Ticker for November 20, 2001

                
MESONET TICKER ... MESONET TICKER ... MESONET TICKER ... MESONET TICKER ...
November 20, 2001  November 20, 2001  November 20, 2001  November 20, 2001



Cheyenne Mesonet Station Escapes Statewide Freeze

The vast, vast majority of the state observed sub-freezing temperatures
this morning, marking one of the latest first freezes on record for
much of central and southern Oklahoma:



However, at least one location escaped the freeze, due to a phenomenon
we've discussed many times on the Ticker.  The Cheyenne Mesonet station
is a notorious "inversion poker" and that status kept it from freezing
this morning.  Compare the Cheyenne meteogram with nearby Butler:




At Butler (as with most of the rest of the state), winds remained light
and northerly throughout the night.  Temperatures plunged into the 20s
rapidly after sunset.  However, at Cheyenne, overnight winds slightly
brisker and from the west and southwest.  Temperatures hovered in the
mid-30s before recovering into the 50s by early afternoon.

A look at the Norman sounding shows a shallow layer of cold air near
the surface.  This makes sense, because it's the result of yesterday's
cold frontal passage:



Because of its location on a ridge, or variations in the height of the
cold layer (probably both), the Cheyenne station was sampling air above
the shallow cold layer.  A crude representation is attempted in the
following graphic (warning: it's hampered by a severe lack of artistic
talent among the entire Ticker staff):



Medicine Park, the most notorious inversion poker in the Mesonet,
behaved much like Cheyenne.  Its minimum temperature of 31 F was
significantly warmer than neighboring stations.