Global patterns driving weather and climate

• Why does the climate vary over the globe?
• Why do we have weather?
• Why can’t we predict the weather better?

The answers to these questions lie in the unequal distribution and movement of the sun’s energy about the globe. Energy from the sun does not reach all portions of the globe in equal amounts. Time of day, time of year, and latitude all influence the amount of incoming solar energy. The differences in incoming solar radiation across the globe result in heat transport from warmer areas to cooler areas by winds and ocean currents. These currents and patterns, driven by the sun, produce meteorological phenomena, such as cold fronts, thunderstorms, and hurricanes, that we know as weather.

Let’s review some of the factors that lead to unequal heating across the globe, thereby resulting in climate and weather patterns.

Earth’s Rotation

The earth rotates on an axis tilted at 23.5 degrees. This rotation results in the world experiencing day and night. One complete rotation takes 24 hours.

Because the globe is round, the sun’s incoming energy is more concentrated on some portions of the earth than in others. Lower latitudes, near the earth’s equator, receive a greater concentration of incoming energy than higher latitudes. This is because at higher latitudes, the same amount of incoming energy is spread over a larger area of the earth than at lower latitudes.

A great animation demonstrating incoming solar radiation, or insolation.

Earth’s Revolution around the Sun

While it is rotating daily, the earth revolves around the sun; a complete revolution takes 365.25 days (1 year). During its revolution around the sun, the earth's tilt remains fixed. Because of the tilt of the earth, during certain times of the year the northern hemisphere is tilted toward the sun; at other times, the southern hemisphere is tilted toward the sun. Accordingly, the region of the globe receiving the most concentrated rays changes. It is this phenomenon that results in seasons.

The hemisphere tilted toward the sun receives more direct sun rays. The more direct the sun’s rays, the warmer the temperatures. During spring and fall, the equator receives the most direct rays. In July and August, months known as summer in the northern hemisphere, the northern hemisphere is tilted toward the sun and receives the most direct rays. The opposite occurs in December and January, months known as winter in the northern hemisphere. During these months, the southern hemisphere is tilted toward the sun and the northern hemisphere is tilted away, resulting in more diffuse (less concentrated) sunlight reaching this part of the globe.