Weather, climate and ecosystems
Measuring weather, the climates of tropical rainforests and hot deserts, and how their vegetation and ecosystems are adapted and threatened.
📌 Key points
- Weather is the day-to-day state of the atmosphere; climate is the average weather over a long period, usually at least 30 years.
- A Stevenson screen is a white, louvred box raised above the ground: white reflects sunlight, louvres let air circulate, and it holds the maximum-minimum and wet and dry bulb thermometers.
- Instruments: rain gauge (rainfall in mm), anemometer (wind speed), wind vane (wind direction, named after where the wind blows from), barometer (air pressure in millibars or hPa), hygrometer / wet and dry bulb (humidity) and sunshine recorder.
- Relative humidity is the amount of water vapour in the air as a percentage of the maximum it could hold at that temperature; a bigger difference between dry and wet bulb readings means lower humidity.
- Cloud cover is measured in oktas (eighths of the sky covered); common types are cirrus (high, wispy), cumulus (heaped) and stratus (layers).
- Tropical rainforests lie near the Equator, with high temperatures all year (about 26–28 °C), a small annual temperature range and over 2000 mm of rainfall, much of it convectional.
- Rainforest vegetation has layers (emergents, canopy, under-canopy, shrub layer) and adaptations such as buttress roots, drip tips, lianas and epiphytes; soils are often infertile because nutrients are leached or quickly taken up by plants.
- Hot deserts receive less than 250 mm of rain a year and have a large daily (diurnal) temperature range because clear skies let heat escape at night.
- Deserts are dry because of sinking air in high-pressure belts near the tropics, cold ocean currents, rain shadows and distance from the sea; plants such as cacti store water and have long or wide roots and spines.
- Deforestation for farming, logging, mining and roads causes loss of biodiversity, soil erosion and more carbon dioxide in the atmosphere; sustainable management includes selective logging, replanting and protected areas.
📖 Key terms
- Weather
- The day-to-day condition of the atmosphere at a place.
- Climate
- The average weather conditions of a place over a long period.
- Relative humidity
- The amount of water vapour in the air as a percentage of the maximum it can hold at that temperature.
- Convectional rainfall
- Rain formed when the ground heats the air, which rises, cools and condenses.
- Diurnal range
- The difference between the highest and lowest temperatures in one day.
- Okta
- A unit for cloud cover: one eighth of the sky.
- Biodiversity
- The variety of plant and animal species in an area.
- Deforestation
- The cutting down and removal of forest.
✏️ Worked example
A desert town recorded a maximum temperature of 41 °C and a minimum of 14 °C in one day. Calculate the diurnal temperature range and the mean daily temperature, and explain why the range is large.
- Diurnal range = maximum − minimum = 41 − 14 = 27 °C.
- Mean daily temperature = (maximum + minimum) ÷ 2 = (41 + 14) ÷ 2 = 55 ÷ 2 = 27.5 °C.
- By day, clear skies let strong sunshine heat the ground.
- At night, with no cloud cover to trap heat, the ground loses heat quickly.
- Answer: range 27 °C; mean 27.5 °C; the range is large because there are few clouds.
When describing a climate graph, quote the maximum and minimum temperature and the months, the annual range, and the wettest and driest months with figures and units (°C, mm).
Test yourself8 questions from a bank of 15. A new mix every time.
Start the quiz →