Meteorology: Understanding the Atmosphere            Ackerman and Knox



Climate Spatial Scales

Atmospheric circulation patterns are of critical importance in determining the climate of a location.   On a global scale, atmospheric motions transport heat from the topics towards the poles.   Evaporation over the oceans supplies the water molecules that support precipitation over land.   These circulation patterns are in large part driven by energy differences between regions of the globe.   As discussed in Chapter 7, dry climates are associated with the descending branch of the Hadley Cell while moist climates coincide with the ascending branch. On a regional scale, precipitation on the lee side of a mountain is typically less than on the windward side.   On a still smaller scale, the amount of snow downwind of a snow fence is on average larger than the amount upwind.


Global climate is the largest spatial scale.   We are concerned with the global scale when we refer to the climate of the globe, its hemispheres, and differences between land and oceans. Energy input from the sun is largely responsible for our global climate.   The solar gain is defined by the orbit of Earth around the sun and determines things like the length of seasons.   The distribution of land and ocean is another import influence on the climatic characteristics of the Earth.   Contrasting the climate of the Northern Hemisphere, which is approximately 39% land, with the Southern Hemisphere, which only has 19% land, demonstrates this (see the table below).   The yearly average temperature of the Northern Hemisphere is approximately 15.2C, while that of the Southern Hemisphere is 13.3C. The presence of the water reduces the annual average temperature.   The land reduces the winter average temperature while increasing the average temperature during summer.   As a result, the annual amplitude of the seasonal temperature is nearly twice as great for the Northern Hemisphere. The Northern Hemisphere has a large variation in the monthly mean temperature.

The land absorbs and loses heat faster than the water.   Over land, the heat is distributed over a thin layer, while conduction, convection and currents mix the energy over a fairly thick layer of water. Soil, and the air near it, therefore follow radiation gains more closely than water.   For this reason, continental climates have a wider temperature variation.   We observed this in Chapter 3 by comparing the seasonal cycles of temperatures for different regions of the globe.

The average temperatures of the Northern Hemisphere and Southern Hemisphere for winter, summer and the year.   The Annual Range is give as well as the differences between the Hemispheres.   Differences between the Hemispheres are caused by the differences in the distribution of land and water.

Annual Range
8.1C (46.6F)
22.4C (72.3F)
15.2C (59.4F)
14.3C (25.7F)
9.7C (49.5F)
17.0C (62.6F)
13.3C (55.9F)
7.3C (13.1F)
-1.6C (-2.9F)
5.4C (9.7F)
1.9C (3.5F)
7.0C (12.6F)


Regional climates

The major factors that determine global climate also influence climate on a regional scale. Regional climates are influenced by water bodies and mountain ranges. Lakes exert a moderating influence on local climate, in a manner similar to how oceans affect larger climate.   The Great Lakes are a good example for demonstrating the impact of lakes on climate.   We saw in Chapter 7 how the Great Lakes effect snow fall.   The Great Lakes also influence the temperature of the region.   The figure below shows the average land temperature versus the average surface lake temperature in the Southern Lake Michigan region. The temperature of the water is lower than the land from mid-March to August. Largest differences occur from mid-May to early June.   The water temperature is greater than that of the land from late August to mid March, with the largest differences in late November and early-December in late autumn and winter.   Exchanges of heat and moisture above the lakes is the key to weather modification by the Great Lakes.   The influence of large water bodies on the weather of surrounding regions is most marked when the temperature differences are greatest. .

The average land temperature versus the average surface lake temperature in the Southern Lake Michigan region.  

Why is there a lag in the temperature of the lake water?



Large mountains influence regional climates.   They provide barriers for the air. Large mountain ranges that are oriented east-west can block cold air outbreaks from reaching regions that are more southern. You can observe this by comparing the annual mean temperature of a city south of large mountain barrier with a city at a similar latitude but with no mountain barrier.   Lahore, India, (31.5N) located south of the Himalayan Mountain Range has an average temperature of 12.8C, while the temperature of Austin, TX USA (30.25N) has an average temperature of only 10.4C.

Vegetation also affects regional climate, an observation made obvious when comparing the wind speed within a forest with the wind speed at the same height over an open field. Friction reduces the wind speed in the forest, so open areas have greater winds.   The relative humidity is usually greater in a forest than in the surrounding open country. Forests depress the summer temperatures by 1 to 2 C (2-4F) below the annual mean in their vicinity. This temperature difference is driven by heat budget differences; less solar energy reaches the forest floor than the open field.


Variations in climate can be observed over a short distance. Small-scale climates are referred to as microclimates.   Examples of microscale climates include the climate of a cornfield, a house, a patio, or a sand dune.   Microclimates can be very different across a particular region. Topography, presence or absence of water, exposure to the sun, and soil conditions are important factors that determine microclimates.

The presence or absence of snow can be a good indication of differences in microclimates.   Variations in temperature due to differences in exposure to the sun affect accumulation and melting.   South facing slopes generally retain smaller snow amounts than north facing slopes (Chapter 4).   Snowdrifts are generated by rapid changes in wind speed due to the interaction of winds with obstacles.