Have you ever noticed that in advance of a winter storm, an otherwise sunny day may feature a sky criss-crossed with lines? Well these lines are actually ice crystals from jet exhaust, called contrails (short for condensation trails). But the question is, why are they sometimes really, really short, while other times they are super long, with many in the sky at once?
Here’s what’s going on: When they are short, it means that the air at the level where planes cruise is dry, and that any moisture in their exhaust is easily evaporating. However, when the air moistens at plane level, evaporation cannot take place as easily, so the exhaust trail lingers in the sky and thus creates a longer contrail. “Often, right around this time, a ring will appear around the sun or the moon.”
So, what does this have to do with whether or not a snowstorm is approaching? It’s due to the fact that, conveniently, the first place moisture shows up in advance of a winter storm, or any Low Pressure System, is up high, where the planes fly. So a trend toward airplane contrails lasting longer in the sky is a sign moisture is increasing at that level.
That’s clue 1.
The next clues, if indeed a winter storm is on the way, go like this:
• clouds will thicken, sun (or moon) will gradually disappear. If night and no moon, stars will appear to dim.
• wind direction will acquire an easterly component.
• barometric pressure will steadily fall.
The last thing you need to wait for, before the snow flies, is for the lower atmosphere to become sufficiently saturated, so that the snowflake can make it all the way down without first evaporating. A good rule of thumb is that when the relative humidity reaches 85 percent, and the factors above are all in place, then precipitation is imminent. (This means that if you have an hour’s worth of errands to run, and you want to get home before the snowstorm begins, you can usually get them done without slippin’ and slidin’ if the relative humidity is still below 60 percent when you start out.)
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been Chief Meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.
