Seasons

Mid-chapter review The Earth absorbs solar radiation only during the daylight hours; however, it emits infrared radiation continuously, both during the day and at night. The Earth’s surface behaves (almost) as a blackbody, making it a much better absorber and emitter of radiation than the atmosphere. Water vapor and carbon dioxide are important atmospheric greenhouse gases that selectively absorb and emit infrared radiation, thereby keeping the Earth’s average surface temperature warmer than it otherwise would be. Cloudy, calm nights are often warmer than clear, calm nights because clouds strongly emit infrared radiation back to the Earth’s surface. It is not the greenhouse effect itself that is of concern, but the enhancement of it due to increasing levels of greenhouse gases. As greenhouse gases continue to increase in concentration, the average surface air temperature is projected to rise substantially by the end of this century.

RECAP The Sun is the ultimate energy source for our atmosphere The peak of the solar emission is in the visible wavelengths As the solar light travels through the atmosphere it is Reflected (albedo) by clouds and particles Absorbed by air molecules and clouds Scattered by molecules and particles Transmitted to the surface of the Earth The latter two portions heat the Earth’s surface, which, in turn, warms the air above The maximum of the Earth emission is in the infrared (IR) part of the spectrum The Earth is close to thermal equilibrium: the amount of received energy is equal to the amount of emitted energy Jupiter, for example, is not in thermal equilibrium

The solar energy budget

The Earth Energy Balance

Seasons The long-term (monthly) weather on the Earth varies: With time (summer, winter…) With latitude (equator versus middle latitudes) With the geographic location (coastal areas versus continental areas) These variations of the climate are a result of: Different length of the day (the time during which a given latitude receives energy from the sun) Different amount of solar energy that is received at a given location at a given time of the year The different response of the Earth surface to the incoming solar energy (oceans and continents)

How much solar energy do we get? The amount of solar energy received at the Earth’s surface depends on several factors Distance to the sun (not important, varies a few %) How long the sun shines The angle at which the sun rays strike the Earth’s surface All three factors depend on the season

The orbit of the Earth around the Sun

The Orbit of the Earth

The Orbit of the Earth The Earth orbits the Sun on a slightly elliptical (almost circular) orbit. The Earth is closer to the Sun during the northern hemisphere (NH) winter than during the NH summer One would conclude that during the winter the temperatures should be higher since the Sun is closer??? The change of the distance to the Sun is less than 4% This has NO significant impact on the seasonal climate! The Earth axis of rotation is tilted with respect to the ecliptic at 23.5 degrees. The inclination of the Earth axis is the reason for the changing weather during a year. The duration of the day changes The angle at which the sun rays hit the surface of the Earth at a given latitude changes. What would happen if the tilt was larger? What if it was smaller?

The solar energy at different latitudes

Seasonal Variations of the Received Solar Energy

Seasonal Changes on the Earth: vegetation and seawater temperature

Summer in the Northern Hemisphere The noon Sun shines straight overhead at 23.5 N. The length of the day increases moving north North of the Arctic Circle (66.5 N) the sun does not set down (Jun 21) Insolation = Incoming solar radiation The maximum solar insolation is at the pole (sun shines all day!). The maximum surface temperature is at about 30N (Why?) At higher angles the path of the solar light through the atmosphere is longer and more of the light is absorbed, scattered or reflected in the atmosphere. The albedo of the Earth at high latitudes (snow) is also higher.

Midnight Sun! The path of the sun above the horizon around midnight during the polar summer north of the Arctic Circle.

Winter in the Northern Hemisphere The length of the day decreases to the north. The noon sun is lower on the horizon -the average in-coming solar energy is less than during the summer. The polar region is not illuminated for about 6 months.

Seasons in the Southern Hemisphere The seasons in the SH are just the opposite to the seasons in the NH. The thermal response to the incoming solar energy is somewhat different: A larger percentage (81%) of the SH is covered with oceans. The specific heat capacity of water is higher than that of land (soil). Using the same amount of solar energy the temperature of the oceans will increase less than the temperature on the continents. The temperatures during the winters and the summers in the SH are on average somewhat less extreme than in the NH.

The Energy Balance Blue – IR energy emitted by the Earth Red - solar energy received by the Earth Low latitudes receive more energy than they emit- warming Higher latitudes receive less energy than they emit- cooling The energy in the atmosphere is redistributed by heat transfer (convection, conduction, radiation).

The Sun’s position in the sky. In the continental US the sun is never directly overhead In the winter the sun is rising south of east The noon sun is very low In the summer The sun is rising north of east The noon sun is high

The Sun’s position in the sky