The surface air temperature at any location on Earth is the result of a small number of factors. These factors act in concert to create a particular heat budget for this location that varies over time. Some of these factors (like the input of solar radiation) positively influence the budget by providing some type of energy which is converted into heat, which then results in a corresponding increase in surface air temperature. Other factors act to reduce temperature by decreasing the input of heat energy or by taking away heat energy from the atmosphere.
This article describes five of the more important factors in detail.
The most important factor controlling surface air temperature is the input of solar radiation. On the Earth’s surface, the amount of radiation input is largely controlled by the intensity and duration of insolation. As the result of these two factors, the quantity of incoming solar radiation available at the surface varies annually with latitude (Figure 1). From the equator to the North and South Pole, seasonal variations in day length and angle of incidence become greater with increasing latitude. Because of these variations, seasonal deviations in surface air temperature also become more extreme as we approach the poles (Figure 2). Highest temperatures tend to correspond roughly to the summer season when day lengths are the longest and Sun angles are at their maximum. When day lengths are at their shortest and Sun angles are at their minimum (winter), the lowest temperatures occur. Locations near the equator (like Singapore - 1°N) have only small variations in solar input annually because seasonal changes in day length and angle of incidence are slight (Figure 3). Consequently, variations in surface air temperature over the period of one calendar year also are minor.
Figure 1: Daily insolation at the top of the atmosphere in Wm-2 for three locations: equator, 50°N, and 90°N. At the equator little annual variation in insolation occurs because of consistent 12-hour days and relatively high Sun angles all year long. 90°N experiences six months of darkness from the September equinox to the March equinox. A location at 50°N would show large seasonal variations in insolation that peak in the June solstice. Lowest quantities of insolation are received during the December solstice. (Image Copyright: Michael Pidwirny).
Figure 2: Latitude influence on mean monthly temperature. Mean monthly temperatures of five Northern Hemisphere locations with different latitudes. The graph suggests that monthly temperatures generally become higher as one moves toward the equator. Also, note that seasonal temperature variations between summer (June, July, and August) and winter (December, January, and February) become more extreme as latitude increases. (Image Copyright: Michael Pidwirny).
Figure 3: Relationship between mean monthly net radiation and surface air temperature for Singapore. Averages of mean monthly surface air temperatures were calculated for the period 1877 to 1988. Average mean monthly net radiation data from the Global Energy Balance Archive (GEBA). (Image Copyright: Michael Pidwirny).
Surface air temperatures tend to cool gradually with increasing elevation. This phenomenon is well known to anyone living in a mountainous region. Temperatures in the valley bottoms are generally warmer than temperatures on mountainsides and peaks. When observing mountains from a distance, we can determine the elevation that roughly represents a surface temperature of 0°C (32°F) by finding the snow line. Above this boundary temperatures are cold enough to maintain snow and ice in a frozen state. Below the snow line average temperatures are above freezing.
Figure 4 describes the monthly mean temperatures for four locations in equatorial Columbia with similar latitudes but different elevations. Girardot has the lowest elevation (286 m, 938 ft) and a mean annual temperature of about 28°C (82°F). Just above Girardot, is Chinchina (1310 m, 4298 ft) and Tibacuy (1525 m, 5003 ft) with mean annual temperatures of 21°C (66°F) and 19°C (70°F), respectively. Bogota has the highest elevation (2556 m, 8386 ft) and an average annual temperature of about 14°C (57°F). We can calculate the rate of temperature drop between these places to be about 6.1°C per 1000 m (3.3°F per 1000 ft). This drop in temperature is related to the fact that the atmosphere becomes less dense with altitude.
Figure 4: Elevation influence on mean monthly temperature. Comparison of mean monthly temperature patterns for four locations in Columbia with different elevations: Girardot (286 m, 938 ft), Chinchina (1310 m, 4298 ft), Tibacuy (1525 m, 5003 ft), and Bogota (2556 m, 8386 ft). These locations are found within 200 kilometers (125 miles) of each other at a latitude of about 5° North of the equator. (Image Copyright: Michael Pidwirny).
The density of air influences both the heating and cooling of the atmosphere near the Earth’s surface. At high elevations less of the incoming shortwave radiation is absorbed and reflected by particles found in the atmosphere lying overhead. This results in more incoming radiation available for conversion into heat energy. The most obvious outcome of having more heat energy available is a relatively rapid rise in daytime air temperature in high elevation locations. A thin atmosphere also causes less of the outgoing longwave radiation to be reradiated back to the Earth’s surface where it can be converted into heat energy again. This greenhouse effect is dependent on the availability of water and other greenhouse gases. When the air becomes thinner, the quantity of these atmospheric substances declines. The net effect of a weaker greenhouse effect is a rapid and intense nighttime cooling. The magnitude of this nighttime cooling is usually greater than the additional heat produced by more intense solar radiation. Consequently, daily mean, monthly mean, and annual temperatures generally decrease as altitude increases.
The presence of cloud cover has a definite effect on air temperature. During the daytime, cloud cover tends to block the transmission of sunlight to the Earth’s surface. Less solar radiation leads to reduced heat generation and lower surface air temperatures. This process explains why daytime temperatures are cooler along the equator and hotter at the subtropical deserts. At the equator, cumulus and cumulonimbus (thunderstorm) clouds develop almost daily reducing the amount of incoming solar radiation absorbed at the Earth’s surface. In the subtropical deserts, skies are usually clear, maximizing the flow of solar radiation through the atmosphere to the surface. Thus, high heat generation at the surface makes desert temperatures very hot during the day.
