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DAYLENGTH VARIATION

METEOROLOGIST JEFF HABY

One observation of the seasons is that the amount of daylight can vary significantly from season to season. This effect is less significant in the equatorial regions and most significant in the polar regions.

The Earth is tilted approximately 23.5 degrees from being straight up and down. There are 90 degrees from the equator to the pole. Thus, subtracting these numbers yields ( 90 23.5) = 66.5 degrees. This is the approximate latitude (66.5 degrees) that is designated the Arctic Circle in the Northern Hemisphere and the Antarctic Circle in the Southern Hemisphere. Above this latitude (closer to the either pole) is where at least some portion of the year has total darkness (a day or so with 24-hour darkness). The region at the pole that has total darkness is maximized at the solstice for one of the poles. The pole has an entire 6 months of darkness while 66.5 degrees has a day or so of darkness. The amount of days of darkness increases when going from 66.5 to the pole. The region at the pole that has 24-hour sunlight is maximized at the solstice also but one pole will have complete darkness while the other pole will have 24-hour sunlight. The pole has an entire 6 months of sunlight while 66.5 degrees has a day or so of 24-hour sunlight. The amount of days of 24-hour sunlight increases when going from 66.5 to the pole. The consequence of this dramatic shift of sunlight from winter to summer is that the temperature experienced in the polar regions are drastically different between winter and summer.

The middle latitudes also experience a significant change in day length but it is never to the point of having 24-hour darkness of 24-hour sunlight like in the polar regions. An example of a day length difference in the middle latitudes is 14 hours of sunlight in the summer with 10 hours of sunlight in winter. The higher latitudes of the middle latitudes will have more of a difference such as 8 hours of sunlight in winter and 16 hours of sunlight in summer. The lower latitudes of the middle latitudes will have less of a difference such as 11 hours of sunlight in winter and 13 hours of sunlight in summer. The variation is winter weather between the lower and higher portions of the middle latitudes varies dramatically with the higher latitude portions having much colder winters than the much more mild lower middle latitudes. This is because the higher middle latitudes are much more influenced by outbreaks of very cold polar air and the day length is short. In the summer, the weather does not vary as much between higher and lower latitudes in the middle latitudes since day length gets long in the higher middle latitudes in summer to help keep it warm there while the lower middle latitudes have their normal warm weather also.

The equatorial regions have the least amount of day light variation from season to season. Of much more significance is the rainy season and dry season for many tropical locations. In the tropics, the sun alternates summer to winter from being high in the sky with some southern displacement to being high in the sky with some northern displacement. This effect will take place at any latitude closer to the equator than 23.5 degrees latitude. Because of this, there is not a time of the year that the sun at noon is significantly lower in the sky when compared to the dramatic changes that occur in the middle and polar latitudes. The relatively high noon sun angle year round helps keep the equatorial regions warm year round.