Cloud Watching

Whatever you fly it is vital to understand and respect the weather. This knowledge takes away fear and apprehension and makes for a more confident pilot. To this end Tom is going right back to basics, concentrating on one aspect in the following months, starting with clouds. The photos of cumulus building up during the morning were taken by Tom and his drawings enhanced by Steve Longland. Watching the development and movement of clouds can tell you quite a lot about the weather, as this short account will show. Some Basics. The air can only hold a limited amount of water; the warmer the air the more water it can hold in the form of vapour. When the air is cooled the excess water vapour is condensed out. Condensation starts when the air is cooled below its dew point. This is the temperature at which dew forms on a cold surface. Most airfield Met reports give both air temperature and dew point at the end of the message. The cooling is generally due to the air being lifted but it can also occur when the air is in contact with cold ground or sea. In the latter case the cloud forms a layer of fog on or very close to the surface. Cooling by Lifting. Pressure falls with height so when air is lifted it expands. Expansion takes energy and the process cools the air. If the air is dry it cools at the rate of 3°C /1000ft (9.8°C/km). This is known as the Dry Adiabatic Lapse Rate (DALR). This is constant. Adiabatic means that the process takes place without exchanging heat with the environment outside. Lapse Rate is the term for the decrease of temperature with height. Latent Heat and the Saturated Adiabatic Lapse Rate. Latent heat is the heat needed to change liquid water into vapour without raising its temperature. This heat is returned when the vapour condenses to form cloud. As a result cloud filled air cools less than dry air when it rises. The new lapse rate is called the Saturated Adiabatic Lapse Rate (SALR). The SALR varies with temperature. In hot tropical air the SALR can be as little as 1°C/1000 ft but in arctic air and at high altitudes it approaches the DALR of 3°C/1000ft.     Condensation Level and Cloudbase (See Fig 1). When air is lifted it cools along a DALR line until it reaches the dew point line. The condensation level is where the two lines meet. Above this the air is saturated so the lapse rate changes from