2.2 Causes of Earth’s Seasons
During the year, seasons are observed with winter being colder and summer being hotter. What causes these seasons? The Earth’s orbit around the Sun and the tilt of the Earth’s axis are looked at to answer this question. The answer is the tilt of the Earth’s axis causes the seasons. Seasonal solar radiation conditions across the globe are then examined to determine why winter is colder and summer is hotter. These details allow a person to further hypothesize why weather occurs on our planet.
2.2.1 Earth’s Orbit Around the Sun
The Earth orbits the Sun on a flat plane called the elliptic (or orbital) plane. The average distance from the Sun to the Earth is 150 million km. Since this orbit is elliptical, the nearest point from the Earth to the Sun is 147 million km and is called perihelion. The farthest point from the Sun is 152 million km and is called aphelion. Since the closest distance to the Sun occurs in our winter, these variations in distance to the Sun are not the cause of the seasons.
Earth’s Rotation and Axis Tilt
The Earth has a spinning motion called rotation. A complete rotation occurs about every 24 hours. This rotation is not perpendicular to the elliptic plane of the Earth’s orbit around the Sun. Rather the axis is tilted at 23.4o. This axis tilt is also called obliquity. The tilt does not change as the Earth orbits the Sun. Thus, a hemisphere (Northern or Southern) is tilted towards the Sun half a year and away from the Sun the other half of the year. This tilt is what causes the seasons!
Additionally, the spherical shape of the Earth causes beam spreading at higher latitudes. Beam spreading is an increase in the surface area over which radiation is distributed. At higher latitudes, the Sun has a lower angle in the sky, and sunlight is spread out over a larger area. This sunlight also passes through more atmosphere before reaching the Earth. These characteristics cause the poles to be cold and the equator hot. Additionally, beam spreading can occur at any location on the Earth whenever the sun angle is low like sunrise or sunset.
2.2.2 Earth’s Orientation for Solstices and Equinoxes
Summer in the Northern Hemisphere is when the Earth’s axis is tilted towards the Sun and it receives more sunlight. The name given this position is the summer solstice and it occurs around June 21. Likewise, this time of year is winter in the Southern Hemisphere because it receives less sunlight. This diagram illustrates the summer solstice.
During the summer solstice, days are longer and the angle of the Sun is higher in the sky. The North Pole experiences 24 hours of daylight. The Southern Hemisphere has the reverse of these conditions with the South Pole experiencing perpetual night. Labels have been placed at important latitudes to highlight these daylight conditions. The Arctic Circle is at 66.6oN and the Antarctic Circle is at 66.6oS. From the Circles to the pole, this region experiences either 24 hours of day or night depending on which solstice is occurring.
The Tropic of Cancer at 23.4oN is where the Sun is perpendicular to the Earth (directly overhead) during the summer solstice. Therefore, the Tropic of Cancer is getting the most intense sunlight during the summer solstice. Beam spreading and absorption of incoming solar radiation by the atmosphere is minimal.
This next diagram illustrates the winter solstice for the Northern Hemisphere. This solstice occurs around December 21. The Southern Hemisphere receives substantially more sunlight. The Sun is overhead at the Tropic of Capricorn (23.4oS) which is getting the most intense solar radiation. Days are shorter in the Northern Hemisphere and the incoming shortwave radiation is less intense because the Sun angle is lower.
The spring and fall equinoxes are when the Sun is directly overhead at the equator. Every location on Earth gets about 12 hours of day and night. These equinoxes occur around March 21 and September 21.
The angle that the Sun strikes the Earth is important and has specific terminology associated with describing it. The solar noon is when the Sun is highest in the sky or the sun angle is highest. Solar noon is not necessarily at noon (12:00 pm). Solar noon is when a person can get sunburned the fastest.
The solar declination is the latitude the Sun is directly overhead at noon. The solar declination varies between -23.4oS and 23.4oN. For the summer solstice, the solar declination is the Tropic of Cancer. For the equinoxes, the solar declination is the equator.
The angle of the Sun above the horizon varies throughout the day. In the early morning and late evening, the Sun angle is at the horizon or near zero. The highest sun angle during the day is at solar noon.
The sun angle at solar noon for your location can be found by:
- subtracting your latitude from 90o, and
- adding the current solar declination latitude.
For example, the sun angle at solar noon during the summer solstice at Tucson is (90o – 32.2o) + 23.4o = 81.2o.
NOAA solar calculator provides information on the Sun for any location on the Earth for any day.
What Causes Weather?
The changing orientation of the Earth with respect to the Sun (because of the axis tilt) affects the incoming solar radiation received in three ways:
- The length of day (sunshine) at a location. Summer days are longer than winter days.
- The angle at which the Sun strikes the Earth. The summer Sun has higher sun angles than winter.
- The amount of atmosphere sunlight must penetrate to reach the Earth changes with the seasons. Lower sun angles cause the radiation to pass through more atmosphere before reaching the surface.
These effects cause the poles to have a “deficit” of incoming shortwave radiation and the tropics to have a “surplus” of radiation. In other words, the poles are cold and the tropics are hot. The atmosphere tries to equalize these temperatures differences with the weather. Warm air from the tropics is transported north and cold air from the poles is transported south. This map from NASA illustrates the radiation imbalance between the poles and tropics.
