HUMIDITY, CLOUDS AND PRECIPITATION
INTRODUCTION
The purpose of this lab is to explore the factors that influence summer and winter precipitation
patterns in the Sonoran Desert within which Tucson is located. In this lab, we will focus on the
concepts of humidity, monsoon, and adiabatic process.
HUMIDITY MEASUREMENTS
Any measure of water vapor in the atmosphere is referred to as humidity. Humidity can be measured
in various ways:
Specific humidity describes the water vapor content of the air in grams of water vapor per kilogram
of air (g/kg). It measures the actual water vapor content of a mass of air and remains constant as air
temperature and atmospheric pressure change.
The maximum specific humidity (also known as capacity) is the maximum amount of water vapor
that a given mass of air can hold. It is a function of the air temperature of an air mass. Warm air has a
higher capacity than colder air (figure 1).
50
Grams of water vapor per kg of air
45
40
35
30
25
20
15
10
5
0
-40
-30
-20
-10
0
10
20
30
40
Degrees Celsius
Figure 1. Maximum specific humidity versus air temperature
Relative humidity expresses how close the air is to being saturated (100% relative humidity). It is
calculated using the following formula:
RH = actual specific humidity of the air (g/kg)
maximum specific humidity capacity of the air (g/kg) at a given temperature
X 100
Air that is holding all the water vapor that it can (relative humidity = 100%) is saturated. Water vapor
will begin to condense if the air is further cooled.
The dew-point temperature is the temperature at which an air parcel becomes saturated and
condensation begins. Air that has a temperature greater than its dew-point temperature will not be
saturated and condensation will not occur. The dew-point temperature itself is dependent on the
amount of water vapor present in the air parcel (specific humidity).
ADIABATIC PROCESS
The adiabatic process involves temperature changes in a descending (warming of the air) or
ascending (cooling of the air) parcel of air through compression or expansion due to changes in
atmospheric pressure with height. This process occurs without any exchange of heat between the air
parcel and its surrounding environment.
Dry adiabatic lapse rate (DALR): when air that has not reached its dew-point temperature is rising or
sinking, it does so at 10ºC/1000 m
Moist adiabatic lapse rate (MALR): when air that has reached its dew-point temperature is rising, it
does so at 4 to 9.9ºC/1000 m
Figure 2 provides an example of the changes in temperature resulting from the adiabatic process
inside an air mass crossing two mountain ranges (Coastal Range and Sierra Nevada). The elevation
for each location is given.
Air temperature at Monterey = 25°C
Dew point temperature at Monterey = 15°C
A. Monterey
A
Monterey0m
Bay, CA; Elev = 0 m
B. Coastal
Range 1000m
B
Coast Range;
Elev = 1000m
C. Fresno
0m
C
Fresno0m
(Central Valley); Elev = 100m
D.
Sierra
Nevada
4000m
D Sierra Nevada; Elev = 4000m
E. Park Village (Death Valley) 0m
E
Death Valley; Elev = -100
0 mm
D
***Altitudes have
been
modified for this
all values
approximate
example
B
Pacific
Ocean
A
C
E
Figure 2. A topographic cross-section from Monterey, CA to Death Valley, CA.
A to B – The air mass is rising and cooling at the DALR. The temperature inside the air mass at B is
15°C. The air mass has reached its dew point temperature, but condensation is not occurring since
the air mass does not continue to rise.
B to C - The air mass is descending and warming at the DALR. The air temperature at C is 25°C. The
air mass cannot reach its dew point temperature when it is warming.
C to D - The air mass is rising and cooling at the DALR. The air temperature at 1000m is 15°C. The
air mass has reached its dew point temperature and continues to rise, thus condensation occurs. A
cloud starts to form. From this point on, the MALR will be used as the air mass continues to rise. If
precipitation is occurring, it is falling as rain.
At 4000m (point D), the temperature inside the air mass is -3°C. If precipitation is occurring, it is now
falling as snow.
