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Freezing fog produces hoar frost, rime across Iowa

Iowans flocked outside Sunday morning to capture snapshots of the gorgeous winter scenes
Credit: Debbie Granzow

DES MOINES, Iowa — The old saying "early bird gets the worm" couldn't have been more valid for photography enthusiasts in Iowa over the weekend.

A combination of freezing fog and excess moisture in the atmosphere produced a mix of hoar frost and rime early Sunday, making for some incredibly picturesque scenes all over the state.

Many braved the frigid temperatures, stepping outside early to capture the beauty firsthand. 

By the early afternoon, much of the hoar frost and rime had already melted due to abundant sunshine. If you happened to miss it, there's another chance to see the pretty sight on Monday morning, too.

According to the National Weather Service, hoar frost is defined as a "deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to air, such as tree branches, plants, wires, poles, etc." 

This typically occurs on relatively clear nights when air with a dewpoint below 32° is brought to saturation by cooling. 

RELATED: WEATHER LAB | Facts about fog

On the other hand, the National Weather Service explains the development of rime as a similar process to hoar frost, with the exception of the moisture's source. 

In the case of rime, the object and the air are both below freezing with the presence of fog. The water vapor in the existing fog serves as the source for the moisture, whereas the hoarfrost forms on clearer nights. 

RELATED: LOCAL 5 FORECAST: Mild start to the week ahead

So you may be wondering whether Iowa experienced hoar frost or rime on Sunday morning? Frankly, the answer could actually be both

Although the majority of the ice buildup on trees and powerlines was likely rime due to the presence of freezing fog, some areas did not experience much fog and remained fairly clear at the surface. In this case, they recorded less fog despite the presence of moisture. 

It would be nearly impossible to pinpoint what areas recorded rime and what areas recorded hoar frost, especially since they are so visually similar. 

With more fog in the forecast this week, it's possible that we'll see even more hoarfrost or rime development. The best opportunities to view the beauty is early in the morning, just after sunrise.

RELATED: WEATHER LAB | How snow cover affects temperatures

For the latest forecast, visit WeAreIowa.com/Weather or text WEATHER to 515-457-1026. 

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