Weather and climate
Weather is the state of the air at one place and time: hot or cold, wet or dry, windy or calm. It can change in an hour.
Climate is the usual pattern of weather in a place, worked out from about 30 years of records. It tells you what to expect, not what will happen today.
The elements of weather and climate are temperature, rainfall (precipitation), air pressure, wind and humidity. A climate graph shows average monthly temperature as a line and rainfall as bars.
Factors that control climate
Latitude
Near the equator the Sun is high, so its light is concentrated on a small area. Towards the poles it is low, so the same light spreads over a larger area and passes through more air. That is why temperature falls as latitude rises.
Altitude
Air gets thinner and cooler as you go up: about 6.5 °C per 1000 m. Quito in Ecuador is almost on the equator but is mild because it is 2.8 km high.
Distance from the sea
Water heats and cools slowly. Coastal places have a small yearly range (maritime climate). Places far inland have hot summers and cold winters (continental climate).
Winds, pressure belts and ocean currents
Winds that blow from the sea bring moisture. Warm currents (like the North Atlantic Drift) warm nearby coasts; cold currents make coasts cool and dry, which helps form deserts such as the Atacama and Namib.
Relief
Mountains force moist air to rise and rain on the windward side; the other side, the rain shadow, stays dry.
The main climate zones (Köppen groups)
In 1900 Wladimir Köppen sorted climates by temperature and rainfall, matched to the plants that grow there. A simple version has five groups:
- A Tropical (about 0°–15°): hot all year (every month above 18 °C). Rainforest, monsoon and savanna types. Life: rainforests, rice and plantation crops.
- B Dry (about 15°–35° and inland): evaporation is more than rainfall. Hot deserts (Sahara, Thar) and semi-arid steppes. Life: oases, herding, drip irrigation.
- C Temperate (about 35°–50°): warm summers, mild winters. Mediterranean (dry summer), humid subtropical and marine west-coast types. Life: wheat, olives, grapes, mixed farming.
- D Continental (about 50°–66°, mainly northern lands): warm summers, long freezing winters. Coniferous forest (taiga). Life: timber, short growing season.
- E Polar (above about 66°): every month below 10 °C. Tundra and ice caps. Life: very few people, reindeer herding, fishing.
- H Highland: on high mountains the climate changes with height, like a climb from tropics to poles in a few km.
The latitudes are rough guides; coasts, mountains and currents bend the bands. Natural vegetation zones (rainforest, savanna, desert, steppe, forest, taiga, tundra) follow the same pattern, and they shift when climate changes.
Monsoon climates and life
A monsoon is a wind that changes direction with the season. In summer land heats faster than sea, low pressure forms over land, and moist sea winds blow in and bring heavy rain. In winter land is colder, so dry winds blow from land to sea.
South Asia, South-East Asia and East Asia (for example India, Bangladesh, Korea and Japan) depend on monsoon rain for rice farming. Korea has hot, wet monsoon summers and cold, dry winters with winds from the continent. A late or weak monsoon can mean drought; a very strong one can mean floods.
Climate and travel; Try it at home
Travellers check climate to choose when to go: a tropical beach in its dry season, a Mediterranean city in spring, a mountain town in summer.
Try it: pick two cities: one coastal (for example Chennai or Lisbon) and one inland at a similar latitude (Nagpur or Madrid). Look up their average monthly temperatures and draw both as lines on one graph. Which line has the bigger gap between summer and winter? That is the continental effect.
Key formulas and definitions
- Temperature drop with height ≈ 6.5 °C per 1000 m
- Annual range = warmest month average − coldest month average
- Climate = average weather over ~30 years
- Köppen groups: A tropical, B dry, C temperate, D continental, E polar, H highland
Worked examples
1. A town at sea level has an average temperature of 26 °C. Estimate the temperature at a hill station 2000 m above it.
Drop = 6.5 × 2000 ÷ 1000 = 13 °C. Temperature ≈ 26 − 13 = 13 °C.
2. City X: warmest month 30 °C, coldest 24 °C. City Y: warmest 32 °C, coldest −4 °C. Which is coastal and which is inland?
Range X = 6 °C (small) → coastal/maritime. Range Y = 36 °C (large) → far inland/continental.
3. A place has every month below 10 °C, low trees cannot grow and the ground is frozen below the surface. Name its Köppen group.
E Polar (tundra type).
Common mistakes
- Saying weather and climate are the same: climate is a long-term average, weather is today.
- Thinking every place near the equator is hot: high mountains there can have snow.
- Thinking deserts are only hot: B means dry, and cold deserts also exist (Gobi, Ladakh).
- Thinking the Sun is closer to the equator: the equator is warmer because the light falls more directly, not because of distance.