Measuring weather: pressure, clouds, and humidity

It’s early on a summer morning. Jenny is at a weather station. She wants to take readings for air pressure, cloud, and humidity to help her predict the day’s weather. Which of these instruments should she use? And how can they help her predict the weather?

Let’s start with pressure. The air in Earth’s atmosphere is pulled down by gravity and exerts pressure on the Earth’s surface. Pressure is measured using a barometer. This barometer contains a small, flexible metal box. The box has the air pumped out of it so that when the atmospheric pressure outside the box changes, the metal expands or contracts.

These movements drive levers which turn a needle left or right around the barometer’s dial. The dial displays pressure in hectopascals. One thousand and thirteen hectopascals is the average air pressure at sea level. When air pressure is high, air falls, and few clouds form. In summer, high pressure often means clear skies and dry, sunny conditions.

In winter, high pressure means clear skies but colder weather, because there are no clouds to trap heat in. When the pressure is low, air rises, and clouds form. So low pressure often brings rain or snow. We can also predict the weather by observing the clouds directly. To determine how much cloud there is, we imagine the sky divided into eight boxes.

These boxes are called oktas. If all eight oktas are empty, the sky is clear. If all eight are full, the sky is overcast. When there is only some cloud, we imagine how many boxes it would fill if it were all clumped together. In this way, we estimate the cloud cover.

We can test how high clouds are with a ceilometer. The ceilometer shines a laser up to the sky. The laser is reflected from the base of the clouds back down to the ceilometer’s receiver. This gives a reading of cloud height, in metres. Clouds’ height, along with their shape and colour, tell us what kind of clouds they are.

These low, white, cauliflower-shaped clouds are cumulus clouds. They are often spotted in fair weather. And these mid-level, dark grey, thick clouds are nimbostratus clouds. They bring heavy rain. We can also predict rain by measuring how much water vapour is in the air — the humidity.

One way we can measure humidity is with an instrument called a psychrometer. A psychrometer uses two thermometers attached to a handle to measure the amount of water in the air compared to the maximum water the air can hold at a certain temperature. One of the thermometers has a wet piece of cloth on the end of it. We call this the wet-bulb thermometer. We swing the psychrometer around, to help the water in the wet cloth evaporate.

This cools the temperature on that thermometer. Next, we compare the temperatures on the two thermometers. The smaller the difference, the nearer the air is to its maximum capacity for holding water. This comparison gives us the relative humidity. The higher the relative humidity, the more likely it is that rain is coming.

When relative humidity reaches 100 percent, the clouds can’t hold any more water, and it begins to rain. So, Jenny, what’s the weather looking like today? Let’s see… the barometer is showing 1020 hectopascals — it looks like we’re experiencing a high pressure front. The cloud cover is three oktas… and according to the ceilometer, they are 500 metres high. They look like small cumulus clouds.

And the psychrometer shows the relative humidity is 10 percent. It’s going to be a clear, dry, sunny day!