SAINT MARY’S UNIVERSITY GEOG 1200 DEPARTMENT OF GEOGRAPHY FUNDAMENTALS OF PHYSICAL GEOGRAPHY Module 2: HUMIDITY Objectives 1. To study the concept of humidity. 2. To understand use of the saturation curve graph. 3. Learn about how relative humidity is measured using a sling psychrometer. Section 1: Understanding Humidity Terminology Air can hold up to a certain amount of water vapour (water in a gaseous state) but the amount varies depending on the temperature. Humidity is a general term that refers to the amount of moisture in air. Some other important terms to know when dealing with moisture in the atmosphere are: Specific Humidity (SH): the actual quantity of water vapour in the air, in grams per kilogram (g/kg). Maximum Specific Humidity (MSH): the maximum quantity of water vapour that could be held in the air at a given temperature (g/kg). If the air is unsaturated, the SH is less than the MSH. Relative Humidity (RH): the ratio of SH to MSH, expressed as a percentage: Equation 1: Specific Humidity RH (%) = ----------------------------------Maximum Specific Humidity x 100 Dew-Point Temperature (DT): the temperature at which air saturation and condensation occur for a given value of specific humidity. Condensation is the change of water from a gaseous state to a liquid state. Saturation Curve: a graph (on a separate sheet) showing the relationship between air saturation and temperature. Once the saturation point is reached, the RH is 100% and no more water vapour can be evaporated into the air. Example For reference, an example using the Saturation Curve Graph is given. Follow the example of how to correctly read the graph. a. A sample of air is collected and determined to lie at Point X on the graph. b. The air temperature is 30C. c. The SH is 10.0g/kg (grams of water vapour per kilogram of air). d. What is the MSH? e. What is the RH, rounded to the nearest %? f. What is the DT? 1 Answers Air Temperature (C) Specific Humidity (g / kg) X 10 30 Maximum Spec. Humidity (g / kg) Relative Humidity (%) Dew-Point Temperature (C) 27.5 36 12.5 2 Section 2: Using the Saturation Curve Graph • Recall from the previous section that relative humidity is the ratio between specific humidity and maximum specific humidity expressed as a percentage. • It is possible for the specific humidity to be lower than or equal to, but not higher than, the maximum specific humidity. A point on the graph can only lie on or below the line. • A series of values will be used to demonstrate the relationships between temperature and humidity of air. • In this example, the temperature of the air changes, forcing changes in the relative humidity values. A starting point was selected with an air temperature of 20C and a specific humidity of 10 g / kg. Air Temperature (C) Specific Humidity (g / kg) A 20 10 B 13 10 C 0 4 D 32 5 Maximum Spec. Humidity (g / kg) Relative Humidity (%) Dew-Point Temperature (C) Use the Saturation Curve Graph to complete the table (above or attached). Section 3: Measuring Relative Humidity with a Sling Psychrometer Background • A device for measuring relative humidity is called a sling psychrometer. • The paragraphs below describe the sling psychrometer and its operation – For the on-line course you will not be using the instrument, however you will learn about it . The Sling Psychrometer • The sling psychrometer contains two thermometers housed in a plastic casing attached to a handle. • The casing is designed to be spun around the handle, vigourously. • The upper thermometer is a standard mercury thermometer with a Celcius temperature scale – this is the dry-bulb thermometer it measures the air temperature. • The lower thermometer is similar, except it is covered by a wick at the far end- this is the wet-bulb thermometer. If the wick is dry, unscrew the plastic cap and add water to the reservoir. • The difference between the dry bulb and wet bulb temperatures is called the wet-bulb depression (or depression of the wet bulb). How the Sling Psychrometer Works • When the psychrometer is spun, evaporation causes the wet-bulb temperature to be lowered. 3 • The amount of evaporation from the wick is related to the relative humidity (RH). The amount of evaporation is determined by the amount of water vapour already in the air (the specific humidity) compared to the maximum amount of water vapour that can be held at that temperature (maximum specific humidity). • If the RH is high, there will be relatively little evaporation and the dry and wet bulb temperatures will be close to each other. • If the RH is low, there will be relatively more evaporation and the dry and wet bulb temperatures will be further apart. Using the Psychrometer • To use, spin the psychrometer vigourously for 30-40 seconds. • Read off the two temperatures to the nearest half degree and calculate the wet-bulb depression. • On a separate sheet is a chart that will tell you the relative humidity for the air temperature you measure (dry-bulb temperature) and the corresponding wet-bulb depression. • Read the chart down to the dry-bulb temperature you recorded, and across to the wet-bulb depression to obtain the relative humidity value. • For example, if you measured a dry-bulb temperature of 34C and a wet-bulb temperature of 24.5C, the wet-bulb depression is 9.5C and the relative humidity is 46%. Use the chart to confirm how this value of relative humidity is obtained. • On the chart, if the exact dry-bulb temperature you recorded is missing, you must interpolate between given values. Before Moving On • It is essential to understand the information provided to this point before moving on. Three points that frequently require emphasis or clarification are: o Dry-bulb temperature on the psychrometer measures the air temperature. o Wet-bulb temperature is not the same as wet-bulb depression. o Wet bulb depression is the