Eugene weather: Why is the sky usually clear at the coldest temperatures? (Meteorology question) (Portland: to live)

[Those Who Squirm]

Having lived here about a year and five months, I've noticed that we usually get clear skies when the weather is particularly cold, last February's snow event being an exception. It seems that, for rain to happen, it usually needs to be a little warmer.

What gives? Where I used to live, it was more typical to have overcast skies when it was colder, although the climate there was much warmer overall.

[karlsch]

The cold doesn't cause clear skys, clear skies increase the cold.

When the sky is clear, heat radiates out into space. The lower the humidity, the higher the heat radiation.

Cloud cover acts as a blanket holding the heat in.

[Cloudy Dayz]

Originally Posted by Those Who Squirm

Having lived here about a year and five months, I've noticed that we usually get clear skies when the weather is particularly cold, last February's snow event being an exception. It seems that, for rain to happen, it usually needs to be a little warmer.

What gives? Where I used to live, it was more typical to have overcast skies when it was colder, although the climate there was much warmer overall.

Simple, the clouds trap the warm air to the ground. If it's clear the heat rises up into the atmosphere. It's one of the reasons I LOVE cloudy days.

We also know, however, that a clear, starry night usually feels colder than a cloudy one. This is because clouds not only block sunlight but also trap the Earth's heat and prevent it from escaping into space.

Some cloud types, like low, thick stratus clouds, are very good in blocking the sun but do little to trap heat, while others, like high thin cirrus clouds, let most of the sunlight in but prevent large amounts of heat from escaping into space. During nightime, when there is no sunlight available to reflect, all clouds are a warming influence on the planet. Whether, then, the clouds that form during the passage of a storm warm or cool the Earth depends on the types of clouds that storms produce and on the amount of sunlight available for them to block

.

NASA GISS_ Science Briefs_ Do Stormy-Weather Clouds Cool or Warm The Earth_

[Those Who Squirm]

Thanks for the answers so far; now here's a related question. It seems that the more arid places in the state, for example Bend, get more reliable anowfall every winter, so ir seems like a different mechanism is in play over there.

Speaking of snow, Accuweather says we're in for a couple of inches tomorrow but I'll believe it when I see it.

[Parnassia]

Originally Posted by Those Who Squirm

Thanks for the answers so far; now here's a related question. It seems that the more arid places in the state, for example Bend, get more reliable anowfall every winter, so ir seems like a different mechanism is in play over there.

Speaking of snow, Accuweather says we're in for a couple of inches tomorrow but I'll believe it when I see it.

That's an easy one. Its not really a different mechanism, it is a later phase of the same mechanism. Obviously, most weather moves from west to east. As you are probably learning, the Cascades create quite a barrier to weaker Pacific storms which is why areas east of them are drier; they're in the rain shadow. And, because they have a lot less moderating cloud cover, they're colder in winter, hotter in summer. Storms hit the Coast Range, Willamette Valley, Eugene and consequently dump a lot of their moisture on the west side of the Cascades. Some still get pushed up over the mountains to reach those more arid inland areas. As moist air rises it cools. The storm moisture that is rising is also slowing down, which is why the mountains get a lot of snow. It sits over them longer. What water vapor does make it over the crests to the east side will hit that much colder air. Those inland areas tend to be higher altitude too...more cooling. The water vapor will freeze, eventually get heavier than the air and fall as snow, not rain. Snow contains water vapor of course, but it is mostly air.

[pnwguy2]

Originally Posted by karlsch

The cold doesn't cause clear skys, clear skies increase the cold.

When the sky is clear, heat radiates out into space. The lower the humidity, the higher the heat radiation.

Cloud cover acts as a blanket holding the heat in.

This is the best and most true answer. I lived in Eugene for several years and enjoyed the climate. A bit warmer than Portland into the summer, but could get cold at night during the winter. Not immune from heavy snow, had one new years day with a foot on the ground I think around 1999, though that is not normal. Generally a mild but wet climate.