The word "climate" is used to describe conditions in the atmosphere over a long period. Like weather, climate consists of things like temperature, wind speed, precipitation and humidity. However, weather involves short-term variations in atmospheric conditions. Climate refers to conditions over extended timespans - average values as well as extremes and the reliability and frequency of particular climactic features. Short-term variations in atmospheric conditions constitute weather, not climate.

Although climate is much more stable than weather, and can seem to be unchanging in the context of human lifespans, if we look at the Earth's long geological history, we can see that climate has changed many times in the past.

For example, the Earth has experienced warming and cooling periods throughout its history, as glaciers expanded and then retreated.

Many parts of the world experienced considerably higher average temperatures in the period 1931-60 than they had probably had for hundreds of years.

Factors that influence climate include distance from the equator (latitude), elevation (altitude), and distance from large bodies of water, proximity to mountain ranges, ocean currents, air currents and vegetation.

In general, climate becomes warmer as you get closer to the equator, and it becomes colder as you move toward the poles. This is because the sun is at a higher angle in the sky above the equator and at a lower angle near the poles. In addition, ice near the poles has a high albedo. (It reflects sunlight well.)

Temperature tends to decrease with elevation because air becomes thinner at higher altitudes. Temperature decreases by about 3.6 degrees Fahrenheit for every one thousand foot increase in height (6.5 degrees Celsius for every one thousand meter increase in height).

Wind and water currents affect climate by carrying heat and moisture to different parts of the globe.

Water that evaporates from the oceans, seas and lakes travels over land as water vapor, where it increases humidity and can eventually cause precipitation.

Because water conducts heat more slowly than land does, regions of the Earth that are near large bodies of water tend to have milder climates, with less variations in temperature, than regions that are further inland.

The difference between air temperatures over water and over land can have a dramatic effect on climate. This difference is responsible for monsoons.  In monsoon regions, during the warmer part of the year, the land heats up more quickly than the ocean does.  This causes the air over land to have a lower pressure than the over the ocean.  Consequently, moist air from above the ocean blows over the land, bringing rain.

When the weather becomes cooler, the land loses heat faster than the ocean does.  The air over the land becomes cooler, and has a higher pressure, than the air over the land. This causes the air current to reverse, blowing from the land to the ocean.

Mountains affect climate by blocking moist air. When moist air reaches a mountain, it rises in order to pass over the mountain. The air cools as altitude increases.  Cool air is not able to retain as much water vapor as warm air, so it rains on the windward side of the mountain. The air that reaches the leeward side of the mountain is very dry. The region on this side is known as a rain shadow. Many of the Earth's deserts are located in rain shadows.

The highest mountains affect wind currents in the upper troposphere. For example, the westward-flowing jet stream rises and veers northward over the Rocky Mountains of North America, then turns southward again on the far side. The effect is to maintain relatively warm air over the Rockies at a high altitude.

Plants emit water vapor into the atmosphere and therefore affect humidity levels. They also have a low albedo - they reduce the ability of sunlight that reaches the Earth's surface to be reflected back into the atmosphere.

While climate is affected by plant life, climate also determines which types of plants can survive in a given region of the Earth. Thus, climate and vegetation are interdependent.

Today, most of the systems that are used for classifying the different climates that exist on Earth are based on the Köppen Climate Classification System, which was developed by Wladimir Köppen, a Russian-German scientist, at around the turn of the twentieth century. According to the Köppen Climate Classification System, there are five major climate types. These are tropical climates, dry climates, temperate climates, continental climates and polar climates.

Microclimates

A microclimate is a climate that covers a relatively small area and differs from the climate of the surrounding area.

Microclimates are influenced by factors that operate locally, such as local winds.

Local soils can also affect microclimate by affecting albedo. Dry, sandy soils have a higher albedo than dark, clayey soils.

Some microclimates are affected by human activity.

Cities are often warmer than surrounding areas because surfaces such as asphalt and concrete reflect heat back into the air. However, urban air pollution can counter this effect by preventing sunlight from reaching the grown.

The presence of many tall buildings in a city tends to cause a reduction in wind speed but an increase in turbulence.