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Unit 5 lab Moisture in the Atmosphere Part 1: Measures of Atmospheric Humidity As you know, the atmosphere has only a limited ability to store water vapor. When that limit is reached, the atmosphere is said to be saturated. More than anything else, the saturation point is determined by temperature. For example, Table 1 shows water vapor capacity on a mass basis. Relative Humidity (RH) is the proportion of the amount of water vapor present at a given temperature relative to the maximum quantity that could be present. In other words, relative humidity is the ratio of the specific humidity (SH) of a given volume of air to the saturation specific humidity (SSH) of a volume of air at the same temperature. We express relative humidity as a percentage. We can determine relative humidity by using the following equation: RH = SH/SSH (100) We can find SH and SSH values as follows: SH = RH (SSH) SSH = SH/RH Using the equations from the last page, work out and answer the following questions: 1. If a parcel of air at 15 ⁰C has a specific humidity of 4.6 g/kg, what is the relative humidity of that parcel? 2. If the parcel were warmed to 25 ⁰C with no change in water vapor content (SH), what would be its new relative humidity? 3. Considering your answer for question 2, describe the relationship between relative humidity and temperature, assuming constant specific humidity. The dew point temperature of a volume of air is the temperature to which the air must be cooled to reach a relative humidity of 100 %. Said another way, the dew point is the temperature at which a parcel of the air becomes saturated. 4. What is the dew point temperature of an air parcel with a specific humidity of 11.2 g/kg? 5. A parcel of air at 20 ⁰C has a relative humidity of 40 percent. What is its dew point temperature? 6. Considering your responses to the questions above, do you need to know an air parcel’s temperature or its specific humidity to determine its dew point temperature? Part 2: Adiabatic Processes Condensation occurs when air is cooled to its dew point temperature. The most common and important way that this occurs in the atmosphere is through the adiabatic process-that is, through temperature changes brought about by changes in pressure. As an air parcel rises, the overlying pressure on that parcel decreases. This causes the parcel to expand, to cool, and ultimately, if conditions are right, to reach saturation. Dry and Wet Adiabatic Lapse Rates If condensation is not occurring in a rising air parcel, that air parcel will cool at what we call the dry adiabatic lapse rate. This is equal to approximately 1⁰C per 100 meters. An air parcel will cool at this dry adiabatic lapse rate until it reaches its dew point temperature. At this point, condensation begins and the rising parcel now cools at a slower rate due to the release of latent heat into the surrounding atmosphere. This wet (or saturated) adiabatic lapse rate is variable, and can range from 0.5⁰C per 100 meters for warm parcels to almost 1⁰C for cold parcels. For this exercise, use a value of 0.6⁰C per 100 meters for the wet adiabatic lapse rate. Orographic Lifting Air rises and falls in response to a number of different lifting mechanisms, including convective uplift and frontal uplift. In this lab exercise we explore adiabatic temperature changes associated with orographic lifting. This process occurs when air is forced to rise over topographic barriers such as mountain ranges. As it does so, it cools adiabatically, until it reaches its dew point. Here condensation begins and we have the potential for precipitation. Figure 1: Cross section from Eureka to Red Bluff In the following exercise you will track the changes in air temperature, dew point temperature, specific humidity, saturation specific humidity, and relative humidity for a parcel of air that originates near the surface at Eureka, California. The prevailing westerlies force this air parcel to rise orographically over the 2400-meter crest of the Coast Range, and to descend to Red Bluff at an elevation of 100 meters in the Sacramento Valley below. Use the following guidelines to get started: · The air temperature at Eureka is 18⁰C · The dew point at Eureka is 13⁰C · Recall that dry adiabatic lapse rate is 1⁰C per 100 meters · We will use a wet adiabatic lapse rate of 0.6⁰C per 100 meters for the sake of this exercise · You may also assume that water condensing in the atmosphere falls out as precipitation Eureka 1. Using the dew point and the air temperatures provide above, return to Table 1 and find the specific humidity value and the saturation specific humidity value of Eureka. SH______________g/kg SSH_____________g/kg 2. Using these values and the equation for relative humidity, find the relative humidity of this air parcel. RH______________% Point of Condensation 3. Recall that the air temperature and the dew point must equal