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 * 106 /(Eccentricity)0.5meters

Figure 3.1, page #60 (Ahrens)

The earth is tilted by an angle of 2327' 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 2327'N (Tropic of Cancer), its northern position, summer solstice, at noon on June 21. The daylight is continuous north of 6633'N (Arctic Circle), while no daylight is present south of 6633'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 2327'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 6633'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 (2327'N) on June 21 to its southern position (2327'S) on December 21 and back again.

The solar radiation reaches its maximum where the sun is directly overhead at local noon between 2327'N and 2327'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 0.3C (0.5F) 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 F (18 C). It is computed by subtracting the average daily temperature from 65 F.

Cooling degree-days: a measure of needs for air conditioning when the average daily temperature is above 65 F (18 C). It is computed by subtracting 65 F 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 C to 600 cal/g at 0 C. The latent heat of sublimation and deposition is 680 cal/g at 0 C. The specific heat of ice is about 0.5 cal/g C, while the specific heat of liquid water is 1 cal/g C.

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 N and 38 S and a net radiative cooling between the poles and 38 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.