Earth’s illumination is the main factor shaping the planet’s temperature. Along with zonal variations in illumination, the annual air temperature at sea level varies zonally. As discussed earlier, only 43% of the solar energy that reaches the upper layers of the atmosphere reaches the Earth’s surface. The rest of it is reflected from clouds in the atmosphere, or absorbed and dissipated. The heating of the Earth’s surface depends on the length of the day, and the height of the sun above the horizon. This is why, on a planet-wide scale, it is coolest in the polar zone and warmest in the intertropical zone.
The temperature at the ground surface depends mainly on:
- terrain moisture, dry ground heats up during the day and cools down at night more strongly than wet ground.
- thermal conductivity, which for water is almost 25 times greater than for air.
- The absorptive capacity of a surface, from its color and roughness. Smooth and light surfaces are characterized by a high albedo, while dark and rough surfaces absorb heat energy intensely
- Land cover by vegetation, or snow cover. The vegetation cover impedes both the inflow and loss of heat from the land surface. It therefore mitigates diurnal temperature fluctuations. Snow cover, on the other hand, protects the land from too much heat loss. However, it itself loses large amounts of heat as a result of its high albedo, cooling its surface
- The degree of cloud cover.When the sky is fully covered with clouds, the area heats up,
and also cools down weakly, hence the daily temperature fluctuations of its surface are smallclear skies, on the other hand, as a result of strong heatingday and cooling during the night, daily temperature fluctuations are large
The temperature of the area is characterized by a distinct diurnal pattern. The lowest values of air temperature during the day occur just before sunrise. Its highest values are recorded from 13:00 to 14:00.However, the influx of air masses with different properties, the passage of atmospheric fronts, and changes in cloud cover cause, sometimes very large deviations from the normal course.
Average daily temperature differences in temperate latitudes show a seasonal dependence. In winter they are 5-10°, in summer they increase to 10-20°. The highest air temperatures in temperate latitudes are recorded in July, and the lowest in January.
The instrument used to read air temperature is a mercury thermometer. It is a type of liquid thermometer that uses mercury to measure temperature. As the temperature increases, the liquid expands and is pushed up the tube. Highly sensitive electric thermometers with low inertia are used to determine minute and rapid changes in temperature.
The daily amplitude of the air temperature is the difference between the highest and lowest temperature values during the day. It depends on several factors:
- From the season: daily temperature amplitude increases from winter to summer, in spring it is higher than in autumn, as the area is still strongly cooled in spring, and also spring minimum air temperature values are lower than autumn ones
- from cloud cover: on clear days the daily amplitude of air temperature is much higher than on cloudy days
- From latitude: Daily air amplitudes should be highest in the intertropical zone; however, heavy cloud cover in periglacial areas reduces the daily temperature amplitude by several degrees. Around the tropics, the highest daily temperature differences are recorded. The reason for this is that these areas have cloudless weather. In deserts, daily temperature differences exceed 30°.
In the circumpolar latitudes, two periods of highest temperatures associated with the zenithal elevation of the sun during the equinoxes are marked, as well as two periods of minimum temperatures during the summer solstice, and winter solstice. At extra-tropical latitudes, the annual course of temperature observes one maximum during summer and one minimum during winter. As latitude increasesfrom the tropics, maximum air temperatures steadily decrease. In circumpolar areas during polar days and nights, there is no clear diurnal course of temperature. - From the ground: The daily amplitude of air temperature is small over the oceans as a result of certain properties of heat uptake and heat release by the waters. Its value is within 1°. The difference in question increases as you move away from the water bodies, reaching values in the depths of the continents up to 15-20°.
- From altitude: As the absolute altitude increases, the daily temperature amplitude decreases. as the influence of the ground, which strongly influences the minimum, and maximum temperatures at the land surface, weakens in the thinned air.
- From the landforms: Over convex formations, temperature amplitudes are higher than over plains because air movement over upland areas reduces diurnal temperature changes. Over concave formations, temperature amplitudes are highest because air heats up faster during the day in valley bottoms, while it cools more at night due to the accumulation of heavy and cold air that has flowed in from the slopes.
The annual amplitude of air temperature is between the average air temperature in the warmest and coolest month. It depends mainly on:
- Latitude: In the intertropical zone, the annual amplitude of air temperature is small. This is due to the high altitude of the sun and thus the high angle of the sun’s rays. The annual temperature difference between the warmest and coolest months increases as latitudes increase, and as we move away from bodies of water. In higher latitudes, high annual air amplitudes are mainly due to low temperatures, and short days during winter.
- From the type of substrate: The annual amplitude of air temperature is small over oceanic surfaces. Over land, the value of the difference in question increases from a dozen degrees on the coast to dozens in the depths of the continents. Average air temperatures in summer change little, while winter temperatures decrease very significantly, which increases the annual amplitude.
