Two important things determine the Earth's climate. One is the amount of heat the sun delivers to the Earth. Also important is where the Earth's continents are located relative to the equator. Continental ice sheets cannot develop unless one or more continents are located at high latitudes.

Thin cores of ice, thousands of meters deep, have been drilled in the ice sheets of Greenland and Antarctica. They are preserved in special cold-storage rooms for study. Glacier ice is formed as each year's snow is compacted under the weight of the snows of later years. Light bands correspond to the relatively fresh, clean snows that fall in the summer when warmer conditions bring more moisture and precipitation. Dark bands mark the polar winter season, when little new snow falls on these frigid deserts and blowing snow is mixed with dust, discoloring the white snow.

The Earth's climate has changed greatly through geologic time, and even in recent centuries. The study of past climates is called paleoclimatology ("paleo-" means "early" or "past").

It is easy to understand how climate can vary over very large areas, because of slight changes in temperature or rainfall. Climates can also vary over very short distances. Local differences in climate are described by the term "microclimate." Low-lying areas are often colder at night than higher ground nearby. On clear nights, the ground is chilled as its heat is radiated out to space. The cold ground then chills the air near the ground. The chilled air is slightly denser than the overlying air, so it tends to flow slowly downhill, in the same way that water flows downhill.

The plant community in an area is the most sensitive indicator of climate. Areas with moderate to high temperatures and abundant rainfall throughout the year are heavily forested (unless humans have cleared the land for agriculture!). Areas with somewhat less rainfall are mainly grasslands, which are called prairies in North America. Humans have converted grasslands into rich agricultural areas around the world. Even in areas with high yearly rainfall, trees are scarce if there is not much rainfall during the warm growing season.

The two most important factors in the climate of an area are temperature and precipitation. The yearly average temperature of the area is obviously important, but the yearly range in temperature is also important. Some areas have a much larger range between highest and lowest temperature than other areas. Likewise, average precipitation is important, but the yearly variation in rainfall is also important. Some areas have about the same rainfall throughout the year. Other areas have very little rainfall for part of the year and a lot of rainfall for the other part of the year.

Weather is the state of the atmosphere from day to day. It includes temperature, atmospheric pressure, clouds, wind, and precipitation. Climate is the long-term average of the weather. It is observed over periods of many years, decades, and centuries. In many areas of the United States, the daily high temperature or the daily low temperature can vary by as much as 30 degrees F from day to day. In contrast, the average temperature for a whole year seldom varies by more than one degree.