D to E - The air mass is descending and warming at the DALR. The air temperature at E is 37°C. A
descending air mass always warms up at the DALR since condensation cannot occur when you warm
the air.
UPLIFT AND CLOUD FORMATION
Clouds generally form as a result of one of the four air mass uplift processes:
Convection
Orographic uplift
Frontal uplift
Convergence
The importance of these lifting processes in southern Arizona varies both spatially and seasonally.
Air can be lifted through heating of the Earth’s surface, in a process called convection.
Air that is heated becomes less dense and has a tendency to rise, resulting in lower surface air
pressure. This occurs on a large scale in the case of the formation of the equatorial low. This can also
occur on a small scale.
To help visualize this concept, imagine a typical late July day in Tucson. As the desert and
surrounding mountains start to heat up, the overlying air is heated and begins to rise. If it is
unsaturated, it cools at the dry adiabatic lapse rate until it reaches its dew-point temperature. At this
point, small cumulus clouds will start forming. This level is called the condensation level. If the
unstable air continues to rise, it cools at a slower rate, the moist adiabatic lapse rate.
The rising air continues to expand, making the cloud tops appear puffy (and sometimes even wispy, if
water vapor at the top of the cloud freezes to form ice crystals). When the air parcel's temperature is
equal to the temperature of the surrounding air, it will stop rising and is then stable.
The process of convection leads to the formation of cumulonimbus clouds that result in late afternoon
and evening thunderstorms during the summer Monsoon.
In the process of orographic uplift, the air is forced up by a mountain barrier. The side of the mountain
along which the air is rising is called the windward side. After the rising air reached the summit of the
mountain, it will descend along the leeward side of the mountain. The cooling of the rising air on the
windward side allows water vapor to condense and form clouds and, sometimes, will lead to
precipitation.
Orographic uplift is the process leading the high peaks surrounding Tucson to receive nearly 3 times
the amount of precipitation that we get here in the city of Tucson.
MEXICAN MONSOON AND TUCSON SUMMER PRECIPITATION
Most of the year in Arizona, the wind direction is from the interior and hence the weather is
predominantly dry and clear. In summer, the temperature soar dramatically, days with temperature of
110°F (43°C) are common. Then comes the Mexican Monsoon—water vapor laden air get blown in
from the Gulf of Mexico, the Gulf of California and the Pacific Ocean, usually starting in July.
The technical definition of a monsoon is: a seasonal reversal in the prevailing wind direction caused
by the formation of a low-pressure center resulting in a large body of warm air rising. This low
pressure center draws in air from its surrounding. If the air that is drawn in was located over large
water bodies, it will contain a large amount of water vapor. As this warm and humid air rises, the
weather will get unstable, triggering spectacular thunderstorms and torrential rain.
If surface heating causes air to rise, condense and create rain, why doesn't it usually rain in May and
June, when air temperatures are just as hot as they are in July? The answer has to do with the
amount of water vapor in the atmosphere.
In May and June the 500 mb subtropical ridge (18,000 feet above sea level) is located over northwest
Mexico (figure 3, June conditions). As a result, the flow of air across Arizona is usually from the
southwest. This air is very dry. Also, there are no major storms generated over the Pacific Ocean
since the Hawaiian High is very strong and expansive. This results in the hot and dry weather
conditions experienced across Arizona during the months of May and June.
The Mexican Monsoon is a regional-scale circulation that develops over southwestern North America
during the months of July through September. It is associated with a dramatic increase in rainfall that
occurs over what is normally an arid region of North America. The term "Mexican Monsoon" is used
because of similarities to the Asian Monsoon. The similarities between the two monsoons include a
shift in the mid-level flow from westerly to easterly.
