AWIS FORECAST // Dew Point and Air Temperature

— Written By

Dear Strawberry Community,

today I had several conversations about dew point and air temperature. Here an email conversation I had earlier today:

Hey Mark,

I am a strawberry grower in XXX. […]We use 1.5oz row covers only as our cold weather/frost protection. What is the correlation between dew point and frost? I had thought it was the temperature at which frost would form, if below 32-degrees.
We try to cover if the predicted temps are below 40-degrees. We can have frost even at 36-degrees here. I know the temps can be lower at bloom height and figure that what causes this to occur. But, on Sunday we had to squeeze in our last Select spray and then our first fungicide spray. So, we didn’t cover til Monday afternoon, due to the REI. There was a light frost at 36-degrees (thermometer at the house). 🙄 We lost some blooms and had some damaged. 
We covered on Monday afternoon and have protected them from two days of hard frosts. I guess we are just wanting to predict what seems to be unpredictable a little better.
Thanks for your time!
My answer:
Thank you for reaching out to us. Several points here. First, the thermometer at the house probably not necessarily reflects the air temperature in your field. If it’s 36 degree at your house, it might only be 34 in the field, and most likely around 32 at plant-level. So now, an air temperature itself of 32 does not harm the flowers, but as you correctly stated, it’s the frost which can build on the flowers which will cause damage.
The dew point describes the temperature at which the air is saturated with water and morning condensation (dew) will start forming on surfaces (such as flowers). The colder the air, the less water it can hold. So the forming of dew is related to the amount of water in the air (humidity) and temperature. Dew always forms if the air temperature reaches the dew point.
Dew usually forms close to the surface, since this usually is the coldest area. That can be different if there is a lot of wind (more than 6-10 mph), because colder air can move in.
However, under low winds, and colder nights, especially when the soil is saturated, it’s very likely to experience dew in the morning.
Frost builds up when the air-temperature reaches a dew-point below freezing. That can happen gradually over night, with dew is building on the plants in a liquid phase, which then freezes over night (frozen dew). Or ice can directly deposit on the plant out of the vapor phase in the air (white frost, hoar frost). 
So if your minimum temperature is predicted to be 40, your dew-point 34, you most likely don’t see frost over night. But if your minimum temperature is predicted to be 34, and your dew point is 25, there is a good chance that you will see frost over night!
In your case, I would recommend to measure temperatures directly in your field, maybe with an affordable weather station and use our weather forecasts to see what the general differences between min air temp. and dew point are in your area.
I hope that makes sense,

Over the next few days, the dew point will play a major role on whether or not there will be frost building up on your plants. Please check your 10 day forecast:

10-day HOURLY forecasts for the Southeast:

North Carolina:

Ahoskie-Tri-Cou Albemarle_AP Andrews Asheboro
Asheville_Munic Beaufort-Michae Boone_NC Burlington-Burl
Cape_Hatteras_AG Chapel_Hill-Hor Charlotte Cherry_Point
Clinton_NC Concord_NC Currituck_NC Edenton-Northea
Elizabeth_City Elizabethtown_NC Erwin-Harnett_C Fayetteville_AP
Fort_Bragg/Simm Franklin/Macon_C Gastonia-Gaston Goldsboro_AFB
Goldsboro_NC Greensboro Greenville_AP_NC Hickory
Jacksonville Jacksonville_(A Jefferson_AP_NC Kenansville-Dup
Kill_Devil_Hills Kinston_AP Lexington-David Lincolnton_AP
Louisburg_NC Lumberton_AP_NC Mackall_AAF Manteo/Dare_Co
Maxton Mcalf_Bogue_Fiel Monroe_AP_NC Morganton-Morga
Mount_Airy-Moun New_Bern North_Wilkesbor Oxford_NC
Piney_Island_Bom Pope_AFB Raleigh-Durham Reidsville_AP
Roanoke_Rap_AP Roanoke_Rapids_R Rockingham_NC Rocky_Mount-Wils
Roxboro_AP Rutherfordton Salisbury-Rowan Sanford_NC
Shelby_AP_NC Smithfield_AP Southern_Pines Southport_NC
Statesville-Sta Tarboro_AP Wadesboro_AP Washington-Warr
Whiteville_NC Wilmington_AP_NC Winston-Salem

