267
Lecture Notes
on
Climatology
For
Integrated Meteorological Training Course
By
Gaurishankar Sawaisarje
Scientist D
India Meteorological Department
Meteorological Training Institute
Pashan, Pune-8
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Lecture Notes
on
Climatology
For
Integrated Meteorological Training Course
By
Gaurishankar Sawaisarje
Scientist D
India Meteorological Department
Meteorological Training Institute
Pashan, Pune- 8
CHAPTER 1
CLIMATOLOGY AND ITS APPLICATIONS
1. Climatology: An Atmospheric Science
Atmospheric scientists often subdivide study of complexity of gaseous envelope that surrounds the earth into specific areas of interest. One such division identifies the fields of meteorology and climatology. Meteorology is a science that deals with motion and the phenomena of the atmosphere with a view to both forecasting weather and explaining the processes involved. It deals largely with status of atmosphere over a short period of time and utilizes physical principles to attain its goal. Climatology is the study of atmospheric conditions over a longer period of time. It includes the study of different kinds of weather that occur at a place. Dynamic change in the atmosphere brings about variation and occasionally great extremes that must be treated on the long term as well as the short term basis. As a result, climatology may be defined as the aggregate of weather at a place over a given time period. There is diversity of approaches available in climate studies. Figure 1. Illustrates the major subgroups of climatology, the approaches that can be used in their implementation, and the scales at which the work can be completed. Figure 1. Subgroups, Analytical methods and scales of climatic study. (From J. E. Olive 1981, P4 used by permission of V. H. Winston and Sons.)
2. Temperature structure of the atmosphere
In general terms, the atmosphere can be considered as a series of concentric layers or shells surrounding the earth. The most commonly used method to describe atmospheric layering uses temperature as the variable. This is illustrated in Figure 2. , which shows temperature changes with height. The troposphere is the lowest level, where “weather” occurs. In this layer, there is generally uniform decrease of temperature with height. The lowest part of the troposphere, up to 1.5 km or 2 km is called friction layer. The upper limit of the troposphere is the tropopause. It is zone where generally decrease of temperature ceases and temperature remains fairly constant with height (isothermal-equal temperature). The tropopause also represents the upper limit of large scale turbulence and mixing of the layer. The stratosphere extends from the 10 km to 45 km above the surface. In this lower section, temperatures are fairly constant, but at an elevation of 30 km they increase toward the upper limit of this zone, the stratospause. Air circulation in the stratosphere is characteristically persistent, with winds blowing at high velocities. The mesosphere , above the stratopause, is identified by a marked temperature decrease with altitude. Beginning at an elevation of about 80 km, the decline continues until the mesopause is reached. The thermosphere lying above the mesopause has no defined upper limit. It is so named because a very high thermodynamic temperature attained. Figure 2. Thermal Structure of Atmosphere
Figure 3. Schematic cross section of Earth's Atmosphere showing broad chemical dividions
CHAPTER 2
CLIMATIC ELEMENTS
2.1 Weather elements
The weather elements that are used to describe climate are also the elements that determine the type of climate for a region. The most important elements of weather which in different combinations make up the climate of a particular place or area are: solar radiation, air temperature, air pressure, wind velocity and wind direction, humidity and precipitation, and amount of cloudiness. The climatic elements of temperature, precipitation, and wind are the most significant elements used to express the climate of a region. Temperature: The temperature of an area is dependent upon a) latitude or the distribution of the incoming and outgoing radiation, b) the nature of the surface (land or water), c) the altitude and the d) prevailing winds. The air temperature normally used in climatology is that recorded at the surface. Moisture or lack of moisture modifies the temperature. The more moisture in a region, the smaller the temperature range, and the drier the region, the greater the temperature range. Moisture is also influenced by temperature. Warmer air can hold more moisture than a cooler air, resulting in increased evaporation and a higher probability of clouds and precipitation. Moisture, when coupled with condensation and evaporation, is an extremely important climatic element. It ultimately determines the type of climate for a specific region. Precipitation: Precipitation is the second most important climatic element. In most studies, precipitation is defined as water reaching Earth’s surface by falling either in a liquid or a solid state. The most significant forms are rain and snow. Precipitation has a wide range of variability over the earth’s surface. Because of its variability, a longer series of observations is generally required to establish mean or an average. It often becomes necessary to include such factors as average number of days with precipitation and average amount per day. Precipitation is expressed in millimeters. Since precipitation amounts are directly associated with amounts and type of clouds, cloud cover must also be considered with precipitation. Cloud climatology also includes phenomena such as fog and thunderstorms.
Wind: Wind is the climatic element that transports heat and moisture into a region. Climatologists are mostly interested in wind with regards to its direction, speed, and gustiness. Wind is therefore usually discussed in terms of prevailing direction, average speeds, and maximum gusts. Some climatological studies use resultant wind, which is the vectorial average of all wind directions and speeds for a given level, at a specific place, and for a given period.
