METR 104:
Our Dynamic Weather

(Lecture w/Lab)
Meteorologically and
Astronomically Significant
Latitude Zones
Dr. Dave Dempsey
Dept. of Geosciences
SFSU

"Regular"
Latitude
Zones
Astronomically Defined
Latitude Zones


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Latitude

(See article on "Latitude and Longitude" at http://www.nationalatlas.gov for more background.)

The latitude of any particular location on earth is defined as the angle (expressed in degrees and measured from the center of the earth) between that location and the equator.

The latitude of the equator itself is 0°. The angle between the North Pole and the equator is 90° (a right angle), so the latitude of the North Pole is 90° (the maximum possible latitude). The same is true of the South Pole.

The farther you are from the equator, the greater your latitude will be. Hence, latitude is a measure of distance from the equator.

To distinguish latitudes north of the equator (i.e., in the Northern Hemisphere) from those south of the equator (in the Southern Hemisphere), an "N" or "S" is attached to the latitude. For example, San Francisco lies at approximately 37.6°N latitude.

In a particular hemisphere, all of the places that lie equally far from the equator (and hence have the same latitude) define a circle, called a latitude circle. Latitude circles are parallel to each other and to the equator (which is just another latitude circle) and are smaller the farther they are from the equator. The North and South Poles are just latitude circles with zero radius—that is, points.

Meteorologically Significant Latitude Zones

Meteorologically significant latitude zones (see Figure 1) include the following:

  1. low latitudes: 30°S to 30°N latitude (which includes the equator).

  2. middle latitudes (or midlatitudes for short): 30° to 60° latitude (in each hemisphere).

  3. high latitudes: 60° to 90° latitude (in each hemisphere).

As a broad generalization, the nature of the weather and climate differs in significant ways between each of these three latitude zones, though it can differ greatly within them too.

Astronomically Defined Latitude Zones

Other meteorologically significant latitude zones, partly overlapping with those above, can be defined in terms of astronomically significant latitudes (see Figure 2), which are associated with the orientation of the earth's axis of rotation relative to the sun and how that orientation varies as the earth revolves around the sun over the course of the year (giving rise to the seasons). Astronomically significant latitudes include:

  1. the Tropic of Cancer (23.5°N) and Tropic of Capricorn (23.5°S). These are the latitudes where the sun strikes the earth directly at the time of the June and December solstices, respectively. At latitudes farther from the equator, the sun never appears directly overhead.

  2. the Arctic Circle (66.5°N) and Antarctic Circle (66.5°S). These are the latitudes where it first becomes possible for the sun to be up all day (at the summer solstice) or down all day (at the winter solstice). At latitudes closer to the equator, the sun rises and sets every day. At latitudes farther from the equator, the sun will never rise or set for longer periods centered around the solstices.

Meteorologically significant latitude zones defined at least partly by these astronomically significant latitudes include:

  1. the tropics: latitudes from 23.5°S to 23.5°N (including the equator). The tropical rain forests lie in the tropics, so you can tell it often rains a lot there.

  2. the subtropics: latitudes from 23.5° to those in the low 30's (in both hemispheres). Most of the worlds deserts lie in the subtropics.

  3. the arctic and antarctic (also referred to as the polar regions): latitudes greater than 66.5° (in the Northern and Southern Hemispheres, respectively). Characterized by very long days (summer) and very long nights (winter); low sun angles above the horizon; and chronically cold temperatures.

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