CHAPTER #03
Seasonal and Daily Temperatures
The earth makes one complete rotation on its axis approximately once
every 24 hours. As a result, at any point in time, half the planet is in
darkness (night) and the other half is illuminated by solar radiation (day).
The earth makes one complete rotation around the sun in one year, 365.25
days. The earth's orbit departures from a circular orbit, eccentricity,
therefore, the earth-to-sun distance varies by about 3.3% through the year.
Earth is closest to sun, 147 million km (91 million mi). on about January
3, perihelion, and farthest from the sun, 152 million km (94 million
mi), on about July 4, aphelion.
Eccentricity = [1 + 0.033 cos (2 pi * day of the year /365)]
Earth sun distance in meters = 150 * 109 /(Eccentricity)0.5meters
Figure 3.1, page #60 (Ahrens)
The earth is tilted by an angle of 23û27' with respect to normal to
the sun, obliquity. This tilt causes the earth's orientation to
change continually as the planet revolves about the sun and explains the
seasons. The Northern Hemisphere leans away from the sun during winter
and leans toward the sun during summer. When the Northern Hemisphere leans
away from the sun, the Southern Hemisphere leans toward the sun and vice
versa.
The sun is directly over the equator at noon on March 21, Vernal
equinox, and on September 23, Autumnal equinox. Day and night
are equal length (12 hours) everywhere.
The sun is directly over 23û27'N (Tropic of Cancer), its northern position,
summer solstice, at noon on June 21. The daylight is continuous
north of 66û33'N (Arctic Circle), while no daylight is present south of
66û33'S (Antarctic Circle). Elsewhere days are longer than nights in the
Northern Hemisphere, where it is summer, days are shorter than nights in
the Southern Hemisphere, where it is winter.
The sun is directly over 23û27'S (Tropic of Capricorn), its southern
position, winter solstice, at noon on December 21. The daylight
is continuous south of 66û33'S, while no daylight is present north
of 66û33'N. Elsewhere days are shorter than nights in the Northern Hemisphere,
where it is winter, days are longer than nights in the Southern Hemisphere,
where it is summer.
Figure 3.3, page #61 (Ahrens)
The total amount of solar radiation received at the earth's surface
varies seasonally. For the same intensity of solar radiation, any given
location on Earth accumulates more solar energy during long days of summer
than during short days of winter. The intensity of solar radiation, itself
also varies seasonally as the sun moves from its northern position (23û27'N)
on June 21 to its southern position (23û27'S) on December 21 and back again.
The solar radiation reaches its maximum where the sun is directly overhead
at local noon between 23û27'N and 23û27'S.
The seasonal (winter-to-summer) contrast in length of day increases with
increasing latitude. The lenght of the day (Lday) is calculated as a function
of latitude and declination angle.
Lday = (2/15) cos-1 (-tan(L) * tan (d))
Solar constant: the rate at which the solar radiation is received
at the top of the atmosphere perpendicular to the sun's rays when the earth
is at a mean distance from the sun. It is approximated as 1.97 calories
per cm2 in energy units or 1372 watts per m2 in power
units. The intensity of solar radiation is inversely proportional to the
square of the earth-to-sun distance such that the planet earth receives
about 6.7% more radiation at perihelion than at aphelion. Therefore, the
solar constant ranges from 2.04 cal/cm2 at perihelion and 1.91
cal/cm2 at aphelion. The solar constant is related to the total amount of energy that has been output from the sun,
luminosity,(3.827 x 10 26 ) Watts.
solar constant = luminosity / (4 pi earth sun distance 2)
As a result of perihelion/aphelion contrast in solar energy, the Southern
Hemisphere receives more radiation in summer and less radiation in winter
than the Northern Hemisphere. This may lead a greater winter-to-summer
temperature contrast in the Southern Hemisphere than in the Northern Hemisphere.
However, the relatively larger percentage of ocean surface area in the
Southern Hemisphere exhibits greater thermal stability, which modifies
the seasonal temperature and largely offsets the greater seasonal contrast
in insolation.
