An earthquake is an intense shaking of Earth's surface. Earth's outer layer is made of giant puzzle pieces. They are called tectonic plates. They slowly move. Scientists use an instrument called a seismometer. It measures the shaking waves. If the ground shakes, drop down. Cover your head and neck. Hold on until the shaking stops. Families can fasten bookcases, televisions and other heavy things so they stay put in an earthquake.

Here is how a tsunami starts. An earthquake under the ocean can push the water above it. A tsunami can be started by an earthquake or by a volcano in the ocean. Here is a sign to know. If the water pulls far back from the shore after an earthquake, a big tsunami may be on its way.

A tornado is a spinning column of air. It reaches from a storm cloud down to the ground. It often looks like a funnel hanging down from a dark cloud. Experts rate a tornado by studying the damage it caused. The Enhanced Fujita Scale has six ratings. They go from EF0 to EF5.

Two words matter in tornado weather. A tornado watch means tornadoes are possible in and near the area. A tornado warning means a tornado is near and it is time to move to a safe place. Go to a small room with no windows. Pick the lowest floor of a sturdy building.

A hurricane is a big storm that forms over warm ocean water. When a storm's winds reach 74 miles per hour, it is called a hurricane. The Saffir-Simpson scale rates hurricanes from 1 to 5 by their wind speed. Hurricanes usually get weaker when they move over land. A hurricane has a calm, clear center called the eye. Each hurricane is given a name so people know a new storm is on its way.

A flood is water that covers land that is usually dry. Floods can come from heavy rain. They can come from fast-melting snow. They can come when ocean waves come on shore. Flash floods can happen within minutes of heavy rain. They can come without warning.

A flood watch means a flood is possible. A flood warning means flooding is happening or will happen soon. Never walk, swim or drive through flood water. A family can make a plan together. A disaster kit holds basic things a family may need, like water, food, a radio, a flashlight and a first aid kit.

Words to know

• tectonic: of Earth's plates
• seismometer: shake measurer
• scale: rating system
• watch: might happen
• funnel: cone shape

The short version

Earthquakes, tornadoes, hurricanes and floods are natural disasters. An earthquake is an intense shaking of Earth's surface. A tornado is a spinning column of air from a storm cloud to the ground. A hurricane is a big storm over warm ocean water. A flood is water covering usually dry land.

Quick facts

• Earth's outer layer is made of giant tectonic plates that slowly move.
• Scientists use a seismometer to measure the shaking waves from an earthquake.
• The Saffir-Simpson scale rates hurricanes from 1 to 5 by their wind speed.
• The Enhanced Fujita Scale rates tornadoes from EF0 to EF5 by studying the damage they caused.
• A flood watch means a flood is possible; a flood warning means flooding is happening or will happen soon.