What is Precipitable Water?
In meteorological terms, precipitable water (PW) is the measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb. In simpler terms, PW is the amount of moisture in the troposphere. PW is usually measured in inches or millimeters and generally speaking:
PW values are measured in a number of different ways, but usually the most accurate is by way of radiosonde, otherwise known as an upper air sounding. Upper air balloons are launched at the NWS Birmingham office twice a day, the results of which can be examined here.
Record Values of PW
On the morning of August 14, 2010, the preliminary 12Z upper air sounding data indicated there were 2.66 inches of PW present in the atmosphere above BMX. According to Matt Bunkers of NWS Rapid City, SD, this is by far the highest PW total ever recorded at BMX. The next two highest values were 2.53 inches on July 11th, 1965 and 2.52 inches on September 2, 1950. Climatologically, 2.66 inches is more than 150% of normal values in Alabama for this time of year.
How Does this Affect Me?
PW values affect many aspects of weather, especially when it comes to the severity of certain weather events.
Flooding potential: 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.
Lightning potential: In a high CAPE environment, high PW will lead to storms that produce an abundant amount of lightning.
Hail potential: High PW tends to reduce hail size.
Wind gusts from thunderstorms: 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.