Pyranometer: the instrument that measures the intensity of solar
radiation striking a horizontal surface. It consists of a sensor enclosed
in a glass bulb that transmits total short-wave insolation. The sensor
is a disk consisting of alternating black and white segments. The temperature
difference of the black and white segments for the same intensity of solar
radiation is calibrated as a flux in units of cal/cm2 or W/m2.
The pyranometer should not affected by shadows, by any highly reflective
surfaces nearby, or by other sources of radiation. The glass bulb must
also be kept clean and dry.
Thermometer: the instrument that monitors the changes in air
temperature. A liquid-in-glass thermometer uses either mercury which freezes
at -39degC (-38degF) or alcohol which freezes at -130degC (-202degF). As the air
temperature rises, the liquid in the glass tube expands and rises, while
the liquid contracts and drops in the tube as the temperature decreases.
Maximum thermometer, reads maximum temperature over a specified
period, usually uses mercury as a liquid and has small construction in
the tube just above the bulb.
Figure 3.29 page#86 (Ahrens)
Minimum thermometer, reads minimum temperature over a specified
period, uses alcohol as a liquid and has a index marker that is free to
slide back or forth within the liquid.
Figure 3.30 page #87 (Ahrens)
Electrical resistance thermometer consists of platinum or a nickel wire and an electrical meter.
As the temperature increases, the resistance of the wire increases. The resistance is measured
by an electric meter and is calibrated to represent air temperature. It is highly accurate and
is used as part of Automated Surface Observing Systems (ASOS).
Thermistor thermometers are made of ceramic material whose resistance increases as temperature
decreases. They are used in radiosondes. Thermocouple, another type of electrical thermometer,
operates as weak electrical current develops in the circuit at the junction of two dissimilar
metals through temperature differences.
Radiometers do not measure the temperature directly, rather they measure the emitted radiation.
They are designed to measure the maximum intensity of the radiant energy at a particular
wavelength. Although they are primarily develop to retrieve the temperature of the atmospheric
layers through satellites, radiometers mounted on research aircrafts are used to validate
satellite remote sensing.
Bimetallic thermometer consists of two different metals (brass and iron) that are welded together side by
side and are linked to a pointer and a calibrated dial via a series of gears or levers. The pointer
may be linked to a pen and a clock-driven drum to give continuous trace
of temperature variations with time. The instrument is called thermograph.
Figure 3.32, page #88 (Ahrens)
The two main characteristics of a thermometer are its accuracy and response
time. A thermometer that is reliable to within 0.3ûC (0.5ûF) is sufficient
for meteorological purposes. Most liquid-in-glass and electrical resistance
thermometers have rapid response time, while, bimetallic thermometers tend to
be more sluggish.
For accurate measurements of air temperature, ideally a thermometer
should be adequately ventilated and shielded from precipitation, direct
sunlight, and the night sky. Thermometers are located inside a white painted
shelter mounted about 1.5 meter above the ground. The shelter should be
located in an open grass area away from trees, buildings, and other obstacles.
It should be no closer than four times the height of the nearest obstacle.
Figure 3.14, page #72 (Ahrens)
Specific heat: the amount of heat required to change the temperature
of one gram of a substance by one Celsius degree. It is first proposed
by Scottish chemist Joseph Black in 1760. The specific heat of all substances
is measured relative to the specific heat of water, which has the highest
value.
Table 2.1, page #34 (Ahrens)
The differences in specific heats is one of the reasons why surface
temperatures over land is more varibale than over ocean. A body of water
exhibits greater resistance to temperature change, thermal stability,
than the land surfaces. In addition, solar radiation penetrates water to
significant depths, but not the opaque land surface. Ocean and lakes circulate
and therefore heat is transported to great volumes of water, while it is
conducted only very slowly through the soil.
