Why Are There Seasons?
Like tides washing regularly across a beach, seasons advance and retreat
across the face of the globe and bring changes that transform the face
of the Earth. Whether it is the arrival of the winter snows, the monsoon
rains, or the summer heat, our environment changes constantly, and these
profound changes occur over relatively short time periods. What helps make
such huge, complex changes comprehensible is that they reoccur in predictable
ways. Many ancient civilizations observed that the Sun's position in the
sky changed throughout the year and were able to construct calendars and
predict seasonal change based on their observations.
All seasonal changes are driven by changes in the amount of the Sun's
energy reaching the Earth's surface (i.e., the amount of insolation).
For example, more energy leads to higher temperatures which results in
more evaporation which produces more rain which starts plants growing.
This sequence describes Spring at mid- latitudes. Since visible light is
the main form of solar energy reaching Earth, day length is a reasonably
accurate way to gauge the level of insolation and has long been used as
a way to understand when one season stops and the next one starts. For
example, the first day of summer (the summer solstice) is the longest
day of the year. Winter starts on the shortest day of the year, the winter
solstice.The first days of fall and spring are when the day and night
are of equal length - roughly 12 hours each.These days are named the vernal
and autumnal equinoxes.
Changing day length implies that the Earth's axis of rotation is inclined
with respect to the plane of its orbit around the sun. The ancient Greeks
knew that the Earth was inclined 23.5°. Figure SE-I-1 shows the inclined
Earth at different positions in its orbit. Notice how at the solstice positions,
each pole is tilted either toward or away from the Sun.The pole inclined
toward the Sun receives 24 hours of sunlight, and the one inclined away
is in the Earth's shadow and experiences 24 hours of darkness.At the equinox
positions, the Earth is inclined in a way so that each pole receives equal
amounts of insolation. This discussion focuses on the poles because they
experience the greatest extremes of insolation.Because of the inclination
of the Earth's axis, insolation levels at every point on Earth change constantly.
We call the effects of these changing levels seasons.
Figure SE-I-1: Positions of Earth in relation to the sun
on the solstices and equinoxes
The Effect of Latitude
Figure SE-I-2 shows how insolation levels vary with latitude. Because of
this variation, latitude has a powerful influence in determining seasonal
conditions and the annual patterns of environmental and climatic parameters
such as precipitation and temperature.
Figure SE-I-2: Areas a, b, and c are all the same size, yet
they all receive different amounts of the sun's "rays."
Different Climatic Zones
The same season can be quite different in the Tropical, Temperate
and Polar zones. These seasonal differences are based on the duration
and directness of insolation. See Figures SE-I-2 and SE-I-3.
Figure SE-I-3
Two Key Factors That Affect Precipitation Levels
Amount of Water Vapor: Evaporation is how most of the water vapor
enters the atmosphere, and air near large bodies of water such as oceans
have the highest levels of water vapor. Also, higher temperatures increase
evaporation rates. Consequently, air in tropical regions downwind from
large sources of water tends to have the highest levels of water vapor,
while air in temperate and polar regions in the center of large continents
tends to have the driest air. In this example, geography influences
amount of water vapor that influences precipitation levels.
Temperature: Though evaporation increases as temperatures rise,
warm air holds more moisture than cool air. Warm air can cool in several
ways. On a local level, the atmosphere cools at night, and the morning
dew is the result of the water vapor condensing on cool surfaces. Warm
air masses can move to cooler locations. Many storms start as warm, humid
air masses that move to higher, cooler altitudes and latitudes. In this
example, latitude influences temperatures which influence precipitation
levels. Finally, increases in elevation cause air to cool. Generally, the
atmosphere cools 1° C for every 150 meter rise in elevation. A considerable
percentage of the water vapor in air rising over mountains condenses and
falls as precipitation. In this example, elevation and geography
influence
temperatures which influence precipitation levels.
Figure SE-I-5
Geographical Features
Geographical features have profound impacts on nearby regions. For example,
mountain chains can cause moist air to rise and precipitate out almost
all of its moisture. When this desiccated (depleted of moisture)
air descends to the regions behind the mountain chain, it creates a rain
shadow. See Figure SE-I-5. Many deserts are found in such rain shadows.
