4.1 Heating, pressure and wind
The atmosphere is heated mostly not directly by the Sun but by the warmed ground. That is why, in the troposphere, temperature falls on average by about 0.65 °C for every 100 m of height. Warmed air expands and rises, leaving low pressure at the surface; cooled air sinks, creating high pressure. Wind blows from high to low pressure, and the bigger the pressure difference, the stronger the wind.
Earth's rotation deflects moving air to the right in the Northern Hemisphere and to the left in the Southern (the Coriolis effect). Air warmed at the Equator rises and sinks again around 30° latitude (the Hadley cell), where the subtropical high-pressure belt and the great deserts lie. Between the dynamic low around 60° and the polar highs lie the Ferrel and polar cells. This circulation drives the trade winds, westerlies and polar easterlies.
FormulaAverage lapse rate ≈ 0.65 °C / 100 m · Dry adiabatic ≈ 1 °C / 100 m · Saturated adiabatic ≈ 0.5 °C / 100 m
4.2 Humidity, cloud and rain
Absolute humidity is the amount of water vapour in the air (g/m³). Maximum humidity is the most vapour air can hold at that temperature, and it rises with temperature. Relative humidity is absolute humidity divided by maximum humidity. If air cools, its maximum humidity falls and its relative humidity rises; the temperature at which it reaches 100% is the dew point. Beyond that, condensation begins and clouds or fog form.
Rising air expands, cools and drops rain. In relief (orographic) rain, moist air is forced up a mountain slope: in the eastern Black Sea region, moist northerly winds hit the Kaçkar Mountains, so Rize gets about 2,200 mm of rain a year. Convectional rain comes from warmed air rising (afternoon showers in Central Anatolia in spring); frontal rain forms where warm and cold air masses meet.
FormulaRelative humidity (%) = absolute humidity ÷ maximum humidity × 100
4.3 Föhn winds and Türkiye's climates
As moist air climbs a mountain it first cools at the dry adiabatic rate (about 1 °C per 100 m). Once it reaches its dew point and condenses, the latent heat released slows the cooling to the saturated rate (about 0.5 °C), and it drops rain. Descending the far side, now dry, it warms at the dry rate. So a warmer, drier wind arrives behind the mountain at the same height: the föhn. That is why the sheltered slopes of mountains stay dry (the rain shadow).
Türkiye has three main climates. In the Mediterranean climate, summers are hot and dry and winters mild and wet; the natural vegetation is maquis. The Black Sea climate is wet in every season and supports dense forests. In the continental climate, the gap between summer and winter temperatures is large and rainfall is low; because Central Anatolia is ringed by mountains, the sea's influence is blocked and steppe forms.
Key terms
- Coriolis effect
- Earth's rotation deflecting moving air and water.
- Dew point
- The temperature at which air becomes saturated and condensation begins.
- Föhn
- A warm, dry wind descending the far side of a mountain.
- Rain shadow
- The dry, sheltered side of a mountain.
End-of-unit problems
Solve each one yourself first, then open the solution.
When it is 25 °C on the coast, roughly what is the temperature on a plateau at 2,000 m, using the average lapse rate?
Show the solution
2,000 m ÷ 100 = 20 steps; 20 × 0.65 = 13 °C cooler. 25 − 13 = 12 °C. This is why people move up to highland pastures in summer.
Moist air at 20 °C climbs a mountain from sea level. It reaches its dew point at 1,000 m, and the summit is at 3,000 m. What is its temperature when it descends to sea level on the far side? (dry 1 °C/100 m, saturated 0.5 °C/100 m)
Show the solution
0–1,000 m: dry rate, 10 °C cooler → 10 °C. 1,000–3,000 m: saturated rate, 2,000 m × 0.5/100 = 10 °C cooler → 0 °C at the summit. Descent of 3,000 m at the dry rate: 30 °C warmer → 30 °C. The air arrives 10 °C warmer than it started at the same height, and much drier: the föhn effect.
In a room, absolute humidity is 8 g/m³ and the maximum at that temperature is 20 g/m³. What is the relative humidity? What happens if the room is cooled until the maximum falls to 8 g/m³?
Show the solution
Relative humidity = 8 ÷ 20 × 100 = 40%. When the maximum falls to 8 g/m³, relative humidity is 100%: the air has reached its dew point. Any more cooling causes condensation — droplets appear on cold windows.
Why are the world's great deserts (the Sahara, Arabia, Australia) clustered around 20°–30° latitude? Explain with the general circulation of the atmosphere.
Show the solution
Air rising at the Equator drops its moisture as rain, then flows poleward high up, now dry, and sinks around 30° latitude (the Hadley cell). Sinking air warms, its relative humidity drops and clouds can't form, creating the subtropical high-pressure belt. Constant high pressure and dry air make the deserts.