As a result of thermal stability differences between land and water,
climate of inland location exhibits substantial differences from climate
of coastal location. An average monthly temperature of St. Louis has a
wider variation around the annual mean than the average temperature of
San Francisco.
The degree of maritime influence on average temperature, index of
continentally, is used by climatologists. It is based on the difference
between average winter and summer temperatures. In North America, the maritime
influence is more pronounced in western part of the US and Canada since
prevailing winds are westerly.
Figure 3.25, page #81 (Ahrens)
Heating degree-days: a measure of needs for heating on days when
the average daily temperature falls below 65 ûF (18 ûC). It is computed
by subtracting the average daily temperature from 65 ûF.
Cooling degree-days: a measure of needs for air conditioning
when the average daily temperature is above 65 ûF (18 ûC). It is computed
by subtracting 65 ûF from the average daily temperature.
Figures 3.23, 3.24, page #80 (Ahrens)
Figures 3.27, 3.28, page #83 (Ahrens)
Growing degree-days: a guide to planting and for determining
the approximate dates when a crop will be ready for harvesting. It is 40 F for wheat
and peas, 50 F for beans and corn, and 60 F for rice and cotton.
Table 3.2, page #83 (Ahrens)
Wind chill index (Apparent wind chill): the perceived temperature
to the human body based on both air temperature and wind speed. The lowest
wind chill was observed in Antarctica on August 25, 2005 when the temperature
was -99 degF and the wind speed was 113 mi/hr resulting in wind chill well
below -100 deg F.
Table 3.3, page #85 (Ahrens)
Table 3.4, page #85 (Ahrens)
The global distribution of incoming solar radiation and outgoing infrared
radiation implies net warming of the Earth's surface and net cooling of
the atmosphere. The rate of cooling due to infrared emission exceeds the
rate of warming due to absorption of solar radiation in the atmosphere
and vice versa at the Earth surface.
In reality, the atmosphere is not cooling relative to the Earth's surface.
Heat is transferred from the Earth's surface to the atmosphere via sensible
and latent heating in 31 units. Sensible heating is responsible for 7 units
(about 23%) of heat energy transfer, while 23 unit (about 77%) of heat
energy is transferred by the latent heating.
Figure 2.16, page #50 (Ahrens)
Sensible heating: the transport of heat from one location or
object to another via conduction, convection, or both. Because air is a
poor conductor of heat, convection is much more important than conduction
in heat transfer within the atmosphere.
Latent heating: the transport of heat from one location to another
as a result of changes in phase of water. The quantity of heat that is
involved in phase changes is known as latent heat. It is required
during the process of melting, vaporization, and sublimation and is released
during the process of freezing, condensation, and deposition.
The latent heat of melting and freezing is 80 cal/g, while the latent
heat of vaporization and condensation varies from 540 cal/g at 100 ûC to
600 cal/g at 0 ûC. The latent heat of sublimation and deposition is 680
cal/g at 0 ûC. The specific heat of ice is about 0.5 cal/g ûC, while the
specific heat of liquid water is 1 cal/g ûC.
Bowen ratio: the ratio of heat energy used for sensible heating
to heat energy used for latent heating. It is 0.30 at the global scale
meaning that latent heat is more significant than the sensible heat on
a global scale. Bowen ratio varies from about 0.10 for oceans to about
2.00 for dry region like desert interiors. The drier the land surface is,
the less important is latent heating and the more important is sensible
heating. Bowen ratio is 0.62 for Europe, 1.14 for Asia, 0.74 for North
America, 0.56 for South America, 1.61 Africa, 2.18 Australia. Overall,
It is 0.96 for all land and 0.11 for all oceans.
The heat transfer also occurs from atmosphere to Earth's surface under
specific conditions. For example, a mild air flows over cold snow-covered
ground, or when warm air blows over relatively cool ocean or lake surface.
It occurs frequently at night when radiational cooling causes the Earth's
surface to become colder than the overlying air.
