Relative humidity varies with
GEO 101, Feb. 27, 2014
1. Availability of moisture
Relative humidity = ratio of amount of water vapor in air to
amount of water vapor air can hold at that temperature
2. Temperature changes for a given amount of water vapor
Relative humidity = content / capacity x 100%
Processes that put water into the air
1. Evaporation
2. Transpiration by plants
Evapotranspiration
Evaporation requires
Heat Source:
Energy to change water from liquid to vapor
Usually comes from the sun
Vapor pressure gradient:
Air not saturated
Relative low humidity
Transpiration requires plants
ACTUAL EVAPOTRANSPIRATION (AE) affected by…
1. Availability of water
Plants lose
water through
their leaves
2. Temperature of air
3. Relative Humidity
4. Wind Speed
5. Vegetative cover
Potential evapotranspiration … PET
Plants take in
water through
their roots
Stomates allow CO2 in, O2 and H2O out
the amount of water that would be added to the
atmosphere if there were NO LIMITS on the
availability of water. Function of 2, 3, 4, and 5
above…mostly related to latitude (insolation).
6 CO2 + 6 H20 + sunlight = C6H12O8 +6 O2
1
Average Monthly Water Budget: Jackson, MS
If cool saturated air, decrease capacity, air becomes over
saturated, water vapor has to get out of air, condensation
occurs.
PET
Precipitation
AE
Moisture
surplus
Moisture deficit
Temperature at which condensation occurs = DEWPOINT
CONDENSATION
Soil
moisture
utilization
affected by…
1) Relative humidity
2) Degree of cooling
3) Availability of nuclei
Soil
moisture
recharge
CONDENSATION DEW AND FROST
Water vapor condenses or sublimates on a surface
CONDENSATION CLOUDS AND FOG
Water vapor condenses on nuclei
Clouds form when air cools below its dewpoint
meters
Types of fog
feet
Radiation
Advection
Upslope
Evaporation
2
Temperature structure of atmosphere = normal lapse rate
“Normally” gets colder as you go up at ≈ 6.5°C per 1000 m
Adiabatic processes: internal to a parcel of air
PROCESS that cools air below dewpoint  clouds
Occurs when parcel of air is LIFTED up into
atmosphere.
Remember...
meters
°C
Altitude in meters
Surface inversion
(starts at surface)
Degrees Celsius
Altitude in meters
Temperature inversion: it gets warmer as you go up
instead of getting colder like it is supposed to do.
The normal lapse rate
would be like having
thermometers tied to a
tall tower that reached
to the top of the
troposphere and
taking a temperature
reading at each
thermometer at the
same time.
Inversion aloft
starts above
surface
Degrees Celsius
Atmospheric pressure decreases with increasing altitude
500
312 mi
Adiabatic processes: internal to a parcel of air
400
Altitude (km)
Air molecules
300
200
Air density
100
62 mi
Air
pressure
90% in first
10 mi
10 mi
3
Dry adiabatic rate ≈ 10°C per 1000 m
-20
Vertically moving air parcels that are NOT SATURATED
follow the dry adiabatic rate of temperature change when
ascending or descending.
0
-10
10
Subtropical Highs
air descends
warms adiabatically
0
capacity to hold water vapor increases
20
relative humidity decreases
Horse latitudes
meters
30
10
°C
Reminder: water vapor contains latent heat
Dry adiabatic rate vs.
normal lapse rate
If air parcel is cooler
than surrounding air, it
will not rise = stable.
If air parcel is warmer
than surrounding air, it
will rise = unstable.
20
24 25
25 27
30
meters
°C
As air cools, it can hold less water vapor. If
cooled below saturation point, condensation
occurs at dewpoint temperature. The altitude
where this occurs is called the lifting
condensation level. Clouds start here.
Condensation releases energy to the
surrounding environment.
Dry adiabatic rate
≈ 10°C/1000m
Wet (moist,
satuated)
adiabatic rate ≈
5-6°C/1000m
(Varies with
moisture content,
temperature, and
pressure)
Dewpoint
Dewpoint = 20°C
LCL = 1000 m
meters
°C
4
If parcel stays
warmer than
surrounding
air, it will
continue to
rise
Dewpoint
meters
°C
WHAT YOU MUST KNOW ABOUT THE ADIABATIC PROCESS
Normal or environmental lapse rate ≈ 6.5°C/1000m
Can be inversions or variations
Dry adiabatic rate = DAR ≈ 10°C/1000m
Wet or saturated adiabatic rate = SAR ≈ 5-6°C/1000m
Adiabatic rate is not the same as the normal (environmental)
lapse rate.
Normal (environmental) lapse rate = temperature structure of
the atmosphere
Adiabatic rates = temperature changes in a rising (cooling) or
falling (warming) parcel of air.
Parcel will only rise if it is warmer than the surrounding air!
When rising parcel cools to dewpoint temperature
Adiabatic cooling  Condensation
condensation starts and clouds form
Condensation  Clouds
lifting condensation level
Clouds  Precipitation
latent heat is released into the parcel
Rain
Sleet
continued cooling is at the slower wet adiabatic rate
Descending parcels warm at the dry adiabatic rate.
5
Hail
Glaze
Snow
Snowflakes
For adiabatic cooling to occur, air must rise
via atmospheric lifting!
Types of atmospheric lifting
Reminder: Precipitable water vapor by latitude
cm
Frontal = warm goes up over cold air
Convective = bubbles of warm wet air
Capacity of air to hold moisture directly related to temperature
Orographic = up a mountainside
Convergent = air meets in a LOW
6
Latitudinal variation in precipitation
Wind and pressure belt review
Only every other person gets a worksheet
Work WITH a partner to fill it out
Put both of your names on the worksheet
Try to reason your way through it without your notes
Use your notes as a last resort
Turn it in
Know the lifting mechanisms, or lack thereof, that
explain this latitudinal pattern
7