Outline

  • Distribution of water in the atmosphere

  • Evaporation, precipitation, and saturation

  • Measures of humidity

  • Why water vapor is important?

Interaction between atmosphere, biosphere, and hydrosphere

Figure 1. The hydrological cycle.

 

Distribution of water vapor in the atmosphere

Figure 2. Vertical distribution of humidity

 

Figure 3. Horizontal distribution of humidity: depth-integrated water thickness

Evaporation, precipitation, and saturation

evaporation: transition from liquid to gas

precipitation: transition from gas to liquid

saturation: equilibrium between evaporation and precipitation.

Figure 4. a) surface covered (no evap or cond), b) evap > cond, c) evap = cond, saturation

 

Measures of humidity

vapor pressure: part of the total atmospheric pressure due to water vapor

saturation vapor pressure: maximum possible vapor pressure. Saturation vapor pressure increases nonlinearly with temperature.

Figure 5. Saturation vapor pressure vs. temperature.

specific humidity: mass of water vapor (g) per 1 kg mass of air.

q = mv/(mv +md)

where q = specific humidity, mv = mass of water vapor, md = mass of dry air.

mixing ratio: mass of water vapor (g) per 1 kg mass of dry air.

r =  mv/md

relative humidity: amount of water vapor in the air as a fraction of the saturation.

rh = q/saturation specific humidity x 100%

dew point: temperature at which saturation occurs

 

Why is water vapor important?

  • source of latent heat
  • radiative effects (water is a "greenhouse" gas)
  • source for clouds
  • source of precipitation

sensible heat: energy in the atmosphere which we sense as temperature.
Examine the average monthly sensible heat (from the University of Oregon Geography Dept.)

latent heat: energy present in water vapor. Released to the atmosphere upon condensation.
Examine the average monthly latent heat (from the University of Oregon Geography Dept.)

latent heat of condensation or vaporization = 2.5 x 106 J kg-1

latent heat of fusion or melting = 3.34 x 105 J kg-1

latent heat of deposition or sublimation = 2.83 x 106 J kg-1

As water vapor is transported, so is latent heat. This represents a redistribution of energy.
See the transport of water vapor over North America!

 

diabatic process: energy is added or removed.

adiabatic process: temperature changes, but no heat is added or removed. Adiabatic process are common in the atmosphere.

 


We can understand adiabatic processes by referring to the First Law of Thermodynamics, which states that when heat is added or removed from a gas there will be some combination of an expansion of the gas and an increase of temperature. An adiabatic process represents a special case where no heat is added or removed. Thus, the first law of thermodynamics becomes:

0 = pDv + cpDT
or
pDv = -cpDT

where p = pressure, Dv=change in volume, cp = specific heat, and DT = change in temperature. This equation states that work performed by the air (expansion of the gas) causes a decrease in internal energy (decrease in temperature). Work performed on the gas (compression) leads to warming. Or, more simply, air undergoing expansion cools and air undergoing compression warms.

dry adiatatic lapse rate: rate at which a rising parcel of unsaturated air cools

saturated adiabatic lapse rate: rate at which a rising parcel of saturated air cools. A saturated parcel of air will cool less rapidly than a rising unsaturated parcel of air.

environmental lapse rate: rate at which the ambient temperature decreases

 

 

STORM CLOUDS BREWING