2.2 Expression of climatic elements
Climatic elements are observed over long periods of time; therefore, specific terms must be used to express these elements so they have a definite meaning. Mean (Average): The mean is the most commonly used climatological parameter. The term mean normally refers to a mathematical averaging obtained by adding the values of all factors or cases and then dividing by the number of items. For example, the average daily temperature would be the sum of the hourly temperatures divided by 24. The mean temperature of 1 day has been devised by simply adding the maximum and minimum temperature values for that day and dividing by 2. In analyzing weather data, the terms average and mean are often used interchangeably. Normal: In climatology, the term normal is applied to the average value over aperiod of time, which serves as a standard with which values (occurring on a date or during a specified time) may be compared. These periods of time may be a particular month or other portion of the year. They may refer to a season or to a year as a whole. The normal is usually determined over a 20- or 30-year period. Absolute: In climatology, the term absolute is usually applied to the extreme highest and lowest values for any given meteorological element recorded at the place of observation and are most frequently applied to temperature. Extreme: The term extreme is applied to the highest and lowest value for a particular meteorological element occurring over a period of time. This period of time is usually a matter of months, seasons or years. The term may be used for a calendar day only, for which it is particularly applicable to temperature. At time the term is applied to the average of the highest and lowest temperatures as mean monthly or mean annual extremes.
Average and standard deviations In the analysis of climatological data, it may be desirable to compute the deviation of all items from a central point. This can be obtained from a computation of either the mean (or average) deviation or the standard deviation. These are termed measures of dispersion and are used to determine whether the average is truly representative or to determine the extent to which data vary from the average. Average deviation : Average deviation is obtained by computing the arithmetic average of the deviations from an average of the data. Average deviation = ∑ d / n where d (the deviations) and n is the number of items. Standard deviation: Standard deviation is the measure of the scatter or spread of all values in a series of observations. Standard deviation = SQRT( ∑ d^2 / n) where d^2 is the sum of the squared deviations from the arithmetic average, and n is the number of items in the group of data.
CHAPTER 3
CLIMATIC ZONES AND CLIMATIC CONTROLS
3.1 Climatic zones
The basic grouping of areas into climatic zones consists of classifying climates into five broad belts based on astronomical or mathematical factors. Actually they are zones of sunshine or solar climate and include the torrid or tropical zone, the two temperate zones, and the two polar zones. The tropical zone is limited on the Tropic of Cancer and on the south by the Tropic of Cancer which are located at 23 1/2º north and south latitude, respectively. The temperate zone of the Northern Hemisphere is limited on the south by the Tropic of Cancer and on the north by the Arctic Circle located at 66 1/2º north latitude. The temperate zone of the Southern Hemisphere is bounded on the north by the Tropic of Capricorn and on the south by the Antarctic Circle located at 66 1/2º south latitude. The two polar zones are the areas in the Polar Regions which have the Arctic and Antarctic Circles as their boundaries. Technically, Climatic zones are limited by isotherms rather than by parallels of latitude. A glance at any chart depicting the isotherms over the surface of the earth shows the isotherms do not coincide with latitude lines. The astronomical or latitude zones therefore differ from the zones of heat. The Climatic zones are shown below :
Figure 4. Eectromagnetic Spectrum
The cause of seasons is due to orbit of the earth round the sun. This is shown below. Winter - December to February. Spring - March to May. Summer - June to August. Fall(autumn) - September to November Figure 5. Orbit of the Earth round the Sun However, On the basis of climate, the period of year has been divided into four seasons in India. They are:-
- Cold weather season (winter season) – January and February
- Pre-monsoon or Hot weather season – March to May
- SW or Summer Monsoon season – June to September
- Post monsoon season – October to December
Figure 6. How Sun Angle decreases the intensity of radiation Figure 7. The tilted axis of the earth results in variations of the angle of overhead sun and length of daylight. Diagram shows conditions at the equinoxes and solstices
Topography: Topography plays an extremely important role in determining the climate of a region. The height of an area above sea level exerts a considerable influence on its climate. All climatic values are affected by surface elevation. An important influence on climate is mountainous terrain, especially, the long, high chains of mountains that act as climatic divides. The orientation of the mountain range may block certain air masses and prevent them from reaching the lee side of the mountains. For example, the Himalayas which are east-west orientations prevent polar air masses from advancing southward. Therefore the climates of India are warmer in winter than are other locations of the same latitude. The most noted influence of mountains is the distribution of precipitation (higher values of precipitation on windward side than on the leeward side). Another important topographical feature is the presence of lakes. The lake effect can be notable for large, unfrozen bodies of water. The lee sides of lakes show considerable diurnal and annual modification in the form of more moderate temperatures; increased moisture, cloud and precipitation; and increased winds (due to less friction) and land and sea breeze effects. Ocean currents: Ocean currents play a significant role in controlling the climate of certain regions. Ocean currents transport heat moving cold polar water equator ward into warmer waters and moving the warm equatorial water pole ward into cooler waters. Currents are driven by the major wind systems; therefore, cold southward moving currents flow along the west coasts of continents, and the warm northward moving currents flow along the east coasts of the continents. This is true in both hemispheres. Basically, this results in cooler climates along the west coasts and the warmer climates along the east coasts.
Before examining the large macroscale circulation for Earth, let us turn to some mesoscale winds: Land and Sea Breezes Land is heated more intensely during day light hours than is an adjacent body of water. As a result, the air above the land surface heats and expands, creating an area of low pressure. A sea breeze then develops, as cooler air over the water moves onto the land. At night, the reverse may take place; the land cools more rapidly than the sea and a land breeze develops.
Mountain and Valley Breezes During the day, air along the mountain slopes is heated more intensely than air at the same elevation over the valley floor. This warmer air glides up along the mountain slope and generates a valley breeze. These can often be identified by the isolated cumulus clouds that develop over the adjacent mountain peaks. During night time, rapid radiation heat loss along the mountain slopes cools the air, which drains into the valley below and causes a mountain breeze.