Temperature: Heating, Cooling, Lapse Rates and Inversions
This domain tests how the air is heated and cooled, the three lapse rates, the shape of the diurnal temperature curve, and the recognition and consequences of inversions. Expect questions that give a surface temperature and require a temperature at height, or that describe a night and ask what forms.
The four heat-transfer processes
- Radiation: incoming shortwave solar radiation and outgoing longwave terrestrial radiation. The atmosphere is largely transparent to shortwave but absorbs longwave, mainly through water vapour - the greenhouse effect.
- Conduction: significant only in the lowest few centimetres of air.
- Convection: sensible heat carried upward as thermals; the reason the surface layer can become superadiabatic on a sunny afternoon.
- Latent heat: absorbed on evaporation, released on condensation - the reason saturated rising air cools more slowly.
Radiation balance
- Albedo controls how much solar energy is reflected: fresh snow is very high, dark land low, clouds reflect strongly but also trap longwave.
- The tropics have a net radiation surplus and the poles a deficit; the resulting transport drives the general circulation.
- On clear, dry, calm nights radiative loss is strong, giving frost, radiation fog and a pronounced ground inversion.
The three lapse rates
- DALR: 3°C per 1,000 ft (9.8°C/km), for unsaturated rising or sinking air.
- SALR: about 1.8°C per 1,000 ft (0.6°C/100 m), but varies roughly 0.5-0.9°C/100 m with temperature and moisture.
- ELR: the actual measured environmental rate, typically 0.6-0.8°C/100 m in the lower troposphere.
Diurnal variation
- Maximum near 14:00-15:00 local, minimum at or just after sunrise.
- Range is large over land, dry soil, clear skies, light wind and low latitude; almost nil over the sea; it shrinks rapidly with height and is small above about 5,000 ft.
Inversions
- Radiation (nocturnal, clear, calm, long night, especially in valleys), subsidence (sinking air in an anticyclone, warming adiabatically), advection (warm air over a cold surface), frontal, and katabatic.
- Consequences: exceptional low-level visibility in haze and smoke, mist or fog, smooth air, nil icing in the inversion, and marked wind shear at the top of the layer.
Worked example
Surface at sea level 24°C; a summit at 8,000 ft reports 12°C. ELR = (24 − 12)/8 = 1.5°C/1,000 ft. This is less than the SALR of 1.8, so the layer is absolutely stable: expect stratiform cloud, steady precipitation and smooth air rather than showers.
Traps that cost marks
- Treating the SALR as a fixed constant.
- Forgetting that in an inversion temperature rises with height.
- Mixing up advection and radiation inversions when a question mentions warm air moving over a cold sea.