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Old 08-27-2013, 11:01 PM
 
Location: Wyoming
9,396 posts, read 17,338,873 times
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Here's my understanding of dew point/relative humidity:

As Sky pointed out, dew point is the temperature when the moisture in the air becomes visible, as fog or cloud. Relative humidity is based on that. If the dew point is 60, like EH mentioned, then when the air temp approaches 60 degrees you'll see fog (100% humidity). We don't typically see that too much in Montana and Wyoming except in river valleys, but when we do it means the humidity is pretty high.

My wife spent most of her life in Medford, OR where summers are even drier than here. Shortly after she moved here we took a camping trip back to Iowa in August. She awoke and looked outside. It was foggy. She was surprised because it had been hot when we'd gone to bed, and because of the fog she assumed it was really cool out. She opened the camper door and was hit by HOT HUMID air. HOT FOG! It must have been at least 80 degrees, so the dew point had to have been 80 degrees or higher. The temperature in Iowa that weekend peaked at about 98 with humidity at about 95%.

I don't think I did a good job explaining the relationship between dew point and humidity, but they're mathematically linked -- exactly how I don't know. (Maybe that's why I didn't explain it well.)
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Old 08-29-2013, 09:41 AM
 
Location: Spots Wyoming
18,696 posts, read 36,389,137 times
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See, there ya go. I knew that if a Pilot weighed in here, we'd get an answer. Pilots tend to be one step up on the rest of us when it comes to weather. Not sure what it is, maybe flying into the thunderstorm more than once? Thanks Newk. Right or wrong, I now understand it better than I have in the past. But don't get a big head, cause I just barely know more than I did in the past. It's still progress though. What's the old saying, you learn something new every day? Well, this morning, I learned that yesterday I had entirely too much sun. Paying for it today.

I watch KELO land news every morning, and they don't mention humidity, but rather Dew Point. They then explain that with the the dew point at 65, it's going to feel muggy today. Being an old sailor that had to do weather forecasting for underwater (i know, confusing) in 3 days of advance of the ship, Humidy was a big factor and we never touched on Dew Point. Maybe it doesn't matter out at sea. I was a Sonar Tech, or ping jokey, and in order to figure out what kind of ranges I could get, I had to do weather forecasting every 4 hours and I had to predict what the weather was going to do for the next 3 days. Just as soon as I figured it out, they'd turn and then I'd have to figure it all out again. Hey, in 3 days, after a turn, your in a whole nuther part of the ocean. Then, based on what the weather was going to do, I could predict what the water was going to do down to about 3,000 ft. See, sub skippers are like Pilots, they know that stuff and they know how to hide in it. But we never ever discussed Dew Point. We measured Humidity Wet bulb and dry bulb, temp wet and dry, pressure and such, but never measured, nor computed Dew Point. Like you 1949, I stayed at a Holliday Inn Express by listening to a second rate news channel every morning. (Kelo Land is over 600 miles from me, but with my Satelite, it's a "Local Channel". Surprisingly, they do better with my weather than Billings, 127 miles away.) My best weather predictor is Billings, after the fact. If a storm hits Billings, I know it's going to hit here in an hour or so.
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Last edited by ElkHunter; 08-29-2013 at 09:52 AM..
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Old 08-29-2013, 02:54 PM
 
Location: Lost in Montana *recalculating*...
11,843 posts, read 15,458,529 times
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All I know is that back in WV when they said the temps would be 90deg with 90% relative humidity, you shut the house down, ran the A/C and never set foot outside.

Now 65 degrees and 80% humidity? Doable. But anything above 80-85deg with 75% or more relative humidity. Nah-ahh. Ain't going into that misty mass of mysery.
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Old 08-29-2013, 05:33 PM
 
Location: Pluto's Home Town
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Disclaimer: I've never lived in Montana, but I've loved the one visit I had.

I just installed an evaporative cooler in my house out here in Oregon (Ashland), and I watch the dew point like a hawk. When the dew point rises above about 55 or so, the swamp cooler is less effective. When it is in the 40s and low 50s, it works like a charm. Based on this ,I would propose that when the Dew Point exceeds 60 degrees, it is pretty humid. Now, this is pretty rare in the West, though some places in Southern California, Arizona, and New Mexico approach it in summer.

