ATMOSPHERIC MOISTURE

Vocabulary: hydrologic cycle; evapotranspiration; glaciers; ground water; soil moisture; water vapor; change of state; latent heat; evaporation; condensation; sublimation; absolute humidity; specific humidity; relative humidity; sling psychrometer; dry bulb thermometer; dry bulb temperature; wet bulb thermometer; wet bulb temperature; depression; saturation; supersaturation; condensation nuclei; hygroscopic nuclei; dew point temperature; convection; frontal; convergence; orographic lifting; normal lapse rate; dry adiabatic lapse rate; wet adiabatic lapse rate; windward; leeward; rainshadow; foehn or chinook; environmental lapse rate; equivalent sea level temperature; stability; instability; Collision-Coalescence Model; Bergeron Model; cloud types; fog types, precipitation types



hydrologic cycle


small amount but water vapor in air is extremely important to us

four states of matter: solid; liquid; gas; plasma
water--unique--only common substance that occurs naturally on earth in the three states of matter naturally found on earth
solid-ice liquid-water gas-water vapor

to change from one state to another energy is involved
either absorbed, removed from the environment
released, given off to the environment
CHANGE OF PHASE OF WATER
(latent heat of water is greater than that of other substances at the earth's surface
specific heat--amount of heat energy needed to raise the temperature of 1 gram of a substance at normal atmospheric pressure by 1C
heat capacity--ability of a substance to absorb heat in relation to its volume
water absorbs 5 times more energy to reach the same temperature as soil or air, but with density considered: water changes temperature slower than air [stores energy])

absolute humidity (specific humidity)--amount of water vapor in the air (absolute uses weight per volume of air)

capacity of air to hold moisture depends on temperature so its capacity varies

WATER VAPOR
CAPACITY TABLE
per one cubic foot of air

temperature grains
30 1.9
40 2.9
50 4.1
60 5.7
70 8.0
80 10.9
90 14.7
100 19.7


Relative Humidity--ratio of amount--percent

RH = (amount of water vapor the air has/what could have) x 100 = RH

OR

RH = (has/could have) x 100

example: temperature 70F; has 2 grains of water vapor

reading from the table, the capacity is 8
so

(2/8) x 100 = .25 x 100 = 25% RH

as temperature increases the air can hold MORE water vapor

example: cool the air above to 40F

(2/2.9) x 100 = about 69%


continue to cool this air

eventually has all the water vapor it can hold--saturated
then condensation occurs

continue to cool this air to 30F
What will be the relative humidity? 100%
What will be the absolute humidity? 1.9 grains

Now, if the air warms up to 40, what will be the absolute humidity? 1.9 grains
What will have happened to the relative humidity? it will be lower

Go to Relative Humidity Practice

sling psychrometer

evaporation is a cooling process

dry air causes more rapid evaporation

The Relative Humidity Table

The Dew Point Table

air temperature 70F
wet bulb temperature 62F
depression 8F
RH 64%
dew point temperature 57F


dry bulb temperature 70F
wet bulb temperature 52F
depression 18F
RH 25%
dew point temperature 33F


air temperature 5F
wet bulb temperature 2F
depression 3F
RH 20%
dew point temperature -20F


if SAME TEMPERATURE, equal evaporation and condensation -- dew point

dew point temperature

  • temperature at which the air becomes saturated
  • the air has all of the water vapor it can hold
  • contains its total capacity
  • RH = 100%
  • condensation = evaporation


Go to the Atmospheric Moisture Exercise which contains a more detailed explanation and lots of practice materials

droplets need condensation nuclei to form on

hygroscopic [hydroscopic]nuclei--have an affinity (attraction) for water

  • cool the atmosphere by:
  • radiation
  • contact with colder surface
  • mixing with colder air
  • expansion when it rises


  • 4 ways to make air rise:
  • convection
  • frontal--over other air, due to different densities (temperatures)
  • convergence
  • orographic--force over barrier like a mountain


