DEFINITION OF SEASONS

Variations in the average weather at different

times of the year are known as seasons, controlled by the

average amount of solar radiation received at the surface in a

specific place for a certain time period. The amount of radiation

received at a particular point on the surface is determined

by several things, including the angle at which the

Sun’s rays hit the surface, the length of time the rays warm

the surface, and the distance to the Sun. As the Earth orbits

the Sun approximately once every 365 days, it follows an

elliptical orbit that brings it closest to the Sun in January (91

million miles, or 147 million kilometers) and farthest from

the sun in July (94.5 million miles, or 152 million kilometers).

Therefore, the Sun’s rays are slightly more intense in

January than in July but, as any Northern Hemisphere resident

can testify, this must not be the main controlling factor

on determining seasonal warmth since winters in the Northern

Hemisphere are colder than summers. Where the Sun’s

rays hit a surface directly, at right angles to the surface, they

are most effective at warming the surface since they are not

being spread out over a larger area on an inclined surface.

Also, where the Sun’s rays enter the atmosphere directly they

travel through the least amount of atmosphere, so are weakened

much less than rays that must travel obliquely through

the atmosphere, which absorbs some of their energy. The

Earth’s rotational axis is inclined at 23.5° from perpendicular

to the plane it rotates around the sun on (the ecliptic plane),

causing different hemispheres of the planet to be tilted

toward of away from the sun in different seasons. In the

Northern Hemisphere summer, the Northern Hemisphere is

tilted toward the Sun so it receives more direct sunlight rays

than the Southern Hemisphere, causing more heating in the

north than in the south. Also, since the Northern Hemisphere

is tilted toward the sun in the summer it receives direct sunlight

for longer periods of time than the Southern Hemisphere,

enhancing this effect. On the summer solstice on June

21, the Sun’s rays are directly hitting 23.5°N latitude (called

the tropic of Cancer) at noon. Because of the tilt of the planet,

the sun does not set below the horizon for all points north

of the arctic circle (66.5°N). Points farther south have progressively

shorter days, and points farther north have progressively

longer days. At the North Pole, the Sun rises above

the horizon on March 20, and does not set again until six

months later on September 22. However, since the Sun’s rays

are so oblique in these northern latitudes, they receive less

solar radiation than areas farther south, where the rays hit

more directly but for shorter times. As the Earth rotates

around the Sun, it finds the Southern Hemisphere tilted at its

maximum amount toward the Sun on December 21 and the

situation is reversed from the Northern Hemisphere summer,

so that the same effects occur in the southern latitudes.

Seasonal variations in temperature and rainfall at specific

places are complicated by global atmospheric circulation

cells, proximity to large bodies of water and warm or cold

ocean currents, and monsoon type effects in some parts of the

world. Some seasons in some places are hot and wet, others

are hot and dry, cold and wet, or cold and dry.

See also ATMOSPHERE; CLIMATE; HADLEY CELLS.

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