Some of you may be wondering why the shortest day of the year is not normally the coldest day of the year. There is a lag between the shortest day of the year and the coldest average temperatures not only across New Mexico but for most of the mid and high latitude locations. In the graph at the end of this section, the length of daylight is plotted in red while the average daily temperature is plotted in blue, and the date of the solstice is depicted by yellow shading. The sun angle is low before and after the winter solstice with a minimum number of daylight minutes and as the sun climbs higher in the sky the length of daylight increases. In Albuquerque, the minimum daily temperature occurs around 2 weeks later in early January. This lag in temperature occurs because even though the minutes of daylight are increasing, the earth's surface continues to lose more energy that it receives from the sun. Average temperatures continue to drop until the sun climbs higher in the sky. While the effect is evident in the daily temperature plot, it is more readily apparent by looking at changes in the monthly average temperature (below and to the right). In Albuquerque, January averages 11.7F degrees less than December, with February also colder than December by 6.0F, even though the days are longer.

We all know that the Earth makes a complete revolution around the sun once every 365 days, following an orbit that is elliptical in shape.  This means that the distance between the Earth and sun, which is 93 million miles on average, varies throughout the year.  During the first week in January, the Earth is about 1.6 million miles closer to the sun. This is referred to as the perihelion.  The aphelion, or the point at which the Earth is about 1.6 million miles farther away from the sun, occurs during the first week in July.  This fact may sound counter to what we know about seasons in the Northern Hemisphere, but actually the difference is not significant in terms of climate and is NOT the reason why we have seasons.  Seasons are caused by the fact that the Earth is tilted on its axis by 23.5°.  This can be seen graphically in the picture below.

The summer solstice occurs when the sun is directly over the Tropic of Cancer, which is located at 23.5° North, and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China.  Because of the Earth's tilt, the sun was directly over the Tropic of Cancer at 12:06 pm MDT on June 21, 2007.  For every place north of the Tropic of Cancer, the sun is at its highest point in the sky and this is the longest day of the year. As stated above, the winter solstice occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° South of the equator.

There are two times of the year when the Earth's axis is tilted neither toward nor away from the sun resulting in an equal amount of daylight and darkness at all latitudes.  These events are referred to as equinoxes and occur near March 21 (Vernal Equinox) and near September 21 (Autumnal Equinox).  At the equator, the sun is directly overhead at noon on the two equinoxes. The Vernal Equinox occurred at 6:07 pm MDT on March 20, 2007.  The Autumnal Equinox occurred at 3:51 am MDT on September 23, 2007.

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