difference between dry-bulb and wet-bulb temperatures. (That is, how much lower, or “depressed”, is the temperature of the wet-bulb compared to the dry-bulb?) Section 4: Practice Exercises Using Given Sling Psychrometer Values Work out the relative humidity assuming the following temperatures were measured off a sling psychrometer: Dry-Bulb Temperature (C) Wet-Bulb Temperature (C) 30 26 5 2 45 27.5 9.5 4.5 Wet-Bulb Depression (C) Relative Humidity (%) 4 4 Section 5: Relative Humidity Measurements • I completed these measures for the class using the sling psychrometer to measure the relative humidity in three locations on the campus: 1. In room B205 2. In the lobby of the Atrium, near the entrance to the library 3. Outside, well away from any buildings or building entrances • Spaces are provided below to about the humidity conditions (using the relative humidity results and the saturation curve graph). • Equation 1 - used to calculate RH when SH and MSH are known - can be rearranged into Equation 1a to calculate SH, if RH and MSH are known. Equation 1: RH = (SH / MSH) x 100 Equation 1a: SH = (RH / 100) x MSH Make sure you include appropriate units for all values on p. 4 and 5. Location 1: In Room B205 Dry-Bulb Temperature _21.5 C ______________ Wet-Bulb Temperature 16.0 C _______ Location 2: In the lobby of the Atrium Dry-Bulb Temperature _19.5 C Wet-Bulb Temperature __18.5 C ______________ Location 3: Outside, away from buildings Dry-Bulb Temperature 7 C ________________ Wet-Bulb Temperature __2 C ______________ Location 1: In Room B205 Wet-Bulb Depression Relative Humidity ________________ ________________ Maximum specific humidity for this air temperature ________________ Specific humidity (SH = [RH / 100] x MSH) ________________ Dew-point temperature ________________ Location 2: In the Atrium, on the second floor beside the green living wall Wet-Bulb Depression Relative Humidity ________________ ________________ Maximum specific humidity for this air temperature ________________ Specific humidity (SH = [RH / 100] x MSH) ________________ Dew-point temperature ________________ 5 Location 3: Outside, away from buildings Wet-Bulb Depression Relative Humidity ________________ ________________ Maximum specific humidity for this air temperature ________________ Specific humidity (SH = [RH / 100] x MSH) ________________ Dew-point temperature ________________ For each of the three locations, plot your values of specific humidity vs. air temperature on the Saturation Curve graph. • At which location is the relative humidity highest? ________________ • Which location has the greatest amount of water vapour in the air? ________________ • In general, is it possible for the highest relative humidity and the greatest amount of water vapour in the air to have occurred at different locations (not considering these three specific locations)? 6 ASSIGNMENT #2 HELP Water and the Hydrosphere Humidity • The amount of water vapor in the air • The maximum volume of water vapor, or humidity, of a mass of air increases sharply with rising temperature • Air at room temperature (20°C [68°F]) can hold about three times as much water vapor as freezing air (0°C [32°F]) © John Wiley & Sons, Inc. Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. HUMIDITY Specific Humidity (SH): Maximum Specific Humidity (MSH) Relative Humidity (RH): the ratio of SH to MSH, expressed as a percentage: Dew-Point Temperature (DT): Saturation Curve: Measuring Relative Humidity with a Sling Psychrometer Putting them together: Using both the Depression_RH table and the Saturation curve Module #2 Mapping Activity Isolines Introduction Isolines : are lines which connect all stations having an equal value of the variable being shown on the map. Drawing isolines : The key to drawing isolines is interpolation which is the determination or estimation of values between two known values. These known elevation values are called spot heights. The value 75 can be interpolated between the pairs of adjacent points 72, 79; 70, 79; 69, 81; 68, 81; 74, 79 in Figure # 1. ** It is assumed in the drawing of all isolines that there is a uniform rate of increase/decrease in value between each pair of points. For the pair of points 72, 79 we could measure the distance between points to find the location of the value 75. This procedure is repeated for each pair of adjacent points. While actual measurement is the most accurate method, experience and judgment allow the placement of the interpolated point without using the more time consuming method of actual measurement. Isolines have the following characteristics : 1. Drawn between 2 points of known value. 2. Smooth firm lines, not angular or sketchy. 3. Closed continuous lines, unless they go off the map, not branching, crossing or discontinuous. 4. The interval between isolines is constant and the series can include zero. 5. The value of the isoline is placed in a small break in the line, usually on the south slope. The number of values placed on the map is governed by the desire for legibility, ease of interpretation and reduction in cluttering. Question 1: Identify the following contour values from the map. Contour interval equals 50 metres. Hint water flows downhill . A _______ B _______ C _______ D 350 metres E _______ F _______ G _______ Complete the following. Remember the principles from above. Nova Scotia Weather Map https://www.weather-forecast.com/maps/Nova-Scotia From the Weather Map below ( displayed isobars – Lines of equal pressure) Identify the following: Location of High Pressure (general description of province, state or ocean): ________________ Highest barometric pressure:______________ Location of Low Pressure (general description of province or ocean): ___________________ Pressure at Halifax ; ________________________
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