METEOROLOGIST JEFF HABY
1. What is PW?
PW stands for Precipitable Water. It is a parameter which gives the amount of
moisture in the troposphere.
2. How is PW determined?
PW is determined by taking all the mass of
water vapor in the troposphere and depositing it on the earth's
surface. The depth of moisture that would be on the earth's surface is the PW value. The mass of water
vapor is determined by the
dewpoint (saturation mixing ratio) of the air integrated over the troposphere.
Higher dewpoints lead to higher PW values, especially if the relatively high dewpoints extend through a
significant vertical depth. The scale below gives an indication of the moisture content of the troposphere
0.50 inches or less = very low moisture content
0.50 to 1.25 inches = low moisture content
1.25 to 1.75 inches = moderate moisture content
1.75 to 2.00 inches = high moisture content
2.00 inches or above = very high moisture content
At the bottom is a sounding from Grand Junction, CO. This is an
inverted-V sounding with a low PW. The PW, as can be seen
from the parameters given to the right of the sounding, is 0.64 inches.
3. Operational significance of PW:
Severe Storms: Tropospheric moisture determines whether a
supercell storm will be HP (High Precipitation),
Classic, or LP (Low Precipitation). HP supercells often have PW of 1.75 inches or above and LP supercells
often have a PW value of less than 1 inch. For more information on these supercell types, go to link below:
Flooding potential: A forecast area has a climatological normal PW for a certain time of year. In cases where
the PW value is 2 to 3 or greater times more than the climatological value, flooding becomes more likely when
a heavy precipitation event occurs. This often occurs in drier climates whose vegetation and land is not
adapted to absorbing large amounts of precipitation.
Lightning: In a high
CAPE environment, high PW will lead to storms that produce an abundant amount of
Updraft velocity: PW is the most significant contribution to water loading. Water loading reduces updraft
strength since gravity tries to push the precipitation mass downwards-- against the momentum of the updraft. High PW
also produces a heavier downdraft. The
updraft being reduced is especially evident in a weak shear environment
where the downdraft locates very near the updraft, thus destroying the updraft.
Hail: High PW tends to reduce hail size since the
updraft velocity is reduced. Classic supercells
are often better large hail producers compared to HP supercells (HP supercells often form in very moist low elevation
Convective wind gusts: A high PW often occurs when the troposphere is fairly saturated. This can reduce
convective wind gusts since convective wind gusts require dry mid-level air to add to their significance.
Winter weather: It is more important for the troposphere to be near saturation than it is for the PW to
be high in a winter weather situation since lifting, rather than instability release, is most important
in precipitation generation. Cold temperatures limit the amount of PW. Winter soundings can have a
low PW and still produce significant precipitation events if lifting and saturated conditions exist.
PW should not be used directly to forecast precipitation amounts (i.e. 1.25 inches of PW does not mean
1.25 inches of precipitation will occur). Factors such as storm motion,
moisture convergence, areal coverage of
precipitation, amount of lifting and instability release, etc. also determine precipitation amount
other than the amount of moisture in the air.