CHAPTER 3 THE CLIMATE SYSTEM: CONTROLS ON CLIMATE
Latitude
 Latitude lines are imaginary lines that run from west to east around the earth, paralleling the
equator and each other
o Locations with higher latitudes generally experience lower temperatures than
locations at lower latitudes.
o By contrast, lines of longitude are are imaginary north-south lines running through the
north and south poles perpendicular to the equator and every other parallel of latitude
 Variations in latitude (in addition to variations in time of day and time of year) cause solar
energy to strike the surface at some varying angle between 90° (when the ray is directly
overhead) and 0° (when the sun is on the horizon)
Earth-Sun Relationships
 Revolution is earth’s orbit around the sun; it creates a distinct impact on temperature on
earth when combined with other factors
o The earth-sun distance varies from about 147.09 million km (91.36 million mi), on
or about January 3 – termed perihelion – to about 151.92 million km (94.36
million mi), on or about July 4 – termed aphelion
 Rotation refers to the spin of the earth on its axis; rotation causes differing longitudes to
experience the most direct ray of the sun that they will receive at different times within the
day
 Axial Tilt and Parallelism
o The 23.5° axial tilt can be thought of as being “fixed” because it is independent of
the time of year; the axis is pointed “parallel to itself” as earth revolves around the
sun
 The Combined Effect of Revolution, Rotation, and Tilt
o In June, more than half of any parallel of latitude in the northern hemisphere is in
the daylight sector and more than half of any parallel of latitude in the southern
hemisphere is in darkness
o In December, more than half of any parallel of latitude in the northern hemisphere
is in darkness while more than half is in daylight in the southern hemisphere.
o The closer to the pole the line of latitude is (in either hemisphere), the greater the
difference in day length from June to December.
o Day length changes throughout the year as the proportion of a parallel of latitude
inside the circle of illumination changes
o The tilt and revolution cause all lines of latitude to receive differing amounts of
solar radiation throughout the year and the direct rays of the sun to strike at
latitudes varying from 23.5°N (the Tropic of Cancer) to 23.5°S (the Tropic of
Capricorn)
Distance to Large Bodies of Water
 Large water bodies (especially oceans) are capable of storing huge quantities of energy
during high-energy times (summer) and releasing this energy slowly to the atmosphere
during low-energy times (winter)
o The specific heat of water is about five times greater than that of land
 Assuming that all other factors are equal, summers over or near oceans are not as warm as
they are in the interiors of continents
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Likewise, winters in coastal and oceanic locations are generally not as severe as they are in
inland locations
In addition, the onset of seasons is delayed significantly over and near oceans because of
the oceanic absorption of the energy through the summer and slow release of it to the
atmosphere from autumn through winter.
Water in the form of clouds also plays a significant role in daily temperature variations
Circulation
 The mission of the atmospheric circulation is to balance energy inequalities across the
latitudes
 Pressure
o A measure of the amount of force applied to a given surface area
o Comparison of pressures on a map is usually done by connecting locations that
have the same sea-level-corrected pressure using isobars – lines of equal
pressure
o In general, “high pressure” indicates pressures above that of mean sea level, while
“low pressures” are considered to be those pressure readings below that of mean
sea level
 Wind
o Air moves in relation to pressure imbalances in either the vertical or horizontal
o Pressure imbalances are caused directly by thermal inequalities occurring at all
spatial scales
o The pressure gradient force (PGF) is the force that initially causes air motion;
induces air to move laterally from one location to another
o Wind speed is proportional to the pressure difference between the high- and lowpressure areas
o Coriolis Effect (CE) alters wind direction and is caused by differences in the
earth’s angular velocity about the local vertical
o Coriolis deflection is to the right of the direction of flow in the northern hemisphere
and to the left of the direction of flow in the southern hemisphere, with deflection
proportional to wind speed and latitude
o Centrifugal force (CF) is an apparent outward-directed force on an object moving
along a curved trajectory and affects winds that flow in a turning motion
o Friction not only slows air, but it also indirectly alters the trajectory because both
the CE and CF are dependent on wind speed
 Surface Versus Upper-Level Winds
o The varying effect of friction with height creates great differences in the
representation in computer models of winds at the surface compared to those in
the free atmosphere
 Vertical Motion
o The tendency for air to rise from the surface (a high-pressure area) to lowerpressure regions aloft (buoyancy) is balanced by the downward force of gravity
 Cyclones and Anticyclones
o If a cyclone – an enclosed area of low pressure – exists at the surface, it is
characterized by a convergence of winds into that low-pressure center which
initiates rising motions
o Because of friction near the surface, air crosses closed isobars at some angle in a
surface cyclone, but in the free atmosphere, air flows parallel to the isobars in a
counterclockwise motion above the cyclone
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o Anticyclones, or enclosed areas of high pressure, have opposite circulation
patterns from cyclones; they are characterized by surface divergence because the
PGF will initially pull air outward away from the high-pressure core in an effort to
eliminate the pressure gradient
o In cyclones surface convergence is linked to upper-air divergence, while with
anticyclones upper-air convergence is associated with the surface divergence
below it
Oceanic Circulation
o Surface ocean currents are considered to be those that extend to approximately
100 m (330 ft) below the surface; they are driven primarily by surface winds
o The momentum involved in moving that surface column of water is then
transferred downward to the next few meters of water, with increasing friction and
therefore a decrease in CE; the result is a spiral of water motion extending from
the surface downward to a depth of approximately 100 m – the Ekman spiral
o Deep water ocean currents are initiated by thermohaline inequalities, ultimately
resulting in a globally-connected deep ocean conveyor belt that greatly affects
climatic conditions with global implications
Topography
 If a location is mountainous, the climate will vary widely across short distances because of
rapid changes in altitude (and therefore temperature) and the amount of exposure to the sun
through the year
 Because of mass and density decreases with height, it is common to see fairly high diurnal
temperature ranges at high elevations compared to other locations that are lower in elevation
but occur along the same parallel of latitude
 Mass and density changes also affect the behavior of gases such that at high elevations
water evaporates more quickly than in a denser atmosphere
 When air is forced up the side of a slope, an orographic effect may enhance precipitation
totals
Local Features
 Such factors include the presence of lakes (both large and small), wetlands, forests,
agricultural areas, urban areas, and many other local features
Putting It All Together: Spatial and Seasonal Variations in Energy
 Isotherms, which are lines of constant temperature, can be used to map the influence of the
various controls on climate
 In July, isotherms push poleward over the warm continents of the northern hemisphere as a
direct result of continentality influences
 In January, the northern hemisphere shows very pronounced continental impacts as
isotherms dip dramatically toward the equator over the continents