The Star-Herald

Observational weather forecasting

This week, I’m going to trot out some useful weather clue rhymes that I developed, and then explain them a bit. I’m using rhymes because they are easy to remember, giving you the knowledge needed to make a good short range forecast on your own.

Here’s rhyme number one:

“When it’s humid and the sun comes out, if you’re going boating, have some doubt.”

Here’s the explanation. First of all, by “going boating,” I’m talking about any outdoor activity, and by “have some doubt,” I’m simply talking about remaining sky-aware.

Now the reasoning: quite often, very humid days will start out with slate-gray skies in the morning, before the sun will start to show itself. This leads many people to think it’s going to be a fine summer day. But the combination of high levels of low-level moisture, indicated by the high dew points, along with the developing sunshine, increases instability, and thunderstorms “like” instability. The best way to describe instability is that air that is lifted from the the ground, will want to continue to rise.

The visual representation of that is those towering cumulus clouds, which sometimes build into thunderheads, with the flat anvil-shaped top, the official name of which are cumulonimbus clouds.

Here’s rhyme number two: “When a day is breezy and bright, the wind will settle by the time it’s night.”

This is a handy rhyme when you need to know if the wind is going to quit by dark (which many times it will).

Here are the conditions that cause the sudden “turning off” of the wind near nightfall.

Clear skies and dry air will allow for cooling in a layer near the ground late in the day, into the early evening, such that you end up with what we call decoupling. The best way to think about that is to think about the way oil and vinegar separate in salad dressing in your fridge. Think of the lower layer of the dressing as the near surface layer, which is experiencing the most rapid cooling. That layer becomes colder and denser, while the layer above is warmer. What happens is that even if the wind is still blowing after dark, it essentially cannot mix down to the surface since the sun it no longer up. When the sun is up, it promotes vertical mixing, which, during the day, can help bring stronger winds down to the surface.

And finally, here’s rhyme number three: “When it’s cool and the water’s warm, be on your guard since fog will form.”

This one applies to a clear, cool evening with calm winds in late summer and fall. When it is clear and calm, and the air is dry (dew points 40s or lower) there will be good radiational cooling, leading to chilly temps.

The fog forms as a result of the chilly air overlaying the still-warm water.

As the water evaporates, it will be immediately chilled to its dew point, by the overlaying chilly air. The water vapor from the evaporation will immediately condense, and this is what generates fog over our rivers and lakes.

So there you go. Three rhymes this week, all of which you can use to forecast your local weather.

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.  

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