Humidity

Meteorology · Chapter 5 · DGCA CPL and ATPL

The invisible water in the air: how much there is, how much the air could hold, and what happens when it is cooled until it can hold no more.

Remember

Air is like a sponge: warmer air can hold more water vapour. Relative humidity (RH) = vapour present ÷ vapour it could hold, as a percentage.
Dew point: the temperature to which air must be cooled, at constant pressure, to become saturated. It changes only when water is added or removed; RH changes with temperature too.
Wet bulb: the lowest temperature reached by evaporating water into the air (the desert-cooler principle). Unsaturated: T > Tw > Td. Saturated (fog, rain): T = Tw = Td.
Mixing ratio (g/kg) stays constant when air is lifted adiabatically. Absolute humidity is g/m³.
Cloud base ≈ (T − Td) × 400 ft. Saturation vapour pressure over water is higher than over ice. Water drops stay supercooled down to −40 °C (−45 °C in CB).

Where is the water in the air you breathe?

Invisible water

Five real photographs. Swipe or tap Next.

From the sea to the sky and back Water evaporates from oceans, lakes, rivers and plants, rises with the air, condenses into cloud and falls again as rain or snow. That loop is the water cycle. Water exists in three phases: gas (water vapour), liquid (cloud drops, rain, drizzle) and solid (ice crystals, snow, hail). Only the gas is invisible.
Warm air holds more Air is like a sponge: it can take up only so much water vapour, and no more. How much depends mainly on its temperature. Warm air can hold far more than cold air. When air holds all it can, it is saturated. Normal air, holding less than that, is moist (unsaturated) air.
Fog is the sponge overflowing Cool moist air and the sponge shrinks. At the dew point it is exactly full; cool it further and the extra vapour must condense out as tiny drops. On a clear night the valleys cool first, so the fog appears there first. For saturated air (in fog, or during rain): air temperature = wet bulb = dew point.
Contrails: a jet's own cloud A jet engine's exhaust is full of water vapour. In the very cold air at cruising levels the air cannot hold it, so it freezes almost at once into a line of ice crystals. Where the air up there is already close to saturation, the trails spread and last for hours; in dry air they vanish quickly. Saturation vapour pressure over water is higher than over ice. Where ice crystals and supercooled drops mix, the drops evaporate and the ice grows.
What happens to the dew point? After sunrise the same air warms from 15 °C to 30 °C. No water has been added or taken away.

Takeaway: The air always carries invisible water vapour. Cool it enough and the vapour shows itself as dew, fog, cloud or contrails.

Where does the water in the air come from, and where does it go?

The water cycle

Follow the water round. Tap Next.

Evaporation There is always some water in the lowest layer of the air, the troposphere, as invisible vapour. It gets there by evaporation from oceans, lakes, rivers and plants.
Rising and condensing The moist air rises, and rising air cools. Cooler air can hold less vapour, so some of it condenses into tiny drops: a cloud. In the picture the wind pushes the air up the mountain slopes, which is one common way to lift it.
Precipitation and back The cloud drops grow and fall as rain or snow. Rivers carry the water back to the sea, and the water cycle is complete.

Takeaway: Water evaporates from oceans, lakes, rivers and plants. It rises, condenses into cloud and falls as rain or snow, then runs back to the sea. In the air it comes in three forms: gas (vapour), liquid (drizzle, rain, showers) and solid (snow, hail).