- tends to change gradually in horizontal or vertical directions
- also changes with time for the same weather system →
diurnal cycle
- the only place there are rapid changes in temperature is in
the vicinity of fronts
Units:- °C, °F, K
Moisture: water vapor
- Two common ways to express the amount of water vapor in
the air
- Relative Humidity:- is the amount of water vapor in the air
relative to the maximum amount that could be present in the
air. Units: %
- Dew Point Temperature:Td > 15°C is humid.
Td > 20°C is very uncomfortable.
Td < 5°C is dry.
Surface Map:
Pressure
Systems:
L
low pressure
H
Fronts:
Cold air
Cold air
Cold air
Warm air
Warm air
Warm air
Cold front
Warm front
Stationary front
high pressure
Upper-level Maps:
500 mb (hPa)
(5 – 6 km or 16000 – 19000 ft)
Upper-level Maps:
300 mb (hPa)
(9 – 10 km or 30,000 – 33,000 ft)
Weather Map Symbols
Ref:- pg 525, Appendix C, Aguado and Burt
ff
CH
TT dd CM PPP
VV ww N ppa
TdTd CLNh WRt
h
RR
N – total cloud cover
dd – wind direction
ff – wind speed (kts)
ww – present weather
Z (UTC) = MST + 7
CH
34
CM 247
VV  
ppa
30
CLNh WRt
h
RR
PPP – barometric pressure (hPa)
(add a 9 or 10 and place a decimal point
to the left of last number)
TT– air temperature in °F
TdTd - dewpoint temperature in °F
i.e., 0000 UTC = 5:00 pm MDT
1200 UTC = 5:00 am MDT
Some basics of Frontal Systems
(Chapter 9)
1. Fronts are boundaries that separate air masses with
differing temperature and other characteristics.
2. Often represent boundaries between polar and tropical air
- marked by sharp temperature changes over a relatively
short distance.
3. Cold air is typically more dense than warm air → no mixing.
Instead, the denser air forces the warmer air upward.
4. This lifting of air upward can cause cloud formation and
precipitation.
5. Fronts are marked by wind shifts.
6. Fronts are marked by pressure and pressure changes.
Types of fronts:
Cooler
air
Cold air
Warm air
Occluded front
Fronts:
Cold air
Cold air
Cold air
Warm air
Warm air
Warm air
Cold front
Warm front
Stationary front
Cold Fronts
Cold
air
Cold fronts occur when a cold
air mass “catches up” with a
warm, generally unstable, air
mass.
Cold air advection
Cold
air
Warm
air
Day
Minneapolis St. Louis Birmingham
1
-5°C
0°C
5°C
2
-25°C
-2°C
3°C
3
-28°C
-12°C
-5°C
Warm
air
(relatively)
Cold Fronts
The cold air mass moves
in a different direction (W
through N) than the warm
air mass ( SW through S).
It is also moving faster.
The cold air catches up with the warm air and it…
3. the
1.
2.
pushes
unstable
coldthe
air
rising
has
warm,
air
a steep
unstable
massslope,
produces
air because
up because
cumulo-type
friction
the cold
causes
air is
the
denser winds toclouds
lowest
(thunderstorm)
slow down compared with winds higher up
- The thunderstorms can produce very intense precipitation.
- They only form right along the frontal boundary,
- the fast movement of these fronts means that the
precipitation is usually of short duration and clearing skies will
soon follow.
So what would our observer on the ground
expect to see and feel with the passage of a
“classic” cold front?
Weather element
Before Passing
While Passing
After Passing
Winds
South-southwesterly
Gusty, shifting
West-northwesterly
Temperature
Warm
Sudden drop
Steadily dropping
Pressure
falling steadily
Minimum, then
sharp rise
Rising steadily
Clouds
Increasing cumulustype clouds
Strong cumulus
clouds
Often cumulus
Precipitation
Short period of
showers
Heavy showers of rain
or snow, sometimes
with hail, thunder, and
lightening
Decreasing intensity,
then clearing
Dew Point
High: remains
steady
Sharp drop
lowering
Warm Fronts
Warm fronts occur when a warm,
stable air mass “catches up” with
a colder air mass.
Warm air advection
Cold
air
Warm
air
Warm
air
Cold
air
(relatively)
The warm air is
moving faster than the
cold air.
The warm air is less
dense than the cold air
The warm air “runs up” along the cold air boundary, which is
not as steep as in the cold front case (over-running).
There are three consequences of this.
1. The clouds
3.
2.
precipitation
warm
air
and
is stable
precipitation
out ofand
thisso
stratiform
form
it doesn’t
welltype
ahead
form
cloud
cumulo-type
of the
tends to
surface
be light
clouds
front.
as
and
in continuous,
the cold frontbut
case.
owing
Instead,
to the large
as it is
horizontal
forced to
area itofcondenses
rise,
the cloud, and
gradually
slow movement
forming a series
of these
of stratiform
kinds of
fronts, the
clouds,
in arain
broad
canarea.
persist for days.
So what would our observer on the ground
expect to see and feel with the passage of a
“classic” warm front?
Weather element
Before Passing
While Passing
After Passing
Winds
South-southeasterly
variable
south-southwesterly
Temperature
Cool – cold slowly
warming
Steady rise
Warmer, then steady
Pressure
Usually falling
Leveling off
Slight rise, followed
by fall
Clouds
Ci, Cs, As, Ns, St,
fog
Stratus-type clouds
Clearing with
scattered Sc
Precipitation
Light-to-moderate
rain, snow, sleet, or
drizzle
Drizzle or none
Usually none,
sometimes light rain
or showers
Dew Point
Steady rise
steady
Rise, then steady
Stationary Fronts
Stationary fronts occur when the
front stalls.
The structure is the same as in other
fronts, with the front sloping over the
cold air mass.
No air
advection
Cold
air
Warm
air
There is no air movement
across the frontal boundary,
thus, there is no real
weather.
Occluded Fronts
Are associated with midlatitude
cyclones that have both a cold
front and a warm front associated
with them.
Occlusion refers to “closure”. In
this case, a faster moving cold
front closes with the warm front.
As the cold front reaches the
warm front, (and thus the cooler
air in front of the warm front), the
warm air mass is separated from
the surface.
Because at the surface, the cold
air mass is now “catching up” with
a cooler air mass rather than a
warm air mass the temperature
change observed at the surface is
not as dramatic.
An additional change is that now
the warm, unstable air is no longer
being strongly lifted by the cold
front.
The cooler air that has replaced
the warm air at the surface is not
unstable.
Thus, where the occlusion has
occurred, only stable stratiform
cloud develops accompanied by
light but persistent rain similar to
the warm front.
Summary:- Types of fronts:
L
Cooler
air
Cold air
Warm air
Occluded front
Fronts:
Cold air
Cold air
Cold air
Warm air
Warm air
Warm air
Cold front
Warm front
Stationary front