Lab 4 – Atmospheric Humidity

Part I:
Atmospheric moisture
The amount of water present in the air, whether it is in the solid, liquid, or gaseous state, plays an important role in the weather experienced on Earth. Atmospheric moisture refers to water vapor and precipitation in the Earth’s atmosphere. This amount of water in the atmosphere varies considerably from place to place and, therefore, it is measured and reported. The amount of water vapor in the atmosphere is called the humidity. Specific humidity measures the mass of water vapor in a given mass of air. Relative humidity (RH), a more common measure of atmospheric moisture, is a ratio between the amount of water vapor in the air of a given temperature and the maximum amount that air could hold at that temperature. Relative humidity is expressed as a percentage. So, if the air holds half the moisture possible at the present temperature, the RH is 50%. When the humidity is 100%, the air holds the maximum amount possible. When the air is cooled, the capacity is reduced and has a higher percentage of total capacity. So, temperature and relative humidity are inversely related. As temperature increases, the amount of water vapor a parcel of air can hold increases; therefore, as the temperature of an air parcel increases, the relative humidity (or degree of saturation) decreases. Conversely, relative humidity increases in a cooling parcel of air. Figure 1 shows the relationship between temperature and relative humidity at 5 A.M., 11 A.M., and 5 P.M. The temperature at 5 A.M. is the coolest temperature of the day, so this temperature has less capacity to hold water vapor (in blue). The RH is 100%. During the day, the water vapor content does not change much, however the temperature does. So, the warmer temperature has a greater capacity to hold water vapor and the relative humidity will be less.
Figure 1
Lab 4 – Atmospheric Humidity
Dew point is the temperature at which the air is saturated (relative humidity is 100%), water vapor then changes from water vapor into precipitation; clouds form at this point. Condensation is the result of the air reaching saturation and relative humidity is 100%. For any given temperature, the dew point can be calculated. Note: If the dew point temperature and the air temperature are the same, the RH will be 100%. The lower the dew point temperature compared to the air temperature, the RH will be less.
A sling psychrometer is a tool designed to allow you to determine dew point temperature and relative humidity. This is an instrument consisting of two thermometers mounted together on a plate (Figure 2). One of the thermometers has a cloth wick tied around the end of the thermometer. This is called the wet bulb thermometer. The wick is saturated with water, and then the instrument is swung through the air. As it swings through the air, the water evaporates from the wick and cools the wet-bulb thermometer (evaporational cooling due to the latent heat of vaporization). The thermometer without the wick is called the dry bulb thermometer. The air temperature and the amount of moisture in the air will influence how quickly water will evaporate or change into water vapor and enter the air. As water evaporates, it removes heat to make this change of state and this heat is stored in the water vapor in the air. This heat will be released when the water vapor returns to its liquid or its solid state by the processes of condensation or sublimation respectively. The sling psychrometer instrument makes use of these principles. The wick is dampened and as water evaporates from this cloth, heat is taken with it. The wet-bulb thermometer will register the wet-bulb temperature which is usually lower than the air temperature (the dry-bulb temperature), registered by the dry-bulb thermometer where no evaporation has taken place. This difference in temperature is called the wet bulb depression. The dry-bulb temperature is always higher than the wet-bulb temperature except for when the air is saturated or the relative humidity is 100%. In this situation, as water evaporates from the wet-bulb, an equal amount of condensation is returning the heat which was lost. Therefore, the two temperatures will be the same. A person would usually do about 5 to 10 trials until both thermometers stay consistent in their temperatures. Once the final temperature is determined, both thermometers, in either Celsius or Fahrenheit are recorded. After both thermometers have registered their temperatures and the depression has been determined, two tables, one in Celsius or Fahrenheit can be used to find relative humidity.
Wet Bulb Thermometer with wick tied around the end of the thermometer
Dry Bulb Thermometer
Figure 2
Lab 4 – Atmospheric Humidity
Again, the amount of cooling (wet-bulb temperature depression) that takes place is directly proportional to the amount of water in the air, i.e. the drier the air, the more the cooling. Therefore, the larger the difference between the temperature of the dry-bulb and the wet-bulb thermometers, the lower the relative humidity in the air. If the air is saturated, no evaporation can occur, and the two thermometers will have the same reading. To find the wet bulb depression, you would subtract the wet bulb temperature from the dry bulb temperature (dry-bulb – wet-bulb). Once you determine the wet bulb depression, if your value is negative, take the absolute value and make it positive to find the RH and dew point in the tables below. To find the relative humidity, you will use Table 1, relative humidity, in percent (%) in Celsius, or Table 2, relative humidity, in percent (%) in Fahrenheit.
To understand how to determine relative humidity from the wet-bulb and dry-bulb temperatures, you will refer to Table 1, RH in Celsius. Below is an example of temperature readings taken from a sling psychrometer during the spring months in the state of Texas. After a series of trials using the instrument, the dry bulb reading is 22ºC and the wet-bulb temperature is 18ºC. The first thing you should do is find the wet-bulb depression. You find this reading by subtracting the wet-bulb reading from the dry-bulb reading. In this case below, 22ºC minus 18ºC is 4ºC. So the wet-bulb depression is 4ºC. After you obtain this information, you can now find the relative humidity (RH) using Table 1.
Table 1 shows air temperature down the left side of the table and the depression of the wet-bulb thermometer across the top of the table. To find RH, you cross reference the dry-bulb temperature, (which is the air temperature) with the wet-bulb depression (in yellow highlight). You will see that the RH is 68%. To find the dew point (see Table 3), you cross reference the air temperature with the wet-bulb depression (in yellow highlight). The dew point is 20ºC.
Dry-Bulb Temperature Wet-Bulb Temperature Wet Bulb Depression (Dry-Bulb –Wet-Bulb) Relative Humidity (RH) 22ºC 18ºC 4ºC 68 % Dry-Bulb Temperature Wet-Bulb Temperature Wet-Bulb Depression (Dry-Bulb –Wet-Bulb) Dew Point 22ºC 18ºC 4ºC 20ºC
Lab 4 – Atmospheric Humidity
Fill in the table below using Tables 1 through 3 to determine dew point (°C) and RH (%). You will notice that the blank cells have numbers encased in parentheses. The numbers will correspond to the lab assessment when you are ready to submit your answers online. Credit will not be given if proper units are not used when submitting your answers online. Each response given in the table below in parentheses is worth .5 points for a total of 9 points.
19. Write a paragraph below and compare changes in relative humidity as temperature increases. How does a change in relative humidity with no change in water vapor between 6 AM to 6 PM on a typical day result, for example, for a mid-latitude location in the United States during the warm months? (1.5 pts)
Air Temperature (°C) Wet-Bulb Temperature (°C) Wet-Bulb Depression (°C) Dew point Temperature (°C) Relative Humidity %

