The change of temperature with height is called the lapse rate. A positive lapse rate represents the normal condition with cooler air above, the temperature falling with increased height. The opposite, a negative lapse rate (ie the temperature increases with height) is called an inversion (Chapter 7). Temperature profiles show the actual lapse rate (or environmental lapse rate) at each level, i.e. the tangent to the profile at that level.
Typical profiles in Figure 1.9 show positive lapse rates at most places in the lowest 10 km or so, and we will see in Chapter 7 that positive lapse rates promote convection and turbulence. This churning has led to the layer being called the troposphere, from the Greek word 'tropos', meaning to turn or change. The layer contains about 80 per cent of the mass of air and almost all the clouds. It is deeper in the tropics (about 17 km) than in temperate climates, where it is higher in summer (about 12 km) than winter (10 km), though there are also significant changes from day to day, e.g. from 8 km to 14 km. Convection is rare at the poles, where the surface is so cold and consequently the troposphere is relatively shallow at high latitudes, especially during the polar winter.
A surprising consequence of the temperature patterns in Figure 1.9 is that the average temperature of all the air above the pole is higher, especially in summer, than that of the column above the equator, where the coldest air occurs, at about 15 km elevation. Therefore, the relative lightness of warm air leads to lower sea-level pressures at high latitudes, as seen in Figure 1.8.
Plate 1.1 Releasing a balloon carrying a radiosonde to measure temperature and humidity conditions in the troposphere and lower stratosphere. The radiosonde equipment is in a box in the meteorologist's left hand. The box is attached by a cord to the pyramidal radar reflector which hangs beneath the balloon. The reflector enables location of the balloon's position by means of the radar equipment seen behind, and the change of position shows the sideways displacement (i.e. the wind) at each height. Notice that the meteorologist is wearing protective clothing in case the hydrogen in the balloon ignites, which is possible in dry conditions.
Measurements at top and bottom of a kilometre-thick layer of the troposphere might show values of 10°C at the top and 16.5°C at the bottom, for instance. Such a positive lapse rate is written as 6.5 K/km, or 6.5 milliKelvins per metre (written as 6.5 mK/m), in Système Internationale units (Note 1.J). Note that temperature differences or changes are preferably described in degrees Kelvin (Note 1.K). Thus, the difference between 5°C and 10°C
is written as 5 K, and is equal to the temperature difference between 41°F and 50°F, whereas a temperature of 5°C does not equal 9°F.
A standard atmosphere is a nominal relationship between altitude and mean temperatures. For instance, that adopted by the International Civil Aviation Organisation (ICAO) is based on measurements over the USA but applied widely in calibrating aneroid barometers for measuring the altitude of light aircraft. (Larger planes use radar to measure their height above the ground.) The ICAO atmosphere has the following features: a mean pressure of 1,013.25 hPa and an air temperature of 15°C at sea-level, a lapse rate of 6.5 mK/m to 11 km, then isothermal conditions at—56.5°C up to 20 km, followed by a negative lapse rate of 1 mK/m to about 32 km (Chapter 7).
So far we have been discussing only lapse rates in the free air. The change of temperature with elevation on the side of a mountain may differ (Chapter 3) on account of the local effect of heat from the ground, which is warmed by the daytime sun and cooled by the radiation of heat to the night sky (Chapter 2).
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