How moisture affects convection?

Dry convection is virtually always shallow. Deep convection extending all the way up to the tropopause level
invariably involves the condensation of water (i.e., the phase transition from the vapor state to the liquid state).

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). Mixing ratios range as high as 2 grams per
kilogram (g/kg) in humid tropical air masses. The amount of water vapor that air is capable of holding,
referred to as its 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 in a thermal, 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

1. Why do thermals 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. At what level of the ocean is the pressure on a diver equal to 2 atmospheres?
5. 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?
6. A moist air mass is more likely to undergo convection than a dry one. Explain.