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Chapter 23 – Water in the Atmosphere
The three processes in the water cycle are evaporation, condensation, and precipitation.
Changing from a lower energy liquid state to a higher energy gas state – evaporation - requires
the input of much energy to break the polar bonds of the water molecule. To change from a
higher energy gas state to a lower energy liquid state – condensation – releases a tremendous
amount of energy. The amount of energy released or absorbed by a change of state is called
latent heat. If a solid changes directly to a gas, the process is called sublimation.
Humidity is the amount of water vapor in the air. There are two types of humidity: absolute
humidity, and relative humidity. Absolute humidity (AH) is the direct measured amount of
water vapor in the air. Saturation value (SV) is the maximum amount of water vapor air can
hold at a particular temperature. Hot air can hold more water vapor than cold air. Relative
humidity (RH) is the ratio of water vapor in the air (AH) to the maximum amount that could be
in the air at a certain temperature (SV). Therefore, the equation we can use to calculate relative
humidity is: RH = AH/SV. The instrument we will use to measure relative humidity is the
psychrometer. The psychrometer has two parts: the dry bulb (which is a regular thermometer)
and the wet bulb (which is a thermometer with a piece of gauze wrapped around it). As
evaporation takes place from the wet bulb, its temperature will decrease. This occurs because
evaporation is a cooling process requiring energy as discussed above. The drier the air, the
easier for evaporation to take place, and the lower the temperature of the wet bulb will drop.
To calculate relative humidity using the psychrometer, we find it in a relative humidity chart
using the dry bulb temperature and the difference between the dry and wet bulb
temperatures.
A better measure of moisture than relative humidity is dew point. Dew point is the
temperature at which condensation first appears. If the dew point temperature is close to the
air temperature, the air contains a lot of moisture. Since cold air can not hold much water
vapor, a parcel of cold air could have a high relative humidity even though its relative humidity
might be low.
The altitude at which clouds form – called condensation level – is dependent upon dew point
and air temperature. Starting from the surface, air temperature of unsaturated air drops 10C for
every 100 meter increase in altitude. Air temperature of saturated air drops between .50 C and
.90C for every 100 meter increase in altitude. There is less of a temperature decrease in
saturated air because energy from condensation offsets some of the cooling. Dew point
temperature decreases by .20C for every 100 meter increase in altitude, whether the air is
saturated or not. Condensation level is determined where dew point equals air temperature.
Clouds are composed of liquid and solid particles. We classify clouds using characteristics such
as their type of development (horizontal or vertical), the altitude where they form, and their
composition (liquid, solid, or both). Three upper level clouds are cirrus, cirrostratus, and
cirrocumulus. Two middle level clouds are altostratus and altocumulus. Three low level clouds
are status, nimbostratus, and stratocumulus. Two vertically developed clouds are cumulus and
cumulonimbus. It is helpful in identifying cloud types to remember the meanings of these root
words: stratus – layered, cumulus – piled, cirro – curly, alto – middle, nimbus – rain. The lower
the cloud, the thicker it is and the darker it appears. The very thin cirrostratus clouds allow the
sun’s light to refract through and create what is called a halo effect. During the night it also
allows for a ring around the moon.
There are different types of fog. Radiation fog forms when the cold surface of the Earth lowers
air temperature to dew point and the water vapor condenses. This is usually thickest is valleys
as cold air sinks. Advection fog forms when warm, moist air moves over a cold surface. The cold
surface lowers the air temperature to dew point and fog forms. This type of fog is common
along coastal areas as warm moist air is moving over cold ocean water. Upslope fog forms as air
is forced up a mountain. The air temperature lowers as it rises and fog forms. Steam fog forms
as cool air moves over an inland body of water. Here the cool air gathers moisture which can
not be kept as vapor due to the low air temperature, so the moisture condenses as fog.
Precipitation can be the result of coalescence or supercooling. Coalescence occurs as larger
droplets fall faster and combine with smaller droplets on the way to the surface. Supercooling
occurs when water droplets do not condense due to a lack of condensation nuclei. Most of the
supercooled droplets evaporate and then condense on already formed ice crystals. If these ice
crystals melt as they fall through warmer air, large raindrops are formed.
There are different types of precipitation. Precipitation types depend not only on the surface
temperatures, but also the temperature as mid and upper levels. When precipitation freezes on
the way to the surface we get sleet. When precipitation freezes on contact with the cold
ground we get glaze ice (also called freezing rain). If precipitation starts to fall, but is pushed
back up to higher altitudes where it freezes, then falls, is pushed back up, refreezes, falls, is
pushed back up, refreezes, falls,…until the lump of ice cannot be supported by the updrafts, a
hailstone forms. When breaking open a hailstone, it looks very similar to an onion with all its
layers. Larger hailstones imply stronger winds. Drizzle is similar to rain but is made of smaller
droplets requiring less air movement.