Winter Precipitation Processes - Part 1: Pressure

I thought, while we have a break, that I would post a bit about the science of meteorology - especially as it pertains to the types of precipitation we see in the winter.  Perhaps you have wondered why sometimes what falls from the sky is rain, sometimes sleet, sometimes snow.  Maybe you've seen the nature of the snow that is falling change from tiny fluffy flakes (bad for snowballs) to wet, large flakes (great for snowballs) and wondered why that's happening.

And, maybe you don't care.  If that's the case, I'd stop reading here.

But for the curious, let's talk for a moment about pressure.  Hopefully at the end of this series, you'll start to see how this ties in to some of those forecast model maps I've posted in the past month or so.

ATMOSPHERIC PRESSURE AND UNITS

What is atmospheric pressure?  Simply: it's the weight of the air in the column above any given point.

This can be measured in all sorts of ways.  In the old days, somebody figured out that if you have put a pool of mercury into a reservoir and the reservoir extended upwards into a vacuum tube, you could measure how "heavy" the air above the mercury reservoir was by observing how high the mercury was forced up the tube.

For lack of anything better, this deflection was measured in inches - which is why you hear, even today, people referring to atmospheric pressure in "inches of mercury".  Crazy and archaic, no?

Meteorologists switched to the metric (really, the "SI" - " le Système international d'unités" - or International Standard) system of units for a lot of things a long time ago.  Atmospheric pressure is no exception.

The SI unit  for pressure is a "Pascal".  Sea level pressure is typically in the neighborhood of 100 kilopascals.  Like other metric units, the prefix "kilo-" means "1000".  So, we can say that sea level pressure is typically 100 X 1000 pascals.  Or 100,000 pascals.

In the early 1900's, some dude named William Napier Shaw decided that nobody wanted to deal with a unit of measurement where the base value was going to be so high.  Cleverly, he created a substitute unit called a bar.  Since he was British, it's unlikely this has anything to do with him having a pint at the time he came up with it.  Otherwise, we'd be walking around with pressure units called "pubs".  But I digress...

A bar is equal to 100 kilopascals or, conveniently, typical sea level pressure.

Pressure decreases as you go higher into the atmosphere.  Simple enough: there's less air between you and the top of the atmosphere (where air eventually runs out and the vacuum of space approaches), therefore the air column weighs less, therefore the pressure is lower.

Because so much of our weather is defined by what happens in the atmosphere, meteorologists knew they would have to use a fractional version of the bar to still be able to have manageable values as they spoke of different pressures in the atmosphere.  So, they settled on the millibar as the unit of pressure measurement.  The prefix "milli-" means  1/1000 for SI units.  So, a millibar (abbreviated "mb") is 1/1000 of a bar and that, in turn, means that 1000 millibars is the same as 1 bar - or typical surface pressure.

Anytime you see pressure measurements on a forecast model map, they will always be in millibars.  Some TV meteorologists have even started to introduce this standard to their audiences (although you'll still always see the pressure given in "inches of mercury").



CONSTANT PRESSURE SURFACES

Pressure is very important to what mets call the "sensible weather" - or the weather that you experience outside while walking down the street.  In meteorology, we have defined what are called "constant pressure surfaces" as a way of understanding and modeling atmospheric processes that cause our weather.

Imagine, for a moment, an "idealized" or "perfect" atmosphere.  This mythic world is one where the pressure at sea level is 1010 millibars (mb) and pressure decreases in a way that is perfectly proportional to how high you are in the atmosphere.

Now, let's say you are hooked onto a big balloon and you go straight up into the atmosphere.  You're armed with a notepad and pencil, thermometer, a barometer, an altimeter (tells you how high up you are) and, hopefully, a parachute.  Here's what you're asked to do by a meteorologist:

- "When your barometer reads 1000 mb, take a measurement of altitude and temperature".

- "Take the same measurements when your barometer reads:  850 mb, 700 mb, 500 mb, 250 mb and, if you can, 100 mb" (we'll assume, in the mythic world that we're discussing, that you don't need air to breathe and that you're impervious to cold).

Shortly after takeoff, your barometer reads 1000 mb. You take a measurement of 100 meters (300 feet) and a temperature of 15 C (59 F).

As you rise into the atmosphere on your balloon, you find that your barometer reads 850 mb at about 1500 meters (5000 feet) and that it's getting cold, about 5 C (41 F).

Up you go.  The barometer reads 700 mb and you take a measurement:  3000 meters (10000 feet) and -5 C (23 F)

You continue taking measurements at the pressure markers the meteorologist mentioned.  As you do, you notice that your balloon is getting larger and larger.  Why?  The pressure in the balloon has remained constant from sea level.  But the pressure outside of the balloon is getting lower and lower.  So the balloon is expanding outward.  A short time after your barometer measures 100 mb and you take your measurements, your balloon pops.  You release your parachute, fall gently to the surface and hand the meteorologist your pad which shows something like this:

PRESSURE (mb)	APPROXIMATE ALTITUDE	TEMPERATURE
1000	100 meters (300 feet)	15 C (59 F)
850	1500 meters (5000 feet)	5 C (41 F)
700	3000 meters (10000 feet)	-5 C (23F)
500	5000 meters (18000 feet)	-20 C (-4 F)
300	9000 meters (30000 feet)	-45 C (-49 F)
200	12000 meters (40000 feet)	-55 C (-67 F)
100	16000 meters (53000 feet)	-56 C (-69 F)

Meteorologists think of the atmosphere as a series of constant pressure levels.  In a perfect atmosphere, these would be standardized, i.e. your barometer would always read 850 mb at 5000 feet.

But in the real world, that's not true.  And it's the difference in the height of those pressure levels, the amount of space between the pressure levels (referred to as "thickness") and the temperatures that make a difference.

For the purposes of winter precipitation, we'll concern ourselves with pressures from the surface up to 500 mb.

Next post: how the heights and thicknesses change and what that means for us on the ground.

Oh - and guess what?  In my little example above, you were essentially a weather balloon.  These are released by National Weather Service upper air stations (Dulles/Sterling is our closest one) twice a day (morning and evening) and record the information from the example as well as other information (dewpoint, winds, etc) on their way up.  More about this in the next post, too!