Weather
Map Your Gaps
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Weather vs climate
Weather and climate describe atmospheric conditions over different timescales.
"Climate is what you expect; weather is what you get."
Weather vs climate — Key Knowledge
- Weather short-term, day-to-day conditions — temperature, precipitation, wind, cloud cover
- Climate average weather conditions measured over 30+ years for a particular area
Stevenson Screen
A white wooden box on legs used to house weather instruments in controlled conditions.
Without a Stevenson Screen, temperature readings would be unreliable — direct sunlight or ground heat would distort them.
Stevenson Screen — Key Knowledge
- White reflects sunlight — prevents heating
- Louvred sides allow air to circulate freely
- Raised off the ground avoids ground-level heat and moisture
- Door faces north in Northern Hemisphere — avoids direct sunlight when opened
- Standardised design so readings from different locations can be compared fairly
Thermometers
Instruments used to measure temperature.
All temperature instruments are kept inside the Stevenson Screen to ensure shade and ventilation.
Thermometers — Key Knowledge
- Maximum-minimum thermometer records the highest and lowest temperatures over a set period
- Wet-and-dry bulb thermometer two thermometers side by side — one has a wet muslin wick; used to calculate humidity
Humidity measurement
The wet-and-dry bulb thermometer (hygrometer) measures humidity.
The bigger the gap between wet and dry readings, the drier the air.
Humidity measurement — Key Knowledge
- Dry bulb reads actual air temperature
- Wet bulb always reads lower — evaporation from the wick cools it
- Large difference between readings air is dry — lots of evaporation
- Small difference air is humid — little evaporation
- Same reading air is saturated — 100% relative humidity
Rain gauge
Measures the amount of precipitation over a period of time.
Rainfall is measured as depth — 1 mm of rain means 1 litre of water per square metre.
Rain gauge — Key Knowledge
- Funnel collects rain into a measuring cylinder, Sunk into the ground to reduce wind effects and evaporation
- Read at the same time each day usually 0900
- Measured in millimetres mm
Wind measurement
Wind speed and direction are measured by different instruments.
Don't confuse anemometer (speed) and wind vane (direction) — a common exam error.
Wind measurement — Key Knowledge
- Anemometer measures wind speed — rotating cups; faster spin = higher speed
- Wind vane measures wind direction — points into the wind; wind is named by where it comes from — a westerly wind blows from the west
- Beaufort scale classifies wind speed from 0 = calm to 12 = hurricane force
Barometer
Measures atmospheric (air) pressure.
Falling pressure often means bad weather is approaching; rising pressure suggests improving conditions.
Barometer — Key Knowledge
- Atmospheric pressure weight of air pressing down — measured in millibars, mb
- High pressure usually associated with dry, settled weather
- Low pressure usually associated with wet, unsettled weather, clouds and rain
Sunshine recorder
Measures the number of hours of sunshine in a day.
Sunshine hours affect temperature, evaporation and plant growth.
Sunshine recorder — Key Knowledge
- Campbell-Stokes recorder glass sphere focuses sunlight onto a card, burning a trace; length of burn = hours of sunshine
Cloud cover
The amount of sky covered by cloud, estimated by observation.
Cloud cover affects temperature — clouds trap heat at night but block sunlight during the day.
Cloud cover — Key Knowledge
- Measured in oktas eighths of the sky
- 0 oktas completely clear sky
- 4 oktas half the sky covered
- 8 oktas completely overcast
- Recorded using standard symbols on weather maps
Climate graphs
A combined chart showing average monthly temperature and rainfall for a location.
Climate graphs allow quick comparison between locations — look at the shape of the line and the height of the bars.
Climate graphs — Key Knowledge
- Temperature shown as a line graph scale on one axis, usually °C
- Rainfall shown as a bar chart scale on the other axis, usually mm
- Annual temperature range highest monthly average minus lowest monthly average
- Total annual rainfall sum of all 12 monthly averages