Relative humidity fall crocus

Jane McGary
Fri, 13 Aug 2004 10:35:29 PDT
Jay Yourch's points were well taken. I was citing the weather report the 
previous day, which stated 19% humidity at 92 degrees F.

Jay wrote,
The case for 100% or near 100% relative humidity overnight is common for many
>locations when winds are light and cloud cover is not extensive. Without 
>the sun
>to heat it up, the air temperature in many areas areas cools until it reaches
>the dew point. At this point further cooling is difficult, because a whole
>lot of heat is released by the water vapor condensing into liquid (dew), 
>or ice (frost) if it is cold enough.

>With overnight lows in the 50's, Jane's dew point is probably in the 50's
>and with a daily maximum RH of 100%.

Well, I was outdoors this morning at 2 am, hoping to see the Perseid meteor 
shower, and it was 54 degrees, and no dew had yet formed, nor was there 
evidence of dew when I went out 6 hours later. What Jay doesn't know is 
that high heat in my area is usually accompanied by strong east wind, the 
movement of an extremely dry air mass through high mountain passes from the 
semi-arid Columbia Plateau. This wind tends to diminish at night. Moreover, 
at this time of year the vegetation and soil here are usually quite dry -- 
many native grasses and herbaceous plants have withered, and most of the 
trees are conifers, not hardwoods.

There are always things about the weather in another part of the world that 
suddenly surprise us. For instance, it was a revelation to me that the 
summer rainfall in eastern North America falls in very brief periods. I 
knew this about the monsoon areas of the arid West, but the notion of a 
drenching rain in 2 hours was new to me. Here on the Pacific coast, most of 
the time, either it doesn't rain or it rains for days on end. Brief 
thunderstorms, if they occur, bring only enough moisture to settle the dust.

Jane McGary
Northwestern Oregon (and formerly living in northern California and 
interior Alaska) 

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