Hoar Frost of Rimed Ice???

Monday and Tuesday were both pretty cool days weather wise, in the Red River Valley. It has been calm (wind wise) but cooler than average. We also had a difficult forecast with the clouds being so spotty!! So on Tuesday... we had a little fog in areas of the city. This fog turned everything into a winter wonderland!! White and serene.

Photo by me. Taken 12/7/10
Outside the Vista Building on the Microsoft Campus

Now, way back when, in my Severe and Hazardous WX class, we went over the difference between two types of weather phenomena that cause everything to turn white. The two different phenomena are Hora Frost and Rimed Ice.

The only difference between the two really is how they form!

How Frost Forms:
1) The conditions have to be cold, calm and clear
2) usually forms over night or in the early morning hours.
3) The dew point is at or below freezing

 During the night if the temperature cools down below the dew point (which is below freezing), water vapor will change into ice directly, skipping the liquid stage completely. (This is known as deposition)

Image Courtesy of Google images.

Hoar Frost!!!

Hoar Frost is the loosely deposited frosty ice crystals that form on trees or side walks or even buildings on calm, cold and cool nights, the same way that normal frost does.

It forms the same way as frost (see above) and usually collects and forms on objects that are cooler than the surrounding air. (aka objects that lost their heat do to radiational cooling). It has a very feathery look to it. It is almost like looking at a snowflake that formed on an object.

Fog Formation:

Fog Forms in much of the same way as frost does however instead of the water vapor going straight to a solid, it condenses. However, the condensation happens on tiny little particles in the air. Once they get big enough they are visible to the human eye and create visibility issues. When the visibility lowers to less than about 3/4 a mile, we consider this hazy condition fog. Basically it is a cloud touching the ground... Pretty cool huh??

If the fog forms in temperatures below zero, then we call it freezing fog!

Rime or Rimed Ice
Image Courtesy of Wikipedia.

Rimed Ice:

Rime or Rimed Ice forms from the freezing fog. The little "supercooled" water droplets freeze on contact with an object, in the below freezing/foggy conditions. This usually happens on calmer nights but the water droplets freeze on the windward side of the objects.

The difference is in the fact that Rime is formed formed after the water vapor has condensed into a water droplet. Where as Hoar Frost goes straight from the water vapor to the ice crystal.

 
I think what the Red River Valley experienced on Tuesday was Rime. The reason I believe that is because it was a foggy morning in most of the Red River Valley area. This seems to be a common happening in the colder climates of this flat state. Here are some neat pictures I took from the parking lot at work :)