Uneven Heating by the Sun

Earth is heated by energy from the sun.  However, the sun doesn't heat all of the Earth's regions evenly.

The diagram shows how the same amount of light lands on three parts of the planet.  When sunlight hits Earth at an angle of 90°, the sun-light spreads out over the smallest possible area.  But when it hits Earth at an acute angle, and angle that measures less than 90°, the  same amount of light spreads out over a larger area.  Because the same amount of energy covers an larger area, each part of the area gets less energy.  The more acute the angle of the light hitting Earth's surface, the less thermal energy a particular place gets.  The Earth gets less energy from the sun at the poles than it does at the equator.

There is another reason that the Earth is heated unevenly.  Light loses some of it's energy as it travels through air.  When the sun is directly overhead, light travels the shortest distance through the atmosphere before reaching Earth's surface.  The smaller the angle the light makes, the farther it has to travel through the atmosphere.  More of its energy is absorbed or reflected before it reaches Earth's surface.

The uneven heating of Earth's surface has a great effect on the air in the troposphere.  For example, the air in the troposphere above a Canadian forest is colder and drier than the air above the ocean near the equator.  A large body of air that has the same characteristics throughout is called an air mass.  Air masses form in  the troposphere and help determine the weather.  The heat and humidity of an air mass are determined by the conditions where it forms.

However, air masses can travel thousands of kilometers.  In this way they bring their temperature and humidity to other regions.