Clear + Calm = Cool

You will hear us talk about this a lot in the next few months as we head into the weekend.  Since it’s the first time this season that the rule applies, its worth explaining in detail.  Bri did an awesome job of explaining what meteorologists call “raditational cooling” in her blog yesterday.  The basic idea is that when you don’t have any clouds in the sky to help trap the heat, or any wind to mix it up, whatever heat was around during the day just radiates out to space at night.  To put it simply, the temperature drop likes a rock as soon as the sun sets.  But why tonight?!?

It has to do with high pressure, the big blue H you see on weather maps.  High pressure is another way of saying that the air is heavier than normal.  If the atmosphere stepped on a bathroom scale it would be overweight.  Heavy air sinks, as you can imagine.  Sinking air makes it difficult for clouds and rain to form so skies are usually clear around an area of high pressure.  The winds always swirl around in a clockwise direction around the edge of an area of high pressure.  That means you get gusty northwesterly winds out ahead of it.  That will be us today.  But, near the center of the high pressure the winds are very light if not completely calm.  That’s what happens tonight.  So there you go, clear skies and calm winds, the formula for “radiational cooling.”



Because of all of that, tonight will likely be a few degrees cooler than last night.  Not surprisingly, the National Weather Service (NWS) has expanded the frost advisory for tonight.  “But Rob, I don’t see any temperatures below 32 in your forecast, how can there be frost?!?”  The standard weather observations are measured at 2 meters above ground level, or roughly 6 feet, or very roughly head high for an average adult.  In this type of “radiational cooling” situation, the ground will actually cool off more than the air above it will.  So even though the temperature might be 38 F in the air 2 meters above ground, the ground could easily be below freezing.  Another complicating factor is the lack of moisture.  You need water to freeze in order to generate frost.  The air will be so dry, frost might be hard to find.  So for both of those reasons, marginal temps and lack of moisture, any frost will be patchy.  Side note…the NWS rules for frost and freeze advisories are more complicated than the US tax code.  It depends in part on the length of the growing season in each town, the date, whether a frost or freeze had already occurred, and which NWS office issues the advisory.  This will lead to some very confusing situations in the coming months where one town has a frost/freeze advisory and the next town over does not even though both areas are expecting a frost/freeze.  So pay attention!

Speaking of dry air, the air on Sunday will be so dry, that combined with gusty northwesterly winds, the risk for wildfires will be particularly high on Sunday.  So it won’t be a good to burn brush or carelessly discard a cigarette butt.   Otherwise enjoy the bright sunshine and crisp feel of fall!

After a brisk start to the day on Monday, temperatures rebound to near seasonal levels (upper 60s) by late afternoon.  Clouds move in late Monday.  With clouds around Monday night, it will be MUCH warmer…and you now know why…no “radiational cooling!”

We’ve got a good chance of beneficial rain on Tuesday.  Despite the clouds and rain, temperatures should still climb to the low 70s.

After Tuesday the forecast is admittedly a bit uncertain and a bit “iffy.”  Right now it looks like a mixture of clouds and sun with a very slight (20% or less) chance of showers for the rest of the week.  It all depends on if and when the system driving Tuesday’s rain decides to clear out, so stay tuned!