Tacna, Arizona

An excessive heat warning/watch is issued when extreme heat or
the combination of extreme heat and very low or very high humidity...
will compromise the health of an average adult person if avoidance
of direct exposure is not taken to preclude the onset of heat stroke.
The warning is issued when the threat is imminent... and the watch is
issued when notification precedes the event by 12 to 72 hours.

Summertime in the lower Arizona and southeastern California deserts
has occasional extremely hot days. People who live in the desert
areas... or those who visit... are vulnerable to significant health
problems that may lead to death if protective action is not
immediately taken. Drinking abundant amounts of water is not enough
to protect against dire health conditions due to the exposure to
excessive heat and very low or very high humidity. The average
adult person must take action to reduce direct exposure to the sun
by staying inside cool sheltered accommodations. The goal is to
keep the body core temperature near normal... or the condition of
hyperthermia may occur... followed by heat exhaustion or worse...
heat stroke.

The National Weather Service forecasters use many factors to
determine when extreme heat conditions are about to occur in the
lower deserts. The rarity of an excessive heat episode is a leading
factor in determining the issuance of a warning and watch. Air mass
analysis schemes developed by the University of Delaware compare
daily meteorological parameters with known health statistics of the
region. Subtle changes in wind... sky coverage... temperature...
humidity and the calendar date of the episode contribute to the
determination of issuance necessity. Thus... a temperature of 112
degrees in a very dry air mass will trigger the issuance of an
excessive heat warning... but the 112 degree temperature in July
may not. Similarly... that same 112 degree temperature may trigger an
excessive heat warning for one location but not at a different
location.