Frost definitions and approx dates

FIRST FROST (FALL):
October 8th according to the NV Coop Ext
September 16th according to the National Weather Service

LIGHT FROST:
A frost where the air temp has dropped below freezing but the ground has not. Many plants can survive a light frost but cannot survive a hard frost.
GardenWeb Glossary of Botanical Terms

Generally when air temperatures fall to less than 33° Fahrenheit but remain above 28° Fahrenheit; typically for only a few hours; See also hard frost

HARD FROST:
Defined as temperatures below 28 degrees for a few hours

May 30th for everything but tomatoes, peppers, and melons

June 20th for tomatoes, peppers, and melons