In addition to arid land, typical desert regions lack the atmospheric moisture
that acts as insulation between the Earth's surface and space (water is
the major greenhouse gas on Earth). Consequently, desert areas easily radiate
their heat energy out to space, and day and night temperature differences
are considerable.
Other Geographical Factors: Continental and Marine
Climates
Marine climates have larger amounts of moisture and smaller temperature
changes from summer to winter than continental climates. However, the size
of a continent affects both the temperature range and the amount of moisture
in the interior - the larger the continent, the larger the effect. See
Figure SE-I-4.
Figure SE-I-4
Elevation
Changes in elevation can affect the environment as much as changes in latitude.
Temperature falls approximately 1°C for every 150 meter increase in
elevation, and, in terms of growing season, every 300 m increase in elevation
is roughly equivalent to moving toward the nearest pole by 400-500 km (roughly
four to five degrees of latitude). Mountain tops can be thought of as climatic
islands where, in the Northern Hemisphere, northern species extend their
ranges southward onto mountains where conditions resemble those of more
northern latitudes. Plants growing on the top of New Hampshire's Mt. Washington
(1,935 m) would feel right at home growing at sea level in the Arctic tundra,
2,400 km to the north in Canada. See Figure SE-I-6.
Figure SE-I-6: A comparison of elevation, temperature, biome and latitude
on Mt. Washington
Global Energy Transfer Systems
As illustrated in Figure SE-I-2, the tropics receive more energy from the
sun per unit of surface area than temperate or polar zones. In fact, even
though the warmer tropics radiate more heat to space than high latitude
regions, the tropics receive more energy from the sun than they radiate
away! Where does this excess energy go? The circulation of the atmosphere
and the oceans carries this energy, in the form of heat, to higher latitudes.
See Figure SE-I-7.
Figure SE-I-7: The rising of heated air and the sinking of
cooled air drives atmospheric convection cells.
If we consider the average north-south motion of the atmosphere, warm air
from near the equator rises and moves toward the poles. At roughly 30°
latitude, the air cools, falls and moves equatorward near the surface.
A similar pattern exists in the polar zones, with air rising at roughly
60° latitude and falling at the poles. Since the tropical and polar
zones bracket the temperate zones, the tropical and polar circulations
drive the circulation patterns of the temperate zones. As a result, the
air in temperate zones moves poleward at low altitudes, rises at roughly
60°, returns equatorward aloft and falls at roughly 30°.
In the oceans, strong currents such as the Gulf Stream, the Brazil,
the East Australia, and the Kuroshio carry warm water from the tropics
to latitudes of roughly 50°. Less prominent currents also contribute
to this heat transport. Consequently, regions at high latitudes adjacent
to an ocean, such as Ireland, have climates typically associated with regions
at lower latitudes.
How Are Components of the Earth System Impacted by Seasonal
Changes?
The atmosphere is perhaps the most obvious in its seasonal changes.
There are annual cycles in temperature and precipitation. Hurricanes and
tropical storms are season-dependent, as are droughts and monsoons. Storm
systems result from large-scale movements of air masses that are strongly
affected by seasonal changes.
Earth's ecology has adapted to Earth's seasonal changes in some
remarkable ways. Animals migrate during the year to avoid extreme conditions.
Most species have annual reproductive cycles. Plants have their highest
photosynthesis levels in the summer when the sun is highest, and then some
drop their leaves so that they do not drain their energy resources during
the winter. Seeds germinate when soil temperature and moisture are favorable.
Soil conditions vary seasonally. For example, seasonal biological
changes such as leaves falling enrich the soil. Soil conditions also vary
seasonally as a result of changes in precipitation patterns, and your students
might find differences in the rate at which rain soaks into the ground
at different seasons.
The hydrologic cycle shows seasonal changes in all aspects of
the water cycle. Rainy and dry seasons affect the quantity and quality
of water in rivers and lakes. Catastrophic flooding can occur in spring
as winter snows melt. Seasonal monsoons are essential for the replenishment
of water reserves in many parts of the world.
NOAA National Geophysical Data Center, Boulder, Colorado
This page modified from materials in the online GLOBE Teacher's Guide
http://www.globe.gov/sda-bin/wt/ghp/tg+L(en)+P(seasons/Welcome)