With the exception of Hawaii, no state extends farther into the tropics than Florida. Key West is located 24 degrees 30 minutes N of the equator, the same latitude as Tampico, Mexico; Sao Paulo, Brazil; and Hong Kong. The angle of the sun has much to do with the degree of heating (insolation) at the earth's surface: the higher the angle of the sun, the more intense the insolation. The closer a place is to the equator, the higher the average angle of the sun throughout the year. Furthermore, the closer a place is to the equator, the less the difference between the angle of the sun at the time of the winter solstice (December 21), when at noon it is lowest on the horizon than at any other time of the year, and that of the summer solstice (June 21), when at noon it is highest. At Key West, on June 21 the angle of the sun is almost perpendicular to the earth, while on December 21 it is 42 degrees. The sun, consequently, can do a much more effective job of heating here than along latitudes farther north, where the angle is lower.
The lines on the maps are called isotherms and connect places of equal temperature. Latitude is not the only variable to affect the temperature at a specific place. If it were, the isotherms would be parallel to the equator. Altitude and distance from a large water body are also very important. For example, both the Appalachian Mountains and the Atlantic Ocean influence temperature. In the winter there is a greater difference (35 degrees F) between the average temperatures of, for example, New York and Orlando, than in the summer when the angle of the sun in both cities is sufficiently high to intensely heat the earth's surface.
