Weather 101 Lesson 4 Air Masses

By: Randy Bynon , 2:59 PM GMT on January 29, 2007

The podcast for this lesson can be found at

http://www.bynon.cc/blog/weather101_lesson_4.mp3

In the last few lessons we've talked about how the earth is heated and the reason for our global circulation. We also talked about the primary properties of our atmosphere that help to make our weather. But the global circulation theories only explain how weather forms at the equator or at 30 and 60 degrees north. What about the rest of the globe? We get weather everywhere. Well, that's what we'll get into in this lesson.

If you recall, in our last lesson on global circulation, we talked about the semi-permanent pressure systems that are established in the high and low pressure bands around the globe near the Equator, 30N and 60N, systems such as the Bermuda High and the Aleutian Low. These systems are "semi-permanent" because they exist in the zones of rising or falling air that result from the 3-cell global circulation theory and those circulations never change other than to shift north or south during the change in seasons.

An air mass is essentially a large bubble of air, usually at least 1000 miles across, that sits stationary over an area that has a uniform surface (i.e. large uniform land area with no mountains, large body of water, or a large region of snow covered land) for a long period of time. The region has to have a fairly uniform and gradual temperature, moisture, and pressure gradient surface and aloft. We call these regions source regions. As a bubble of air sits over this source region long enough, it acquires the temperature and humidity characteristics of the region. What these characteristics are depends on the source region. Meteorologists assign labels to these characteristics. The moisture content is addressed first and abbreviated with a small letter. A dry air mass is considered Continental (abbreviated c). A moist air mass is Maritime (abbreviated m). Then temperature is addressed and abbreviated with a capital letter. A warm air mass is called Tropical (abbreviated T). A cold air mass is called Polar (abbreviated P). Some common source regions for Continental Polar (cP) air masses are Siberia, Canada, and Greenland. Common source regions for Continental Tropical air masses are the Sahara Desert, Australia, Southwest Asia, and the US desert SW. Maritime Polar air masses form over the North Atlantic, the North Pacific and the Antarctic Oceans. Maritime Tropical air masses form in the tropical Atlantic, Pacific, and Indian Oceans.

So lets put all this together! An air mass that sits over a large snowfield, like the continent of Greenland, would most likely be a dry, cold air mass. We would call that a Continental Polar air mass (abbreviated cP). An air mass that sits over a warm ocean, like the tropical Pacific, would be a Maritime Tropical air mass (abbreviated mT).

So we have an air mass that has a particular set of temperature and humidity characteristics. What now? The air mass itself doesnt really cause any weather. Afterall, all the weather characteristics of the air mass are uniform by nature. So how do they cause weather? Weather happens when the air mass starts to move. Any air mass forms because pressure gradients in the source region were light meaning there was little air flow in the region either at the surface or aloft. But those conditions dont last forever. At some point, upper level flow at the jet stream level will pick up and begin to move the air mass. As soon as the air mass moves out of its source region, it begins to modify but the air mass is large enough that it will take time to significantly modify its characteristics. In the meantime, its running into air with different characteristics. The boundary where these air masses meets is where the weather usually occurs and we refer to these boundaries as fronts.

And fronts will be the topic for our next lesson!

Air Mass (LRandyB)
Air mass lesson
Air Mass

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33. EmmyRose
6:36 PM GMT on February 01, 2007
mas margaritas...
Member Since: July 15, 2005 Posts: 347 Comments: 76406
32. Patrap
6:32 PM GMT on February 01, 2007
Whats the weekend to bring?..
Member Since: July 3, 2005 Posts: 449 Comments: 139984
31. EmmyRose
4:03 PM GMT on February 01, 2007
Always the men......
mas maragaritas?
Member Since: July 15, 2005 Posts: 347 Comments: 76406
30. Randy Bynon , Dropsonde SysOp/AVAPS PM
3:51 PM GMT on February 01, 2007
You can thank El Nino! Gracias!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
29. EmmyRose
3:35 PM GMT on February 01, 2007
LOL We're hoisting the RED FLAG OF NO MERCY OR QUARTER to the rain - we want our sun back
arghhhhhhhhh
Member Since: July 15, 2005 Posts: 347 Comments: 76406
28. Randy Bynon , Dropsonde SysOp/AVAPS PM
2:58 PM GMT on February 01, 2007
I stopped counting here at 4.. but we are getting some periods of heavy rain this morning!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
27. EmmyRose
2:37 PM GMT on February 01, 2007
one billion raindrops have fallen
in Houston
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26. Raysfan70
12:06 PM GMT on February 01, 2007
Thanks Randy.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
25. Randy Bynon , Dropsonde SysOp/AVAPS PM
12:02 PM GMT on February 01, 2007
Good Morning!!!!!

I wouldn't expect any severe weather for you Rays but you definitly have some rain coming!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
24. Raysfan70
11:18 AM GMT on February 01, 2007
Good Morning {{Randy}}!

MySpace Comments Graphics
Looks as if there might be some severe stuff my way tonight.
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23. Redhead
7:44 AM GMT on February 01, 2007

Personalized Myspace Comments
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22. Raysfan70
10:40 AM GMT on January 31, 2007
{{Randy}}
myspace layouts, myspace codes, glitter graphics
Have a WUnderful Wednesday. :-)
Member Since: July 28, 2005 Posts: 138 Comments: 57354
21. LowerCal
2:26 AM GMT on January 31, 2007
A Pacific high off the SW US Coast sounds right. :)
Member Since: July 26, 2006 Posts: 59 Comments: 10288
20. LowerCal
2:00 AM GMT on January 31, 2007
Thanks Randy. Actually I was thinking of the times of the year when we have absolutely no weather -- just day after day after day of clear skies. There's a very large expanse of water in the 14-18C range and at least the weather is stagnant. It seemed a little odd (to a non meteorologist) to call it Polar though (or Tropical for that matter). I appreciate the entire clarification.

