Temperature PDF

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Summary of the lecture given on the topic of Temperature....

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TEMPERATURE Temperature is a quantity that described how hot or cold an object is with respect to some standard measure. It is hence the degree of hotness or coldness. It can also be described as a measure of the average kinetic energy of the atoms or molecules in a substance. When a substance gains energy, its particles move faster and its temperature rises. By contrast, when energy is lost, the atoms and molecule vibrate more slowly and its temperature drops. Temperature is one of the basic elements of weather and climate. Temperatures recorded daily at thousands of weather stations world-wide provide much of the temperature data compiled by meteorologists and climatologists.  The daily mean temperature is determined by averaging the 24 hours readings or more frequently by adding the maximum and minimum temperatures for a 24 hours period and dividing by 2.  The daily temperature range is computed by finding the difference between the maximum and minimum figures.  The monthly mean temperature is calculated by adding together the daily means for each day of the month and dividing by the number of days in the month.  The annual mean temperature is an average of the 12 monthly means.  The annual temperature range is computed by finding the difference between the warmest and coldest monthly means.  The annual temperature range is computed by finding the difference between the warmest and coldest monthly mean temperature.  Mean temperatures are especially useful for making daily monthly or annual comparisons. To examine the distribution of air temperature over large areas, isotherms are commonly used. As isotherm is a line that connects points on a map that have the same temperature. Isotherm is the combination of two words; Iso meaning equal and therm meaning temperature. Therefore, all points through

which an isotherm panes have identical temperatures for the time period indicated.  The amount of temperature change per unit of distance is called temperature gradient. Closely spaced isotherms indicate a rapid rate of temperature change, whereas more widely spaced lines indicate a more gradual rate of change. The Controls Of Temperatures

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The controls of temperature are the factors that cause temperatures to vary from place to place and from time to time. The factors are as follows : Latitudes and angle of sun. Differential heating of land and water. Oceans currents. Altitude. Geographic position. Cloud cover and albedo.

Latitudes and Angle of Sun This is the most important cause for temperature variation. Variation in sun angle and length of the day depend upon Latitude, they are responsible for warm temperatures in the tropics and colder temperatures pole ward. Of course, seasonal temperature changes at a given latitude occur as the sun’s vertical rays migrate toward and away from a place during the year. The places on earth receiving direct sun rays changes as the result of earths revolving nature around the sun. The earth keeps receiving the perpendicular sun rays once in a year on the different places between 23 degree south and north latitude. Rest of the places have slant rays due to which the heat impact reduces and the temperature gets its rising limit.

Differential Heating of land and water Heating of the earth’s surface controls the heating of the air above it. Therefore, to understand variations in air temperature, we must understand the variations in heat properties of the different surfaces that the earth presents to the sun, soil, water, trees, ice and so on. Different land surfaces reflect and absorb varying amounts of incoming solar energy, which in turn cause variations in the temperature of the air above. The greatest contrast however, is not between different land surfaces but between land and water. Land surface temperatures are clearly much higher than water surface temperatures. 1. Surface temperature of water rises and falls much more slowly than the surrounding surface temperature of land. It is because water is highly mobile. When it is heated, convection distributes the heat through considerably larger masses. Whereas, heat does not penetrate deeply into soil on rock. It remains near the surface. Here, heat is transferred through conduction which is a slow process. 2. Because land surfaces are opaque, heat is absorbed only at the surface. Water being more transparent, allows some solar radiation to penetrate to a depth of several meters.

3. The specific heat (the amount of heat needed to raise the temperature of 1 gm of a substance 1 ° C) is more than three times greater for water than for land. Thus, water requires considerably more heat to raise its temperature the same amount as an equal quantity of land. 4. Evaporation (a cooling process) from water bodies is greater than from land surface. All these factors collectively cause water to warm more slowly, store greater quantities of heat and cool more slowly than land.

Ocean Currents

Surface currents in the ocean are set in motion by the wind. At the water surface where the atmosphere and ocean meet, energy is passes from moving air to the water through friction. As a consequence, the drag exerted by winds blowing steadily across the ocean causes the surface layer of water to move. Thus, major horizontal movements of surface waters are closely related to the unequal heating of earth by the sun. Surface currents have an important effect on climate. It is known that for earth as a whole, the gains in solar energy equal the losses to space of heat radiated from the surface. When most latitudes are considered individually, however this is not the case. There is a net gain of energy in lower latitudes and a net loss at higher latitudes. Because the tropics are neither becoming progressively warmer, nor the Polar Regions colder, there must be a large scale transfer of heat from areas of excess to areas of deficit. This is indeed the case. The transfer of heat by winds and ocean currents equalizes these latitudinal energy imbalances. Oceans water movements account for about a quarter of this total heat transport and winds the remaining three quarters. The moderating effects of pole wards moving warm ocean currents are well known. The North Atlantic drift an extension of the warm Gulf Stream, keeps wintertime temperatures in Great Britain and much of Western Europe warmer than would be expected for their latitudes. Because of the prevailing winds (westerly), the moderating effects are carried for inland.

