Climatic and Environmental Processes Annual
Cycle & Variability(100
Hydrograph & Water Budget
Forcing Factors Reservoir Management Decision Calendar
fact that Earth's 23.5 degree tilt off a vertical axis causes seasonal
variations as the planet orbits the sun is not something obvious from
everyday experience. In our everyday life, it is hard to tell that the
orientation of the axis remains fixed in space, producing changes in the
distribution of solar radiation over the course of the year. Nevertheless,
because of the tilt of the axis, these changes in the distribution of
radiation reaching earth's surface do cause the succession of the seasons,
with more extreme seasonal changes generally occurring at high latitudes,
and less variation happening closer to the equator.
In the figure above, summer occurs in the northern hemisphere when the
northern hemisphere is tilted towards the sun, receiving more direct radiation.
(Contrary to the illustration, however, the Earth does not get significantly
closer to the sun during any time of year). Winter in the northern hemisphere
occurs when there is less direct radiation. The Equinox (when the sun's
direct rays hit the equator) occurs around March 21st and September 23rd,
give or take a day or two depending on the year.
Learn more about other key
cycles of extraterrestrial dynamics such as solar variability (sunspots
11 years.), Obliquity (~41k years. change), Precession (wobble on axis
due to gravity of sun and moon in 23k yearrs. cycle) and Eccentricity
(96k yrs. change) by visiting the Paleoclimatology
Branch online slides series on the Ice Ages and Climate
Science 100,000 Years
hydrograph is an important tool
in tracking climate variability over the course of a year and
beyond. In watersheds that are not heavily managed with dams and
reservoirs, the hydrograph shows an annual climate signature generated
by the seasonal runoff pattern of streams and rivers in a region.
Also, the hydrograph is important in developing a water budget
(the input and output of water) for a region such as a watershed.
See the Decision Calendar below for an overview of how reservoir
management decisions are made or click
here for more information on a water budget for the Boulder
Creek watershed in Colorado, including a look at the role of potential
evapotranspiration in a seasonal cycle.
The tilt of the Earth's axis creates seasonal
variability, but there are other factors that may impact climate
systems at the annual scale, such as volcanic aerosols which can
cool climate and shorten growing seasons.
Measured Instruments used to track annual variability
and climate patterns include thermometers, rain gauges, and stream
proxies such as tree rings and cores from corals and
ice caps and glaciers also provide information on an annual resolution
in terms of precipitation and in some cases extreme events such
as fires or volcanic activity.
The calendar above developed
by Andrea Ray (2001) and associates for
Water Assessment demonstrates some of the types of decisions that water
managers need to consider in planning for water storage and release over
the course of a year.
Images from NOAA Paleoclimatology Program, EPA and NGDC.