Latent Heat of Fusion / Latent Heat
The heat of fusion and vaporization can most easily be understood
by examining the temperature increase as water is heated. For
example, imagine we heat some ice, starting at a temperature of -20 degrees
Fahrenheit. As the ice is heated, the
temperature will climb smoothly and evenly until the melting point of water, 32
degrees. At this point, the temperature remains at 32 degrees, even though
the ice is still being heated.
The temperature is not climbing, because the
ice is absorbing the heat of fusion. With continued heating, the ice
melts and becomes a liquid at a temperature of 32 degrees. Even though the
liquid is 32 degrees, it contains more heat than when it was ice at 32 degrees,
having absorbed the heat of fusion.
Once all the ice is liquid, the temperature
will again smoothly and steadily climb to 212 degrees, the boiling point of water.
Again, the temperature ceases to climb, while heat is still added. The boiling
water is absorbing the heat of vaporization, as changes states from liquid to gas.
The gaseous water contains more heat than the liquid water, as it has absorbed the
heat of vaporization.
Water does not need to be at 212 degrees and boil to become gaseous, however . It will
evaporate into the air at everyday temperatures from the liquid state.
When it does so, it will still absorb the heat of vaporization. For example, a person's sweat evaporates
from their skin and clothing. When it does so, it takes up the heat of vaporization
from that person's skin and clothing, cooling the skin and clothing of the sweating
Ice also changes directly to the gaseous state. This is known as
Humidity and Relative Humidity
The amount of water vapor which air can hold depends on the air temperature.
Higher air temperature means a higher capacity to hold water vapor. Every 20 degrees Fahrenheit air is warmed, its capacity
to hold moisture roughly doubles, and every 20 degrees Fahrenheit
the air is cooled, its capacity is roughly halved.
Humidity refers to the actual amount of water vapor present in the air, while relative humidity refers to the amount of water vapor in the air
as compared with the air's capacity to hold water vapor.
If an area of warm air has the same humidity as an area of cold air, the area of
cold air has a higher relative humidity. This is because the warm air is capable
of holding more water vapor, while the colder air is closer to its capacity, even
though both contain the same amount of water vapor.
As air is cooled, its capacity to hold moisture is reduced.
As a result, its relative humidity increases. At a certain temperature, the
air is cooled to the point where the amount of water vapor contained is the same
as the air's capacity, and the air has a 100% relative humidity. At this point,
the air is saturated with water vapor. The water vapor will condense, and
liquid water droplets will appear on any
condensation nuclei that
are present. Condensation nuclei may be dust, smoke particles, or tiny rough
edges or imperfections on about any surface.
Some condensation nuclei have properties which cause condensation just below
100% relative humidity. Alternatively, if an insufficient amount of
condensation nuclei exist to support condensation, then the air may become
supersaturated with water vapor. As a result, condensation occurs to
varying levels when the air has a high relative humidity.
Clouds, fog, or dew will always form when water vapor condenses.
Dewpoint is the temperature at which air must
be cooled to become saturated (100% relative humidity).
The relative humidity can be seen in the difference between the temperature and
dewpoint, called the temperature / dewpoint spread. If air has a temperature
of 75 degrees and a dewpoint of 15 degrees, it has a low relative humidity, since
it must be cooled a great deal to reach its dewpoint. If the same 75 degree
air had a dewpoint of 70, however, it would have a high relative humidity.
Say, for example, the temperature on a given evening was 75 degrees, and the air
had a dewpoint of 70 degrees. During the evening, the temperature dropped
to 65 degrees. Water would have begun to condense at 70 degrees. In
the morning, the air at a temperature of 65 degrees would have a dewpoint of 65
degrees, and morning
dew would exist.
If the temperatures involved were colder, and the dewpoint were below freezing,
the water vapor would form frost instead of dew. The change of state from
a gas directly to a solid, such as in the formation of frost, is called
Clouds form as a result of condensation, which is likely when the
temperature/dewpoint spread is small or zero. Since temperature decreases
with altitude, we have a simple method by which to estimate cloud bases.
The standard temperature lapse rate, or temperature decrease with altitude, is
2 degrees Celsius per thousand feet. In fahrenheit, the standard lapse
rate is 4.4 degrees.
Example 1: With a temperature of 82 and dewpoint of 38, we have a 44
degree temperature/dewpoint spread. Assuming a standard lapse rate of 4.4
degrees per thousand feet exists, the temperature dewpoint spread should be zero
at about 10,000 feet above the ground.
Example 2: We're at an airport with a field elevation of 1,000 feet MSL.
The temperature at this airport is 70, and the dewpoint is 48. As a
result, the temperature/dewpoint spread is 22 degrees. This spread should
equal zero at about 5,000 feet AGL, since 22 over 4.4 is five. So, we
estimate the cloud bases should be around 5,000 feet above field elevation
(AGL), or at about 6,000 MSL.
Frost forms on small rough features of objects when the air becomes saturated at
below freezing temperatures. Frost on an airplane wing might appear
harmless to a new pilot. On the contrary, frost is very dangerous.
It may not add very much weight to the airplane, but frost disrupts the smooth
flow of air across the wings. The wings depend on this smooth flow of air
in order to make lift. With frost on the wings, the wings lifting capacity
is adversely affected.
If a takeoff is attempted with frost on the wings, the frost could prevent the
airplane from becoming airborne at normal takeoff speed.
Operating an airplane with reduced lifting capacity, which exists with any
amount of frost on the wings and tail surfaces, is very dangerous and should
always be avoided.