In a word, heat. The energy we get from the sun heats up our atmosphere. Since there is a difference in how much heat we during the night and day, and whether we are at the earth's poles or equator, the heat tends to move from one place to another with the air.

Clouds, the most obvious weather phenomenon, usually form when hot air rises like a balloon. Inside that parcel of warmer air, moisture condenses into tiny droplets as it rises and we see a cloud.

Weather is an interesting topic to study, especially so when the weatherman is so often wrong. Can you predict tomorrow's weather better than the weatherman? Write down your forecast and see how it compares with the real weather the next day.

Weather is caused by differences in the temperature of the air.Warm air rises over deserts and cool air falls so that the air is always moving. But since warm air can hold more water or be more humid than cold air, whenever it begins to rise over a mountain or push up against an area of cold air, the warm air cools and it rains or snows. This is why when warm air moves over the ocean, it picks up evaporated water. When it moves on shore and up a mountain, it cools and rains (or snows).

Scientists who study weather are running huge models on the biggest computers to try to understand this and it is still a hard question. There are some principles that can be described when trying to understand weather, even if these give an imperfect ability to predict long-term weather. Weather is largely driven by temperature, pressure, the shape of the land surface, and moisture in the atmosphere. Some of these are related and weather events, such as storms, usually occur when factors that want to be related become different. For instance, moisture in the atmosphere is usually related to temperature, with warmer air able to hold more moisture. If you suddenly make the air colder by sending it to a high altitude, the air can no longer hold all of its water and the water vapor condenses to form clouds and rain. This is a very simple case but weather can usually be explained because of these factors being locally out of balance or because large masses of air of different temperature, moisture, and pressure run into each other and create weather along their border. This is what is known as a weather front. Based on my example above, why do you think it usually rains more in mountains than near sea level?