Atmospheric Humidity
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Transcript of Atmospheric Humidity
Chapter 4
Circulation of Water in the Atmosphere A general definition of humidity is the
amount of water vapor in the air. Remember, humidity is not constant
through time or space, there is constant circulation of water through
the hydrologic cycle.
Stepped Art
Fig. 4-1, p. 90
The Many Phases of Water Phase is related to molecular motion, an
increase or decrease in motion creates a phase change.Ice is the coolest/slowest phaseWater vapor is the warmest/fastest
phase
Evaporation, Condensation, & Saturation Evaporation is the change of liquid into a
gas -- requires heat. Condensation is the change of a gas
into a liquid and releases heat.Condensation nuclei
Saturation is an equilibrium condition in which for each molecule that evaporates, one condenses.
(a) Water molecules at the surface of the water are evaporating (changing from liquid into vapor) and condensing (changing from vapor into liquid). Since more molecules are evaporating than condensing, net evaporation is occurring.
(b) When the number of water molecules escaping from the liquid (evaporating) balances those returning (condensing), the air above
the liquid is saturated with water vapor.
a)
b)
Humidity
Any of a number of ways of specifying the amount of water vapor in the air.
Absolute humidity: mass of water vapor/volume of airWater vapor densityNot commonly used due to frequent change
of volume
Specific Humidity: mass of water vapor/mass of air
The water vapor content (humidity) inside this air parcel can be expressed in a number of ways.
With the same amount of water vapor in a parcel of air, an increase in volume decreases absolute humidity, whereas a decrease in volume increases absolute humidity.
The specific humidity does not change as air rises and descends – since it is only dealing with mass.
The average specific humidity for each latitude.
The highest average values are observed in the tropics and the lowest values in polar regions.
Humidity
Vapor pressure: the pressure exerted by water vapor molecules in an air parcel More water molecules = high vapor
pressure
Saturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature
Humidity
Relative Humidity: (actual water vapor/saturation water vapor)*100RH can be changed two ways:
○ Change vapor content○ Change saturation
Decrease temperature causes an increase in relative humidity (inverse relationship).
(a) At the same air temperature, an increase in the water vapor content of the air increases relative humidity as air approaches saturation.
(b) With the same water vapor content, an increase in temp-erature causes a decrease in relative humidity asair moves farther away from being saturated.
Humidity
Relative Humidity and Dew PointDew point is the temperature at which
saturation occursCool an air parcel to dew point and liquid
water condensesA good measure of actual water vapor
contentRelative humidity indicates how close to
saturation, dew point indicates the amount of water vapor
When
When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low.
Average surface dew-point temperatures (oF) for (a) January .
Average surface dew-point temperatures (oF) for (b) July.
Relative humidity averaged for latitudes north and south of the equator.
All data represent conditions during a July afternoon at 3 p.m. local time.
Relative Humidity in the Home
T represents temperature
Td, dew point
RH, relative humidity
Humidity
Relative humidity & human comfort“It’s not the heat, it’s the humidity.”High relative humidity equates to less
evaporative cooling.Sweat cannot evaporate and cool the bodyWet bulb temperatureHeat Index
Air temperature (°F) and relative humidity are combined to determine an apparent temperature or heat index (HI).
Humidity
Special Topic: Heavier humid airDue to the molecular weight of water as
compared to nitrogen, humid air is lighter than dry air.
Baseball announcers are incorrect.
Humidity
Humidity is measured by:Sling psychrometer
Hygrometer