Michael Pidwirny
LABORATORY 8: ATMOSPHERIC MOISTURE AND PRECIPITATION
LEARNING GOALS
The objectives of this laboratory are to familiarize you with two important components of the hydrological cycle, atmospheric moisture and precipitation, and the mechanisms that are responsible for the formation of precipitation and its global distribution.
Upon completion of this laboratory you will be able to:
- Mathematically manipulate several different measures of atmospheric moisture.
- Describe the spatial patterns of precipitation on a global scale.
- Understand the mechanisms of precipitation formation.
ATMOSPHERIC WATER
The hydrologic cycle models the course of water over the Earth’s surface and within its atmosphere (Figure 8.1). It follows the precipitation of water in the form of rain and snow to the Earth’s surface, its storage in oceans, lakes, snow, and ice, its transport by rivers and ocean currents, and its subsequent return to the atmosphere through evaporation and transpiration by vegetation. Less than 0.001% of Earth’s water is normally found in the atmosphere. Within the atmosphere, water can move about the globe through advection bringing areas more precipitation than their actual evaporation.
At any given time and place, the amount of water in the atmosphere available for precipitation is precipitable water. On average, over a year this water cycles between the atmosphere and Earth’s surface some 36 times, resulting in a mean annual global precipitation of about 1050 mm. At the global scale, evaporation from the Earth’s land and ocean surfaces balances the precipitation that falls.
In this laboratory, we focus on two components of the hydrological cycle, atmospheric storage of water vapor (humidity) and precipitation.
ATMOSPHERIC MOISTURE, HUMIDITY AND DEW POINT
The term humidity is used in climatology and meteorology to describe the water vapor content of the atmosphere. This term does not refer to the presence of water in liquid or solid forms.
Common expressions of humidity are:
- Vapor pressure (e): the pressure exerted by the vapor alone.
- Absolute humidity (ρ): the mass of water vapor in one cubic meter of air.
- Mixing ratio (r): the ratio of the mass of water vapor to the mass of dry air.
- Relative humidity (RH): the ratio of the amount of water vapor that is present to the amount of water vapor that could be present. Expressed mathematically, we can say that:
RH = e/es x 100 (Units: %)
or
RH = ρ/ρs x 100 (Units: %)
or
RH = r/rs x 100 (Units: %)
where es is the vapor pressure for saturated air at a given temperature; ρs is the absolute humidity for saturated air at a given temperature; and rs is the mixing ratio for saturated air at a given temperature. A relative humidity of 100% indicates saturation and usually results in condensation.
Table 8.1 shows the vapor pressure of saturated air:
Another important expression that can be used as a measure of atmospheric humidity is the dew point (if the dew point is below freezing, it is