About That 540 Line...

Hello Again!

I just want to clear something up for all of you kind enough to read this blog.

About that 540 line...I said earlier that the 540 line is a "rule of thumb" break point between air that is mostly above freezing, and air that is mostly at or below freezing.  It also can be thought of as a reference point of sorts between mostly frozen precipitation (snow, sleet) and mostly liquid precipitation (rain).  In this morning's forecast, on the GFS model map, look at the blue line that extends east and west from where the number 540 is.  Use the red arrow in Minnesota to find the number.  Anything north of that line is colder air that can in some cases support snow.

Let me be clear about this:  Just because you happen to be north of the 540 line doesn't guarantee that temperatures are automatically below 32 degrees.  Here's why.

That 540 line is a measurement of the upper atmosphere; it's taken up where jet aircraft fly, about 18,000 feet above sea level (In Jackson County, for example, that's actually about 17,000 feet above ground level, because if you're sitting in an airplane on the ground, at most airports in the county, you're about 1000 feet above sea level).  You have over 3 miles of air to get through before you touch the ground.

The air is colder and thinner as you go up higher.  You can check that out at numerous sites on the web that have to do with general science, not just weather.  Here's the key:  just because it may be cold enough for snow or something else frozen to form at 18,000 feet, that doesn't mean you're guaranteed to see snow on the ground.  Depending on the air temperature close to the earth's surface, snow that starts somewhere higher up may fall through air that is warmer than 32 degrees on it's own, or the air may have been warmed from the bottom up because the sun warmed the ground under it.

Think back to the last time you experienced snow showers or a rain/snow mix.  Odds are good that you were on or north of the 540 line on the forecast map, but temperature, among other variables, was such that the frozen snow melted, becoming rain, or the snow melted as soon as it touched ground that was above 32 degrees.  That's a big reason why Tuesday's snow is mentioned as a "chance" in my forecast. Different story in the dead of winter, when the probability of snow goes way up due to seasonal changes in the atmosphere and on the ground.  Also, there's a lot of time between now and Tuesday for the atmosphere to change in a way that would bust forecasts and get the computer models arguing amongst themselves.

Bottom line: If you read my blog and see the 540 line mentioned again, or hear a forecaster or meteorologist mention it, you can usually hang your hat on sharply cooler or just plain cold temperatures, but don't go placing bets on snowfall amounts!