Data Tables for the Western US from the Western Regional Climate Center show that only a few sites in Montana approach this. Look at the link for monthly dew points.

Climate Tables | Western Regional Climate Center

Circle and Sidney, MT, way over on the eastern border, are the only places that average dew points over 50 degrees in summer. It makes sense that the far eastern edge would get some Midwestern humidity occasionally.

Another consideration is whether the afternoon relative humidity is above 40% during the hot summer months. Look at the table for average afternoon relative humidity by month. It looks like nowhere in the state approaches this (on average).

So, it seems that nowhere in Montana is humid on average, but that is not to say that some humid monsoon or Midwestern air masses might pass through sometimes. That said, I am not sure how much the averages mean in a continental state like Montana. I suspect the weather whatever it wants.

As an aside, it looks to me like a swamp cooler would usually work in Montana too, so I would say you are not in a humid state, most of the time, that is.

Last edited by Fiddlehead; 08-29-2013 at 06:59 PM..
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Old 08-30-2013, 10:28 PM
 
Location: Millersville, Md and King George, Va
120 posts, read 172,434 times
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Quote:
Originally Posted by Vegabern View Post
No. It's not humid here.

I don't care what anyone says, dry heat is still HOT!!!

I'll still take an Arizona 120* over a Virginia 95* anytime.
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Old 09-17-2013, 08:55 AM
 
13 posts, read 53,757 times
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Quote:
Originally Posted by ElkHunter View Post
...We measured Humidity Wet bulb and dry bulb, temp wet and dry, pressure and such, but never measured, nor computed Dew Point....
I think wet bulb and dry bulb is where you should be focusing your attention. These are used in critical calculations by engineers when they design conditioning systems for buildings.
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Old 09-17-2013, 12:49 PM
 
Location: Idaho
4,600 posts, read 4,428,590 times
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Quote:
Originally Posted by sky1949 View Post
Dew point is the temperature at which moisture/water vapor condense from its gaseous state to its liquid state. In the morning that is commonly known as "dew"! Pilots know this: if the air temperature is very close to the dew point (typically we're interested in the area of the airport) there is likely to be fog, which makes sense since fog = a cloud that is touching the ground, and clouds are simply areas of condensed water vapor.
+1

In other words, the dew point is the temperature at which the air temperature falls for the relative humidity to reach 100%, or saturation. The closer the two temperatures are to each other, the higher the relative humidity.
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Old 09-17-2013, 12:52 PM
 
Location: Idaho
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Quote:
Originally Posted by Reziac View Post
If fog rises off the river, is it still a cloud?
If it doesn't touch the ground, yes. Fog off a body of water is usually called "evaporative fog", and usually evaporates pretty quickly. It doesn't rise very far or get very thick.
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Old 09-25-2013, 06:57 PM
 
201 posts, read 486,120 times
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Quote:
Originally Posted by sky1949 View Post
Dew point has little to dew with it. (I couldn't help myself.). Humidity makes high air temperature miserable because your perspiration doesn't evaporate as easily in humid air as in dry air, hence your body feels cooler in the dry air where the evaporating perspiration carries away heat. That is also why your sweat beads up and stays on your skin in humid air and you feel sticky.
After living in the heat of northern Montana and south Saskatchewan, when I moved to Minnesota I had to learn to wipe my brow so the sweat didn't run into my eyes. As you said in the west it evaporates. In the east you can wring you shirt out on a hot day if you working hard at all.
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Old 10-01-2013, 02:54 PM
 
Location: SW MO
1,116 posts, read 986,099 times
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Quote:
Originally Posted by 13levine View Post
After living in the heat of northern Montana and south Saskatchewan, when I moved to Minnesota I had to learn to wipe my brow so the sweat didn't run into my eyes. As you said in the west it evaporates. In the east you can wring you shirt out on a hot day if you working hard at all.
Ha! Here in the Ozarks, I CHANGED my shirt three times in as many hours while framing a porch floor on the NORTH side of my house(started at 7 AM, finished before 10)! 90+ F and 90%+ humidity are not uncommon here in the summer. And I have seen -20 F with a thirty MPH breeze in the winter. Missouri is spelled wrong. Much of the time, I live in Misery. Ah, but from the last half of October through the first half of November, this is God's country, I tell you...
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