Lightning is a giant spark of electricity in the sky. It heats the air around it to about five times hotter than the surface of the Sun. Thunder is the sound lightning makes. You see the flash first because light travels much faster than sound. Count the seconds between the flash and the thunder, then divide by 5 to get the distance in miles.

Thunder can also tell you how close the lightning was. A sharp crack means the lightning was near. A low rumble means it was farther away. Do not wait for it to get close, though. Thunder can usually be heard only about 10 miles from the flash, so if you can hear thunder, you are close enough to be struck by lightning.

You may have heard that a tent or a picnic shelter keeps you safe. It does not. Picnic shelters, dugouts, sheds and tents give no protection from lightning. A building with wiring and plumbing does. So does a car with a metal top and closed windows. A convertible or an open vehicle is not safe.

Once you are inside, stay away from plumbing and plugged-in electronics. Lightning can travel through wires and pipes. Then be patient. After the last rumble of thunder, wait 30 minutes before you go back outside. A storm can still strike after the rain slows or stops. The rule is short: when thunder roars, go indoors.

A tornado is a spinning column of air that reaches from a storm cloud down to the ground. Tornadoes come in many sizes. A typical one is 20 to 100 meters wide, and the biggest can be up to two miles wide.

A hurricane is a huge spinning storm that forms over warm ocean water. It is also huge in size. The average one is about 340 miles wide, so a single storm can cover a very large area. The same kind of storm has different names around the world. It is a hurricane in the Atlantic. In other parts of the world it is called a typhoon or a tropical cyclone.

How do forecasters know where storms are going? A forecast begins with knowing what the weather is doing right now. Forecasters use that to work out where storms will be tomorrow. Weather satellites in space gather information about the air and clouds below, day and night. Meteorologists then tell the public what to expect.

Two words matter most when a storm is on the way. A watch means the weather could turn dangerous, so be ready. A warning means it is happening or about to, so take action.

Words to know

• lightning: sky spark
• tornado: spinning wind
• typhoon: Pacific hurricane
• forecast: weather prediction
• warning: danger now