Meteorology lecture for the civil engineering

LECTURE

1. 2. Meteorology

2. The science related to atmosphere and its phenomena. From book by Aristotle (340 B.C.) called Meteorologica which explored everything known about weather and climate at that time (as well as chemistry, astronomy and geography). At that time, all substances that fell from the sky were called meteors Falling objects from outside the atmosphere (meteoroids) and water and ice particles falling from clouds within the atmosphere (hydrometeors) Meteorology is important for estimation of precipitation and its formation, required for regional climate processes and design.

3. Atmosphere The gaseous envelope around the earth. It consists of dry air, water vapor and various kinds of salts and dusts. 99% of the atmosphere lies within 19 miles (30 km) of earth’s surface Shields surface inhabitants from dangerous radiant energy (e.g. ultraviolet from the sun). Becomes thinner with increasing altitude, eventually merging with outer space

4. Troposphere It is the zone of atmosphere adjacent to earth. It extends approximately up to seven miles above sea level. Almost 100 % of the total moisture contents of the atmosphere are present in this zone and there is comparatively high temperature gradient in this part of atmosphere.

5. Vapor Pressure It is the pressure exerted by the amount of water vapors present in the atmosphere. It is usually denoted by “e or e a ” and expressed in millibars , pascals (Pa) or Kilo- Pascals ( KPa ). (1 millibar =100 pascals) Since the temperature of water vapor is the same as that of the air in the atmosphere, the maximum amount of water vapor may be said to depend on the air temperature. The higher the temperature, the more vapor can the atmospheric air hold. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the kinetic energy of its molecules also increases. As the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor also increases, thereby increasing the vapor pressure.

6. Saturation Vapor Pressure The air can only “hold” a certain number of water vapor molecules before they become so crowded together that they start sticking together to form liquid water droplets When the air reaches this point it is said to be “saturated” The amount of water vapor molecules (vapor pressure) that the air can “hold” or the air’s water vapor capacity is called the saturation vapor pressure (SVP) It is denoted by ‘ e s ’ .

7. In warm air, the molecules are moving fast, so they collide and readily bounce off each other, making it difficult for them to stick together. In cold air, the molecules are moving slow, making it easier for them to stick together when they collide. Therefore, saturation vapor pressure is dependent upon the air temperature, with cold air being able to hold less water vapor before becoming saturated (lower saturation vapor pressure) than warm air

8. Isobars These are the lines joining points of same atmospheric pressure at a given elevation. The horizontal distribution of pressure is generally shown on weather charts by isobars. The spacing between isobars is a measure of the pressure gradient.

9. Relative Humidity Relative humidity does not measure the actual amount of water vapor in the atmosphere. It is a measure of how close the air is to being saturated. Relative Humidity (RH) is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature

10. In other words, RH is a ratio of air’s water vapor content to its capacity: R H = ( e a /e s )x 100 Air with 50% RH contains only half the amount of water vapor necessary for saturation and air with 100% RH is fully saturated

11. Dew Point When the air is cooled at a constant atmospheric pressure, the temperature at which air becomes saturated is called Dew point. The dew point is the temperature at which water vapor present in the atmosphere will condense if the air is further cooled.

12. Measurement of Relative Humidity The instruments used for measurements of relative humidity are: Psychrometer Hair hygrometer Hygrograph Thermo-hygrograph

13. Measurement of Relative Humidity Psychrometer consists of two thermometers - Dry bulb thermometer and Wet bulb thermometer. The mercury bulb of wet bulb thermometer is covered by a jacket of clean muslin (plain woven cotton fabric) cloth saturated with water. This is done by putting a beaker with distilled water underneath so that the bulb is not submerged in water but only the cloth. Then water rises due to capillary action.

14. The thermometers are ventilated by whirling (rotate) or by use of a fan. As a result of evaporation cooling takes place. Readings are taken on both the thermometers simultaneously https://www.bing.com/videos/search?q=how+to+use+Psychrometer&&view=detail&mid=97DD1FCB1A746D94E63397DD1FCB1A746D94E633&&FORM=VRDGAR

15. Measurement of Relative Humidity The dry bulb reading is denoted as T a and wet bulb reading as T w . The difference of these two temperatures is called the wet bulb depression . i.e. T a - T w =Wet bulb depression Using these readings, the relative humidity can be found from the psychrometer tables.

16. Measurement of Relative Humidity The value of ‘e a ’ for air temperature ‘T a ’ may be obtained by the relation: ( e s – e a ) = γ ( T a - T w ) or e a = e s – γ (T a – T w ) Where γ = psychrometer constant and its value is 0.660 when ‘e a ’ is measured in millibar units and 0.485 when it is measured in units of ‘mm of Hg’.

17. Measurement of Relative Humidity The hair hygrometer consists of a frame in which a strand of hair is kept at approximately constant tension. Changes in length of the hair corresponding to changes in relative humidity are transmitted to a pointer. This instrument is seldom used for meteorological purposes, but it is an inexpensive humidity indicator and is often found in homes and offices.

18. Measurement of Relative Humidity The hair hygrograph is essentially a hair hygrometer, but having automatic recording instrument. The movement of hair activates a pen, which records on a rotating drum. The hygrothermograph combines the registration of both relative humidity and temperature on one record sheet.

