Wednesday, September 25 notes

Wednesday, September 25

Pages from the ClassNotes that we will be using today are: page 40c, page 40d, page 148a, page 148c, page 148b, page 148d, page 39c, page 41, and page 42

We first briefly review material covered at the end of class on Monday.

Temperature patterns and fronts
The pressure pattern causes the wind to start to blow; the wind can then affect and change the temperature pattern.

The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all or if the wind was just blowing straight from west to east. The bands of different temperature are aligned parallel to the lines of latitude. Temperature changes when moving from south to north but not moving from west to east.

This picture gets a little more interesting if you place a center of high or low pressure in the middle.

In the case of high pressure, the clockwise spinning winds move warm air to the north on the western side of the High. The front edge of this northward moving air is shown with a dotted line (at Pt. W) in the picture above. Cold air moves toward the south on the eastern side of the High (another dotted line at Pt. C, it's a little hard to distinguish between the blue and green in the picture). The diverging winds also move the warm and cold air away from the center of the High. Now you would experience a change in temperature if you traveled from west to east across the center of the picture.

The transition from warm to cold along the boundaries (Pts. W and C) is spread out over a fairly long distance and is gradual. This is because the winds around high pressure diverge and blow outward away from the center of high pressure. There is also some mixing of the different temperature air along the boundaries.

Now we'll replace the H pressure center with a L pressure center.

Counterclockwise winds move cold air toward the south on the west side of the Low. Warm air advances toward the north on the eastern side of the low. This is just the opposite of what we saw with high pressure.

There's another difference - converging winds in the case of low pressure will move the air masses of different temperature in toward the center of low pressure. The transition zone between different temperature air gets squeezed and compressed. The change from warm to cold occurs in a shorter distance and is sharper and more distinct. Solid lines have been used to delineate the boundaries above. These sharper and more abrupt boundaries are called fronts.

Warm and cold fronts, middle latitude storms (aka extratropical cyclones)

A cold front is drawn at the front edge of the southward moving mass of cold air on the west side of the Low. Cold fronts are generally drawn in blue on a surface weather map. The small triangular symbols on the side of the front identify it as a cold front and show what direction it is moving.

A warm front (drawn in red with half circle symbols) is shown on the right hand side of the map at front edge of the northward moving mass of. A warm front is usually drawn in red and has half circles on one side of the front to identify it and show its direction of motion.

The fronts are like spokes on a wheel. The "spokes" will spin counterclockwise around the low pressure center (the axle).

Both types of fronts cause rising air motions. Fronts are another way of causing air to rise. That's important because rising air expands and cools. If the air is moist and cools enough, clouds can form.

The storm system shown in the picture above (the Low together with the fronts) is referred to a middle latitude storm or an extra-tropical cyclone. Extra-tropical means outside the tropics, cyclone means winds spinning around low pressure (tornadoes are sometimes called cyclones, so are hurricanes). These storms form at middle latitudes because that is where air masses coming from the polar regions to the north and the more tropical regions to the south can collide.

Large storms that form in the tropics (where this mostly just warm air) are called tropical cyclones or, in our part of the world, hurricanes.

3-dimensional structure of cold fronts

A 3-dimensional cross-sectional view of a cold front is shown below. We've jumped to page 148a in the online version of the ClassNotes.

The person in the figure is positioned ahead of an approaching cold front. Time wise, it might be the day before the front actually passes through. There are 3 fairly important features to notice in this picture.

1. The front edge of the approaching air mass has a blunt, rounded shape. A vertical slice through a cold front is shown below at left.

Friction with the ground causes the edge to "bunch up" and gives it the blunt shape it has. You'd see something similar if you were to pour something thick and gooey on an inclined surface and watched it roll downhill. Or, as shown in class, you can lay your arm and hand on a flat surface.

Slide your arm to the right.

Your fingers will drag on the table surface and will curl up and your hand will make a fist.

2. A cold front, the leading edge of a cold air mass is kind of like a fist slamming into a bunch of warmer air. Because it is denser, the cold air lifts the warm air out of the way.

The cold dense air mass behind a cold front moves into a region occupied by warm air. The warm air has lower density and will be displaced by the cold air mass. In some ways its analogous to a big heavy Cadillac plowing into a bunch of Volkswagens.

At this point, just 15 to 20 minutes into today's class, we're in a position to better appreciate a video recording of the cold front passing through Tucson. The first video is a time lapse movie of a cold front that came through Tucson on on Easter Sunday morning, April 4, 1999. Click here to see the cold front video (it may take a minute or two to transfer the data from the server computer in the Atmospheric Sciences Dept., be patient). Remember this is a time lapse movie of the frontal passage. The front seems to race through Tucson in the video, it wasn't moving as fast as the video might lead you to believe. Cold fronts typically move 15 to 25 MPH.

The 2nd video was another cold front passage that occurred on February 12, 2012.

In the past I've had trouble playing the videos using Firefox on the classroom computer. If that is the case, you can right click on each link, then click on the Save Link As... option, and choose to save to the Desktop. Then double click on the icon on your desktop to view the video. If you use Chrome or Internet Explorer you should be able to watch the videos.

