Exercise 4-B: Orographic Uplift and the Rainshadow Effect When an air mass is cooled to its dew point, it becomes saturated. Any further cooling of the air mass results in condensation, in which water vapor Question: Exercise 4-B: Orographic Uplift And The Rainshadow Effect When An Air Mass Is Cooled To Its Dew Point, It Becomes Saturated. Any Further Cooling Of The Air Mass Results In Condensation, In Which Water Vapor Changes State To Liquid Water. During Condensation, Latent Heat Is Released To The Atmosphere In The Form Of Sensible Heat (540 Calories / Gram Of
Exercise 4-B: Orographic Uplift and the Rainshadow Effect When an air mass is cooled to its dew point, it becomes saturated. Any further cooling of the air mass results in condensation, in which water vapor changes state to liquid water. During condensation, latent heat is released to the atmosphere in the form of sensible heat (540 calories / gram of water). One of the principal mechanisms of atmospheric cooling is the uplift of air masses. Because pressure decreases in the atmosphere from the surface upward, uplifted air masses expand as they rise. This expansion results in a decreased rate of collisions between air molecules, which in turn causes the air mass to decrease in temperature. A temperature decrease due to uplift of an air mass is called adiabatic cooling. Conversely, a temperature increase due to downward movement of an air mass is called adiabatic warming. The elevation at which condensation occurs during adiabatic cooling is called the lifting condensation level (LCL). It is the elevation at which the air mass is cooled to the dewpoint by adiabatic cooling. When the temperature reaches the dewpoint, the air is fully saturated and further cooling will result in condensation. One way that air masses are uplifted is by being "pushed" up over a topographic obstruction, such as a mountain range. This is referred to as orographic uplift. Remember, when air is lifted, it will cool down due to adiabatic cooling. If there is enough moisture in the air and the temperature is lowered to the dewpoint, condensation in the form of clouds and precipitation may occur. Thus, air can loose moisture as it moves over a mountain range. Often, locations on the lee side of the mountain range are very dry because of this phenomenon. We call this the rain shadow effect. This exercise allows you to calculate changes in temperature and humidity of an air mass as it passes over a mountain range and will illustrate the rain shadow effect.
The situation: An air mass over Seattle, WA has a temperature of 15°C and a dewpoint of 12°C at sea level. As it travels from west to east it is pushed up over the Cascade Mountains and then down the eastern slope toward Spokane. Use your understanding of adiabatic cooling and heating, dry and wet/moist adiabatic lapse rates and condensation to answer the following questions. Use the following rates in your calculations: DAR = 10°C / 1000 m; MAR = 6°C / 1000 m. Assume that all water condensing in the atmosphere falls as precipitation.
Please show all work
1) What is the specific humidity of the air mass at Seattle? What is its relative humidity?
2) By how many degrees must the air mass be cooled before it reaches saturation?
3) What is the elevation of the lifting condensation level? What is the air temperature, specific humidity and relative humidity at the lifting condensation level? Show your work.
4) What is the air temperature, specific humidity and relative humidity at the summit? Show your work.
5) What is the air temperature, specific humidity and relative humidity at Spokane? Show your work.
6) Why is the relative humidity lower in Spokane than in Seattle? Remember the definition of relative humidity.
7) Why is the air temperature warmer in Spokane than it is at the summit?
8) Which side of the Cascade Mountains would you expect to receive more precipitation? What term do we give to the side of the mountain that receives less precipitation as a result of orographic uplift?