Latent Heat of Fusion / Latent Heat of Vaporization
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 person.
Ice also changes directly to the gaseous state. This is known as sublimation.
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.
Condensation
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
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 deposition.
Cloud Formation
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
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.