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LAPSE RATES AND
STABILITY
We investigate the relationship between pressure and
temperature in further detail.
As air rises in the atmosphere, the pressure falls, the air expands and the temperature of the air falls. If a dry ‘parcel’ of air is forced to rise through the atmosphere – maybe as it travels up a hillside
- then it cools at the ‘dry adiabatic lapse rate’: 9.8°C/1000m. ‘Dry adiabatic lapse rate’ is a scientific term which sounds complicated but is really quite simple. ‘Lapse rate’ refers to the rate of change of a quantity–in this case the temperature of the air parcel–with height. ‘Dry’ means that the parcel is not saturated with water vapour. ‘Adiabatic’ is a term that scientists use meaning that no heat either enters or leaves the system they are describing - in this case the rising air parcel. So any changes in temperature are due purely to changes in pressure as the parcel rises. The sloping line (Predicted Lapse rate) in Figure 1 shows the air cooling with height at 9.8°C for every 1000m it rises. But what if the air isn’t dry? What if it has moisture in it? At some height, the air will reach the temperature at which condensation exceeds evaporation (the dew point) and cloud droplets form, releasing latent heat. This released heat will make the air warmer than it would otherwise have been at that height.
Figure 1: A diagram showing how the temperature of a rising parcel of air would cool with height, if no cloud droplets were condensing (green line). In Figure 2, the red, wavy line shows the actual temperature of the air, which is warmer than it would otherwise have been above the cloud base (horizontal black line); the level at which cloud starts forming. The air now cools at the ‘saturated adiabatic lapse rate’ also known as the ‘moist adiabatic lapse rate’. This varies with pressure and temperature, and although always lower than the dry adiabatic lapse rate, it can be very similar or as low as 3°C/1000m. Figure 2: A diagram showing how the temperature of a rising parcel of air would cool with height, if no water vapour were condensing (green line) and, for comparison, how it would cool if cloud droplets did start condensing above a certain height (red line).
This makes the stratosphere extremely stable, as any air rising from the troposphere below and cooling as it rises, will always be cooler than the surrounding air, and fall back to where it came from. It’s virtually impossible for air to move from the troposphere to the stratosphere; usually this only happens in the Tropics, where air ascending very quickly, in a cumulonimbus cloud, can literally punch its way through. This is why deep thunderclouds can have a spreading, ‘anvil’ shaped top (Figure 4). The rising air in the unstable troposphere reaches the top of the troposphere (the tropopause) and, because it can’t cross into the stratosphere above, spreads out sideways. Figure 4: A thundercloud, or cumulonimbus, showing how the convection or rising air in the troposphere cannot continue up in to the stratosphere, so the cloud spreads out sideways. © RMetS Fog is another example of stability in the atmosphere (Figure 5). On clear, winter nights, when there are anticyclonic or high pressure conditions, the ground loses heat to space. In turn, it cools the air directly above which can end up colder than air higher in the atmosphere. Such a temperature ‘inversion’, with colder air below warmer air rather than the more normal cooling of air with height, is very stable and the air remains trapped near the ground, unless there is some wind to blow it away. Temperatures can fall low enough for cloud droplets to form, giving fog or low cloud.
Figure 5: Low cloud or fog showing an inversion in the English Lake District © Sophie Richards You can read more about lapse rates and stability in the atmosphere on the Metlink website
