What is extreme weather?
Weather is what the air does today: hot, wet, windy. Extreme weather is weather that is rare for a place and strong enough to harm people, crops or buildings. What is extreme in one place may be normal in another: 40 °C is a heatwave in a cold country but a usual day in a desert.
The main kinds are heatwaves and cold waves, heavy rain and floods, droughts, cyclones, thunderstorms with hail, heavy snow and dust storms.
Heatwaves and cold waves
A heatwave is several days in a row when the temperature is far above normal for that place. Hot air sits still under a high-pressure area and the ground bakes. Heatwaves cause heat stroke, water shortage and power cuts. Drink water, stay in shade and check on old people and children.
A cold wave is the same idea on the cold side: several days far below normal, with frost that harms crops and people. Drag the heat slider in the 3D to see how a warmer average makes red days (heatwave days) more common.
Heavy rain, floods and droughts
Warm air can hold about 7% more water vapour for every 1 °C of warming. So when it rains, it can rain harder. A flood happens when water flows over land that is normally dry. Causes: very heavy rain, rivers filling up, melting snow, storm surge from the sea, or blocked drains and built-up ground that cannot soak water.
A drought is the opposite: a long time with much less rain than normal. Many places now get longer dry spells and sharper downpours, both at once.
Cyclones, typhoons and hurricanes
These are the same kind of storm with different names in different oceans. They form over warm sea (about 26.5 °C or more). Warm moist air rises, rotates and builds a spiral with a calm eye in the middle and the strongest winds around it.
The dangers are strong wind, heavy rain and a storm surge: sea water pushed onto the coast. Early warning and shelters save many lives. Try the sea-temperature slider: cool sea, no cyclone.
Thunderstorms, hail, heavy snow and dust storms
A thunderstorm comes from tall clouds where warm air rises fast. It brings lightning, thunder, gusty wind, sudden heavy rain and sometimes hail (balls of ice). Heavy snow can block roads and break roofs. A dust storm happens when strong wind lifts dry soil from a desert or bare field and carries it far, harming breathing and visibility.
Safety rule for lightning: go indoors, stay away from trees, water and metal poles.
Why are extremes rising?
Burning fossil fuels adds carbon dioxide, which traps heat, so the world has warmed. Scientists compare the pile of days before and after. The average moved, and the rare hot end grew a lot more than the average itself. See Climate change and Disaster management for causes and how to prepare.
Remember: one storm is weather; a changing pattern over many years is climate.
Try it: your own weather pile
Ask an adult or look online for last month's daily maximum temperature in your town. Make a tally chart: one mark per day in each 2 °C box. Do you see a pile with a few hot days at the end? Then in the 3D, predict how many red days appear if the world warms by 2 °C, and check.
Key formulas and definitions
- Warm air holds about 7% more water vapour per +1 °C
- Cyclone needs sea surface temperature of about 26.5 °C or more
- Rain total = rain rate × time (mm/h × h = mm)
- Key terms: heatwave, cold wave, flood, drought, cyclone, storm surge, hail, dust storm
Worked examples
1. A town has 2 very hot days out of 100. After warming it has 11. How many times more is that?
11 ÷ 2 = 5.5, so about 5 to 6 times more very hot days, even though the average rose only 3 °C.
2. Rain falls at 40 mm per hour for 3 hours. How much rain fell?
Total = 40 × 3 = 120 mm of rain.
3. By how much can the water vapour in air grow if the air warms by 2 °C? (about 7% per °C, simple addition)
About 7 × 2 = 14% more water vapour (a more exact compounding gives about 14.5%).
4. Sea temperatures are 25.8 °C, 27.1 °C and 29.4 °C in three places. Where can a cyclone form?
A cyclone needs about 26.5 °C or more. So it can form over 27.1 °C and 29.4 °C, but not over 25.8 °C.
5. A drain can take 30 mm/h. A storm gives 75 mm/h for 1 hour. How much water is too much for the drain?
Extra rate = 75 − 30 = 45 mm/h. In 1 hour this is 45 mm of water that cannot go down the drain and floods the road.
6. A hot week has maximum temperatures of 36, 39, 41, 40, 38, 37, 35 °C. The heatwave line is 38 °C. How many heatwave days are there?
Count days at or above 38 °C: 39, 41, 40 and 38. That is 4 days.
Common mistakes
- Thinking one hot day proves climate change. Climate is a pattern over many years; look at how often extremes happen.
- Mixing weather and climate: weather is today's air, climate is the usual pattern over 30 years or more.
- Believing cyclones form over land. They need a warm sea and weaken quickly over land.
- Standing under a tree or in open water during a thunderstorm. Go indoors instead.