News - Blogs - What causes vehicle windshields to frost

What causes vehicle windshields to frost?
Published: 1/9/2013 12:53 PM
Last Modified: 1/9/2013 12:58 PM

In this Jan. 12, 2012 file photo, Janet Kellogg scrapes snow and ice from her car window. MIKE SIMONS/Tulsa World File

In this Jan. 12, 2012 file photo, snow covers a group of taxicabs near Sixth Street and Peoria Avenue in Tulsa. MATT BARNARD/Tulsa World File

Elisha Green, of Tulsa, but who grew up in Chicago, clears snow from her windshield, near Country Club Drive and West Edison Street, amidst cold and snowy conditions in Owen Park, on Feb. 13, 2012. "Look at the streets. They are clear," she said. "I don't know why they cancelled school today. This wouldn't happen in Chicago." CORY YOUNG/Tulsa World File

In this Feb. 11, 2011 file photo, Larry Smith, lead jailer at the Delaware County Sheriff's Office, in Jay, shovels vehicles out from two snow storms that left approximately 50 inches of snow over a two week period in the area. CORY YOUNG/Tulsa World File

Frosty the windshield... will never be a Christmas carol. However, for many of the days in the past week before our recent uptick in temperatures, I found myself getting out the ice scraper before the morning commute.

What causes this? A Q&A with Tulsa National Weather Service meteorologist Karen Hatfield:

Q: What weather conditions are necessary for windshield ice/frost?

Hatfield: Windshield ice can come in two forms, the most common being "normal" frost and the other coming from freezing rain or drizzle.

Frost forms at night when the relative humidity is high, winds are light or calm, and the temperature of a surface is below the frost point.

The most typical scenario is when surface high pressure is overhead, providing lighter winds and usually, clear skies.

Windshield frost can form in this manner. It can also form via the same concepts but in a little more unique fashion. When the outside of the windshield gets cold, but the interior of the vehicle is relatively moist (from your breath, etc.), the water vapor can condense and freeze on the outside of the window. Think about what happens if you get
in the car when it's very cold and begin to drive before letting the
vehicle heat up; your breath fogs up the window. It's that concept.

Q: What causes some sides of a vehicle to be free of ice/frost, and other sides not? The west side of my car this morning was ice-free, the east side had light ice.

Hatfield: As far as your vehicle goes, something as simple as one side of the vehicle being exposed and others closer to a building (likely providing warmer air) could affect it. The wind direction (even if the wind was light) could affect it, as well.

Q: Why do vehicles parked inside garages not get ice/frost, even if it's just as cold outside as it is inside the garage? Do carports prevent ice/frost also?

Hatfield: Any kind of covering (i.e. - parking garage, carport) keeps radiation frost from forming because heat is trapped closer to the ground (even if it doesn't feel like it). The roof over the vehicle will also keep ice
crystals from depositing on the vehicle, causing windshield frost.

Q: Are there any materials/products that will prevent windshield ice/frost, besides garage parking?

Hatfield: When I lived in Minnesota, I had a physical covering for the windshield that would collect any frost rather than it depositing on the windshield.

All I had to do was remove the cover when I wanted to drive. There are other things like de-icers that will remove the frost easily.

Althought it was unrelated to windshields, I also noticed my breath in the air late last night, so a follow-up question:

Q: In what weather conditions can you see your breath in the air, almost making it look like you're smoking?

Hatfield: Usually, it's when temperatures are colder because the air can hold less water vapor at colder temperatures. Therefore, the moisture in your breath can saturate easier at lower temperatures.

Tulsa is forecast to nearly hit 70 in a few days, but will likely be back to the 20s for overnight lows by the weekend. You can check Tulsa's latest forecast information, courtesy of the National Weather Service, at tulsaworld.com/weather.

--Althea Peterson

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Almanac

Jan	Feb	Mar	Apr	May	Jun
Jul	Aug	Sep	Oct	Nov	Dec

	Temperature		Precipitation
Date	High Temp	Low Temp	Total	Month to date	Historical average
1	44°	16°	0	0.00	0.05
2	59°	24°	0	0.00	0.11
3	57°	33°	0	0.00	0.16
4	68°	37°	Trace	0.00	0.21
5	69°	29°	0	0.00	0.26
6	66°	33°	0	0.00	0.32
7	59°	38°	0.05	0.05	0.38
8	51°	34°	0	0.05	0.44
9	44°	36°	0.01	0.06	0.51
10	62°	37°	0.07	0.13	0.57
11	54°	28°	0	0.13	0.64
12	44°	30°	0.25	0.38	0.70
13	55°	40°	0.01	0.39	0.76
14	°	°			0.83
15	°	°			0.89
16	°	°			0.95
17	°	°			1.02
18	°	°			1.09
19	°	°			1.16
20	°	°			1.23
21	°	°			1.31
22	°	°			1.38
23	°	°			1.46
24	°	°			1.53
25	°	°			1.61
26	°	°			1.69
27	°	°			1.77
28	°	°			1.85

TEMPERATURE

RAINFALL

SNOWFALL

Weather World

Follow Jerry Wofford on Twitter for updates during severe weather conditions.

Tulsa weather milestones of 2013 (as of Feb. 12)

Highest temperature: 70 on Jan. 11 (Record: 115 on Aug. 15, 1936)
Lowest temperature: 15 on Jan. 16 (Record: Minus-16 on Jan. 22, 1930)
Hottest month (average): 40.5 degrees in January (Record: 91.7 degrees on July 1980)
Coldest month (average): 40. 5 degrees in January (Record: 21.7 in January 1918)
Most snowfall (day): 0.1 of an inch on Feb. 12(Record: 13.2 inches on Feb. 1, 2011)
Most snowfall (month): 0.1 of an inch in February(Record: 22.5 inches in February 2011)
Most rainfall (day): 0.91 of an inch on Jan. 29 (Record: 9.27 inches on May 26-27, 1984)
Most rainfall (month): 1.54 of an inch in January (Record: 18.18 inches on September 1971)
Highest wind speed: 30 mph on Jan. 30
Previous day with any rain: Feb. 12
Previous day with 1 inch or more of rain: Oct. 17, 2012
Previous day with any snow: Feb. 12
Previous day with freezing temperatures: Feb. 12
Read regular updates on Oklahoma's unpredictable weather and learn more about meteorology from the Tulsa office of the National Weather Service.

>> Visit the main weather page
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Contributors
Staff Writer Althea Peterson started writing for the Tulsa World in March 2007 after previous stops at the Norman Transcript in 2006 and the Oklahoma Gazette in 2005. She followed her older brother from rural Wisconsin (with a public school that never seemed to call snow days) to the University of Oklahoma, but did not follow his pursuit to study meteorology. However, she tries to find as many opportunities to report on the weather as possible.

Staff Writer Jerry Wofford came to the Tulsa World in 2010 from The Manhattan Mercury in Manhattan, Kan. Originally from western Arkansas and a graduate of the University of Oklahoma, Jerry has lived in Tornado Alley his entire life and is one of those people who goes outside when the sirens go off.

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