|UNDERSTANDING LATENT HEAT
METEOROLOGIST JEFF HABY
A complete understanding of latent heat will add greatly to your
analysis and forecasting skills. Latent heat is nothing magical but can be very confusing to
understand. Water molecules can move in three ways. They can move by vibration, rotation, and
translation. Ice is not very free to move. Ice can vibrate but ice remains rigid. Liquid water moves
freely but since the molecules are still very close together they do not move as
freely as air.
Solid water (ice) is the most ordered
state of H20 while gas is the least ordered. In order for ice to go from an ice to a liquid state, energy must be
added to cause the ice to go from a higher state to a lower ordered state. When ice melts or water evaporates,
energy must be taken from the environment in order for the ice or liquid to move to a less ordered state. Energy
is needed to weaken the individual hydrogen bonds between H20 molecules. When water (in any of the three phrases)
moves from a higher to a lower ordered state, the air surrounding the H20 will have heat subtracted from it. The
three processes that subtract heat from
the surrounding air are evaporation, melting and sublimation (solid to gas). When water (in any of the three phrases)
moves from a lower to a higher ordered state, the air surrounding the H20 will have energy added to it. This
is called a release of
latent heat (e.g. when heat is subtracted from liquid water, the individual water molecules will slow down. They
eventually slow down to the point at which the hydrogen bonds do not allow the liquid to rotate anymore. Ice now
develops. The energy the water molecules once had to rotate has been given up to the surrounding air). The three
processes that add heat to the surrounding air are condensation, freezing and deposition (gas to solid).
processes of evaporation and condensation take 7.5 times as much energy as melting or freezing. This is why evaporational
cooling will cool the air much more than the melting of snow. For example, let's say snow is falling and the outside
temperature is 40 degrees Fahrenheit. As the snow falls into the warmer air it will begin to melt and some of it
will be evaporating. The evaporation from the wet snow will cool the air 7.5 times as much as the melting of the snow.
If the temperature drops from 40 to 32 degrees as the snow falls, about 7 of those 8 degrees of cooling is caused by
the evaporation process. Melting cools the air also, just not near as much as evaporation does. When water undergoes
a phase change (a change from solid, liquid or gas to another phase) the temperature of the H20 stays at the same
temperature. Why? Energy is being used to either weaken the hydrogen bonds between H20 molecules or energy is being
taken away from the H20 which tightens the hydrogen bonds. When ice melts, energy is being taken from the environment
and absorbed into the ice to loosen the hydrogen bonds.
The energy taken to loosen the hydrogen bonds causes the surrounding air to cool (energy is taken away from the
environment: this is latent heat absorption). The temperature of the melting ice however stays the same until
all the ice is melted. All hydrogen bonds must be broken from the solid state before energy can be used to increase
the H20's temperature.
Energy always flows from a warmer object toward a colder object. An ice cube
at 32 degrees F absorbs energy from air that has a temperature warmer than freezing. Energy flows from the room toward
the ice cube. Throw enough ice cubes in your kitchen and you may notice the temperature of the air cooling slightly.
Energy is taken from your warmer room and moved into the ice cubes to melt them; A subtraction of energy causes
cooling. The same holds when comparing freezing to condensation. The condensation process will warm the surrounding
air 7.5 times as much as when the freezing process occurs. When a thunderstorm develops, the release of latent heat
by condensation is 7.5 times as much as the release of latent heat by freezing. Now let's do some application of
this latent heat process with regard to forecasting.
1. Evaporational cooling from rain (in the absence of downdrafts) will cause
the temperature to decrease but the dewpoint to
increase. The dewpoint will always (in the range of normally
observed temperatures) increase more than the temperature falls (e.g. suppose the temperature is 70 F with a
dewpoint of 50 F, after a persistent rain the temperature will cool to about 63 and the dewpoint will rise to
2. Temperatures have a difficulty warming significantly on days
when there is surface snow cover. The
melting and evaporation from the snow continuously cools the air.
3. Condensation releases latent heat. This causes
the temperature of a cloud to be warmer than it otherwise would have been if it did not release latent heat. Anytime
a cloud is warmer than the surrounding environmental air, it will continue to rise and develop. The more moisture a
cloud contains, the more potential it has to release latent heat.
4. The amount of cooling experienced during melting
or evaporation is a function of the dewpoint depression. If
the air is saturated, evaporation will be minimized.
Evaporational cooling can not take place once
dew forms on the ground but can start to take place when the sun begins
to warm the surface (dewpoint depression becomes greater than 0).
5. Dry climates tend to have a larger diurnal
range in temperature than moist climates. The primary reason is because of latent heat. In a dry climate,
evaporational cooling is at a minimum and there is little water vapor to trap longwave radiation at night. Therefore,
in a dry climate the highs will be higher and the lows lower as compared to a moist climate at the same altitude and
latitude (all else being equal).