atmospheric temperature

atmospheric temperature In the atmosphere, temperature is one of the most sensitive indicators of dynamical and physical processes. It is affected by interactions between the air and the land or ocean, by the radiation received from the Sun and emitted by the atmosphere and the Earth's surface, by chemical interactions (particularly in the upper atmosphere), by changes in state of water from gas to liquid to ice and back again, and by upward and downward motion.

A knowledge of the current temperature in all parts of the atmosphere is crucial to weather forecasting. A massive international effort yields detailed temperature (and wind and pressure) observations every few hours. Because many factors influence the temperature at any given place great care must be taken in making these observations to ensure that they are truly representative of the atmosphere at that location.

Physical meaning of temperature

Temperature is one of the fundamental concepts of physics, like mass, length, and time. Our human senses perceive temperature only when heat is being transferred to or from our bodies; for example, when we stand in front of a roaring fire or we hold an ice cube in our hand. Heat flows from objects which have a higher temperature to objects with a lower temperature; it does not flow in the opposite direction. This is a statement of the second law of thermodynamics.

Heat is a form of energy. All substances, whether solids, gases, or liquids, are composed of molecules moving more or less randomly at different speeds. If we took two solid objects, say the top of a stove and the bottom of a cooking pot, one being much hotter than the other, then the molecules in the hotter object would be moving much faster. When the objects are brought into contact, the faster-moving molecules collide with the slower-moving molecules and transfer some of their energy. The slower-moving molecules in the colder object increase their energy and their temperature. Temperature is a way of measuring the average energy of molecules in a substance. If we pour a glass of hot water and a glass of cold water (each containing the same amount of water) into a jug, then the internal energy of the molecules in the mixture will be the average of the internal energies of the initial two glasses of water—and the temperature of the mixture will be the average of the temperatures of the two initial glasses.

In these examples all the heat transferred to an object results in an increase in the temperature. When this happens the heat is described as ‘specific heat’. Another form of heat, ‘latent heat’, does not immediately result in a temperature change. This applies when materials change from one state to another: for example, from a gas to a liquid (condensation) or from a solid to a liquid (melting). When it rains and a road surface is wet, the temperature of the water on the road and the temperature of the air will be approximately the same. If heat is supplied, for example by sunshine, then some of the energy supplied to the water will be used to break the molecular bonds that hold liquids together and will enable the molecules to move freely as they do in a gas; the liquid water then evaporates to become water vapour. Because the energy is not used to increase the motion of the molecules, the temperature is not affected during this process: the water changes from a liquid to a gas without changing temperature. The heat used in this process is latent heat. It is latent because the heat is potentially available and will be released when the water vapour condenses at some time in the future to form a liquid. The water which evaporated at the road surface may be carried about in the atmosphere until eventually it becomes part of a cloud and condenses into water drops. When this condensation occurs, the heat originally used to break the molecular bonds is released and warms the air and the water droplets, thus raising their temperature. Latent heat is a very important factor in the atmosphere, for it enables heat received at one location to be released at another location. We can consider the large-scale movement of water vapour carried by winds in the atmosphere to be a large-scale movement of latent heat.

Temperature scales

In common with the other fundamental physical properties, temperature is measured in arbitrary units. To devise a temperature scale, scientists in the eighteenth century chose two situations which were easily reproducible as critical points on which a scale is based. They then divided the interval between these points into intervals called degrees. The two scales still in common use are those named after Fahrenheit and Celsius. Fahrenheit used the boiling point of water as his upper reference point, and the lowest temperature achievable by mixing water, ice, and salt as the lowest. He defined the lower point as 0 degrees and the upper point as 212 degrees (following a temperature scale earlier proposed by Newton). On this scale the freezing point of water lies at 32 degrees. The Celsius scale, for many years called the centigrade scale, uses the freezing point of water as the lower reference point (0 degrees) and the boiling point of water as the upper point (100 degrees). Several other scales were suggested in the eighteenth century but only these two have stood the test of time.

Because temperature is a measure of the motion of molecules there is a theoretical absolute zero temperature at which all molecular motion would cease. A temperature scale can accordingly be devised which has absolute zero as its lower reference point. This scale, known as the Kelvin scale, has the same intervals as the Celsius scale. Zero on the Kelvin scale is –273.15 Celsius. The Kelvin scale is used widely for scientific purposes since it has the advantage that there are no negative values.

Measuring temperature

Thermometers are the most commonly used instrument for measuring temperature, but other instruments are also in regular use for making remote measurements in the atmosphere. Several varieties of thermometer are used in meteorology. The maximum thermometer is similar to a clinical thermometer used by medical staff. It has a small constriction near the reservoir (Fig. 1), which prevents the fluid from returning once it has expanded; it therefore records the highest temperature reached until it is reset. To record minimum temperatures, some thermometers have a small metallic marker inside the fluid. When the temperature drops, this marker is dragged down the tube by the meniscus of the fluid (Fig. 1). When the temperature rises, the fluid moves along the tube but leaves the marker to indicate the lowest temperature reached.

