SEASONS

Seasons are caused by the 23.5 degree tilt of the earth's rotational axis to the plane defined by the orbit of the earth

This tilt causes the earth's orientation to the sun to change continually as the planet revolves about the sun. The Northern hemisphere thus leans away from the sun during winter and towards the sun in summer.

As the earth's orientation to the sun changes, so too, do the solar altitude and length of day and hence the amount of solar radiation received. In the winter hemisphere, solar altitudes are lower, days are shorter, and there is less solar radiation. In the summer hemisphere, solar altitudes are higher, days are longer and there is more solar radiation. Less solar radiation in summer means that winters are colder than summers.

Because the earth is circular and the sun can be considered a point source of radiation only one half of the globe can be illuminated at any given time

Because the earth rotates some areas of the earth are losing sunlight and others are gaining.

Over the course of a year all points on earth get the equivalent of 6 months of daylight and 6 months of darkness.

The quantity of radiation received at any given latitude depends upon whether the sun's rays strike the earth at a high (90 degrees or directly overhead) or at a low (0 degrees) angle. In the former instance more radiation is received at a point than when the sun is low and on the horizon.

The angle of the sun above the horizon is referred to as the solar altitude and ranges from 0 (horizon) to 90 (directly overhead.)

Higher solar altitudes are found at solar noon (all locations) and lower solar altitudes at sunrise and sunset (all latitudes)

On any given day there is only one latitude on earth that receives solar radiation with a solar altitude of 90 degrees. Because of the tilt of the earth's axis this varies a bit from day to day but is never poleward of 23.5 deg. N or 23.5 deg. S which means the Tropics gets the most intense solar radiation.

Even though polar regions get their 6 months of daylight they get less intense solar radiation than the tropics.

Another result of this axial tilt is that there is considerable seasonal variation in daylength and solar altitude as you move away from the equator. In fact, the farther you are away from the equator the more unequal is the period of day and night. At Omaha, for instance, on June 21 we get a noon solar altitude of over 70 degrees and 15 hours of daylight while on December 21 we get only 9 hours of daylight and a solar altitude of about 25 degrees.

This variation in daylength with season is the primary reason for differences in winter and summer temperatures in mid latitude and polar regions

Much of our variability in weather from day to night and from winter to summer is associated with the axial tilt of the earth's axis and its effect in the distrubution of solar radiation over the earth