Water in the air, and the changes of state
There is always some water vapour in the troposphere, and it drives almost everything that follows in this book: cloud, fog, precipitation, icing, thunderstorms. This page is about the water itself, in its three phases, and the heat that moves in and out every time it changes phase.
Latent heat
Latent heat is the heat absorbed or released when a substance changes state, with no change in its temperature. The heat goes into rearranging the molecules rather than into making them move faster.
| Change of state | Name | Latent heat |
|---|---|---|
| Liquid to gas | Evaporation | Absorbed |
| Gas to liquid | Condensation | Released |
| Liquid to solid | Freezing | Released |
| Solid to liquid | Melting | Absorbed |
| Vapour to ice, or ice to vapour, directly | Sublimation, also called deposition | Released going to ice, absorbed going to vapour |
One extra detail Oxford asks directly: because warm air holds more vapour, the higher the temperature, the greater the amount of latent heat released when condensation takes place.
Nuclei
Vapour does not condense in genuinely clean air. It needs condensation nuclei, tiny hygroscopic particles, and these are plentiful in the troposphere. Freezing nuclei are far rarer, so water droplets can survive well below 0°C without freezing. These are supercooled droplets, and they persist down to about −40°C, or about −45°C in a cumulonimbus. They are the direct cause of airframe icing, covered in chapter 16.
Measuring how much water the air holds
There are several measures of humidity, and the exam expects you to tell them apart. The one thing they all sit on: the amount of water vapour the air can hold depends almost entirely on temperature. Warm air holds much more than cold.
The measures
- Vapour pressure, VP. The part of the total air pressure contributed by the water vapour. If the total pressure is p and the vapour pressure is e, then p minus e is the pressure of the dry air.
- Saturation vapour pressure, SVP. The vapour pressure when the air is holding all the vapour it can at that temperature.
- Absolute humidity. The mass of water vapour in unit volume of air, in g/m³.
- Humidity mixing ratio, HMR. The mass of water vapour per unit mass of dry air, in g/kg. In unsaturated air the HMR stays constant during ascent even though temperature and pressure are falling, because no vapour is being added or removed.
- Saturation mixing ratio, SMR. The maximum HMR the air could hold at that temperature.
- Relative humidity, RH. HMR divided by SMR, as a percentage, or in plain words the water vapour actually present divided by the most the air could hold at that temperature.
Dew point, wet bulb and frost point
Three temperatures describe the same moist air. They spread apart when the air is dry and close together as it approaches saturation, and knowing how they relate is worth several marks.
| Temperature | Definition |
|---|---|
| Dry bulb | The actual air temperature |
| Wet bulb | The lowest temperature the air can be cooled to by evaporating water into it. A wet muslin round the bulb loses heat as the water evaporates |
| Dew point | The temperature the air must be cooled to, at constant pressure, for saturation with respect to water |
What moves each one
- Dew point changes only with water content. Warming or cooling the air does not move it. A higher dew point means more water vapour is present.
- Relative humidity changes with both water content and temperature. Cooling the air at constant vapour raises RH, warming it lowers RH.
- Dew point lapse rate is 0.5°C per 1000 ft.
Frost point and cloud base
Frost point is the temperature the air must be cooled to for saturation with respect to ice. Cooling below it deposits hoar frost. Joshi also gives the standard empirical rule for the height of the base of convective cloud:
Diurnal variation of humidity
Because RH depends on temperature, it runs opposite to the temperature curve: lowest in the mid afternoon when the air is warmest, highest about 30 minutes after sunrise when it is coldest. The high night time RH is why mist and fog form after dark in autumn and winter. The dew point itself barely changes through the day.
Saturation over water and over ice
Below freezing, water and ice do not behave the same way. This difference, small on paper, is the main reason it rains and snows at all from cold clouds, and it comes up again in the icing and precipitation chapters.
The Bergeron process
More fully the Wegener Bergeron Findeissen theory. Below 0°C, the water molecules in a supercooled droplet have more energy than those locked in ice, so:
In a cloud that holds both supercooled droplets and ice crystals, this means the air is supersaturated relative to the ice but not relative to the water. The droplets evaporate, and that vapour deposits onto the ice crystals, which grow until they are heavy enough to fall. This is the principal way precipitation forms in clouds colder than 0°C, and it also matters in the formation of fog.
Humidity and aircraft performance
A water vapour molecule is lighter than the nitrogen or oxygen molecule it displaces, so moist air is less dense than dry air at the same pressure and temperature, the same point made in chapter 3. The operational effect at a hot, humid airfield:
- Lift at a given speed is reduced, so the takeoff run is longer and the climb out is weaker.
- Joshi's practical allowance is to add about 10 per cent to the computed takeoff distance at hot, moist airfields such as those in the coastal tropics.