February 29, 2008

Water Vapor in the Atmosphere (continued)

Using the definitions of mixing ratio (U) and saturation mixing ratio (US), we can now determine relative humidity (RH) as

  • RH = U / US
  • Go over Table 4.1 (Saturation Mixing Ratio) on in-class handout. Describe what happens in a closed system.
  • Do a few examples for computing Relative Humidity using the concept of mixing ratio.

Most people use Relative humidity to describe the water vapor content of the air, but it is widely misunderstood. Relative humidity in itself does not indicate the actual amount of water vapor in the air because it depends on temperature. It only tells you how close the air is to being saturated. For example, an air parcel at a temperature of 10°C with RH = 100% contains the same concentration of water vapor as an air parcel at a temperature of 20°C with RH = 50%. You should be able to convince yourself of this by using the Table 4-1 from the class handout.

  • In fact RH can be changed in two ways:
  • Change the amount of water vapor in a parcel (changes U)
  • Change the air temperature in the parcel (changes US)

The dew point temperature (or dew point) (Td) is the temperature to which an air parcel would have to be cooled for the parcel to become saturated with water vapor.

  • The dew point temperature is the answer to this question: Given the amount of water vapor that is in a parcel, what would the air temperature have to be for the parcel to be saturated with that amount of water vapor?
  • Unlike RH, the dew point temperature does indicate the actual amount of water vapor in the air … the higher the dew point, the higher the water vapor content of the air.
  • We will also use table 4.1 from the class handout to perform calculations involving the dew point temperature.
  • Describe the correspondences between
  • Air temperature and saturation mixing ratio
  • Dew Point Temperature and mixing ratio
  • Go over some example calculations
  • The following statements should make sense to you:
  • When the air and the dew point temperatures are far apart, the relative humidity is low
  • When the air and the dew point temperatures are close to the same value, the relative humidity is high
  • When the air and the dew point temperatures are the same, the air is saturated and the relative humidity is 100 percent
  • When comparing the absolute amount of water vapor in the air between different locations, we must use the dew point temperature rather than relative humidity.
  • Look at figure 4.9 from textbook, which compares cold, humid air with hot, dry air.
  • Which air contains more water vapor? Use table 4.1 to estimate how much more water vapor.
  • Under which condition, will the ground dry out faster? Why?

Summary for how water behaves

  • If RH = 100%, there will be no net evaporation or condensation, since the rate of evaporation is equal to the rate of condensation
  • If RH < 100% (Td < T), any liquid water present will evaporate with time, since the rate of evaporation is greater than the rate of condensation.
  • If RH > 100% (Td > T), water vapor will condense to liquid water until RH falls back to 100%, since the rate of condensation is greater than the rate of evaporation. When this occurs, it is only a temporary situation until sufficient water vapor condenses out of the air. Condensation will take place:
  • 1st onto other existing liquid surfaces, if available
  • 2nd onto solid surfaces (dew or frost), if available
  • 3rdonto tiny aerosol particles called cloud condensation nuclei. These will become tiny droplets of liquid water that remain suspended in the air (clouds).

Heat Index

  • The human body must regulate its internal core temperature within a narrow operating range, normally (35-39)° C or (95.0 – 102.2)° F in order for the body to function properly.
  • When the body becomes heat stressed, due to being exposed to warm conditions and/or exercising, it must take actions to cool itself. The most important way the body does this is by sweating.
  • Keep in mind that the sweat itself does not directly cool the body. It is only when sweat evaporates that the body realizes any cooling.
  • Thus, if the body is exposed to conditions of high temperature and high humidity, it can become difficult to remove enough excess heat because the sweat will evaporate slowly. Under these conditions a person can develop heat cramps, heat exhaustion, and possibly heat stroke, which can result in death.
  • To alert people to the possible dangers associated with the combination of high heat and high humidity, the weather service has developed a heat index chart. The numbers in the chart are sometimes referred to as a “feels like” temperature. Really the numbers should be interpreted as a measure of how fast a typical human body can lose heat under the specified conditions of air temperature and relative humidity. The higher the heat index, the slower the rate of heat loss from the body and the greater chance of the body overheating.
  • Take a look at figure 4.10 from textbook
  • There are other factors to consider beside just the heat index.
  • Obviously, someone exercising hard is under more danger because they are generating a lot of internal heat.
  • Someone who does not drink enough water can overheat simply because they are unable to produce enough sweat.
  • Dehydration is another potential problem. Dehydration is common under hot, dry desert conditions even when the heat index value is low. Because sweat evaporates so quickly, people sometimes do not notice how much sweat they are producing (they remain dry instead of dripping with sweat, which happens in more humid conditions), so they fail to drink enough water to replenish lost sweat.

Dew and Frost

Normally, near the Earth’s surface, RH is less than 100% and there is net evaporation taking place. Occasionally, though, conditions can develop where net condensation takes place near the surface of the Earth. When water vapor condenses onto objects located on the surface of the Earth, we call this dew or frost.

Dew or frost will occur whenever an object becomes colder than the dew point temperature of the air. Physically this happens because a thin layer of air in contact with the cold surface will become the same temperature as the object. This air is then cooled below its dew point temperature. Whenever this happens, net condensation must occur, since the cooled air cannot hold all the water vapor it contains … in other words, the air’s mixing ratio is higher than its saturation mixing ratio, and the excess water vapor must condense onto the surface of the object.

  • The condensation will be liquid if the temperature of the object is above the freezing point of water (> 0° C). This is dew.
  • The condensation will be ice (water vapor to ice, which is called deposition) if the temperature of the object is below the freezing point of water (<0° C). This is frost. Frost is not frozen dew.

The formation of dew is somewhat rare in the desert because the air is typically very dry in terms of relative humidity. Or in other words, the dew point temperature is often much lower than the air temperature, so objects will not commonly become colder than the dew point temperature.

The most common time of day for dew or frost formation is in the early morning near the time of the low air temperature for the day. Because the ground (and objects on the ground) can cool very rapidly by emitting radiation, at night the ground temperature is often colder than the reported air temperature (taken a few meters above the ground).

An everyday example of dew formation is condensation that happens on the outside of a cold can of soda or a glass of ice water. The water that you see came from the air as water vapor condensed into liquid. It tells you that the can must be colder than the dew point temperature of the air.

End of Quiz 3 material