Measurement of Air Temperature

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Measurement of Air Temperature
The freezing point of water in Fahrenheit scale
is 32 F, and the boiling point is 212 F. This
represents a range of 180 F. Converting from
Fahrenheit to Celsius and the reverse can be done
using these formulasC 5/9 (F-32)F 9/5
C32 Try converting 50F to Celsius.
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Measurement of Air Temperature
A comparison of the Celsius and Fahrenheit
temperature scales. At sea level, the freezing
point of water is at Celsius temperature (C) 0,
while it is 32 on the Fahrenheit (F) scale.
Boiling occurs at 100C, or 212F.
This white wooden louvered box houses
maximum-minimum thermometers and other
instruments.
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The Daily Cycle of Air Temperature
The earth rotates on its axis, so consequently
incoming solar energy varies during each day. We
can compare insolation at each solstice and
equinox. What to look for in these graphs
Notice how insolation peaks at the same time but
its magnitude varies. Notice how net radiation is
positive at noon but may be negative in the early
morning or late night. Notice how temperature
maxima occur later than noon, due to a time lag
effect.
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The Daily Cycle of Air Temperature
a).
b).
c).
Daily cycles of insolation, net radiation, and
air temperature. These three graphs show
idealized cycles of insolation, net radiation,
and air temperature for a midlatitude station in
the interior United States. Insolation (a) is a
strong determiner of net radiation (b). Air
temperatures (c) respond by generally increasing
while net radiation is positive and decreasing
when it is negative.
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Urban and Rural Surfaces
In rural areas, the land surface is normally
covered with layer of vegetation. In the process
of transpiration water is taken up by plants and
moved to the leaves, where it evaporates. Since
evaporation cools a surface by removing heat, we
would expect the rural surface to be cooler.
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Urban and Rural Surfaces
Urban surfaces (left) are composed of asphalt,
concrete, building stone, and similar materials.
Sewers drain away rainwater, keeping urban
surfaces dry. Rural surfaces (right) are composed
of moist soil, covered largely by vegetation.
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Urban Heat Islands
Air temperatures in the central region of a city
are typically several degrees warmer than those
of surrounding suburbs and countryside.
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Urban Heat Islands
On a summer afternoon, temperatures over an urban
area are highest, while temperatures remain lower
over surrounding rural land. The lines of equal
air temperature delineate a heat island.
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Temperature Structure of the Atmosphere
How does air temperature change with
altitude? In the troposphere, temperatures
decrease with increasing altitude. This is
referred to as the lapse rate. It is usually
described as the drop in temperature in degrees C
per 1000 m (or degrees F per 1000 ft.) At high
elevations we can expect temperatures to be
colder than at sea level, as at these elevations
the air is less dense and is more distant from
the potential warming influence of the surface.
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Temperature Structure of the Atmosphere
A typical environmental temperature lapse-rate
curve for a summer day in the midlatitudes. The
rate of temperature decrease with elevation, or
lapse rate, is shown at the average value of
6.4C/1000m ((3.5F/1000ft). The tropopause
separates the troposphere, where temperature
decreases with increasing elevation, from the
stratosphere, where temperature is constant or
slightly increases with elevation.
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Temperature Changes With Altitude
What to look for Notice the decrease in
temperature as altitude increases except for
Vincocoya (Vincocoya has an urban heat island).
Notice the increase in the daily temperature
range with altitude (as the air is less dense,
there is less water vapor and carbon dioxide to
warm the surface at night).
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Temperature Changes With Altitude
Daily maximum and minimum air temperatures for
mountain stations in Peru, lat. 15S
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Temperature Inversion
When air temperature increases with altitude, an
inversion is present. This can develop on clear
nights when the surface loses longwave radiation
to space. What to look for in this graph Notice
that there is now a bend in the temperature
profile. Notice that the temperature at the
ground is below zero degrees Celsius. Should
water condense and freeze on surfaces, this will
lead to frost.
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Temperature Inversion
A low-level temperature inversion with frost.
Instead of temperature decreasing with elevation
(dashed line) from, say, 4C (39F), the surface
temperature is at -1C (30F) and temperature
increases with elevation (solid line) for several
hundred meters (1000 ft. or so) above the ground.
Since the surface is below the freezing point,
frost can form
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The Annual Cycle of Air Temperature
As the Earth revolves around the Sun, the tilt of
the Earth's axis causes an annual cycle of
variation in insolation. This cycle produces an
annual cycle of net radiation, which, in turn,
causes an annual cycle to occur in mean monthly
temperature.
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The Annual Cycle of Air Temperature
Net radiation (a) and temperature (b) cycles for
four stations ranging from the equatorial zone to
the arctic zone. Aswan (Egypt), Manaus (Brazil),
Hamburg (Germany), and Yakutsk (Siberia).
What to look for Notice how temperature changes
vary according to the latitude of each location.
Notice how net radiation can become negative in
high latitude locations. Notice how net radiation
at Manaus has two peaks because of its close
proximity to the equator.
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Land and Water Contrasts
Ocean temperatures vary less than land
temperatures because water heats more slowly,
absorbs energy throughout a surface layer, and
can mix and evaporate freely. As a result,
temperatures at maritime locations are less
variable from day to night and from summer to
winder when compared to continental locations.
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Land and Water Contrasts
A recording thermometer made these continuous
records of the rise and fall of air temperature
over a period of a week in summer at San
Francisco, California, and at Yuma, Arizona. The
daily cycle is strongly developed at Yuma, a
station in the desert. In contrast, the graph for
San Francisco, on the Pacific Ocean, shows a very
weak daily cycle.
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Land and Water Contrasts
Four differences that illustrate why a land
surface heats more rapidly and more intensely
than the surface of a deep water body. Locations
near the ocean have more uniform air temperatures
- cooler in summer and warmer in winter - because
of these differences.
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Mapping Surface Temperatures
The distribution of air temperature is often
shown on a map by isotherms - lines drawn to
connect locations having the same temperature.
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Mapping Surface Temperatures
Isotherms are used to make temperature maps. Each
line connects points having the same temperature.
Where temperature changes along one direction, a
temperature gradient exists. Where isotherms
close in a tight circle, a center exists. This
example shows a center of low temperature.
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Mean Monthly Air Temperatures for January and July