one another for condensation to occur. If this air parcel is lifted orographically up the windward side of the Coast Range, at what altitude will condensation begin? (Hint: you need determine which is the proper lapse rate to use for this question: Is it the wet or the dray?) ALT__________m 4. Summarize the air temperature, dew point temperature, and the relative humidity of the air parcel at this elevation. AT_____________C DP_____________C RH_____________% The Crest 5. Using the following procedures, determine the air parcel’s temperature at the 2400 meter crest. To find the air parcel temperature at a specific elevation: · Choose the appropriate lapse rate to use in your calculations based on whether or not condensation is occurring in this air parcel · Find the difference in elevation between the elevation in question and the elevation that corresponds to the point at which you begin using the appropriate lapse rate for these conditions · Multiply this difference in elevation by the lapse rate you have chosen for these conditions · Take this number and add it to, or subtract it from, the parcel’s original temperature at the place where you began using the appropriate lapse rate. This will give you the air parcel temperature AT_____________C Recall that dew point is a function of an air parcel’s specific humidity. If specific humidity values remain constant, for example, so too do dew points. In a parcel where condensation is not occurring and where no moisture is being added, we assume for this lab that specific humidity values (and thus, dew point temperatures) remain constant with elevation. You saw this in questions 3 and 4 above. By contrast, however, we can assume that dew points fall in a parcel of air where condensation is occurring. This, after all, is because the specific humidity value of that air parcel is also changing. For the sake of this lab, you can assume that the dew point temperature in a condensing and rising air parcel falls at the same rate as our wet adiabatic lapse rate (0.6 degrees C per 100m). 6. Considering the information above, what must the dew point temperature be at the Coast Range’s 2400 meter crest? DP_____________C 7. Based on your answer to questions 5 and 6, what is the relative humidity at the crest? RH_____________% Red Bluff As you track the parcel’s descent to Red Bluff, keep in mind that descending air parcels warm as they are compressed. With no addition of water vapor, the actual specific humidity of a parcel remains constant while its saturation specific humidity, a function of parcel temperature, increases. Thus, the relative humidity of a descending parcel of air decreases. This is what is happening to this air parcel as it descends to Red Bluff. For this lab we can assume that relative humidity starts decreasing immediately as the parcel begins descending from the crest. 8. Returning to the procedures outlined in question 5, what is the parcel’s air temperature at Red Bluff? AT________________C 9. As our air parcel descends down the eastern flank of the Coast Range, its temperature rises and condensation ceases to occur. Assuming that we add no moisture to the parcel of air, then we know that our specific humidity values remain constant as our parcel descends. What, then, is the dew point temperature at Red Bluff? DP________________C 10. What are the parcel’s specific humidity and saturation specific humidity values at Red Bluff? SH________________g/kg SSH_______________g/kg 11. Using these values, determine the parcel’s relative humidity at Red Bluff. What does this suggest about the moisture content of the parcel here as compared to its origin in Eureka? RH_______________% Rainshadows 12. Refer to Table 1 below. Describe the differences in the annual range of precipitation between the two sites, as well as the differences in total precipitation received at the two stations. Table 1: Average precipitation (inches): Eureka and Red Bluff Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Eureka 7.4 5.2 4.8 3.0 2.4 0.7 0.1 0.3 0.6 3.2 5.8 6.6 39.8 Red Bluff 2.3 3.2 2.5 1.8 1.0 0.5 T 0.2 0.3 1.2 3.1 3.9 19.9 13. The area around Red Bluff is said to be in a “rainshadow”—an area of dry conditions located on the downslope (or “lee” side) of a mountain range. Search online for an elevation map (or the so-called relief map) of the United States. Where else would you find similar conditions that lead to a rainshadow? What role does prevailing winds play in your decisions? In Summary 14. Fill in the chart below as a way to summarize and contrast the conditions along the path of this air parcel. You can use this chart to summarize and review a wide range of relationships between air temperature, dew point, specific humidity, saturation specific humidity, and relative humidity. Table 2: Conditions at all stations Air Temp Dew Point Temp Specific Humidity Saturation Specific Humidity Relative Humidity Eureka Crest Red Bluff 7