- Latitude lines, or parallels, divide the earth from east to west; Longitude lines, or meridians, divide the earth from north to south.

- We assume that the rays of the sun are all parallel.

- Insolation is the amount of radiation received at surface.

- Equinox: equal day, equal night; sun over the equator.

- Soltice: the sun is furthest from the equator. 23½° N is the Tropic of Cancer; 23½° S is the Tropic of Capricorn.

- The path of the earth's orbit around the sun is elliptical. Its nearest point in January is 91 million miles away and is called the perihelion (near sun). Its farthest point in July is 96 million miles and called the aphelion.

- During the aphelion the earth travels the slowest, while during the perihelion, the earth travels fastest.

- The amount of insolation varies with season and latitude.

- Averaged, the northern and southern hemispheres receive the same amount of insolation each year as proved by Alexander von Humboldt, despite the variations in earth-sun distance.

- There is about a one month lag between the longest day and the warmest day (or shortest and coldest days). This is caused by the temperature responding to the balance of incoming and outgoing radiation.

- The onset of spring warming season is delayed because of the energy needed to first melt the snow and ice.

- The basic cause of the weather is that the poles are cold and the equator is warm (due to angle of incidence) and the atmos- phere attempts to find a balance.

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