In summary, the Earth's surface is cooled via three processes: i) sublimation
or melting ice, and vaporization of water, ii) conduction + convection,
iii) emission of infrared radiation. The latent heating (23 units) is more
important than the radiative cooling (21 units), which in turn, is more
important than sensible heating (7 units).
The imbalance between radiative heating and radiative cooling also occurs
horizontally with latitude. Both solar and infrared radiation decreases
with latitude, but the latter declines more slowly than the former. As
a result, over the course of a year, the rate of radiative cooling exceeds
the rate of radiative heating at high latitudes and vice versa is true
at lower latitudes.
Averaged over all latitudes, incoming solar radiation must equal the
outgoing infrared radiation following global radiative equilibrium. Satellite
measurements indicate that there is a net radiative warming between 38
ûN and 38 ûS and a net radiative cooling between the poles and 38û latitudinal
circles.
Poleward heat transfer: the flow of heat from tropical to middle
and high latitudes in response to latitudinal imbalances in radiative heating
and cooling. It is accompanied by air mass advection (50%), release of
latent heat in storms (30%) and ocean currents (20%).
Figure 2.17, page #51 (Ahrens)
Imbalances in rates of radiative cooling and heating give rise to both
meridional (north-south) and vertical temperature gradients, which in turn,
causes the heat to transfer within the earth-atmosphere system via conduction,
convection, cloud development, air mass exchange, and storms.
The sun, which is source of kinetic energy, drives the circulation of
the atmosphere by causing the heat imbalances within the earth-atmosphere
system. The kinetic energy of atmospheric circulation is dissipated as
frictional winds blow against the earth's surface.
The rate of heat redistribution within the earth-atmosphere system varies
with season, and, atmospheric circulation and weather also change through
the year. In the presence of strong temperature gradients across the US,
the weather tends to be more energetic in winter, while weather tends to
be calm in summer when air temperatures varies gradually. However, intense
solar heating can trigger strong convection and the development of thunderstorms
in summer afternoon.
The variation in air temperature is controlled by radiation balance
and air mass advection. The radiation balance and local air temperature
is affected by i) time of day, day of the year ii) cloud cover, and iii)
the nature of surface cover. Air temperature is generally higher in June
than in January in the Northern Hemisphere, during the day than at night,
under clear rather cloudy afternoon skies, when the ground is bare instead
of snow covered, and when the ground is dry rather than wet.
Air temperatures are lower in mountain tops than in low lands even though
the former location is closer to the sun. This is because i) more infrared
radiation escapes to space at mountain tops where the concentration of
water vapor is very low, ii) temperature decreases with altitude.
The annual temperature cycle reflects the systematic variation in incoming
solar radiation over the course of a year. In the tropics, a little variation
in solar radiation varies results uniform monthly mean temperatures through
the year. The temperature difference between day and night is often greater
the winter-to-summer temperature contrast in these latitudes.
In midlatitudes, solar radiation exhibits a pronounced annual maximum
and minimum and the seasonal difference in solar radiation is extreme in
polar latitudes. As a results, a distinct winter-to-summer temperature
contrasts are observed in middle and high latitudes.
A lag in mean monthly temperatures is observed in middle and high latitudes.
Typically, warmer temperatures occur about a month after summer solstice
and colder temperatures occur about a month after winter solstice. In the
US, the temperature lags the solar cycle by an average of 27 days. In the
coastal regions, the lag time is up to 36 days. The maritime climate reduces
the amplitude of the annual pattern of monthly mean temperatures meaning
less winter-to-summer temperature contrast in coastal regions.
The day-to-night (diurnal) temperature variation indicates a lag of
several hours such that the warmest temperatures occur mid to late afternoon
even though the insolation peaks around noon. This also explains why in
summer greatest risk of sunburn is about noon but not during the warmest
time of day.
In addition to radiation balance, air temperature is also modified by
air mass advection. Cold air advection occurs when winds blow from colder
to warmer regions, while warm air advection occurs when winds blow from
warmer to colder regions. In some cases, cold or warm air advection overwhelms
the local radiation balance.
