Elements of weather

Weather, climate and their elements

Weather is the state of the atmosphere at a place over a short time (hours or days). Climate is the average weather of a region over a long period, usually about 30 years. The main elements of weather are rainfall, temperature, humidity, wind and sunshine (solar radiation).

• Rainfall: measured in millimetres with a rain gauge. Its amount and distribution decide which crops can be grown without irrigation.
• Temperature: measured with thermometers; a maximum–minimum thermometer records the day's highest and lowest values. Every crop has three cardinal temperatures: minimum, optimum and maximum. Growth is fastest near the optimum.
• Humidity: the amount of water vapour in air. Relative humidity (RH) = (actual vapour pressure ÷ saturation vapour pressure) × 100. It is measured with a hygrometer or a psychrometer (wet- and dry-bulb thermometers). High RH favours many fungal diseases.
• Wind: speed is measured with a cup anemometer; direction with a wind vane. Strong winds cause lodging, shedding of flowers and soil erosion; gentle winds help pollination and CO₂ supply.
• Sunshine: hours of bright sunshine are recorded by the Campbell–Stokes sunshine recorder; solar radiation by a pyranometer. Light drives photosynthesis; day length (photoperiod) controls flowering.
• Evaporation: measured with an open pan evaporimeter; used for irrigation scheduling.

Key points

• Weather = short-term; climate = long-term average (about 30 years).
• Rain gauge → rainfall; anemometer → wind speed; wind vane → direction; hygrometer/psychrometer → humidity.
• Campbell–Stokes recorder → sunshine hours; pyranometer → solar radiation.
• RH = (actual ÷ saturation vapour pressure) × 100.
• Cardinal temperatures: minimum, optimum, maximum.

Exam tips

Most questions are instrument–element matches or ask about the effect of an element on crops (e.g. high humidity → disease). Read carefully whether the question asks for wind speed or wind direction.

Common mistakes

• Confusing weather (short-term) with climate (long-term).
• Mixing up the anemometer (speed) and the wind vane (direction).
• Thinking a pyranometer counts sunshine hours; it measures radiation energy.