This work week will be pretty non-descript, with mild afternoon highs in the 50s and overnight lows falling to below the freezing mark, but not dramatically below. The storm track will be far enough to the north so that we aren’t likely to see any precipitation through Friday, but not so far that we won’t see occasional cloudiness through the end of the work week. On Saturday, the ridge over us breaks down to allow a weak to moderate storm to come through the region, giving us a chance of rain showers late Saturday that could turn to some snowflakes by late Sunday.

Air pressure column

Yesterday I told you a one inch column of air to the top of the atmosphere would weigh about 14.7 pounds.  If you cut that column in half, how much would the bottom half weigh? About 7 ½ pounds, right? Nope. It would be somewhere around 14 ½ pounds. The actual answer depends in part on how you define the top of the atmosphere (for our purposes, we’ll ignore the thermosphere), but most is packed in the bottom. Because air compresses, most of the atmosphere’s mass is crammed into the lower 5%.

So just where is the “mass mid-point”? Out of an air column of about 105 miles high, half of all the air molecules are in the lower 3 ½ miles. The 500 millibar (half a bar) surface is located at about 18,000 feet, and is an important layer to watch when forecasting the weather.

500mb

 

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