I'll watch the area later this year to see if an "airmass" does form. Thanks again.
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19. Raysfan70
9:01 PM GMT on January 30, 2007
Good Afternoon {{Randy}}.
AFternoon has warmed up here 61. Now that is the kind of weather I can live with.

Member Since: July 28, 2005 Posts: 138 Comments: 57354
18. code1
12:49 PM GMT on January 30, 2007
29F in Ocala. Can't wait for the warm up. Thanks Randy for both. Very exciting for all.
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17. EmmyRose
12:25 PM GMT on January 30, 2007
Texas Reigns. I mean RAINS AND RAINS AND RAINS
El Nino? La Nina? El Raino.
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16. Raysfan70
10:39 AM GMT on January 30, 2007
Good Morning {{Randy}}!
Have a Great Day. :-)

Temps. Didn't get to 32 thankfully. All is good here. NOw for the warm up.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
15. Randy Bynon , Dropsonde SysOp/AVAPS PM
4:48 AM GMT on January 30, 2007
That is true for the west and NE US because they get to enjoy the strong ridge pushing warm air north on the west coast and the southerly flow of increased Gulf moisture in the NE.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
14. HurricaneMyles
4:43 AM GMT on January 30, 2007
I understand how the ridge would induce the trough, ect. Its just everything I've read on El Nino has said it normally causes warmer then normal winters in the US with a pumped up subtropical jet. Perhaps they were being a bit too simplistic.
Member Since: January 12, 2006 Posts: 5 Comments: 827
13. Randy Bynon , Dropsonde SysOp/AVAPS PM
4:32 AM GMT on January 30, 2007
HurricaneMyles, no, actually during an El Nino winter, the warmer water, and therefor the wamer air above the water off the west coast of North, Central, and South America helps to create a less dense, thicker atmosphere. That in effect helps to create a strong ridge along the west coast of the US. This results in warmer than normal conditions in the Pacific NW into the northern Plains.

The atmosphere works in checks and balances. A stronger ridge along the west coast induces a deeper trough in the eastern half of the US. That causes an increase in storms developing in the Gulf with a stronger jet digging into the Gulf. So we get more rain in the SE US. And bouts of colder weather than we might usually get.

El Nino affects on US climate are not as appearant in the summer because we normally see a strong Pacific high off the SW US Coast anyway. But it does impact upper level global flow patterns and help to reduce the frequency of hurricanes in the Atlantic basin.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
12. HurricaneMyles
2:53 AM GMT on January 30, 2007
Randy...the way you put it you make it sound as if El Nino helps to bring down polar air, but everything I've heard says El Nino makes the jet stream rather zonal. A zonal jet doesnt have the large troughs and ridges, so it doesnt bring down cold air. Perhaps you could elaborate?
Member Since: January 12, 2006 Posts: 5 Comments: 827
11. Raysfan70
2:26 AM GMT on January 30, 2007
Thank You again Randy.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
10. Randy Bynon , Dropsonde SysOp/AVAPS PM
2:01 AM GMT on January 30, 2007
Rays, yes, unfortunately that is what we would expect as the El Nino phase continues. The warmer water off the west coast of Central and South America helps to indude a stronger than usual ridge along the west coast. That, in turn induces a strong trough in the eastern half of th US. This results in warmer than usual weather in the Pacific NW, and wetter and cooler than usual weather in the SE US. .... where we are!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
9. Raysfan70
1:34 AM GMT on January 30, 2007
Thank You. Thank You.
Next ti,me I put the request in I will only ask for cooler temps. Not COLD temps.

With El Nino setting up like this should we continue to see the Polar Express continue to come down like this?
Member Since: July 28, 2005 Posts: 138 Comments: 57354
8. Randy Bynon , Dropsonde SysOp/AVAPS PM
1:18 AM GMT on January 30, 2007
You're welcome Rays! It is modifying even as we type! You'll warm up in the next day or two! :-)
Member Since: July 17, 2001 Posts: 190 Comments: 2012
7. Randy Bynon , Dropsonde SysOp/AVAPS PM
1:17 AM GMT on January 30, 2007
LowerCal.. no, there really isn't anything in between the two. Tropical airmasses generally have a surface air temp above 21C in the winter time. Anything colder than that is considered polar.

If you are referring to the cut off low off S CA, that wouldn't really qualify as an "airmass" in the same sense as we're discussing. That low actually has a pretty strong upper level jet stream associaed with it.

In order to be considered an "airmass", the air would have to be fairly stagnant and stationary for a period of several days. And they generally form in regions of high pressure.

Member Since: July 17, 2001 Posts: 190 Comments: 2012
6. Raysfan70
1:14 AM GMT on January 30, 2007
Thanks for the weather 101 Update {{Randy}}.

By chance can you take this Polar Airmass back up your way? It's alittle to cold.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
5. LowerCal
12:49 AM GMT on January 30, 2007
Thanks Randy.

Is there any designation between Tropical and Polar? For the maritime airmass west of California for instance.
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1. weatherboykris
4:29 PM GMT on January 29, 2007
thanks
Member Since: December 9, 2006 Posts: 125 Comments: 11346

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I was an AF aviation weather forecaster for 12 years, then 15 years as a dropsonde systems operator with the AF Reserve Hurricane Hunters.

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