Daily Archives: May 21, 2009

The Killing Frost






**There is a new poll on the sidebar related to this post**

It is 6:00 AM. The sun is just on the horizon, the sky is clear and the air is cool…a little too cool for mid May. However, there is no room to complain because the sun is out and it is shaping up to be a pretty pleasant day weather-wise. I walk to my car, unlock it and get inside. As has been a common theme since November, I see my breath float aimlessly in my car because of the cold night the night before. I put the key in the ignition and get ready to drive off, except there’s one thing—dew has blanketed my windshield and I need to use the wipers in order to see the road ahead of me.

No problem.

I twist the little knob on the side of my steering wheel and the wipers go back and forth once, making a horrible scratching noise. I shut them off and stare horrified at my windshield.

The dew is frozen. The Killing Frost has arrived unexpectedly.

Indeed, I sat stunned in my car for several seconds staring at my windshield, thinking frustratingly, “How in the world am I sitting in my car with a frozen winshield in the middle of May? FROST SEASON IS OVER! It’s not even technically freezing out! It can’t frost above 32 degrees!” But I quickly stopped myself. How foolish of me! Frost can definitley occur above freezing, and has done it several times this season without a lot of us even knowing it. But how can that be? How can the late-season Killing Frost be active?

Have you ever heard weather anchors use the term “surface temperature?” Surface temperature is the temperature of the air 4-6 feet above the surface which is what most weather observatories use for the surface temperature forecast, so it is a little misleading. The closer to the ground the cooler the temperature, making sense because cold air sinks and is heavier than the air above it. The radiation of heat away from the ground causes the temperature to drop.

So here’s what I THINK happened: it was a still, calm night (1st ingredient for a frost) and the clouds steadily increased through the night hours helping keep most of the moisture from the day previous still intact by the time the clouds parted early this morning. It is important to note the coldest time of day is typically between 4-6 AM, so the clouds couldn’t have parted at a better time to allow quick cooling and a brief glaze of frost on some surfaces. In fact, between midnight and 3 am the low tempertaure was only at 46 degrees. But by 4 am the temperature plummeted to 41. So, even though last night’s official low surface temperature was 41 degrees, the ground temperature (though not official considering I don’t have a ground thermometer) was likely at freezing or below, resulting in an unexpected frost. And WHY the ground temperature was so cold I don’t know. I’m still dumbfounded by that myself!

Now on to the next question…was it the dreaded Killing Frost? I think it would be safe to say no. First of all, the frost wasn’t widespread or heavy. It didn’t take much to scrape the ice of my windshield, though it was enough to prevent my windshield wipers from wiping it off clean in the first place. A heavy widespread frost in mid-late May is not impossible, but is very much unlikely especially over here west of the cascades. So should you plant your vegetables and flowers now? I’d say go ahead. But if Jack Frost startled you this morning with a brief glaze, then by all means wait until after Memorial Day 🙂

Speaking of which, Memorial Day weekend looks absolutely splendid…and frost free! Highs tomorrow will likely get close to 70 with Saturday looking like a cooler day in the mid to upper 60s. I know, I know, I said Friday would be the cooler day but the models have flip-flopped on that idea. Bottom line, it’ll be dry. Sunday through most (if not all) of next week will be partly to mostly sunny and positively, absolutely, no question BEAUTIFUL with highs in the low to mid 70s and a slight sea breeze to keep things from getting too warm.

I have to say, May is one of my favorite months not just because it’s my birthday month but because typically the most comfortable spring (and summer) temperatures of the whole year occur during this time. And you know what else is awesome? We may actually have a full 7 days of dry, sunny and pleasant weather for once without the obnoxious cloudy, rainy intermissions we’ve been experiencing lately.

Have a fantastic day and before you go to bed, make sure you can locate your ice scraper easily just in case 😉

Matthew Leach

Forecasting Kitsap