Table 1. Relative humidity, in percent. Temperature in ºCelsius
Air Depression of the web-bulb thermometer Temperature

Table 2. Relative humidity, in percent. Temperature in ºFahrenheit
Air Depression of the web bulb thermometer Temperature

Table 3. Dew point temperature, in ºCelsius
Air Depression of the web bulb thermometer Temperature

Part II:
Finding Weather Data
Collect present temperature, relative humidity, and dewpoint weather data from: http://www.wunderground.com/. Visual instructions are given to you on the following page. Choose a location near you and gather three days of temperature, relative humidity, and dew point data. You can use degrees Fahrenheit or degrees Celsius, whichever you choose. It is important that you stay consistent for all readings. Try to collect this data at the same time each day. Also, find a location that is further south of your location in another state or to the most southerly location where the weather is warmer and document the temperature, relative humidity, and dew point for three days. Document the weather data for both locations at the same time and day, and be consistent with your units. For example, if you live in Kent, Ohio, you could choose a location along the Gulf of Mexico, such as Mobile, Alabama. Again, you can use degrees Fahrenheit or degrees Celsius and remember to include your units. Points will be taken off if units are not included. You will be coming back to this website for the weather lab. The purpose of this exercise is to show you an informative weather website and for you to compare two locations and their weather data with differing latitudes. (Each question is worth.5 points)
20. What is the location nearest to you and the second location you chose for this exercise? (.5 pts)
21. Provide latitude and longitude coordinates for both locations (i.e. 41 ° N and 81°W) (.5 pts)
22. What were the temperatures (°C or °F), relative humidity (RH%), and dewpoint (°C or °F) temperatures for all three days for both locations? (.5 pts)
23. Compare and contrast the temperatures, relative humidity, and dew point temperatures between these two locations? If there was not a great difference, that is okay because once we cover weather analysis, air masses, and climate, it will become apparent. (.5 pts)
Enter your location and click search:
You will see your location with current weather data.
Part III:
Adiabatic lapse rates
The term ‘adiabatic’ refers to moving air. It is used to describe the changes in air temperature as an air parcel moves within the troposphere. Remember that the higher the temperature of air, the more moisture a parcel of air holds before reaching saturation. When a parcel of air rises, the temperature of the parcel cools at a constant rate. The rate is dependent on whether the air parcel is saturated or unsaturated. A rising parcel of air will cool at an average rate of 10º C/1000 m (5.5ºF per 1,000 feet). This is the dry adiabatic lapse rate (DALR). It is only used when the air is not saturated. When air is forced downward in the atmosphere, the air is being compressed and temperature increases (pressure is increasing). Since the temperature is increasing, the parcel can hold more water vapor and does not reach saturation. Therefore, an air parcel that is decreasing in elevation always follows the DALR rate.
When the air is saturated, condensation begins. This is the level of cloud formation. After saturation, a rising parcel of air will cool at a lower rate of change due to the release of latent heat. A rising saturated parcel of air will cool at an average rate of 5ºC/1000 m (3.2ºF per 1,000 feet). This is the wet adiabatic lapse rate (WALR). It is only used after air is saturated (i.e. the air mass is at 100% saturation).
Using the diagram below, calculate the temperature of air below at each letter using a starting temperature of 25ºC on the windward side at 100 meters (m) elevation.
8. What was the temperature (ºC) at the following letters?
24: A__________________ (.5 pt) 27: D ____________________ (.5 pts)
25: B ____________________ (.5 pt) 28: E____________________ (.5 pts)
26: C____________________ (.5 pt)
29. At what elevation did condensation begin to occur? (1 pt)
30. What was the difference in the temperature at 100 m elevation on the windward and leeward side? Why is there a difference? (1 pts)
100 m
100 m
1100 m
1600 m
2100 m
A
B
C
D
E
2600 m
Windward
Leeward
25ºC
31. What would you expect the landscape on the leeward side of the mountain to look like given your calculations, taking into account the relationship between temperature and atmospheric moisture? (1.5 pts)
32. Give a geographic example of where this phenomenon, with uplift and condensation occurring on the windward side of a mountain and increasing temperature on the leeward side, occurs. (1.5 pts)

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