Name School LAB 3: Atmospheric Pressure & Moisture Our atmosphere is a very dynamic area especially when we see what type of interactions it has with the surrounding environment. This lab will begin discussing the ideas behind atmospheric gasses, pressure, humidity and temperature. Beginning with pressure, the air in the atmosphere is made up of a number of gases; we can differentiate them two ways, either as a permanent or variable gas. What this means are those permanent gasses of the atmosphere do not change, these percentages are constant and will remain so long after we are gone, but for variable gasses these gasses can change, and they do. 1. What are the top two permanent gasses found in our atmosphere? 2. What are the top two variable gasses found in our atmosphere? 3. With variable gasses, why are they variable? What can control the amount emitted? Back to pressure, these permanent and variable gases press down on the Earth s surface, exerting a force that we call atmospheric pressure or air pressure. Although we are usually unaware of this pressure, it actually presses down very hard roughly equivalent to the force of an elephant balancing on your shoulders. 4. T or F: Air pressure is higher at lower altitudes. 5. T or F: Air density is higher at lower altitudes. 6. T or F: There is more space between air molecules at higher altitudes. 7. T or F: There is less oxygen to breathe at the top of a high mountain than there is at sea level. Patrich Physical Geography Lab 1
Temperature & Specifics about the Atmosphere Troposphere Known as the lower atmosphere almost all weather occurs in this region. The troposphere begins at the Earth's surface and extends from 4 to 12 miles (6 to 20 km) high. As the density of the gases in this layer decrease with height, the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height in response. As one climbs higher, the temperature drops to -60 F (-51 C) (NOAA). Stratosphere The Stratosphere extends around 31 miles (50 km) down to anywhere from 4 to 12 miles (6 to 20 km) above the Earth's surface. This layer holds 19 percent of the atmosphere's gases but very little water vapor. In this region the temperature increases with height. Heat is produced in the process of the formation of Ozone and this heat is responsible for temperature increases from an average - 60 F (-51 C) at tropopause to a maximum of about 5 F (-15 C) at the top of the stratosphere. Mesosphere This layer extends from around 31 miles (50 km) above the Earth's surface to 53 miles (85 km). The gases, including the oxygen molecules, continue to become denser as one descends. As such, temperatures increase as one descends rising to about 5 F (-15 C) near the bottom of this layer. The gases in the mesosphere are now thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky. Both the stratosphere (next layer down) and the mesosphere are considered the middle atmosphere. The transition boundary which separates the mesosphere from the stratosphere is called the stratopause (NOAA) Patrich Physical Geography Lab 2
Thermosphere Between about 53 miles (85 km) and 375 miles (600 km) lies the thermosphere. This layer is known as the upper atmosphere. While still extremely thin, the gases of the thermosphere become increasingly denser as one descends toward the earth. As such, incoming high energy ultraviolet and x-ray radiation from the sun begins to be absorbed by the molecules in this layer and causes a large temperature increase. Because of this absorption, the temperature increases with height. From as low as -184 F (-120 C) at the bottom of this layer, temperatures can reach as high as 3,600 F (2,000 C) near the top. However, despite the high temperature, this layer of the atmosphere would still feel very cold to our skin due to the very thin atmosphere. The high temperature indicates the amount of the energy absorbed by the molecules but with so few in this layer, the total number of molecules is not enough to heat our skin. Exosphere This is the outermost layer of the atmosphere. It extends from the top of the thermosphere to 6,200 miles (10,000 km) above the earth. In this layer, atoms and molecules escape into space and satellites orbit the earth. At the bottom of the exosphere is the thermopause located around 375 miles (600 km) above the earth (NOAA). 8. T or F: If you follow the temperature line in the graph above from the surface of the Earth to the top of the atmosphere, moving to your left represents a decrease in temperature, to your right represents an increase in temperature, and straight up represents a constant temperature. 9. T or F: Temperature increases as you gain altitude in the Stratosphere and the Thermosphere. 10. T or F: Temperature decreases as you gain altitude in the troposphere and mesosphere. 11. T or F: Air temperature varies in complicated ways with altitude. Patrich Physical Geography Lab 3
Atmospheric moisture can be seen two different ways, as absolute and relative, but there are many other variables involved, below is a list of vocab that may be useful Specific humidity - measures mass of water vapor in a fixed total mass of air. In general it is greater in the tropics (16 g/kg) than in the Polar Regions (4 g/kg). This definition is used when we are interested in the actual amount of water that is in the atmosphere as a gas. The specific humidity remains constant as long as you do not add or take out water vapor molecules from the volume of air. Mixing ratio - mass of water vapor in a fixed mass of remaining dry air. Since there are so few molecules of water vapor in a volume of air, as compared to N2 and O2, the value of the mixing ratio is similar to the specific humidity. Changing the temperature of the air parcel does not affect the parcels mixing ratio. Absolute humidity - mass of water vapor in a fixed volume of air, or the water vapor density. We don't ever use this definition so don't bother memorizing it! Relative humidity - ratio of the amount of water vapor actually in the air compared to the amount of water vapor required for saturation