Weather is sunshine, rain, snow, wind, and storms. It's what is going on outside right now. The weather is different at different places around the planet. In some places it's sunny right now, while in other places it's snowing. Many things affect the weather including the atmosphere, the Sun, and the season.
The science of weather is called meteorology. Meteorologists study the weather and try to predict it. Predicting the weather is not easy as there are so many factors and variables involved.
Different places in the world tend to have different kinds of weather. Some places, like San Diego, California are warm and sunny for much of the year. While others, like the tropical rain forests, get rain most every day. Still others are cold and snowy most of the year, like Alaska.
Wind
What is Wind?
Wind is the result of air moving around in the atmosphere. Wind is caused by differences in air pressure. Cool air is heavier than hot air. A lot of cool air will create an area of high pressure. A lot of hot air will create an area of low pressure. When areas of low pressure and high pressure meet, the air will want to move from the high pressure area to the low pressure area. This creates wind. The larger the difference in temperature between the two areas of pressure, the faster the wind will blow.
Wind on the Earth
On the Earth there are generally areas of high pressure near the poles where the air is cold. There is also lower pressure at the equator where the air is hot. These two major areas of air pressure keep the wind constantly moving about the Earth. The spin of the Earth also affects the direction of winds. This is called the Coriolis Effect.
Precipitation (rain and snow)
When water falls from clouds it's called precipitation. This can be rain, snow, sleet, or hail. Rain forms from the water cycle. The sun heats up water on the Earth's surface. Water evaporates into vapor and travels into the atmosphere. As more and more water condenses, clouds form. Eventually water droplets in clouds become large and heavy enough that gravity pulls them back to the ground in the form of rain.
We get snow when the temperature is below freezing and small ice crystals stick together to form snowflakes. Each snow flake is unique making no two snowflakes exactly alike. Hail generally gets formed in large thunderstorms where balls of ice get blown several times up into the cold atmosphere. Each time another layer of water on the ball of ice gets frozen making the ball larger and larger until it finally falls to the ground.
Clouds
Clouds are tiny drops of water in the air. They are so small and light that they float in the air.
Clouds form from condensed water vapor. This can occur in a number of ways. One way is when warm air or a warm front, meets up with cold air or a cold front. The warm air will be forced upward and into colder air. When the warm air starts to drop in temperature, water vapor will condense into liquid droplets and clouds will form. Also, warm damp air can blow up against a mountain. The mountain will force the air up into the atmosphere. As this air cools, clouds will form. That's why there are often clouds at the top of mountains.
Not all clouds are the same. There are three main types of clouds called cumulus, cirrus, and stratus.
Cumulus - Cumulus clouds are the big puffy white clouds. They look like floating cotton. Sometimes they can turn into cumulonimbus or tall towering cumulus clouds. These clouds are thunderstorm clouds.
Cirrus - Cirrus clouds are high, thin clouds made of ice crystals. They generally mean good weather is on the way.
Stratus - Stratus clouds are the low flat and large clouds that tend to cover the entire sky. They give us those "overcast" days and can drop light rain called drizzle.
Fog - Fog is a cloud that forms right at the surface of the Earth. Fog can make it very hard to see and dangerous for driving a car, landing a plane, or piloting a ship.
Weather Fronts
A weather front is a boundary between two different air masses, a warm air mass and a cold air mass. There is usually stormy weather at a weather front.
A cold front is where cold air meets warm air. The cold air will move under the warm air forcing the warmer air to rise quickly. Because the warm air can rise quickly, cold fronts can cause cumulonimbus clouds to form with heavy rain and thunderstorms.
A warm front is where warm air meets cold air. In this case the warm air will rise slowly over the top of the cold air. Warm fronts can cause long periods of light rain and drizzle.
Sometimes a cold front can catch up to a warm front. When this happens it creates an occluded front. Occluded fronts can generate heavy rain and thunderstorms.
