Why does dropping air pressure tell you a storm is coming?
After you watchWhy does dropping air pressure tell you a storm is coming?
The short answer
Dropping air pressure tells you a storm is coming because a falling barometer is a sign that the air at that spot is rising, not that there is simply less or gentler air. As air rises it cools, and cool air cannot hold its invisible water, so the water condenses into clouds and rain. High pressure is the opposite: sinking air that warms, dries and clears the sky, which is why a falling gauge warns of a storm and a rising one promises fair weather.
Try this next
- What if the air were already cold and dry when it started rising? In the lab, picture starting with drier air. Predict first: would the rising air still make a cloud, or would it have to climb much higher? Then watch how high the air goes before any droplets appear.
- What if the gauge fell slowly instead of fast? Predict whether the storm would be gentle or rough, then compare a quick drop with a slow drop and watch how hard the rising air pushes the clouds up each time.
- What if you watched a real barometer at home before a rainy day? Check a weather app's pressure number tonight and again tomorrow. Predict rain if it drops, sun if it rises, then see who was right the next morning.
The whole story
How it works
A barometer measures how hard the air is pushing down at one spot, which is the weight of the whole column of air above it. When that reading falls, it marks a low-pressure area where air is rising. Rising air expands and cools as it climbs, and cool air can hold less water vapour, so the hidden water condenses into tiny droplets that form clouds and, with enough lift, rain and thunderstorms. When the reading rises, it marks high pressure where air is sinking; sinking air warms and dries, which squashes clouds away and leaves clear, calm skies. The faster the gauge falls, the more vigorously the air is rising, which is why a quick drop warns of a stronger storm.
What people get wrong
Many people think low pressure just means thin, calm, harmless air and that high pressure means heavy, stormy air. It is actually the other way around. Low pressure is the stormy one, because it goes with rising air that cools and makes clouds and rain, while high pressure is the calm, clear one, because it goes with sinking air that dries the sky out.
The catch
Low pressure and its rising air bring the rain that fills rivers and waters crops, but that same rising air also brings the clouds, wind and storms, and the faster the gauge falls the rougher the weather. High pressure and its sinking air bring calm, clear, sunny days, but that same sinking air can trap haze and cold near the ground, and if it lingers for weeks the rain never comes and a drought can set in.
Questions kids ask
Does lower pressure mean there is less air?
There is a little less air pressing down, but that is not the point. The reason it matters is what the air is doing: a falling gauge means the air at that spot is rising. Rising air cools and turns its hidden water into clouds and rain, which is what brings the storm.
Why does rising air make clouds?
As air rises it spreads out and cools, and cool air cannot hold as much invisible water vapour. The extra water condenses into millions of tiny droplets, and a crowd of those droplets is a cloud. If the air keeps rising and the droplets grow heavy enough, they fall as rain.
Why does high pressure bring sunny weather?
High pressure means air is sinking. Sinking air warms up and dries out as it presses downward, so any droplets evaporate away and clouds cannot form. That leaves a clear, calm, sunny sky, which is why a rising barometer usually means fair weather.
How can the gauge know before the clouds appear?
The pressure drops as soon as the air starts rising, which happens before that rising air has climbed high enough to cool and form visible clouds. So the needle falls first and the clouds show up a bit later, giving you an early warning that a storm is on the way.
Talk about it
- Guess first: when the air at one spot rises, does the weather get calmer or stormier, and why?
- Why do you think a falling needle can warn us before a single cloud even shows up?
- High pressure brings sunny days, so why might too many sunny days in a row be a problem for farmers?
For grown-ups
A barometer reads the weight of the air column above it. A falling reading marks a region of low pressure where air converges and rises; as it ascends it expands and cools adiabatically, relative humidity climbs to saturation, and water vapour condenses into cloud and precipitation. High pressure is subsiding air that warms and dries as it descends, suppressing cloud and giving clear skies. The rate of the pressure fall roughly tracks how vigorous the ascent is, which is why a fast-falling barometer warns of a stronger storm.
Keep going
What else makes you wonder?
- Where does all that invisible water in the air come from in the first place?
- If sinking air makes clear skies, what makes the air start sinking in one place and rising in another?
- How did people warn each other about storms before barometers were invented?