NAME TEMPERATURE AND HUMIDITY I. Intrductin Temperature is the single mst imprtant factr in determining atmspheric cnditins because it greatly influences: 1. The amunt f water vapr in the air 2. The pssibility f cndensatin and precipitatin 3. Air pressure In this exercise we will lk at temperature variatins and the reasn fr temperature variatins. We will als cnsider humidity and the effect f temperature n relative humidity and the ability f air t hld water vapr. II. Lapse Rate The lapse rate is the change in air temperature with altitude. A. Lapse Rate and the Trppause The trpsphere is the lwest layer f the atmsphere. Within this layer the temperature decreases with altitude in a relatively regular manner (n average at the rate f 6 C km ). In the trpsphere vertical cnvectin currents, induced primarily by the uneven heating f the earth s surface, keep the air fairly well stirred. Practically all cluds and weather, and mst f the dust and water vapr f the atmsphere, are fund in this turbulent layer. Its upper bundary is call the trppause. 1. At Chicag, Illinis, n a day in mid-april, the surface air temperature is 10 C. A vertical sunding f the atmsphere by balln reveals a nearly cnstant lapse rate f 6 C km. At the trppause a temperature f -50 C is reprted. Assume the elevatin f Chicag t be sea level. Calculate the altitude f the trppause. 2. Nauru Island is lcated in the middle f the Pacific clse t the equatr. On the same day that the bservatins were made in Chicag a similar sunding was made at Nauru. Here the sea level temperature is 30 C and the lapse rate is 5.9 C km. The trppause is encuntered at 16.2 km. Calculate the air temperature at the trppause. -1-
3. Hw des the altitude f the trppause vary with latitude? Can yu think f any reasns fr this variatin? B. Dry and Wet Adiabatic Lapse Rates. Inversins As air ascends it expands because f the reductin in pressure. In rder fr this expansin t take place a certain amunt f heat must be cnverted t mechanical wrk (the wrk dne by the gas as it expands). Since this is an adiabatic prcess, the expansin results in a temperature decrease since heat is nt transferred t r frm the atmsphere during the prcess. If cndensatin des nt ccur, the air cls at the dry adiabatic lapse rate f apprximately 10 C km. If cndensatin des ccur the energy invlved in the change f state causes the lapse rate t be reduced t abut 5 C km. This is called the wet adiabatic lapse rate. This lapse rate will vary as a functin f temperature since the amunt f water vapr the air parcel can hld is inversely related t the temperature. An inversin is an increase in air temperature with increasing altitude. The fllwing temperatures were recrded at a specific lcatin during the early mrning and midafternn hurs. Altitude (m) Sea level 500 1000 1500 2000 AM 10C 18C 15C 12C 9C PM 22 C 19 C 16 C 13 C 10 C 1. Plt the mrning and afternn temperatures n the graphs belw. AM 2000 m 1500 m PM 1000 m 500 m Sea level 5 C 10 15 20 25 5 C 10 15 20 25-2-
2. What is the mid-afternn lapse rate? 3. Is cndensatin ccurring during the afternn? Hw d yu knw? 4. Explain the early mrning temperature curve. 5. Wuld air pllutin be mre serius during the mrning r mid-afternn hurs? Explain. III. Mean Temperature Mean daily temperature - mid-pint between maximum and minimum temperatures Mean annual temperature - sum f the mean temperatures divided by the number f mean temperatures. A. The fllwing are the mnthly means fr tw lcatins. Temperatures are in F. J F M A M J J A S O N D St. Luis 32 34.5 45 56 66 75 79.5 78 71 59 46 35.5 San Francisc 50 53 54.5 55.5 57 59 59 59 62 61 57 52-3-
1. Plt the annual temperature curve fr each lcatin n the graphs belw. St. Luis 80 70 San Francisc 60 50 40 30 J F M A M J J A S O N D J F M A M J J A S O N D 2. Draw the mean annual temperature as a heavy line n each graph. 3. Cmpute the annual temperature range fr each lcatin. 4. Hw d yu accunt fr the difference in the annual temperature ranges? B. During a typical January day the fllwing temperatures were bserved. Maximum Minimum Farg, ND 25 F -1 F Mbile, AL 59 F 45 F 1. What is the daily mean temperature fr each lcatin? -4-
2. Hw d yu accunt fr this difference in the daily mean? IV. Misture Althugh water vapr cmprises but a small percentage f the atmsphere it is the mst imprtant gas in relatin t the weather. It is restricted t the lwer atmsphere and the amunt fluctuates cnsiderably frm time t time and frm place t place. As a result precipitatin is highly variable in distributin and this is a majr cause f climate variatins. -3 Abslute humidity is the mass f water vapr per unit vlume f air (e.g. - grams cm ). Relative humidity is defined as the rati f the amunt f water vapr present t the amunt f water vapr in saturated air at that temperature. It is expressed as a percent. It can als be defined as the rati f the actual water vapr pressure t the saturatin vapr pressure at that temperature. Dew pint is the temperature at which the air becmes saturated. Thus the saturatin vapr pressure assciated with the dew pint is the actual vapr pressure at any time. 1. Determine the relative humidity and the dew pint fr the labratry using a sling psychrmeter. A sling psychrmeter cnsists f a wet and dry bulb thermmeter which can be rtated rapidly in the air. This speeds up the evapratin f water frm the wick attached t the wet bulb thermmeter. After a cnstant temperature has been achieved the relative humidity and dew pint can be btained frm tables. Dry bulb T = Wet bulb T = Difference = Relative humidity = Dew pint = 2. Human cmfrt depends upn temperature and humidity. The Natinal Weather Service has devised the temperature-humidity index, THI. Any ne f the fllwing frmulas may be used. THI = 0.4 (T dry + T wet) + 15 THI = 0.55 T dry + 0.2 T dew + 17.5 THI = T dry - (0.55-0.55 RH) (T dry - 58) -5-
When the THI reaches 75 at least half f the peple are uncmfrtable. At 80 n ne is cmfrtable and sme businesses use this as a standard fr clsing dwn. What is the THI f the labratry? Dry bulb and wet bulb temperatures are determined fr each f the fllwing fur places. What is the THI fr each lcatin? Based n THI nly indicate which place yu wuld mst like t be. Which place wuld yu least like t be? Why? Lcatin Dry Bulb Wet Bulb THI Nashville, TN 70 50 New Orleans, LA 95 90 Fairbanks, AK 30 22 Flagstaff, AZ 105 77-6-
Data Tables fr Wet and Dry Bulb Calculatins -7-