Precipitation
There are two types of precipitation and four forms of precipitation. Types of precipitation may be identified by the physical responses which cause moisture to come from the atmosphere. Forms of precipitation are identified by the form of moisture as it descends from the sky. The two types of precipitation are convectional and orographic. They are very similar in that both are caused by the rising, cooling, condensing, and falling of moisture within a given body of air.
Convectional precipitation, as the term implies, is caused by a heat transfer at the surface of the Earth. It is most commonly associated with the higher temperatures of summer and spring and the heating of the crust in the late afternoon or early evening. As the Earth’s crust is warmed by the Sun, the air immediately above the crust is also warmed by convection. The warmer air rises, and as it rises, begins to cool at a fairly constant and known rate. Air containing moisture cools at what is called the wet adiabatic rate. At this rate, air temperature decreases approximately 3.5 degrees for every one thousand feet of increased elevation. When the moisture is removed from the air through precipitation, it begins to cool at the dry adiabatic rate. At this rate, air temperature decreases approximately 5.5 degrees per one thousand feet of increased elevation.
As the air rises and cools, the temperature eventually reaches the dew point. The dew point of a given body of air is the temperature at which the moisture in the air will condense. (The relative humidity, mentioned in weather reports is the ratio between absolute humidity and dew point.) Dew point will vary depending upon the amount of water in the air, also known as the absolute humidity. When the air temperature reaches dew point, the water condenses and begins to fall as rain, sleet, or snow depending upon the temperature. If the air continues to rise, its temperature will decrease faster as it has reached dry adiabatic rate status.
Orographic precipitation is very similar; however, in the case of orographic precipitation, the air rises due to an obstruction of the air mass rather than temperature. The only obstructions large enough to cause this are mountains and cold fronts. As warm air moves up a mountain, it cools at the wet adiabatic rate until the air reaches its dew point.* It then cools at the dry adiabatic rate. When the air descends the mountain, it then begins to warm at the dry adiabatic rate because there is very little moisture. If no moisture is added to the air, it will warm all the way to the base of the mountain, creating a usually hot, dry climate on the leeward side (the side opposite from prevailing wind direction).
Algebraic formulas may be utilized to compute the air temperature as it moves up a mountain. The first measurements may be obtained without the use of these formulas. Simply multiply the number of feet (to the nearest 1000) by 3.5 degrees and subtract that product from the original air temperature. Measurements on the leeward side may be obtained by multiplying the distance descended from the crest of the mountain by 5.5 degrees and adding the product to the temperature at the top of the mountain. The formulas may also be used when computing the occurrence of condensation and snow.
Use the following formulas:
Let T1 = the original temperature (at sea level)
Let T2 = the temperature at a given elevation (dew point or freezing point - see hint)
Let x = elevation in feet
To compute the elevation (in feet) at which condensation or snow (see hint) occurs,
• 1.Then: x = (T1 - T2)÷ .0035
To compute the temperature at any given elevation on the windward side of the mountain,
• 2Then: T2 = T1 - .0035x
To compute the original temperature given the temperature at a fixed point and the elevation,
• 3Then: T1 = T2 + .0035x
*For simple models, we assume that the air cools at the wet adiabatic rate to the top of the mountain, and warms at the dry adiabatic rate descending the mountain to its base.
For more complex models, we may assume that the air cools at the wet adiabatic rate to the dew point or to the freezing point. In actuality, it may cool at the wet adiabatic rate far below freezing depending upon the absolute humidity.
HINT:
Remember that when computing the point at which snow occurs on the mountain, you must first change the value of T2 in equation 1 from the dew point to the freezing point Fahrenheit (32o). Or use the equation on the following page on more complex models.
To compute the temperatures on the leeward side of the mountain, a formula is not necessary. Simply multiply the number of feet descended by 5.5 and add the product to the temperature at the top of the mountain. If a formula is desired, use the following:
Let T1 = temperature at the crest or top of the mountain
Let T = temperature at any given elevation on the leeward side of the mountain
Let x = elevation in feet
Then: T = T1 + .0055x
• Your temperatures on top of very tall mountains will be very low. The air on top of a mountain is very cold and very dry.
• Your temperatures on the bottom leeward side will be extremely high. The air here is very warm and dry often causing desert climates to exist.
Now convert temperatures from Fahrenheit to Celsius using the following formula:
Let F = temperature in Fahrenheit
Let C = temperature in Celsius
Then: F = 1.8 C + 32 or C = (F - 32) ÷ 1.8
**
Assuming that the dry adiabatic rate begins when the air reaches its dew point, one must also compute the altitude change between the point of condensation and the point of snow based upon the dry adiabatic rate.
Therefore:S = [(T1 - T2) ÷ .0035] + [(T2 - F) ÷ .0055]
where S = Elevation of snow
T1 = Original temperature
T2 = Dew Point
F = Freezing Point
B 10,000 feet D
A 2000 feet
windward sideleeward side
Look at the diagram of the mountain above.
Determine air temperature (to the nearest degree) at points A, B, C, D, and E given that air temperature at windward sea level is 86o and dew point is 52o.
Temperature at A = ______D = ______
B = ______E = ______
C = ______
Now convert all temperatures from Fahrenheit to Celsius.
Temperature at A = ______D = ______
B = ______E = ______
C = ______
At what height (to the nearest 500 feet) should water first condense and fall as rain?
______
At what height (to the nearest 500 feet) should it begin snowing? Assume W/A rate TO dew point.
______
Describe the climate at the top of the mountain.
Describe the climate at the bottom leeward side of the mountain.