Monitoring the weather

Temperature across the atmosphere
When talking about the 'temperature' we mean the temperature of some object. In the middle of a cricket field various objects will be at different temperatures, depending on their reactions to the sun shining on them. The grass, the pitch, the bat, the ball and the players' clothing will all have different temperatures. If they are exposed to the sun, these objects will usually be hotter than the air.

The temperature of the air often varies greatly, depending on whether you measure it close to the ground or several metres higher.

In the air, like any gas, the temperature is a measure of the average speed of the tiny molecules that make up the gas. The more energy they have, the faster they move and the higher will be the temperature of the gas. If you heat the gas you add energy, the molecules move faster and the temperature rises.

In Australia, as in most other countries, temperatures are measured in degrees Celsius (C). Boiling water has a temperature of 100°C, and water with lumps of ice floating in it has a temperature of 0°C (freezing point).

Temperatures change with height. The atmosphere can be divided into layers or 'spheres' according to their temperature. The troposphere extends from the surface to about 15 km; this is followed by the stratosphere up to 50 km, then the mesosphere to 80 km and finally the thermosphere to over 100 km. Generally speaking, the temperature decreases in the troposphere (where our weather occurs) at an average rate of 6.5°C per 1000 metres. (As far as the weather is concerned, all the action takes place in the troposphere).

This decrease is the 'lapse rate'. There are large variations from place to place, day to day and season to season. Apart from the general lapse rate in the troposphere, there are large temperature variations in the air layers immediately in contact with the ground, particularly on clear days and clear calm nights.

Temperature inversions
One of the more significant changes is a temperature inversion, in which the temperature increases with increasing height. Low-level inversions often occur with fogs, and they also act like a lid to trap pollutants, resulting in smog in our cities. A strong inversion forms at night at the surface due to cooling of the ground through loss of heat by radiation. This nocturnal or surface inversion is best developed on clear calm nights.

Air settling in the subtropical high pressure belts is compressed and warmed as it subsides. This often results in inversions. These inversions, typically 300 to 3000 metres above the surface, put a lid on convection and restrict cloud growth. Above the inversion the air is warm and dry. This is a major reason for the arid weather of the subtropical high pressure belt.

The boundary between the troposphere and the stratosphere, the tropopause, is also an inversion. It restricts movement of air between the troposphere and the stratosphere, and is the effective limit of even the largest thunderstorm clouds.

After sunrise, radiation from the sun is absorbed by the earth's surface, but scarcely any is absorbed by the air. Conduction of heat from the ground warms the air in the lowest few centimetres, but not higher up - air is a poor conductor. If there is no wind, this sets up a large vertical temperature difference in the lowest metre or so of the atmosphere. Cricketers fielding on a hot day may experience temperatures of over 45°C at their feet but only 35°C at their waist.

On windy days the situation is completely different. The air in contact with the ground is constantly being replaced; this distributes the heat through a deeper layer and prevents a large temperature difference from being established close to the ground.

On clear nights the earth's surface cools by radiating heat, and air in contact with it becomes cooler. Being then denser it becomes difficult for any wind to mix it with warmer air overhead. On long clear cold nights a strong temperature inversion forms, and conditions are ideal for the formation of frost.

(left) Vertical temperature profiles just above the ground on a cold night. A strong temperature inversion develops on nights with little or no wind.