South Carolina:

Aiken_AP Anderson_Co_Air Barnwell_AP Beaufort_Cnty_AP
Beaufort_SC Bennettsville_Je Camden_AP Charleston_Exec_
Charleston_Muni Cheraw_AP Chester_AP Clemson-Clemson
Columbia-Columb Columbia/McEnti Columbia_Metro Conway_AP
Darlington Florence_SC Georgetown_AP_SC Greenville_Donal
Greenville_Down Greenville-Spart Greenwood-Green Hartsville_AP
Hilton_Head_AP Kingstree_AP Lancaster-McWhir Laurens_AP
Manning_AP Marion_AP_SC Moncks_Corner_AP Mount_Pleasant_A
Myrtle_Beach Newberry_AP North_Myrtle_Be Orangeburg-Oran
Pickens_AP Rock_Hill Spartanburg_Down Summerville_AP
Sumter_AP Sumter_SC Walterboro_AP Winnsboro_AP


Albany_Municipa Alma/Bacon_Co Americus_AP Athens_Municipa
Atlanta/DeKalb Atlanta/Fulton Atlanta_Intl_Ar Atlanta_Paulding
Augusta/Bush_Fi Augusta_Daniel_F Bainbridge_GA Blakely_AP
Brunswick_GA Brunswick/Glync Canton_GA Carrollton_AP
Cartersville_AP Claxton_AP Columbus_GA Covington_GA_AP
Dalton-Dalton_M Douglas_Muni Dublin/WHBarron Fitzgerald_AP
Fort_Benning Ft_Stewart/Wrig Gainesville_AP_G Greensboro_GA
Homerville_AP Hunter_(AAF) Jesup_AP La_Grange
Lakeland Lawrenceville_AP Macon_AP Marietta
Marietta/Cobb Milledgeville_AP Moultrie_Muni_AP Newnan-Newnan_C
Peachtree_City Rome_GA Savannah Statesboro_GA
Swainsboro_AP Sylvania_AP_GA Thomaston_AP Thomasville_AP
Thomson_GA Tifton_AP Valdosta Valdosta/Moody
Vidalia Warner_Robins_A Washington_GA Washington_GA
Waycross_AP Winder_AP


Abingdon Ashland_VA Charlottesville Chesapeake-Ches
Culpeper Danville Dublin/New_Riv Emporia_VA
Farmville Fentress_Naval_A Fort_Belvoir/Da Fort_Eustis/Fel
Franklin/J_B_Ro Hillsville Hot_Springs/Ing Jonesville_AP
Langley_AFB Leesburg/Godfre Louisa-Louisa_C Lynchburg
Manassas_Muni(A Marion/Wythev Martinsville Mecklenburg/Bru
Melfa/Accomack Newport_News Norfolk_Hampton_ Norfolk_NAS
Norfolk_VA Oceana Orange_VA Petersburg_(AWO
Quantico Richlands_AP Richlands_VA Richmond
Richmond_Chester Roanoke Shannon_Arpt Stafford_AP_VA
Staunton-Shenan Suffolk-Suffolk Tappahannock_AP Virginia_Tech_A
Wakefield_AP Wallops_Island Warrenton_Fauqui Washington_Dulle
West_Point_VA Williamsburg_VA Winchester_Rgnl Wise/Lonesome_P


Aberdeen Andrews_AFB Annapolis_Naval Baltimore_Sci_Ce
Baltimore/Wash Cambridge/Dorche College_Park_AP Cumberland_AP
Easton_MD Fort_Meade Frederick_MD Gaithersburg_AP
Hagerstown_AP Leonardtown Middle_River Oakland_MD_AP
Ocean_City_AP Patuxent_River Salisbury Stevensville_AP
St_Inigoes/Webst Washington_DC_AP Westminster_AP

As always, I hope that helps,