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Image of Earth's tilted axisSummary of 1 Year Time Scale
Salt Lake Seasonal Changes
AnnualCycle Climate Variability

One full orbit around the sun- the period of a year- serves as a fundamental force of climate variability as well as a measure of time. The essential cause of seasonal climate change during the year is the tilt of Earth's axis, currently 23.5 degree tilt off its axis, which alters the angle of solar radiation and thus its intensity over the course of the year.

In Climate Science we dig deeper into how Earth's tilted axis results in climate changes that can be seen in such climate signatures as patterns of precipitation and temperature or the flow of creeks and rivers, which are important considerations for water managers in charge of controlling reservoirs or developing conservation plans. In Climate History, we focus on the annual migration patterns of birds and marine animals in the western hemisphere, while in Resources there are links to websites with information relating to the annual cycle and ideas for further inquiry into our yearly journey around the sun.

Annual Cycle
Time of Year
Climate Variability
Dec. 21 or 22, shortest day in Northern Hemisphere, longest in Southern
Image of Crater Lake from NPSNorthern Hemisphere tilts away from sun, receiving less direct solar radiation, while the Southern Hemisphere tilts towards the sun.

Blizzards and ice storms can occur throughout the boreal (Northern Hemisphere) winter months in higher latitudes and elevations.
Image of Crater Lake by NPS.
Spring (Vernal) Equinox
March 20 or 21 in Northern Hemisphere

Image of New River from NPSWhen the sun's direct rays pass the equator, the length of day and night are the same. (Equinox means " equal night"). Without sufficient spring precipitation, droughts may occur later in the year as evapotranspiration increases. In regions where winter and spring precipitation falls as snow, runoff peaks start in the Spring in the southern regions and progresses northward and upward in elevation as the season progresses.
Image of New River by NPS

June 21 or 22, shortest day in the Southern Hemisphere, longest day in Northern Hemisphere

Image of Death Valley from NPSNorthern Hemisphere tilts toward the sun, receiving more intense direct solar radiation, while the Southern Hemisphere tilts away from the sun and experiences winter.

In regions where winter and spring precipitation falls as snow, there is often a peak in snow melt runoff around the solstice. Droughts, flash floods, forest fires and hurricanes are all climate-related events that usually occur during the summer months and into the fall.

Wet season during the Indian Monsoon generally begins in June and goes through September.
Image of Death Valley by NPs

(Autumnal) Equinox
Sept. 22 or 23
in Northern Hemisphere

Black Rock Mountain State Park by USFSSun's direct rays pass the equator and length of day and night are the same. The seasonal cooling of the climate during autumn may include the end of the growing season in some climates. Hurricane activity may extend well into the Autumn season.

Image of Black Rock State Park, GA by Tom Wilson, USFS.

Seasonal Changes in the Salt Lake region of Utah
Seasonal changes in teh Salt Lake region of Utah
Images of central Utah were taken by NASA Terra satellite using a Multi-angle Imaging SpectroRadiometer (MISR) camera on February 8, 2001 and June 16, 2001.

NASA Notes:

Salt Lake City is surrounded by mountains including the Wasatch Range to the east, and the temperature difference between the Great Salt Lake and the overlying atmosphere enhances the moisture content of winter storms. These factors, in combination with natural cloud seeding by salt crystals from the lake, are believed to result in greater snowfall in neighboring areas compared to more distant locales.

In addition to the obvious difference in snow cover between the winter and summer views, water color changes in parts of the Great Salt Lake are apparent in these images. The distinctly different coloration between the northern and southern arms of the Great Salt Lake is the result of a rock-filled causeway built in 1953 to support a permanent railroad. The causeway has resulted in decreased circulation between the two arms and higher salinity on the northern side.

Also see: What is Variability? and Overview of Climate Processes.
Images from NGDC, NPS, NASA

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