• There are six dominant factors controlling
  surface temperatures
• Temperatures decrease from the equator to the
  poles.
• Large landmasses located in the subarctic and
  arctic zones develop centers of extremely low
  temperatures in winter.
• Temperatures in equatorial regions change little
  from January to July.
• Isotherms make a large north-south shift from
  January to July over continents in the
  midlatitude and subarctic zones.
• Highlands are always colder than surrounding
  lowlands.
• Areas of perpetual ice and snow are always
  intensely cold.

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Mean Monthly Air Temperatures for January and July
Mean monthly air temperatures (C) for January
and July, respectively.
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Mean Monthly Air Temperatures for January and
July (continued)
These Mercator projections show temperature
trends from the equator to the midlatitude
zones. What to look for Notice how the isotherms
bend southwards over North America in January,
but northwards in July. Notice how the isotherms
along the west coast of North America bend
northwards in January, but southwards in July.
Notice how an intense area of heat develops over
Australia in January (their summer), and that in
July a temperature gradient runs from south to
north. The polar projections give the best
picture for high latitudes. What to look
for Notice how the isotherms are much closer
together over the Antarctic in July (Summer) than
in January. Notice how the isotherms bend
northwards over the North Atlantic in January.
This is because of the contrast between cold
winter temperatures and the warming influence of
the Gulf Stream.
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Mean Monthly Air Temperatures for January and
July (continued)
Mercator and polar projections.
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Global Warming and the Greenhouse Effect
One of the most important issues in the study of
climate change is the potential for global
warming as a consequence of a magnified
greenhouse effect. The greenhouse effect is a
natural phenomenon that keeps our planet warm
enough for life to flourish. However, by the
emission of additional greenhouse gasses to the
atmosphere (such as carbon dioxide) we may be
inadvertently increasing that warming and so
causing climate changes. However, as the second
graph shows, the climate has always constantly
changed and it is difficult to differentiate
between that natural "noise" and the
human-induced change.
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Global Warming and the Greenhouse Effect
A forecast of future changes in surface air
temperature (in C) resulting from an effective
doubling of atmospheric CO2 concentration
relative to that of the present.
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Global Warming and the Greenhouse Effect
Mean annual surface temperature of the Earth,
1880-1994. The vertical scale shows departures in
degrees from a zero line of reference
representing the average for the years 1951-1980.
The yellow line shows the mean for each year. The
red line shows a running five-year average.
A reconstruction of the departures of northern
hemisphere temperatures from the 1950-1965 mean,
based on analyses of tree rings sampled along the
northern tree limit of North America.
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