Weather instruments and weather stations

From clues to instruments

Everyday life already tells us about temperature: snow melts quickly when it is warm, coconut oil turns solid in winter and curd takes longer to set on a cold day. Over the last few centuries, scientists built instruments that measure each element of weather precisely, so that the weather can be monitored and forecast.

Thermometers

In Class 6 Science you met clinical and laboratory thermometers and two scales, Celsius and Fahrenheit: 15°C is the same as 59°F. Weather thermometers come in several types. Some show the ambient temperature, that of the surroundings, while others record the day’s maximum and minimum. Many use a coloured liquid that expands as it warms, but digital thermometers are now preferred because they are more precise and store more data. Recorded temperatures give useful statistics:

• Range = maximum − minimum, usually over 24 hours.
• Mean daily temperature = (maximum + minimum) ÷ 2.

For a week in a city in Madhya Pradesh, the highest maximum was 32°C and the lowest minimum 14°C, a range of 18°C for the week.

Rain gauge

A rain gauge measures precipitation. Rain falls into a collecting funnel and gathers in a measuring tube marked with a scale. The depth of water is the rainfall: water 5 mm deep means 5 mm of rain. Place it in the open on a flat, steady surface, away from anything that blocks the rain, and read it at the same time every day; melt any snow before measuring. Extra: if you make one at home, use a straight-sided jar or bottle with an opening as wide as its body, so the depth you read is the real rainfall.

Barometer

A barometer measures atmospheric pressure, usually in millibars (mb). Normal pressure at the coast is about 1013 mb. A reading below about 1000 mb signals a depression, which can become a storm or a cyclone, so pilots, sailors, fishermen and weather forecasters watch the barometer closely.

Wind vane, windsock and anemometer

A wind vane is an arm that can swing round, with a pointer on one end and a broad tail on the other. The wind pushes the tail, so the pointer swings round to face the wind; even a light breeze moves it. A windsock on an airfield streams away from the wind and shows pilots its direction during take-off and landing; industries that release ash or gases use similar socks. An anemometer has three or four cups on a vertical shaft. The stronger the wind, the faster they spin, and a meter turns the spins into a speed in km/h. Extra: the cups measure speed; a vane fixed beside them gives the direction.

Hygrometer

A hygrometer measures humidity as relative humidity. Air with no water vapour at all, which never happens in nature, would be 0 per cent, and air saturated with vapour is 100 per cent. Dry weather is usually 20–40% and humid weather 60–80%. Hygrometers work on different principles, and they matter beyond forecasts: food factories control humidity, and museums keep the air dry to protect their exhibits.

Weather stations

A weather station keeps all these instruments together, and readings taken at regular intervals help map and forecast the weather. An Automated Weather Station (AWS) uses sensors to record temperature, humidity, wind, rainfall and pressure without anyone reading them, and is used in farming, aviation, navigation and environmental monitoring.

Why it matters

Instruments turn “it feels cold” into numbers that anyone can compare and that forecasters can use. The elements themselves are explained in elements of weather, and the step from readings to warnings in weather forecasts and warnings. Worked answers to every textbook question are in the NCERT solutions for Chapter 2; see all of the Chapter 2 notes.