I. Climate Controls, Climate Patterns, Climate
Classification
A. Climatic Controls
1. intensity of solar radiation and its variation
with latitude
2. distribution of land and water
3. ocean currents
4. prevailing winds
5. positions of high- and low pressure areas
6. mountain barriers
7. altitude
B. Pattern of Global Temperatures
· In both hemispheres isotherms are
oriented east-west reflecting the fact that locations at the same latitude
receive nearly the same amount of solar radiation.
· Additionally, the annual solar heat
energy that each latitude receives decreases toward the poles; as a result,
annual temperatures decrease toward the poles.
· Isotherms (lines of constant
temperature) bend along the coastal margins as a result of land water heating
differences, ocean currents, and upwelling. (E.g., along the west coasts
of North and
· Because temperature changes are far less
conservative over land versus large bodies of water, variation in temperature
between summer and winter is far greater over continental interiors than along
the west coastal margins of continents. The climates of the interiors of
continents are much more extreme, having (on average) higher summer
temperatures and lower winter temperatures than their west coast
counterparts. West coast climates tend to be quite mild for their
latitude.
· The highest mean temperatures do not
occur in the tropics, but instead in the subtropical deserts of the Northern
Hemisphere as a result of subsidence associated with the subtropical high
pressure cells. The subsidence produces clear skies and low levels of
humidity. During the summer months the barren landscapes absorb
significant amounts of insolation from the high sun
which leads to very high temperatures.
· The lowest mean temperatures occur over
the continental interiors of the high latitudes.
· The coldest area of the world is the
Antarctic. The extremely cold temperatures there result from the fact
that during part of the year the sun is below the horizon, and during the part
of the year when it is above the horizon, it is so low in the sky that it does
not provide much heating to the surface below. As a result the surface
remains snow and ice covered. The snow and ice reflect 80% of the
sunlight that strikes the surface. That which is not reflected transforms
the ice and snow to water vapor.
· The Antarctic’s low humidity and high elevation allows for strong radiational cooling during the dark winter months producing extremely cold surface temperatures. The extremely cold Antarctic explains in part, why, overall, the Southern Hemisphere is cooler than the Northern Hemisphere. Other contributing factors for a cooler Southern Hemisphere include the fact that there is less land area in the tropical and subtropical areas of the Southern Hemisphere.
C. Pattern of Global Precipitation
· Precipitation varies from place to
place; however, certain regions stand out as being wet or dry.
· Equatorial regions are typically wet,
while the subtropics and polar regions are relatively dry.
· The global distribution of
precipitation is closely tied to the general circulation of the atmosphere and
to the distribution of mountain ranges.
· Precipitation is most abundant where
air rises, and least abundant where it sinks.
As a result copious amounts of precipitation are expected in the
tropics, along the polar front, and very little precipitation is expected near
the subtropical highs and at the poles.
· Because the major wind and pressure
belts shift seasonally northward in July and southward in January, the area
between the rainy tropics and the dry subtropics is influenced by both the ITCZ
and the subtropical highs.
· In the cold polar regions the air is
so dry that there is little precipitation. Any snow that does fall,
however, remains on the ground because of the low rates of evaporation.
· There are exceptions to the idealized
pattern of precipitation. E.g., in the middle latitudes the migrating position
of the subtropical highs has an effect on the west-to-east distribution of
precipitation.
· The sinking motion associated with the
anticyclones is more fully developed on the eastern sides. Hence, the air
along the eastern side of an anticyclone tends to be more stable; it is also
drier, as cooler air moves equatorward because of the
circulating winds around the highs.
· Additionally, along coastlines, cold
upwelling water cools the surface air even more, adding to the air’s
stability.
· As a result,, in the summer, when the
Pacific high moves to a position off the
· On the western side of subtropical
highs, the air is less stable and more moist as warmer air moves poleward. In summer, over the North Atlantic, the
Bermuda high (the same high pressure
system that gives NC hot, humid, and sunny summer weather) pumps moist
tropical air northwards from the Gulf of Mexico into the eastern two thirds of
the United States. The conditionally unstable air is heated by the warm
ground below and becomes even more unstable leading to the development of
thunderstorms.
D. Climate Classification
Defn: "weather"-
the short-term condition of the atmosphere at any given time & place
Defn: "climate"
- the consistent long-term, behavior over time - including its variability (its
extremes)
Defn: "climatology"
- the study of climate & climatic patterns
Defn: "climatic
regions" - areas of similar climate - they contain characteristic
regional weather & air mass patterns
1. Classification of Climatic Regions
a. "genetic classifications" - a type of classification
that uses causative factors to determine climatic regions; e.g.,
one based on air mass dominance
b. "empirical classification" - a classification based
on weather statistics – it’s used to determine general climate
categories
(1) The "Koppen Classification
Scheme" (it’s an empirical classification scheme, and one of the
most commonly used and important classification schemes)
A = Tropical moist climates (found in the equatorial regions)
·
All
months have an average temp above 18 C (64 F) - no real winter
B = Dry climates: have deficient precipitation
·
Most of
the year; POTET (potential evaporation) exceeds precipitation
C = Moist mid-latitude climates with mild winters
·
These
climates have warm-to-hot summers with mild winters. The average temp of the coldest month < 64 & > 27 F
D = Moist mid-latitude climates w/severe winters:
·
These
climates have warm summers w/cold winters. The average temp of the warmest month exceeds 50 F; the avg. temp of the coldest month falls below 27 F
E = Polar climates: extremely cold winters & summers.
·
The avg.
temp of the warmest month is
below 50 F. There is no real summer
season.
H =
Compared to lowlands at the same latitude, highlands have lower temperatures
Each of the above groups contains subregions that describe special regional characteristics, such as seasonal temperature & precipitation. Lowercase letters are used to designate the subregions.
Af = tropical wet - wet all seasons
Aw= tropical wet & dry - winter dry season
To Summarize:
The First Letter = defined on basis of annual temp regime (from hot
tropical, to cold polar (A,C,D,E)
Second Letter = characterizes moisture regime
3rd letter = for a, c, d - further subdivides the temperature regime
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Example: the “Humid Subtropical
Climate” (Cfa) -- hot summers,
without a dry season (this is
· Cfa climates have hot summers, their warmest month must be above 71.6 F (a)
· Their coldest month must be above 32 F, but below 64.4 F
·
They have year round precipitation (f)
Humid subtropical hot summer climates (Cfa climates) are influenced by maritime tropical air
masses generated over warm waters off eastern coasts. This warm moist unstable air produces
convectional showers over land. In fall,
winter, and spring, mT and cp air interact,
generating frontal activity and frequent midlatitude
cyclonic storms. Overall precipitation
averages 40-80 inches a year.
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