Vertical changes in air temperature
The thermal equilibrium of the atmosphere is determined overwhelmingly by the vertical distribution of temperature. However, temperature changes in the atmosphere also take place due to so-called adiabatic transformations, that is, transformations without heat exchange with the surroundings and only due to heat and volume changes. Such changes occur most clearly with vertical air movements. If a given amount of air rises adiabatically upward, its then temperature will decrease because it will expand due to lower pressure on the higher layers of the atmosphere.
For this process, the air consumes its own thermal energy, which manifests as a drop in temperature. If the air descends adiabatically, then its temperature will rise, because compression of the air will then take place, due to the increase in ambient pressure. This compression will occur due to external forces, mainly the pressure of the atmosphere. As a result, the air will receive some internal energy determined by the increase in its temperature.
The increase or decrease in the temperature of dry air as it rises or falls is the dryadiabatic gradient, which is the change in air temperature with a change in altitude. It is equal to 1°C/100m. In the case of rising humid air, which is saturated with water vapor, the temperature drop will be slower, because as a result of cooling, some of the water vapor is condensed, and the latent heat of vaporization liberated in this process remains in this volume of air and thus inhibits the temperature drop caused by expansion. In this case, the measure of cooling will be the moisture-adiabatic gradient, which averages 0.6°C/100m. This value is highly dependent on the temperature of the surrounding air. At higher temperatures, a lower gradient with altitude is observed, while at lower temperatures it is higher.
Thus, the change in air temperature with height without heat exchange with the surroundings is 1°C/100 m for dry air, while 0.6°C/100 m for moist air. Therefore, in the zones of highs, thanks to the descending movement of air (the descent of air), the temperature at the Earth’s surface is higher than it would appear from the illumination of the area alone. However, it is often the case that the actual decrease or increase in air temperature is different from adiabatic. The measure of the change in air temperature in a specific air mass, under specific atmospheric conditions, is the actual vertical gradient. When the actual decrease in air temperature is less than adiabatic, (for example, when the actual decrease in temperature for every 100 m is 1°, with an average decrease in the rolling air of 0.4°C per 100 m), then we are dealing with a steady state equilibrium in the atmosphere. This state w arises when warm air masses flow into higher latitudes.
The lower layers of air then cool from the ground, slowing the temperature drop with altitude. The air will then tend to settle. The equilibrium will be most constant when, with altitude, the air temperature will increase instead of decreasing. Since such a condition is reversed from normal, it is called an air temperature inversion. Temperature inversions are found on warm cloudless nights, when heat from the Earth’s surface is radiated away. This leads to a significant cooling of the ground surface. This type of inversion disappears after sunrise, with an influx of solar radiation. Inversions also form in areas covered with snow, or ice. During daytime melting, heat is drawn from the immediate environment. In addition, there are also inversions associated with the arrival of a warm front, when warmer air flows over the top of the cooler air.
When the actual temperature drop equals the adiabatic one (when the temperature in a given column of air will be equal to the temperature of the surrounding air at any height), then this air does not tend to sink or rise and therefore behaves inertly. Then we speak of inert equilibrium. Such a state in the atmosphere in reality is rarely encountered. When, on the other hand, the actual decrease in the temperature of the air is greater than adiabatic and it is at any height warmer and lighter than the surrounding air, the resulting ascending motion of the air (lifting of the lighter, warmer air) will be more intense, the greater the differences between the temperature of the air in the rising column and the temperature of the surrounding air. In such a case it is said that there is an unstable equilibrium.
This is a situation that is very common in the atmosphere. The air in this state of the atmosphere will, therefore, constantly rise upward, as it will always be warmer than the surroundings. This state of the atmosphere most often occurs on a hot and sunny day. The state of unstable equilibrium can be caused by the warming of air from the ground, the influx of warm air over a given area in the lower layers of the atmosphere, or the influx of cooler air in the higher layers. The unstable equilibrium consequently leads to the formation of clouds and precipitation. This condition arises in air masses arriving from higher latitudes to the surface of lower latitudes. The lower layers of the cooler mass then heat up from the warmer substrate. The equilibrium state of the air is therefore assessed by the value of the ratio of the actual gradient to the adiabatic gradient, i.e. the actual temperature difference of a particular air to the temperature difference of the surrounding air changing with altitude.
In summary, the distribution of air temperature on the Earth depends mainly on: latitude (the amount of solar energy reaching the Earth’s surface depends on the height of the sun above the horizon), absolute altitude, and terrain (the importance of color, type of land cover, slope exposure, as well as altitude n.p.m., the average temperature drop is 0.6°C per 100 m), the distance from sea reservoirs, sea currents (warm sea currents increase the air temperature, while cold ones lower it), as well as the season and time of day (different angles of the sun’s rays occur at different times of the day and seasons). The spatial distribution of air temperature is illustrated by isotherms, which are lines connecting points of equal temperature.
