Climate is the long-term pattern of weather typical of a place. Weather changes over hours and days, while climate stays fairly stable over decades; it is usually defined from 30 years of observations. Climate shapes natural zones, soils, river regimes, farming and people’s lives. Exams ask about climate-forming factors, air masses, climate belts and where they lie on the continents, and how to read a climate graph.
§1. Climate-forming factors
Latitude sets the angle of the Sun’s rays and the length of day. At the equator the rays are steep and heat is abundant; towards the poles the angle decreases and temperature falls. This is the main factor.
Distance from the ocean — water warms and cools slowly, so coasts have a mild, moist (maritime) climate, while continental interiors have hot summers, cold winters and dry air (continental climate).
Relief — mountains block air masses: windward slopes get much precipitation, leeward slopes little; temperature falls with height.
Ocean currents — a warm current warms and moistens the coast (the Gulf Stream gives north-western Europe mild winters); a cold current chills the air from below and reduces rainfall (the coastal Atacama and Namib deserts).
Air masses and atmospheric circulation — trade winds, westerlies and monsoons carry moisture and heat.
The nature of the surface — ice, snow, forest and desert reflect sunlight differently (albedo).
Human activity — greenhouse-gas emissions, deforestation and large reservoirs affect local and global climate.
The factors always act together. London and Irkutsk, for example, lie at roughly the same latitude, yet London’s winter is mild and Irkutsk’s severe. The reason is that London is near the ocean, under the westerlies and a warm current, while Irkutsk is deep inside the continent. Latitude is the main factor, but the others change climate a great deal.
§2. Air masses and fronts
An air mass is a large body of air with uniform properties (temperature, humidity, clarity). It takes on the character of the area where it forms. The main types are equatorial, tropical, temperate (polar) and arctic (antarctic); each can be maritime (moist) or continental (dry). The boundary between two different air masses is an atmospheric front. At a front warm air rises over cold air, cools and forms clouds and precipitation, and the weather changes sharply. Fronts and cyclones pass often in temperate latitudes, so the weather there is changeable.
§3. Climate belts (B. P. Alisov’s classification)
Main belts: equatorial, two tropical, two temperate, arctic and antarctic
Transitional belts: two subequatorial, two subtropical, subarctic and subantarctic
In the main belts one air mass dominates all year; in the transitional belts air masses change with the seasons (in the subequatorial belt, for example, equatorial air in summer and tropical air in winter). The prefix "sub-" means "under, next to".
Equatorial — hot all year (about +25 to +28 °C), humid, with heavy, evenly spread rain.
Subequatorial — wet summer and dry winter; savannas and monsoon forests.
Tropical — dry and hot, with very hot summers; the Sahara, central Australia. On western coasts cold currents create coastal deserts; on eastern coasts the trade winds bring moist air.
Subtropical — the Mediterranean type has hot dry summers and mild wet winters; the monsoon type lies on eastern coasts and the continental type inland.
Temperate — four clear seasons; maritime (Western Europe), continental, extreme continental (Siberia) and monsoon (the Russian Far East) types.
Subarctic and arctic — cold, little precipitation, long winters.
Uzbekistan lies mainly in the southern part of the temperate belt, with a dry continental climate; its southern areas have subtropical features.
Worked example
In which climate belt does most of Australia lie?
The Tropic of Capricorn (about 23.5° S) crosses the middle of the continent.
The interior and the west are under subtropical high pressure; air sinks, so rain is scarce.
The north is subequatorial and the south subtropical, but these cover a smaller part.
Answer: the tropical belt (mostly a tropical desert climate).
§4. Moisture: precipitation and potential evaporation
You cannot judge how moist a place is from rainfall alone: in a hot place water evaporates quickly. So precipitation is compared with potential evaporation — the amount of water that could evaporate under the given conditions.
Moisture coefficient K = annual precipitation / annual potential evaporation
K > 1 — surplus moisture (taiga, tundra, equatorial forest; many swamps);
K ≈ 1 — adequate moisture (mixed forest, forest-steppe);
K < 1 — deficient moisture (steppe, semi-desert, desert).
Worked example
A lowland city gets about 400 mm of precipitation a year, and potential evaporation is about 1,200 mm. How moist is it?
Formula: K = precipitation / evaporation.
K = 400 / 1,200 ≈ 0.33.
K is well below 1, so moisture is deficient.
Answer: K ≈ 0.33 — a dry climate; farming needs irrigation.
§5. Reading a climate graph
A climate graph shows mean monthly temperature (a line) and precipitation (bars). Analyse it in this order:
If the warmest month is July, it is the Northern Hemisphere; if January, the Southern.
Work out the annual range: large (over 30 °C) means continental; small means maritime or equatorial.
Heavy rain all year and about +25 °C means equatorial; summer rain means monsoon or subequatorial; winter rain means Mediterranean subtropical.
Very little precipitation (under 200 mm a year) means a desert climate.
Worked example
A graph shows July as the warmest month (+28 °C), January as the coldest (−4 °C), little precipitation, mostly in spring and winter. Identify the climate.
The warmest month is July — Northern Hemisphere.
Range: 28 − (−4) = 32 °C — continental.
Little rain and a dry summer — the interior of a continent.
Answer: temperate belt, dry continental climate (for example, the plains of Central Asia).
§6. Climate records and climate change
Lowest temperature: Vostok Station in Antarctica (−89.2 °C).
The cold pole of the Northern Hemisphere: Oymyakon and Verkhoyansk in Siberia.
Highest rainfall: the Cherrapunji area in India (monsoon plus a windward slope).
Driest place: the Atacama Desert.
Today’s global warming is a rise in the Earth’s mean temperature of somewhat more than 1 °C compared with pre-industrial times. The main cause is the build-up of greenhouse gases (CO₂, methane) from burning fuel. Consequences include melting glaciers, rising sea level and more droughts and heatwaves. In Central Asia shrinking mountain glaciers threaten river flow. International co-operation (the 2015 Paris Agreement) aims to cut emissions.
Check yourself
Is the subtropical belt a main or a transitional belt?
In which belt are hard-leaved evergreen forests found?
What caused the Atacama Desert?
The warmest month is January. Which hemisphere?
How does climate differ from weather?
Precipitation 600 mm, potential evaporation 300 mm. Moisture coefficient?
July +24 °C, January −16 °C. Annual range?
Where was the lowest temperature on Earth recorded?
Common mistakes
Treating the subtropical and subequatorial belts as main belts — they are transitional.
Thinking the leeward slope gets heavy rain — it gets little.
Treating climate and weather as the same thing.
Dropping the minus sign when working out the range: 28 − (−4) = 32, not 24.
Judging moisture by rainfall alone — evaporation must be taken into account.