|WHAT IS A DIRTY HIGH PRESSURE?
METEOROLOGIST JEFF HABY
High pressure is associated with clear skies and fair weather. This
is ideal, but not always the case. In a
forecast discussion you may come across the term "dirty high". This is in reference to a high pressure that has
clouds and/or precipitation within its domain.
There are several ways clouds can occur in association with high
pressure. A deck of low clouds (stratus) and/or
fog could be within the domain of high pressure. This can occur
as a cool area of high pressure moves over saturated land. Moisture evaporated into the low levels and cooler
conditions cause a quick saturation of air which results in a low deck of clouds and/or fog (high pressure
generally has light wind and clear skies at its core which is conducive to morning clouds and fog over night
soils are saturated). An
inversion produced by the high pressure can trap the
moisture in the
and can cause low level clouds to persist. Cloud can occur with high pressure when
orographic lifting causes cloud
Lifting mechanisms such as
upper level divergence can produce
clouds in close proximity to the
high pressure. Much also depends on the strength of the high pressure and the verticality of the high pressure. A
very dry high pressure is less likely to have clouds located near its core, especially if the land underneath
is fairly dry.