At night, the presence of clouds is associated with less dramatic nighttime cooling. Nighttime cooling is the result of heat energy in the lower atmosphere being converted into outgoing longwave radiation. In a clear atmosphere, most of this emission leaves the Earth and only a small amount of this outgoing energy is reemitted as counter-radiation to partially offset radiative cooling at the Earth’s surface and lower atmosphere. The presence of clouds greatly enhances the greenhouse process. When cloud cover is extensive and thick, most of the outgoing longwave is redirected back to the surface where it again creates heat energy. Therefore, cloudy nights tend to be warmer than cloudless nights.
Maritime and Continental Effects
The two dominant surface types found on the Earth are land and water. These surface types influence the heating and cooling of the air directly above them in different ways. Simply stated, the heating and cooling of air over land is more intense and rapid creating more extreme temporal variations in surface air temperature. The causes of this phenomenon are related to the following factors:
- Because of its solid form and low thermal conductivity, only a small portion of the heat energy created by absorption of solar radiation at the surface of land flows into this substance.
- Most of the heat created by absorption flows almost immediately into the atmosphere by conduction and convection. This transfer of energy is high when insolation input is great. Likewise, when insolation input is small the amount of heat entering the atmosphere is low. This fact explains why extreme diurnal (daily) and annual variations of air temperature occur over land.
Water stores a lot of heat energy because of its fluid nature and high specific heat. Fluid mixing redistributes the heat generated from solar radiation absorption throughout the vertical expanse of the water body. Thus, deep bodies of water like our planet’s oceans contain heat energy from thousands of years of insolation absorption. Some of this heat is available for conductive and convective transfer into the atmosphere. The amount of heat transferred from the surface of the ocean to the lower atmosphere is strongly controlled by the heat stored in the oceans. Annual variations in availability of solar radiation have minimal effect on the quantity of this heat flow. Consequently, the amount of heat energy flowing into the atmosphere is relatively high even when insolation input is low.
Figure 5 illustrates the maritime effect of oceans on surface air temperature. The graphs in this figure describe the annual variations in monthly mean, maximum, and minimum air temperature for four terrestrial locations with similar latitudes but differing proximity to the Pacific and Atlantic Oceans. Vancouver and St. Johns are located along the west and east coast of North America, respectively (Figure 6). Minot and Winnipeg are found at the center of North America and their climates are strongly influenced by the continental effect (also called continentality). Comparing the monthly temperatures of these places reveals that annual variations are almost twice as great for the continental locations. Minot and Winnipeg also have greater differences between monthly maximum and minimum temperatures for most months indicating greater diurnal variations. Of the maritime climates, Vancouver is slightly milder than St. Johns and experiences less annual variation in temperature. The moderating influence of the oceans is greater for Vancouver because weather in the mid-latitudes generally travels from west to east.
Figure 5: Maritime versus continental effects on monthly temperatures. Comparison of monthly mean, maximum, and minimum temperatures of two maritime (Vancouver and St. Johns) and two continental (Minot and Winnipeg) Northern Hemisphere locations with similar latitudes. (Image Copyright: Michael Pidwirny).
Figure 6: Relative location of Vancouver, Minot, Winnipeg, and St. Johns in North America. (Image Copyright: Michael Pidwirny).
The maritime and continental effects also influence surface air temperature variations on a global scale. The latitudinal distribution of land and water on the Earth’s surface is not even. From 30° North to 70° North more land than water covers the Earth’s surface, while from 30° South to 70° South ocean surface dominates. The climatic effects of this difference can be seen in Table 1. For the regions of interest, the data in this table indicate that seasonal temperature fluctuations in the Southern Hemisphere are considerably less than in the Northern Hemisphere.
Ocean waters on our planet are constantly in motion. These motions occur in the horizontal and in the vertical. Horizontal motions of ocean waters produce surface and subsurface flows known as ocean currents. Currents that occur at the ocean’s surface are driven by the winds associated with the large-scale global circulation of the atmosphere. Frictional drag caused by wind blowing over the ocean surface is the primary factor determining the course that these currents will take. The presence of land masses can also influences the direction of ocean currents. These barriers redirect the flow of east or west flowing currents in a north or south direction.
Surface ocean currents can influence the climate of locations because the ocean water that they transport can have a contrasting temperature. In general, currents that flow from the tropics to higher latitudes have relatively warm flows. Cold flows of ocean water are produced when the currents move from the high latitudes to the tropics. The moderating effect of warm ocean currents on land temperatures is well known. The Gulf Stream warms temperatures along the eastern seaboard of the United States from Florida to North Carolina (Figure 7). Also, an extension of this flow, known as the North Atlantic Drift, travels across the Atlantic Ocean warming air temperatures in Britain and Western Europe. This warming is most pronounced in the winter. For example, during this season average January mean temperature in Coimbra, Portugal is 10.1°C (18.1°F) warmer than Philadelphia although these two locations are at roughly the same latitude (40°N).
Figure 7: The Gulf Stream (A) moves a current of warm water from the tip of Florida northward along the coastline of Georgia, South Carolina, and North Carolina. Off the coast of Virginia, the Gulf Stream changes its direction and begins to move more eastward. The California Current (B) starts off the coast of British Columbia and moves cold water down the west coast of North America. The scale at the bottom matches ocean surface temperature in degrees Celsius with the colors found on the satellite image. (Image Source: NOAA).
Cold surface ocean currents originate in polar and temperate latitudes and generally flow towards the equator. Like the warm surface currents, they are driven largely by atmospheric circulation. The climatic effects of cold currents are most obvious in the summer season. For example, the cold California Current off the west coast of the United States moderates air temperatures from the state of Washington to California (Figure 7). The magnitude of this cooling can be seen by comparing the summer temperature records for cities on the east and west coast of the United States with similar latitudes. For example, the average July mean temperature in Santa Cruz, California is 8.9°C (16.0°F) cooler than Norfolk, Virginia. Yet, both locations are located at about 37°N latitude.
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