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Thursday, November 03, 2011

Average First Frost and Freeze Dates In Jacksonville

First Frost
November is climatologically when most inland sites across the NWS Jacksonville forecast area have their first frosty morning. For frost to form, similar conditions conducive to dew formation are needed, including light low level winds and low level moisture. Frost forms when the temperature at the ground reaches freezing, or 32 °F.  If the dew point temperature (which measures the amount of moisture in the air) also nears freezing, then the moisture in the air near the freezing ground changes from water vapor to solid ice crystals and the ice will deposit itself on objects that had also reach freezing, which includes grass, plants and roof tops. 


Many times frost is observed when measured temperatures are in the mid 30s and not at the freezing mark.  Why is this?  Standard temperature measurements are made generally between 4 and 6 feet above ground level.  The coldest air is the densest, which means that it will lingering at ground level. The actual temperatures will sometimes be many degrees below the observed temperature, which is usually measured many feet about ground level. This is why sometimes observed temperatures are warmer than freezing, despite a widespread frost indicating freezing temperatures at ground level. 

The NWS Jacksonville will issue a Frost Advisory for northeast Florida when widespread frost is expected (and a freeze warning is not in effect).  For southeast Georgia, a frost advisory will be issued until the first hard freeze event is experienced, then frost advisories will resume, if necessary, after March 1st. Below is a map of the average date of the first frost (based on temperatures of 36 ° F or lower) across the NWS Jacksonville forecast area. 

First Average Frost On First Coast

First Freeze
NWS Jacksonville will issue a Freeze Warnings when forecast temperatures are expected to remain below 32 ° F for 2 or more hours.  Freeze warnings will continue to be issued throughout the cool season as needed for northeast Florida. Freeze warnings will not be issued for southeast Georgia after the first widespread hard freeze of the season (temperatures 27 ° F or less). Freeze warnings will resume for southeast Georgia as needed after March 1st. 

Hard Freeze Warnings will be issued as needed for the entire forecast area when minimum temperatures are expected to fall at or below 27 ° F for 2 or more hours. Freeze Watches will be issued typically within 24 to 36 hours of a forecast freeze event. Below is a map of the average date of the first freeze across the NWS Jacksonville forecast area.
Average First Freeze

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