Glass thermometers are suitable for measuring temperature at the Earth's surface but would be impractical at higher levels. Between the surface and about 30 km altitude the temperature is measured twice daily at about 300 observing stations by using radiosondes. These are instrument packages lifted by balloons filled with hydrogen or helium. The packages have to be very light and the measuring device has to produce an electrical signal which can be transmitted by radio. Most temperature-measuring devices on radiosondes are electrical resistance thermometers. These rely on the fact that the electrical resistance of several materials, for example platinum and certain ceramics, varies with temperature.

Temperature measurements are also made from satellites. Although pictures of weather systems are the most obvious satellite products, many of these satellites also carry instruments designed to measure temperature at various altitudes through the atmosphere at all points along the satellite orbit. This is a very effective way of obtaining observations around the world on a regular basis. The observations made from space are not as accurate or as detailed as those made by radiosondes, but their much broader range makes them invaluable, particularly over the oceans, which are inadequately observed by the radiosonde network. Satellite measurements of temperature are made using a radiometer. Unlike other temperature-measuring devices, the radiometer makes its measurements remotely. It measures the radiation emitted by molecules in the atmosphere. From the intensity of the radiation it is possible to deduce the temperature of the molecules that were the source of the radiation.

It is vitally important that temperature measurements are made in a way that minimizes errors so that observations made over a tropical ocean or over a polar ice cap are each truly representative of the air at those places. There are many possible sources of error. Factors which can give an erroneously high temperature include sunlight falling directly on the thermometer; observing too close to the ground on a sunny day; insufficient ventilation around the thermometer on a sunny day. Errors leading to a reduction in temperature include exposing thermometers to the sky on a clear night; observing too close to the ground on a clear night; allowing rain to fall on the thermometers. Most of these errors are eliminated by ensuring that thermometers are placed in a well-ventilated box, which is painted white to ensure that it absorbs as little sunlight as possible and is at a height such that the thermometers themselves are exactly 1.5 metres above the ground (Fig. 2). The ground surface should also be grass; the nature of the surface affects the rate at which heat from the Sun is absorbed, and the temperature of the air just above the surface therefore depends on the nature of the surface.

Factors influencing temperature

The temperature at any particular place is influenced by a number of factors: latitude; season; altitude; proximity to a major ocean; time of day; wind direction; present weather conditions. The last three of these control variations in temperature over short periods, of hours to days; the others are important for periods of weeks to months or longer.

On the shorter timescales the diurnal cycle has a major effect. During the day solar radiation is absorbed at the ground and heats the air above it by convection and conduction. Although solar radiation is at its maximum about the middle of the day the ground reacts slowly to the heating and the maximum air temperature lags a few hours behind the maximum radiation. The Earth's surface, in common with all bodies, radiates heat. During the day the incoming heat from the Sun exceeds the radiated heat, except in polar regions, but at night there is a net loss of heat and the air near the surface cools. The cooling process proceeds until the Sun again produces a net input of heat; the minimum in temperature is consequently around dawn. The heat received from the Sun and the heat lost by radiation are, of course, both reduced in cloudy conditions. Local weather systems which determine the direction of the wind influence the day-to-day temperature changes. Air may be directed from warmer regions, from polar regions, or from oceans or continental land masses, each giving different local characteristics.

Seasonal variations exist because the axis about which the Earth spins is tilted in relation to the plane of its orbit around the Sun. This means that during part of the year the southern hemisphere receives more sunlight and six months later the northern hemisphere receives more (Fig. 3). The maximum temperatures in the summer hemisphere are found in a belt between the equator and about 30 degrees, but higher latitudes are also warmer than in the winter hemisphere. The winter hemisphere has no sunlight falling on the polar region and the warm tropical regions are cooler than their summer counterpart.

Since the sunlight falling on tropical regions exceeds that falling on polar regions, there is a variation of temperature with latitude. On a journey in January from the north of Norway (70° N) to southern Spain (40° N), we would experience temperatures from about −12 °C to about 10 °C on average. In North America, the equivalent journey would be from Baffin Island in Canada to New York. The temperatures experienced on this journey would be from about −34 °C to −12 °C. This difference illustrates the effect of the circulation of the Atlantic Ocean. Warm currents are found on the eastern side of the great oceans, and as a result equivalent latitudes in western Europe are considerably warmer than their equivalent on the eastern seaboard of North America. If we travelled away from the coast into the continental land mass, while remaining at the same latitude, we would find that the temperature in January would drop the further we were from the ocean. This is because the thermal capacity of the oceans is very much greater than that of the land. The same amount of heating or cooling will cause a much smaller difference in the ocean temperature than in the land temperature. In summer, the result of this difference is that the continental land masses are heated much more rapidly than the oceans; the coastal regions therefore experience lower temperatures than their inland neighbours.

Altitude is the remaining major factor influencing temperature. Because temperature decreases with height at an average rate of about 6.5 °C for every kilometre it might be expected that, all else being equal, two towns at altitudes one kilometre apart would have average temperatures 6.5 °C apart. This is not the case. Rising up through the atmosphere leaving the land behind is different from rising up a hillside but remaining close to the land. The hillside absorbs solar radiation and is thus warmer than the free atmosphere at the same altitude. The amount of warming depends on the orientation of the hillside. If it is south-facing, then a considerable amount of sunshine may be absorbed, while north-facing slopes can be extremely cold.

Charles N. Duncan

Bibliography

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