By July, the high-pressure ridge has normally shifted northward, with the center of circulation located
over west Texas and New Mexico (figure 3, July conditions). This results in a general onshore flow of
humid air from the Gulf of Mexico, the Gulf of California and the Pacific Ocean. The result is an
increased flow of humid air into northern Mexico and southern Arizona; leading to higher dew-point
temperature as the humidity content of the air increases.
The shift in the location of the subtropical ridge is followed by a dramatic increase in thunderstorm
activity over northwestern Mexico. Arizona, with its high rugged desert, lies on the northern fringes of
this area of enhanced thunderstorm activity. The monsoon phenomenon is particularly active here. It
is during this time that Arizona experiences periodic increase in humidity coming from the south and
east. The increased atmospheric water vapor content, coupled with convectional heating and
orographic uplift, contributes to the development of late afternoon and evening thunderstorms.
Figure 3. Airflow pattern associated to the Mexican Monsoon in June and July
TUCSON WINTER PRECIPITATION
During the winter, the subtropical high-pressure cell that blocks water vapor-bearing winds during the
dry times of the year, shifts southward. This allows maritime air masses to invade the area. Thus,
winter precipitation in Arizona are associated to incursions of humid Pacific Ocean air masses.
Generation of winter storms over the northern Pacific Ocean is more frequent because of the
proximity and increased intensity of the Aleutian Low.
N.B. Refer to the map of global atmospheric pressure in January in your textbook.
Precipitation is low to moderate in the early winter, increasing in February and March.
LAB QUESTIONS
SECTION A: Measuring humidity
Answer the following questions using figure 1 at the beginning of this lab.
1. Estimate the maximum specific humidity for the empty fields in table 1.
Table 1. Maximum specific humidity as a function of air temperature
Maximum
Temperature specific humidity
(°Celsius)
(g/kg)
-40
-30
0.3
-20
0.75
-10
0
3.5
5
5
10
7
15
10
20
25
20
30
35
35
40
Formula for calculating relative humidity:
RH = actual specific humidity of the air (g/kg)
maximum specific humidity capacity of the air (g/kg) at a given temperature
X 100
EXAMPLE:
Air temperature is 10°C
Actual specific humidity is 2 g/kg
RH = 2 g/kg X 100 = 28%
7 g/kg (estimated from figure 1)
2. An air mass at 20°C has a specific humidity of 9 g/kg. What is the relative humidity of this air mass?
3. An air mass at 25°C has a specific humidity of 5 g/kg. What is the dew-point temperature of this air
mass? This is estimated using figure 1. _______________________
4. Assuming the changes in relative humidity displayed in figure 4 are the result of air temperature
changes solely:
a) On which date and at what time of the day (give approximate clock time) did the coolest air
temperature occur? ____________________
b) On which date and at what time of the day (give approximate clock time) did the warmest air
temperature occur? ____________________
Figure 4. Relative humidity changes between August 17th and 21st 2005 in Tucson
00Z = Midnight and 12Z = Noon in Tucson
SECTION B: Annual pattern of precipitation in Tucson
Figure 5. Average monthly precipitation in Tucson, AZ
1. Using figure 5, identify the time period during which the Monsoon was experienced on the list
provided in table 2. Explain why you chose that period.
Table 2. Time period of the Monsoon in Tucson
PERIOD
April to June
July to September
October
November to March
Explanation:
2. Using figure 5, identify the second season of high precipitation besides the Monsoon on the list
provided in table 2. Explain what causes these high precipitation.
3. In Tucson, the Monsoon officially starts when the average daily dew-point temperature is 54ºF or
higher for three consecutive days. The first of these three days is the starting date. Using figure 6,
determine the date the monsoon started in the summer of 2005. ____________________
Figure 6. Dew-point temperatures in Tucson for July 2005
4. In 1987, the monsoon started on July 25th. What information can you deduce from this late start
date in regards to the humidity and atmospheric circulation pattern in the Sonoran Desert during the
1987 summer? Explain.
