Role of moisture in convection

Let 's consider a pan of liquid  water exposed to the atmosphere (Fig. 1).  Water molecules are in constant motion. As a result of this motion, some of the water molecules  molecules will escape from the liquid water and evaporate.  If we let this process take place for a long enough time, all the water will evaporate and the pan will dry out.

Fig 1. Pan of water open to the atmosphere: evaporation of water

Now if we put a lid on the pan of water (Fig. 2), we will prevent the water molecules from irreversibly escaping to the atmosphere. The water molecules escaping from the liquid water eventually bounce off the lid and return to the liquid.  At equilibrium the rate of escape of water molecules from the liquid is matched by the rate at which the water vapor molecules return to the liquid.  When such an equilibrium is reached, the air is saturated.  If one increases the temperature, the water molecules in the liquid will move faster and more of them will escape to the gas-phase.  As a result, the amount of water vapor will increase with increasing temperature.
 

Fig 2. Pan of water with lid: saturation (equilibrium between the evaporation of water molecules from the liquid and condensation of water vapor)
 
 

The amount of water vapor in the atmosphere at a particular place and time is expressed in terms of the mixing ratio (the mass of water vapor per kilogram of dry air), w:

    w = mixing ratio of water vapor = grams of water vapor/kilogram of dry air

Mixing ratios range as high as 25 grams per kilogram (g/kg) in humid tropical air masses. The saturation mixing ratio, w_s, corresponds to the mixing ratio of water vapor at saturation:

Fig. 3   Saturation mixing ratio as a function of Temperature
 

The saturation mixing ratio increases rapidly with temperature, as shown in Figure 3. You can see from this figure that for each 10°C temperature rise the saturation mixing ratio of an air parcel nearly doubles.

The ratio of the actual mixing ratio of an air parcel to the the saturation mixing ratio of air at the same temperature and pressure (x 100) is called  the relative humidity (expressed as a percent).

Relative humidity = RH = 100 x w/ws
 

For example an air mass with an actual mixing ratio of 10 g/kg and a saturation mixing ratio of 20 g/kg has a relative humidity of 50%. If an unsaturated air parcel is lifted by convection, its mixing ratio is conserved, but its relative humidity increases as the parcel cools and its saturation mixing ratio drops. If the air parcel is lifted high enough it eventually becomes saturated, at which point, water vapor begins to condense out to form cloud droplets. The temperature at which condensation begins to occur is called the dew point and the level at which it occurs is called the lifting condensation level (LCL). The LCL corresponds to cloud base. 
 

It may seem surprising at first to learn that most of the air in the earth's atmosphere is not saturated with water vapor.  Most of the underlying surface is oceans and there is no way that molecules can  escape from the top of the atmosphere, so why doesn't the system eventually reach the equilibrium situation pictured in Fig. 2 above, with 100% relative humidity everywhere?  The answer is that (unlike the situation pictured in Fig. 2) water vapor is continually being removed from the atmosphere in clouds, where it condenses onto growing cloud droplets and ice crystals that eventually fall out. And since much of the air that's in contact with the wet underlying surface is unsaturated, water molecules are continually escaping from the underlying surface and entering the atmosphere as water vapor. This recycling of water molecules between the atmosphere and the underlying surface is the atmospheric branch of the so called 'hydrologic cycle'.  Water vapor molecules are evaporated at the earth's surface, they're transported (often over long distances) within the atmosphere, and then they eventually condense in clouds and fall back to the earth's surface as rain or snow.  Ironically, it is the growth precipitation particles in clouds that keeps the atmosphere from getting saturated with water vapor.

An alternative measure the amount of water vapor present at a point in the atmosphere is the dew point.  The dew point is the temperature to which a parcel of air can be cooled at constant pressure (i.e., without lifting it) before it becomes saturated.  The cooling of an unsaturated air parcel decreases its saturation mixing ratio in accordance with Fig. 3, but it doesn't affect its actual mixing ratio.  Hence, as the parcel cools its relative humidity rises until it reaches 100%.  It's temperature at that point is called the dew point.  If the temperature and the dew point of an air parcel are the same, the parcel must be saturated.  The larger the difference between the temperature and the dew point, the lower the relative humidity.  The highest dew points observed in the atmosphere are in the range of 75 F.  Like relative humidity the dew point of an air parcel tends to be conserved unless water vapor is evaporated into it or condensed out of it.  If you know the  dew point of an air parcel it's easy to find the relative humidity and vice versa, provided that you know the temperature and pressure.
 

Now let's consider why clouds form. If an unsaturated air parcel is lifted by convection, its mixing ratio is conserved,
 
 
 
 

 
Review Questions on Convection

1. Why does warm air rise? 
2. Explain how convection transports heat and moisture upward. 
3. Define 'dry adiabatic lapse rate' and state its numerical value. 
4. What is the limiting lapse rate for convection in (a) fresh water, (2) dry (i.e., unsaturated) air? 
5. Explain why the limiting lapse rate for convection is less for moist air is less than that for dry (i.e., unsaturated) air. 
6. Where does the latent heat released in moist convection come from? 
7. Define (a) relative humidity and (b) lifting condensation level. 
 

Critical Thinking Questions

1. Why does relative humidity usually drop during the morning hours and rise during the evening? 
2. Why does convection over land occur much more frequently around mid-day than during the night and during summer than during winter? 
3. Air pollution and convection are rarely observed at the same time and place.  Explain. 
4. If the relative humidity is 25% and it ascends dry adiabatically in a thermal, by approximately how much will it rise before it becomes saturated? 
5. A moist air mass is more likely to undergo convection than a dry one. Explain. 

 Last Updated:
01/17/02