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:

The saturation mixing ratio increases rapidly with temperature: 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).

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. 

If an air parcel is lifted beyond its LCL, its mixing ratio is no longer conserved. Water vapor is condensed out onto growing cloud droplets at a rate of ~50% of the amount remaining for each km the parcel is lifted. The heat of condensation warms the surrounding air at a rate of 3-4 degrees per kilometer that the air parcel rises.  Because of this 'condensation heating' a saturated air mass with a given lapse rate is less stable than an unsaturated air mass with the same lapse rate. This reduction in stability makes it possible for air parcels originating near the Earth's surface to penetrate all the way up to the tropopause before they run out of buoyancy. 

 
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:
10/17/2001