This series of Haby Hints investigates problems that cause a forecast to bust. A bust occurs when a certain weather parameter is expected but one or more factors cause the forecast to be wrong. This particular Haby Hint will focus on how precipitation causes forecast problems.

Precipitation can profoundly influence temperature. Here are a few examples:

a. outflow from a storm cooling the surface temperature

b. evaporative cooling

c. increase of soil moisture influencing temperatures

d. change in albedo of the earth's surface such as a surface snow cover

The theme that is developing here is that precipitation is usually a cooling agent. If precipitation occurs when none is expected then it is likely temperature will be cooler than forecasted. In some situations the influence on temperature is small but in other situations it can result in a huge temperature swing. The high temperature can be much lower than forecasted if a surface snow cover is not foreseen on a clear day. The high temperature can also be much lower when unforeseen precipitation and clouds blocks out solar radiation, absorb latent heat through evaporation and bring cooler downdraft air to the surface.

Precipitation can also result in the temperature being warmer at night than expected. An unforeseen increase in surface dewpoint overnight can result in a low temperature that will be warmer than expected. This can occur when unforeseen rain occurs preceding the night. Especially at high dewpoints, the overnight low will not cool significantly below the initial dewpoint temperature since condensation from dew releases latent heat to the air.