H EATING AND E ARTH ’ S S URFACE AND A TMOSPHERE Lutgens and Tarbuk “The Atmosphere” Chp 2 College Now Meteorology

I NSOLATION Insolation is the amount of sunlight reaching an area of the Earth. Insolation is greatest near the equator, and least near the poles. This is why earth is warm around the equator, and cold at the poles.

W HY DO WE HAVE SEASONS ? If surface temperature is being determined by the amount of sunlight, then why is there a change throughout the year? Is the sun changing? Is the Earth further from the sun in winter?

W HY DO WE HAVE SEASONS ? The Earth’s axis is tilted 23 ½ degress from normal. The axis remains pointed at Polaris, the north star. So during half the year, the northern hemisphere is pointing towards the sun and being especially warmed, and during the other half, it is pointing away and especially cooling.

W HY DO WE HAVE SEASONS ? Because of the tilt of the earth, the focus of the sun is not centered on the equator. The sun is over the Tropic of Cancer, and the tropic of Capricorn in winter. In fact, summer follows the Sun, when it is over the Tropic of Cancer, it is summer in the Northern Hemisphere (boreal summer), when it is over the Tropic of Capricorn, it is summer in the southern hemisphere (austral summer)

T HE SEASONS Because of these effects, the angle at which the sun resides overhead at noontime during the day (its zenith) changes throughout the year. When the sun is at a high angle (directly overhead), the sunlight is very focused and concentrated. It also needs to pass through a single atmosphere’s worth of material. Thus it is warmer. When the sun is at a low angle while at its zenith, its light is diffuse, and it is passing through a greater thickness of atmosphere. Thus it is cooler.

E QUINOXES AND S OLSTICES Four days of the year are given special attention, two solstices, and two equinoxes. Day and Night are equal on equinoxes. Day has reached its greatest length at the Summer Solstice Night has reached its greatest length by the Winter Solstice. Summer Solstice 6/21 or 22, Winter Solstice 12/21 or 22, Autumnal Equinox 9/2 or 23, Spring (vernal) Equinox 3/21 or 22, in the Northern Hemisphere

M ECHANISMS OF HEAT TRANSFER Conduction Convection Radiation All objects emit radiant energy over a range of wavelengths Hotter objects radiate more total energy per unit of area than cooler ones The hotter the radiating body, the shorter the wavelength of maximum radiation Good absorbers of radiation are also good emiters

I NCOMING RADIATION Reflection – light leaves with the same intensity Scattering – light breaks into a greater number of weaker rays in different directions Albedo – brighter objects reflect more solar radiation than darker ones. Fields of ice reflect more sunlight than dark bare ground, which absorbs more and warms.

G LOBAL H EAT BUDGET Keeping track of incoming heat and heat loss. If heat in is less than heat out, the globe cools. It heat in is greater than heat out, the globe warms.

L ATITUDINAL H EAT B UDGET Insolation varies with latitude, so the heat budget varies with latitude

G REENHOUSE EFFECT Some gases trap heat in the atmosphere. Water vapour, ozone, carbon dioxide are examples of greenhouse gases. They cause the atmosphere to warm, which can transport heat to other parts of the planetary system.