Measuring weather: temperature, precipitation, and wind

Ah… a warm, sunny day! Perfect for a cool dip, an icy drink, or for reading a magazine in the sun. If it were snowing, you’d be spending your day quite differently, Jenny. Weather has a big impact on our daily lives. Because weather is so important, we have ways of checking the weather, days or even weeks in advance.

Weather forecasts can help us know what to expect, and plan accordingly. To forecast future weather, experts called meteorologists gather information about the weather today. But how, exactly, do meteorologists get this information? What tools can we use to measure how hot or cold or windy or wet it is? To measure temperature, we use a thermometer.

Traditional thermometers contain a liquid that expands when warm and contracts when cool, causing the liquid to move up and down a tube. Nowadays, meteorologists rely on digital thermometers that use an electronic sensor to measure and record the temperature. Sometimes, a thermometer is placed inside a protective box called a Stevenson screen. The Stevenson screen protects the thermometer from rain, snow, and direct sun but allows air to circulate freely, through slats on the sides. Temperature can be measured in degrees Celsius.

Today, the temperature is a warm 25 degrees Celsius… but look out Jenny! It’s starting to rain. To measure rain, we use a tube that collects rain drops — a rain gauge. The rain gauge is usually anchored in the ground with the top of the tube at least 30cm above ground level. This way, no splashes enter the tube, just falling rain.

As the tube fills, the depth of the rainwater can be read in millimetres on the side. Gauges like these can also measure snow or hail — other forms of precipitation. This rain gauge shows that 6 millimetres of rain has fallen in the last hour. Jenny will be glad she headed inside. It’s beginning to get blustery, too!

When measuring wind, we need to know two things: the direction and the speed. We can observe wind direction on this instrument. It has arrows that spin in the wind and point to north, south, east, and west. This is a weather vane. Weather vanes are often on top of tall buildings, where there are no buildings or trees blocking the wind.

This weather vane is pointing west. That means the wind is blowing from the west, towards the east. When we report wind direction, we always say the direction it is coming from. So today’s wind is a westerly wind. How fast is that wind?

Wind speed can be measured using an instrument with four cups attached to the end of arms mounted on a pole. This is an anemometer. The wind catches the cups and turns the pole. The more turns per minute, the higher the wind speed. Some anemometers have a dial that shows the wind speed in kilometres per hour.

This afternoon’s wind speed has reached 35 kilometers per hour. Go grab your magazine, Jenny, before it flies away! Weather stations around the world use these instruments to measure temperature, precipitation, and wind in specific locations. But sometimes, meteorologists need a bigger picture of what’s happening in the atmosphere. For this, they use images taken far above the earth by weather satellites.

Together, the data from weather stations and satellites help meteorologists track patterns in the weather and predict what’s coming next. Luckily for Jenny, it looks like the wind and rain will soon clear up, so she can get back to her sunbathing.