19. Solar Radiation The only source of heat energy for earth system is the sun. Whatever heat is received by earth or reflected is the solar energy. Depending upon the shape, rotation, angle of inclination of earth, it is visualized that solar energy received by earth is changing from time to time and point to point.

20. Solar Radiation

21. Solar Radiation Solar energy received by sun is the maximum at the equator and decreases pole-wards. The variations in solar energy are the following: Diurnal (during a day) Variation Seasonal Variation Regional Variation.

22. Solar Radiation The units for radiation flux per unit area are either joules per square centimeters or milliwatt hours per square centimeter. In some countries calories per square centimeters is designated as Langley and the corresponding unit of radiation flux is Langley per minute. Pyrheliometer and Pyranometer is used for measurement of solar radiations.

23. Temperature The degree of hotness or coldness is called the temperature. Its units of measurement are Degree Centigrade ( °C ) or Degree Fahrenheit ( °F ). Temperature Variation There are three types of temperature variations. Daily Variation of Temperature Seasonal Variation Regional Variation of Temperature

24. Daily Variation of Temperature The daily variation of temperature is dependent on the elevation of the sun, the cloud cover and the wind speed. The variation of temperature is large in low latitudes and small in high latitudes, with the result that the daily variation decreases from equator towards poles. The clouds reduce the radiations coming down during the day and outgoing radiations during the night. On a cloudy day, the maximum temperature is lower and the minimum temperature higher than on normal bright days. On windy days the temperature on ground surface is lower than on calm days, because the greater mobility of air along the vertical axis results in greater heat exchange with the upper atmospheric layers.

25. Seasonal Variation The seasonal variation in rainfall and wind also affect the temperature. During the rainy season, the cloud cover is large with the result that less radiation is received by the earth. Annual migration of vast masses of air also brings about horizontal heat exchange and thus affects the annual range of temperature variation. Regional Variation of Temperature Since the amount of net radiation decreases with increasing latitude, the temperature tends to be highest at the equator and decreases towards the poles.

26. The temperature is measured with the help of thermometers. There are two types of thermometers - Maximum thermometer (Mercury Type thermometer) and Minimum Thermometer (Alcoholic type thermometer). In order to measure the air temperature properly, thermometers must be placed where air circulation is relatively unobstructed and yet they must be protected from the direct sunrays and from precipitation. Therefore thermometers are placed in white, louvered, wooden boxes, called instrument shelters. These shelters are set about 4.5 feet above the ground.

27. Chitral Meteorological Station in Pakistan

28. Four commonly used terms of temperature are: Mean Daily Temperature Normal Daily Temperature Mean Monthly Temperature Mean Annual Temperature

29. Mean Daily Temperature It is the average of maximum and minimum temperatures during the past 24 hours. Normal Daily Temperature It is the average daily mean temperature for a given day over the past 30-years period i.e. it is the mean temperature for a specific day

30. Mean Monthly Temperature It is the average of the mean monthly maximum and minimum temperatures or it is the mean temperature of the mean daily temperatures during the month. Mean Annual Temperature It is the mean temperature of 12 months.

31. Lapse Rate The lapse rate or vertical temperature gradient is defined as the change in temperature per unit distance in the vertical direction from the Earth surface. The average value of the lapse rate is 6.5 o C per 1000 meters in lower troposphere. The greatest variation in lapse rate is found in the layer of air just above the land surface. The lapse rates are of three types depending upon the type of water vapors. Dry Adiabatic Lapse Rate Wet (Saturated) Adiabatic Lapse Rate Pseudo-Adiabatic Lapse Rate

32. Dry Adiabatic Lapse Rate It is the rate of change of temperature when air is not fully saturated with water vapors. The average value of this is 9.8 o C per km. Wet (Saturated) Adiabatic Lapse Rate The value of the lapse rate is strongly dependent on the amount of water vapor in the air. Hence the value of wet adiabatic lapse rate is not constant and depends on amount of moisture that the air contains. Normally its average value has been found below 6 o C per km.

33. Pseudo-Adiabatic Lapse Rate The lapse rate of a saturated air parcel from which some condensation products are removed as precipitation. The air parcel will cool at a slightly greater rate than the saturated adiabatic lapse rate.

34. Wind Measurement Wind speed is measured with an instrument called Anemometer . This instrument gives continuous record over some graph called Anemograph .

35. A very well known Anemometer is Dynes Apparatus. It gives reading in miles of total wind movement in 24 hours. Wind has both speed and direction. Wind direction is the direction from which wind is blowing. Wind speed is usually given in miles per hour, meters per second or knots. 1m/sec = 2.2 mph

36. The equation of the curve is, V/V o = (  / Z o ) 1/7 Where ‘V’ is wind speed at height ‘Z’ from ground and ‘V o ’ is wind speed measured by the anemometer at height ‘Z ’. V V o Z o Z

37. The speed of air at a height of 15 meter above ground was measured as 10 m/s. Find the speed at 2 m level. Solution V 2 / V 15 = ( Z 2 / Z 15 ) 1/7 or V 2 = ( Z 2 / Z 15 ) 1/7 x V 15 V 2 = ( 2 / 15) 1/7 x 10 = 7.5 m/s