Chapter 10 — Water in the Atmosphere

Absolute humidity is the weight of water vapour per unit volume of air, expressed in grams per cubic metre. Relative humidity is the percentage of moisture actually present in the atmosphere compared to its full capacity at a given temperature.

The dew point is the temperature at which saturation occurs in a given sample of air, meaning the air can no longer hold additional moisture and condensation begins.

Condensation is caused by the loss of heat. When moist air cools to its dew point, excess water vapour condenses into liquid form, usually around hygroscopic condensation nuclei such as dust, smoke, and salt particles.

The ideal conditions for dew formation are clear sky, calm air, high relative humidity, and cold and long nights. The dew point must be above the freezing point.

Frost forms when condensation takes place below the freezing point (0°C), so the dew point is at or below freezing. Excess moisture is deposited as minute ice crystals instead of water droplets.

Fog reduces horizontal visibility to less than 1 km and is drier, forming where warm air meets cold currents. Mist limits visibility to between 1 km and 2 km and contains more moisture; each nucleus in mist has a thicker layer of moisture.

The four types are: cirrus (thin, feathery, white clouds at 8,000–12,000 m), cumulus (cotton-wool appearance, flat base, at 4,000–7,000 m), stratus (layered clouds covering large sky areas), and nimbus (black or dark grey, dense, opaque, formed near the surface).

Orographic rainfall occurs when a saturated air mass is forced to ascend a mountain. As it rises it cools and moisture condenses, giving heavy rain on the windward slopes. The leeward slopes receive little rain and are called the rain-shadow area.

Sleet is frozen raindrops or refrozen melted snow-water that forms when raindrops fall from warmer air into a sub-freezing layer near the ground. Hailstones are rounded solid pieces of ice with several concentric layers, formed when rainwater passes through colder atmospheric layers.

Rainfall generally decreases from the equator towards the poles. Coastal areas receive more rainfall than continental interiors, and oceans receive more than landmasses. Between 35°–40° N and S of the equator, eastern coasts are wetter; between 45°–65° N and S, westerlies bring rain first to western margins.

Convectional rainfall occurs when air is heated, rises in convection currents, expands, cools, and condenses to form cumulous clouds. It is accompanied by thunder and lightning and is common in equatorial regions and interior continental areas, especially during the hotter part of the day.

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