Recording of Weather
ICSE Class 7 · Geography · Chapter 6
What this chapter covers
- Weather instruments
- Thermometer
- Barometer
- Rain gauge
- Hygrometer
- Anemometer and wind vane
- Weather station and forecasts
Key definitions
Meteorology is the scientific study of the atmosphere and its weather.
Key points
- Weather instruments measure the elements of weather, and their readings are recorded at fixed times every day.
- A thermometer measures temperature in °C; a maximum and minimum thermometer (Six's thermometer) records the day's highest and lowest temperatures.
- Mean daily temperature is (maximum + minimum) ÷ 2; daily range is maximum − minimum.
- A barometer measures atmospheric pressure; the mercury barometer uses a column of mercury and the aneroid barometer uses a thin metal box.
- Rising pressure usually brings fair weather; falling pressure brings clouds and rain; a sudden steep fall can warn of a storm.
- A rain gauge collects rain through a funnel into a bottle, and the water is measured in millimetres with a measuring cylinder.
- A wet and dry bulb hygrometer measures humidity: the bigger the difference between the two readings, the drier the air.
- A cup anemometer measures wind speed in km/h; a wind vane points to the direction from which the wind blows.
- A weather station keeps thermometers in a Stevenson screen and records all the elements of weather daily.
- A weather forecast predicts coming weather from records collected by weather stations, satellites, balloons and radar.
Questions answered
Name the instrument used to measure each of these: (a) wind speed, (b) humidity, (c) atmospheric pressure, (d) rainfall.
(a) anemometer, (b) hygrometer, (c) barometer, (d) rain gauge.
Why is alcohol, not mercury, used in thermometers in very cold places?
Mercury freezes at about -39 °C, while alcohol stays liquid at much lower temperatures, so an alcohol thermometer can still work in polar regions and on high mountains.
A maximum and minimum thermometer recorded 36 °C and 22 °C in a day. Find the mean daily temperature and the daily range.
Mean = (36 + 22) ÷ 2 = 29 °C; range = 36 - 22 = 14 °C.
How is an aneroid barometer different from a mercury barometer?
A mercury barometer measures pressure by the height of a column of mercury in a glass tube; an aneroid barometer uses no liquid and measures pressure through the squeezing and springing back of a thin metal box, which moves a pointer on a dial. The aneroid is lighter and easier to carry.
The barometer at a coastal town falls suddenly and steeply. What weather might follow?
A sudden, steep fall in pressure often warns of a storm or a cyclone.
Give two reasons why a rain gauge is placed in open ground with its rim about 30 cm above the ground.
In open ground, trees and buildings cannot block or splash the rain; at 30 cm, raindrops bouncing off the ground cannot jump into the funnel.
In a wet and dry bulb hygrometer, both thermometers show 27 °C. What does this tell you about the air?
No water is evaporating from the wet bulb, so the air is saturated and the relative humidity is 100 per cent.
A wind vane's arrow points towards the north-east. Name the wind.
It is a north-east wind, because the arrow points to the direction from which the wind comes.