WHAT IS AN ISOTHERMAL LAYER?
METEOROLOGIST JEFF HABY
An isothermal layer is defined as a vertical column of air having a constant temperature with height. Isothermal
layers often occur
in the low levels of the troposphere during a
differential advection situation. The image at the bottom of this
Haby Hint shows an isothermal layer. Notice the temperature is near 0 C from the surface up to the 650 mb layer.
That is one deep isothermal layer!
IMPORTANCE TO WARM THICKNESS BIASING- An isothermal layer will cause a warm thickness
biasing of the
1000 to 500 mb thickness. Thus, snow becomes more likely with 1000 to 500 mb thicknesses
of 5,460 or even higher. Climatologically, there is a 50% chance precipitation will be snow if precipitation
does occur at locations within 1,000 feet of sea level and the 5,400 gpm thickness line is overhead. The
thickness can be
higher in high elevation regions (average closer to 5,460 gpm) and have snow still occur since the earth's
surface is closer the colder middle levels of the atmosphere. When an isothermal layer is present
that is below freezing in the lower troposphere, the 50% chance snow 1000 to 500 mb thickness
increases. This is because temperature is not cooling with height like would be normal through
a deep layer. Thickness depends most on the average temperature in a layer of air. See webpage
below for a complete discussion on thickness biasing.
The 540 line and precipitation type
IMPORTANCE TO STABILITY- The air within an isothermal layer is stable (lapse rate less than the MALR). To cause
air in an isothermal layer to rise it must be
forced lifted since it will not rise on its own from positive
buoyancy. A deep isothermal layer will cause the
Lifted Index (LI) and
Showalter Index (SWI) to indicate
very high stability since a rapid cooling with height is needed to create