5. Why do you think the dew-point temperature is used instead of relative humidity to identify the
beginning of the Monsoon? Explain.
SECTION C: Spatial pattern of Monsoon precipitation in Tucson
Figure 7. Percentage of contribution of the Mexican Monsoon to the annual rainfall
1. Using your atlas, identify the longitude and latitude of the meridians and parallels displayed over
the Pacific Ocean on the map displayed in figure 7.
2. Position Tucson on the map by drawing a cross on its approximate location.
3. Give an estimate of the Monsoon contributions to the annual rainfall in Tucson (in percentage).
__________
Figure 8. Spatial pattern of precipitation for the 2003 Monsoon in Tucson
4. Using figure 8, explain what process is responsible for the high precipitation values over the Santa
Catalina Mountains?
SECTION D: Convectional thunderstorms in Tucson
Table 3 shows typical surface temperatures and environmental lapse rate (rate at which temperature
decreases with height in still air) for June and July in Tucson.
N.B. Remember: the environmental lapse rate varies for each location. The value of 6.4°/1000m is an average value
for the troposphere as a whole.
A mass of air starts to rise over a parking lot as it is heated to 38°C.
The air temperature is 33°C.
DALR is 10°C/1000 m and MALR is 6°C/1000 m
Altitude (m)
5000
4000
3000
2000
1000
Table 3. Air temperature and adiabatic cooling
June air parcel
Surrounding air
temperature (°C)
temperature (°C)
Fill this field:
1
8
9
18
17
28
25
38
33
July air parcel
temperature (°C)
Fill this field:
12
18
28
38
1. Give the environmental lapse rate for this specific case. _________________________
2. Using a dew-point temperature of -2°C, calculate the value at 5000m in June. Enter your answer in
table 3 under the June column.
N.B. Remember, if the temperature inside the rising air mass reaches the temperature of the surrounding air, it will
stabilize and stop rising.
3. Does the air mass reach its dew-point temperature in June? Explain.
4. At which altitude will the air mass stabilize (i.e. stop rising) in June? Explain.
5. What does the answer given in question 4 mean in terms of cloud formation? Explain.
6. Using a dew-point temperature of 18°C, calculate the value at 5000m in July. Enter your answer in
table 3 under the July column.
7. At what altitude does the air mass become saturated in July? _____________________________
8. What does the answer given in question 7 mean in terms of cloud formation? Explain.
9. Does the air mass stabilize (i.e. stop rising) at 5000 m in July? Explain.
10. Why are the temperatures inside the rising air masses in June and July different at 4000 m?
EXTRA CREDIT (1 point)
To be completed outside the lab period
SECTION A: Adiabatic process, cloud formation and precipitation
An air mass moves off the ocean and climbs over a mountain range that averages 5000m in
elevation.
The air mass has a temperature of 30C and a dew-point temperature of 10C. The wet adiabatic
lapse rate is 4C/1000m.
Using this information fill the following table and answer the following questions.
Elevation
(m)
3000 Descending
4000 Descending
5000 Ascending
4000 Ascending
3000 Ascending
2000 Ascending
1000 Ascending
Sea level
Temperature
inside the air mass
(C)
30
1. At which altitude will saturation be reached? ___________________________________________
2. What process will start occurring at this altitude? ________________________________________
3. What is the result of this process? ___________________________________________________
4. As the air mass continues to ascend toward the summit of the mountain, at what altitude do you
expect rain to turn into snow if precipitation is occurring? ___________________________________
5. After crossing the summit of the mountain, the air mass descends to an elevation of 3000 m on the
leeward side of the mountain. What will the temperature of the air mass be at this altitude?
_____________________
6. Is it possible to experience precipitation on the leeward side of the mountain? Explain.
7. Will the dew-point temperature of the air mass be higher or lower on the leeward side of the
mountain compared to the windward side dew-point temperature value (assuming that precipitation
has been falling on the windward side)? Explain.