Altitude As we know that temperatures drop an average of 6.5 ° C per kilometer in the troposphere. Thus cooler temperatures are to be expected at greater heights. But at some places, higher altitudes are warmer than the

value calculated using the normal lapse rate. This results from the absorption and radiation of solar energy by the ground surface. In addition to the effect of altitude on mean temperatures, the daily temperature range also changes with variation in height. Not only do temperatures drop with an increase in altitude but atmospheric pressure and density also diminish. Because of the reduced density at high altitudes, the overlying atmosphere absorbs and reflects a smaller portion of the incoming solar radiation. Consequently with an increase in altitude, the intensity of solar radiation increases, resulting in relatively rapid and intense daytime heating. Conversely, rapid nighttime cooling is also the rule in high mountain locations. Therefore, stations located high in the mountains generally have a greater daily temperature range than do stations at lower elevations.

Geographic Position The geographic setting can greatly influence the temperatures experienced at a specific location. A coastal location where prevailing winds blow from the ocean onto the shore (a wind ward coast) experiences considerably different temperatures than does a coastal location where prevailing winds blow from the land towards the ocean ( a leeward coast). The windward coast will experience the full moderating influence of the ocean-cool summers and mild winters-compared to an inland situation at the same latitude. A leeward coastal situation however, will have more continental temperature regions because the winds do not carry the ocean’s influence onshore.

Cloud Cover and Albedo

It can be noticed that clear days are often warmer than cloudy ones and clear nights are usually cooler than cloudy ones. This demonstrates that cloud cover is another factor that influences temperatures in the lower atmosphere. At any particular time, about half of our planet is covered by clouds. Cloud cover is important because many clouds have a high albedo and therefore reflect a significant proportion of the sunlight that strikes them back to space. If the clouds in the sky are absence and the sky is clear, then there will be no absorption of the incoming solar radiation. Therefore, the temperature on the earth’s surface is more. Clouds are an important agent to reduce the amount of solar radiation, which reduces the daytime temperature. At night, clouds have the opposite effect as during daylight. They absorb outgoing earth radiation and emit portion of it toward the surface. Consequently, some of the heat that otherwise would have been lost remains near the ground. Thus, nighttime air temperatures do not drop as low as they would on a clear night. The effect of cloud cover is to reduce the daily temperature range by lowering the daytime maximum and raising the nighttime minimum. Cloudiness is not the only phenomenon that increases albedo and thereby reduces air temperatures. We also know that snow and ice covered surfaces have high albedo. This is the reason why mountain glaciers do not melt away in the summer and why snow may still be present on a mild spring day. In addition, during the winter when snow covers the ground, daytime maximum on a sunny day are cooler than they otherwise would be, because energy that the land would have absorbed and used to heat the air is reflected and lost.

GLOBAL DISTRIBUTION OF TEMPERATURE Global distribution of temperature is represented through isotherms (lines that connect points of equal temperature). It generally trend east and west and show a decrease in temperature pole ward. Moreover, the isotherms illustrate a latitudinal shifting of temperature caused by the seasonal migration of the sun’s vertical rays and also reveal the presence of ocean currents. The north south migration of isotherms is more pronounced over the continents because the temperatures do not fluctuate as much over water.

Annual temperature range is small near the equator and increases with an increase in altitude. Outside the tropics, annual temperature range also increase with an increase in continentality. The primary control of the daily cycle of air temperature is earth’s rotation. However, the magnitude of these changes is variables and influenced by locational factors, local weather conditions or both. In the Northern Hemisphere the greatest intensity of solar radiation occurs at the time of the summer solstice, yet the months of July and August are generally the warmest of the year. Conversely, in the Northern Hemisphere a minimum of solar energy is received in December. At the time of the winter solstice, but January and February are usually colder.

Weather & Climate We observe various atmospheric conditions in a day. Sometimes it is shiny, sometimes rainy or cloudy. For a while we feel more humidity and then normal. Some days are warmer, some colder and some neutral. Some days see flowers and leaves blossoming and some days have the leaf less surroundings. What causes these changes? These changes are caused due to the reactions in the formless and invisible envelop of air surrounding our earth. Acted by the combined effects of earth’s motions and energy from the air reacts by producing an infinite variety of weather, which in turn creates the basic pattern of global climates. Although not identical, weather and climate have much in common. Weather is the day to day atmospheric conditions. It is constantly changing, sometimes from hour to hour and at other times from day to day. It is a term that refers to the state of the atmosphere at a given time and place.

Changes in the weather are continuous and sometimes seemingly erratic. Still it is possible to arrive at a generalization of these variations. Such a description of aggregate weather conditions accumulated over many decades. Climate is often defined simply as “average weather” but this in an inadequate definition. Climate is an important element of the physical environment of mankind. Although man usually thinks of himself as a creature of the land, he actually lives at the bottom of a deep ocean of air that surrounds the earth. Climate is the aggregate of atmospheric conditions involving heat, moisture and air movement. The process of exchange of energy and mass between the earth and atmosphere over a long period of time result in conditions which we call climate. Thus, climate is the Sun of all statistical weather information that helps describe a place or region. The nature of both weather and climate is expressed in terms of the same basic elements, which are measured regularly. The most important are :1. The temperature of the air. 2. The humidity of air. 3. The type and amount of cloudiness. 4. The type and amount of precipitation. 5. The pressure exerted by the air. 6. The speed and direction of the wind. These elements constitute the variables by which weather patterns and climate types are depicted....