ADIABATICS

normal lapse rate = 3.5F/1000'

rises and cools; lower and warms

example:
Denver 5000'; 60F while New York City on coast 0' [sea level]; 70F
Which air contains more heat energy?

convert using the normal lapse rate

either way

Denver to sea level
60 + (5 x 3.5) = ?
60 + 17.5= 77.5F
warmer

New York to 5000'
70 - (5 x 3.5) = ?
70 - 17.5 = 52.5F
cooler

equivalent sea level temperature (ESLT)--temperature of the air if it were at sea level

USE THE NORMAL LAPSE RATE FOR A COMPARISON; THE AIR IS NOT MOVING!!!!!

If air is moving, the pressure changes and makes the temperature change at a different rate--the adiabatic rates


DRY ADIABATIC LAPSE RATE = 5.5F/1000'



Given: A = 1000', 71F, B = 4000'; C =1000'; DRY AIR
What is the temperature at C?

temperature at B = 3 x 5.5 = 16.5; 71 - 16.5 = 54.5

temperature at C = 3 x 5.5 = 16.5; 54.5 + 16.5 = 71F

note: SAME ELEVATION AND TEMPERATURE AT A AND C

Usually always some moisture in the air; as go up, cools; so may reach saturation if cools sufficiently

add moisture to the air in the problem

let's say it becomes saturated at 60F (dew point temperature)

ADD POINT X on mountain--point where the air becomes saturated; may be at any place on the mountain, A, A B, B, or above B

What is the temperature of point X in the problem? 60F

Don't know elevation of X, but know rate at which the air cools.

71F at A
71 - 60 =11F
11/5.5 =2

2000' the air moves from 1000' so = 3000'=X

temperature at B?

WET ADIABATIC LAPSE RATE =3.0F/1000 feet

distance 1000' and 60 - 3.0 = 57F

temperature at C?

distance 4000 - 1000 = 3000'

3 x 5.5 = 16.5
57 + 16.5 = 73.5F

COMPARE THE TWO PROBLEMS AND NOTE THE WARMER TEMPERATURE

rainshadow effect--on the leeward side of the mountain, the air is warmer and drier if condensation occurred on the windward side of the mountain

foehn or chinook--the warm, dry breeze which comes down the leeward side of the mountain, California's Santa Anna winds

practice problem

A=6000'; 89F
X=8000'
B=14,000'
C=4000'
What is the equivalent sea level temperature of the air at C?

answers: X=78F; B=60F; C=115F; ESLT of C=129F

extra practice problem: A=4000'; B=8000'; C=2000'; ESLT of A=75; the air contains 4.1 grains of water vapor. What is the ESLT of C?

answers: A=61; X=6000'; B=44; C=77; ESLT of C=84F.


Go to the Lapse Rate Summary DIAGRAM

Go to the Adiabatic Exercise

Review the steps in the Lapse Rate Problems

THE SERIES OF MOUNTAINS


stability versus instability

stability

6000' 62F 54  
4000' 67F 60  
2000' 72F 66 dew point
0' 77F 77  

ENVIRONMENTAL LAPSE RATE = 2.5f/1000' colder, sinks

instability

6000' 41F 49  
4000' 53F 55 dew point
2000' 65F 66  
0' 77F 77  

ENVIRONMENTAL LAPSE RATE = 6F/1000' warmer, rises

stable air resists vertical movement; upward movement of unstable air may cause it to rise without further assistance

cloud types

types of fog and types of precipitation are also described in detail in the text

precipitation occurs when the droplets become too large to be supported by the air

raindrops may form by:
the Bergeron or ice crystal model in cold clouds when the ice crystals attract moisture and the crystals grow by sublimation
the collision-coalescence model where water droplets merge in warmer clouds because of the electrical charges of the droplets


Go to the Moisture Review Questions

Go to the Moisture-Lapse Rate Review Exercise

Go to the Atmospheric Moisture Exercise
Go to the Adiabatic Exercise