Vietnam Veterans Crash

[Flash]

Too bad it took 'til 8:43 AM for NWS-Nashville to get a SWS out (9:14 AM for the HWO)

[jmundie]

Humidity (or relative humidity) is the measure of saturation of a particular column of air.  Look at it like water in a glass.

The higher the temperature, the bigger the glass... the more water it can hold.  Now, fill a large glass half full of water.  Now, let's say the temperature begins to drop.  Pour that water into a smaller glass.  It goes from 50% to about 80%.  Temps drop even lower, pour into a smaller glass.  Once you hit 100%, any water that doesn't fit, will fall out of the glass.  You cannot exceed 100% humidity simply because the water will 'fall out' of the column.

The dewpoint is a measure of how small the glass (or how low the temp) has to be for the water to fall out in this example.

That is helpful. I think I got confused because the temperature can't fall lower than the dewpoint when dealing with evaporational cooling,but that's because its raining, which is where the excess moisture goes. I guess you have to have other factors involved to push the temp below the dewpoint?

[Eric]

That is helpful. I think I got confused because the temperature can't fall lower than the dewpoint when dealing with evaporational cooling,but that's because its raining, which is where the excess moisture goes. I guess you have to have other factors involved to push the temp below the dewpoint?

The temp CAN go below the dew point, it's just that the extra moisture (i.e. water vapor) changes state and forms fog, and since the condensed moisture changes state, it's not considered part of the water-vapor content of the air. 

Think of it in severe weather terms...when the air parcel cools to the dew point, the cloud base forms (LCL).  The air parcel temp doesn't stop cooling...the extra moisture that's "squeezed" from the air forms the cloud.

[Eric]

Slideshow from @WKRN reposted by @MWN:

http://www.wkrn.com/global/slideshow.asp?widgetId=41338#.TtemYJR35A4.twitter

[kailynleto | Nightwolf]

The atmosphere can become supersaturated.  I don't know a lot about this phenomenon but I know it's possible and a normal occurrence especially higher in the troposphere.  Eric said something about air parcels and how they don't stop cooling even after reaching the dewpoint, and I think that might be part of it. Like I said, I don't remember a lot about this, so I'll have some reading to do after this.

Also, freezing fog was awful out here in Antioch this morning, and there's nothing forming out here yet but by about 2, I think the valley will start seeing it in earnest.  I'll be able to see the valley fogged over in the morning.

[skillsweather]

It didn't take them long today time to issue a freezing fog advisory.

[dwagner88]

We had thick freezing fog here this morning, but I never saw an advisory for it. I don't think the roads were cold enough to cause problems like yesterday in Nashville though.