Introduction
Condensation
Stability and instability
Summary

Anyone who has been up a mountain will have noticed that the higher you go, the cooler it becomes. This fall of temperature is known as the Lapse Rate and, in this case, is a characteristic of the atmosphere around. If, on the other hand, we consider a 'parcel' of air rising through the atmosphere, the situation is rather different. An analogy to the 'parcel' of air is a bubble of gas rising through a fizzy drink. It stays more or less unchanged all the way up.

When this parcel of air rises, it cools. The reason for this is that pressure falls with increasing height and, consequently, the air expands. As air expands, it does 'work' in order to occupy the larger volume. This is a rather difficult concept to understand, so it might be easier to think of its opposite; that is, air becoming compressed. In this case, 'work' is being done on the air to force it into a smaller volume and this causes the temperature to rise.

All air contains some moisture in the form of water vapour, but there is a maximum amount of water vapour that a 'parcel' of air can contain. The amount of water vapour contained within air is related to that maximum by a measure called its Relative Humidity (RH). This is a percentage ranging from 0% to 100%, although the relative humidity of air in the atmosphere is always above 0%. When the RH reaches 100%, the air is said to be saturated.

Warm air can contain more water vapour than cold air. This means that the same amount of water vapour in cold air will have a higher RH than in warm air. If air is cooled, it eventually reaches a temperature where its RH is 100%. Below this temperature, the amount of water vapour has become greater than the air can support, so some is deposited out through condensation or dew. Hence this temperature is known as the Dew Point.

When a 'parcel' of dry air rises, it cools at 9.8 °C per 1000 metres. This is known as the Dry Adiabatic Lapse Rate (DALR). However, as it cools, its RH rises and eventually, it may reach 100% and condensation begins, leading to clouds being formed. The level at which clouds form is called the Condensation Level. The process of condensation causes heat to be released (latent heat of condensation), which means that, as it rises, the air parcel cools at a slower rate, called the Saturated Adiabatic Lapse Rate (SALR).

Fig 2: showing the fall of temperature of a rising parcel of air

Air can contain very little water vapour at very high levels in the atmosphere and the rate of cooling of the air parcel returns to approximately the DALR. The DALR is constant with height, whereas from this, it can be seen that the SALR changes from a rate of around 4 °C per 1000 metres at low levels in the atmosphere to near the DALR at high levels.

In the previous section, we were concentrating on a parcel of air rising through the atmosphere. In this section, we shall be looking more at the atmosphere. The air over the Arctic is very different in character from air over the Tropics; it is much colder. Air over a desert is different from air over the sea; it is much drier. In between these extremes, most noticeably in middle latitudes, the air over a particular spot may have come from different places; it is a mixture. Consequently, although the temperature almost always falls with height, the rate of fall varies, depending on the characteristics of the atmosphere. This is known as the Environmental Lapse Rate (ELR) and, on the average, it is about 6.5 °C per 1000 metres, though it varies a lot.

If we go back to our idea of a parcel of air, it has a particular temperature near the surface. This temperature will differ from the immediate surroundings. If the parcel's temperature is higher than the surroundings, then it will be buoyant and will start to rise. Even though it cools, its temperature may remain higher than the surrounding atmosphere, in which case, it will continue to rise. The atmosphere is then said to be unstable, figure 3 shows an example of a cumulonimbus cloud which forms in an unstable atmosphere . If its temperature falls below the surroundings, the parcel loses buoyancy and gradually comes to a stop. The atmosphere has now become stable, figure 4 shows an example of stratocumulus cloud typically formed in a stable atmosphere.

Fig 3: Cumulonimbus formed in an unstable atmosphere © N. Elkins		Fig 4: Stratocumulus formed in a stable atmosphere © J. Galvin

Take as an example a fine day in spring or early summer. The air may be quite cold, but the sun is warming the land. This causes the air immediately above to become warmer and 'bubbles' of this warm air begin to rise. This process is called convection. This can be seen when water vapour rises from a hot wet roof. Above this lowest surface layer, the atmosphere is much colder, so the 'bubbles' become more buoyant and continue to rise. This lowest part of the atmosphere, where the fall in temperature is greater than the DALR, is called absolutely unstable.

Fig 5: Absolute instability - e.g. sunny day in summer

Above a short distance (around 100 m over the British Isles on a summer's day, but up to 1000 m over a hot desert), the ELR becomes equal to the DALR and the bubbles continue to rise. If the ELR becomes less than the DALR, the bubbles will rise more slowly and will eventually stop if they are too dry to form clouds. This means that the atmosphere is stable and the day will stay fine. However, if clouds are formed, the bubbles of air now cool at the SALR. The ELR may well now be greater than the SALR and convection will continue. The atmosphere is now said to be conditionally unstable. Once the ELR becomes less than the SALR, convection stops altogether and the atmosphere is now absolutely stable.

Fig 6: Instability in dry air - below base of any cloud

Fig 7: Stability in dry air - if this layer deep enough, no clouds form

Fig 8: Conditional instability - clouds can range from small cumulus to deep cumulonimbus (thunderclouds)

Fig 9: Absolute stability above top of cloud - convection stops

An extreme example of stability is the inversion. This occurs when there is no fall or even a small rise of temperature with height. Two processes can lead to the formation of inversions. One is cooling of the surface on a clear night, in which case the air in contact becomes colder than the air above. The other is subsidence of air in an anticyclone. As the air sinks, it warms at the DALR and this can lead to an inversion forming between the subsiding air and the air immediately below.

Fig 10: Cloud will develop when the condensation level is below the inversion

Fig 11: Inversion just above the surface - caused by night-time cooling

A special case of an inversion is the stratosphere. In the lower atmosphere (known as the troposphere), the temperature falls with height until it reaches the tropopause, which marks the boundary with the stratosphere. The temperature then remains more or less constant before a steady rise begins.

Fig 12: Stratospheric inversion

Fig 13: Examples of lapse rate and stability through the atmosphere