at that particular temperature and pressure. This is one method that nightly weather persons report. It is expressed as a percentage. Increasing or decreasing the amount of water vapor in the air changes the relative humidity. A change in temperature will also bring about a change in relative humidity. Dew point temperature - the temperature to which air would have to be cooled (with no change in air pressure or water content) for saturation to occur. When the dew point is below 32F (0C) it is called the Frost point. The Heat Index - combines air temperature with relative humidity to determine an 'apparent' temperature - what the temperature 'feels like'. Let s spend a few moments to discuss relative humidity. Relative humidity is the measure of the amount of water vapor in the air this ratio is a percentage of water vapor in the air compared to the maximum amount the air can hold. 12. Can warm air hold more moisture than cold air? 13. What happens if you have 80% humidity and it cools from 30 C to 10 C? Patrich Physical Geography Lab 4
The temperature and pressures that occur at the earth s surface allow water to change from one state of matter to another. The key is to understand whether energy is absorbed or released. The second piece is to understand the vocab associated with this process. 14. To melt ice, heat energy must be (absorbed or released) by the water molecules. 15. The process of condensation requires that water molecules (absorb or release) heat energy. Let s finish with water-vapor capacity in air. Any measure of water vapor in the air is referred to as humidity- we know this because as mentioned earlier we see humidity two different ways: relative and absolute. The mass of water vapor in a unit of air compared to the remaining mass of dry air is referred to as the mixing ratio. The diagram to the right represents the mixing ratio, (or how much water is required to saturate the air at a specific temperature). Using the mixing ratio table, answer the following questions. What is the water vapor content at saturation of a kilogram of air at each of the following temperatures? 16. 40 C: grams/kilogram. 19. -20 C: grams/kilogram. 17. 68 F: grams/kilogram. 20. 50 F: grams/kilogram. 18. 0 C: grams/kilogram. 21. 35 C: grams/kilogram. Patrich Physical Geography Lab 5
As previously mentioned, relative humidity is a ratio of water vapor content divided by the saturation. The formula looks like this: Water Vapor Content Relative Humidity (%) = Saturation Mixing Ratio x 100 As an example of this function, the mixing ratio of air at 25 C is 20 grams per kilogram. If the actual amount of water vapor in the air was 5 grams per kilogram, the relative humidity would be calculated like this: 5g/kg Relative Humidity (%) = 20g/kg x 100 = 25% Using the mixing ratio table and the formula for relative humidity, answer the following questions. Air Water Vapor Relative Temperature Content Humidity 22. 15 C 2 g/kg % 23. 15 C 5 g/kg % 24. 15 C 7 g/kg % 25. 25 C 5 g/kg % 26. 14 F 1 g/kg % 27. 5 C 5 g/kg % 28. In the winter, air is heated in homes. What effect does heating have on relative humidity inside the home? What can be done to lessen that effect? 29. Explain why the air in a cool basement is humid (damp) in the summer. 30. Write a brief statement describing each of the two ways that the relative humidity of air can be changed. Patrich Physical Geography Lab 6
The Lapse Rate Temperature changes brought about solely by expansion or compression are called adiabatic temperature changes. Air with a temperature above its dew point (unsaturated air) cools by expansion or warms by compression at a rate of 1 C per 100 meters of changing altitude the dry adiabatic lapse rate. After the dew point temperature is reached, and as condensation occurs, latent heat that has been stored in the water vapor will be released. The heat being released by the condensing water slows down the rate of cooling of the air. Please use the diagram below to answer questions 31-33. Please solve all the temperatures on the windward side on the diagram provided above before proceeding. 31. What would the temperature be at 2000 on the windward side? 32. What would the temperature be at 10000 on the leeward side? 33. What would the temperature be at sea level on the leeward side? Please solve all the temperatures on the windward side on the diagram provided. Patrich Physical Geography Lab 7
Measuring Relative Humidity One method to measure relative humidity and to determine the dew point temperature is the use of a simple instrument called a psychrometer. It consists of two thermometers mounted side by side in a small frame. One thermometer measures the temperature of the air; it is the dry-bulb thermometer. The other thermometer is wrapped with a cloth, which is soaked with air-temperature water. It is called the wet-bulb thermometer. The two thermometers are whirled through the air, which speeds evaporation of water from the wet cloth. The evaporation process requires energy (latent heat), cooling the wet-bulb thermometer and causing a lower temperature reading than on the dry-bulb thermometer. The difference in temperature between the two thermometers is the wet- bulb depression. The amount of wet-bulb depression depends on the amount of evaporation from the cloth and this depends on the absolute humidity of the air. Low absolute humidity results in more evaporation (and lower wet-bulb temperature) than occurs in air with high absolute humidity. EXAMPLE After following the above procedure, a dry bulb temperature of 90 F and a wet bulb temperature of 72 F were obtained. Thus, the temperature difference is 18 F. From the Relative Humidity table, locate the column at the top headed with 18. Follow this column down to the row headed with dry bulb temperature of 90 F. The intersection of the column and the row gives the number 41. This number is the percent relative humidity. 34. Dry Bulb Temperature 35. Wet Bulb Temperature 36. Differences in Temperature (+/-) 37. Relative Humidity Patrich Physical Geography Lab 8