3. Note the cool, cold, colder bands of air behind the cold front.

The warm air mass ahead of the front has just been sitting there and temperatures are pretty uniform throughout. Cold fronts are found at the leading edge of a cold air mass. The air behind the front might have originated in Canada. It might have started out very cold but as it travels to a place like Arizona it can change (warm) considerably. The air right behind the front will have traveled the furthest and warmed the most. That's the reason for the cool, cold, and colder temperature bands (temperature gradient) behind the front. The really cold air behind a cold front might not arrive in Arizona until 1 or 2 days after the passage of the front.

Weather changes that precede and follow passage of a cold front
Here are some of the specific weather changes that might precede and follow a cold front.

Weather variable	Behind	Passing	Ahead
Temperature	cool, cold, colder*		warm
Dew Point	usually much drier**		may be moist (though that is often not the case here in the desert southwest)
Winds	northwest	gusty winds (dusty)	from the southwest
Clouds, Weather	clearing	rain clouds, thunderstorms in a narrow band along the front (if the warm air mass is moist)	might see some high clouds
Pressure	rising	reaches a minimum	falling

* as mentioned above, the coldest air might follow passage of a cold front by a day or two.
**nighttime temperatures drop much more quickly in dry air than in moist or cloudy air. This is part of the reason it can get very cold a day or two after passage of a cold front.

Gusty winds and a shift in wind direction are often one of the most obvious change associated with the passage of a cold front in Tucson.

The pressure changes that precede and follow a cold front are not something we would observe or feel but are very useful when trying to locate a front on a weather map.

3-dimensional structure of warm fronts
We've learned a fair amount about cold fronts: cross-sectional structure, weather changes that precede and follow passage of a cold front, and how to locate a cold front on a surface weather map. Now we have to do the same for warm fronts. An older version of page 148c is shown below, a somewhat cleaner version is in the ClassNotes.

Warm air approaching and colliding with a cold air mass is like a fleet of Volkswagens overtaking a Cadillac

The VWs are still lighter than the Cadillac. What will happen when the VWs catch the Cadillac?

They'll run up and over (overrun) the Cadillac.

The same kind of thing happens along a warm front. Warm air is overtaking some colder air that is also moving to the right.
The approaching warm air is still less dense than the cold air and will overrun the cold air mass.

There's one key difference between cold and warm front boundaries

1. The back edge of a retreating cold mass that the warm air overtakes has a much different shape than the advancing edge. The advancing edge bunches up and is blunt. The back edge gets stretched out and has a more gradual ramp like shape.

You can use your hand and arm again.

You start with your fingers curled up then move your arm and hand to the right.

As your arm moves to the right, friction uncurls your fingers.

The warm air rises more slowly and rises over a much larger area out ahead of the warm front. A variety of cloud types form and spread out over a large area ahead of the warm front. This is an important difference between warm and cold fronts.

Weather changes that precede and follow passage of a warm front

Here are the kinds of weather changes that usually precede and follow passage of a warm front.

Weather Variable	Behind (after)	Passing	Ahead (before)
Temperature	warmer		cool
Dew point	may be moister		drier
Winds	SW, S, SE		from the East or SE, maybe even the S
Clouds, Weather	clearing		wide variety of clouds that may precede arrival of the front by a day or two clouds may produce a wide variety of types of precipitation also (snow, sleet, freezing rain, and rain)
Pressure	rising	minimum	falling

Locating a cold front on a weather map

In the next figure we started with some weather data plotted on a surface map using the station model notation. We'll try to make a little more sense of this data and eventually locate a cold front. Study this example carefully because you will have an opportunity to do a surface weather map analysis of your own and will be able to earn some Extra Credit points.

Step #1 - draw in some isobars and ocate the low pressure center

In some respects fronts are like spokes on a wheel - they rotate counterclockwise around centers of low pressure. It makes sense to first locate the center of low pressure. To do that we need to draw in a few isobars and map out the pressure pattern.

Isobars are drawn at 4 mb increments above and below a starting value of 1000 mb. Some of the allowed values are shown on the right side of the figure (992, 996, 1000, 1004, 1008 etc). The highest pressure on the map is 1003.0 mb, the lowest is 994.9 mb. You must choose from the allowed list of isobar values and pick only the values that fall between the high and low pressure values on the map. Thus we only need to draw in 996 mb and 1000 mb isobars.

Step #1 cont'd
Color coding the plotted pressure values may be helpful. In the figure below stations with pressures lower than 996 mb have been colored in purple. These will be enclosed by the 996 mb contour. Pressures between 996 and 1000 mb have been colored blue. These stations will lie outside the 996 mb contour but inside the 1000 mb isobar. Finally stations with pressures greater than 1000 mb have been colored green. The 1000 mb isobar will separate the blue stations from the green stations.

End of Step #1
The map below shows the same picture with the 996 mb and 1000 mb contours drawn in (it is always a good idea to label the isobars when you draw them in).

Step #2 - locate the warm air mass
The next step was to try to locate the warm air mass in the picture. I'll start with a new map for clarity that keeps the isobars. The colors now will represent different air temperatures.

Temperatures are in the 60s in the lower right portion of the map; this area has been circled in orange. Cooler air to the west of the Low pressure center has also been identified. Do the green, blue, purple (cool, cold, colder) bands look familiar? Based on just the temperatures we have a pretty good idea where a cold front would be found.

Step #3 - draw in a tentative location for the cold front
Locating and drawing in the cold front.

Step #4 - double check the front location
We should double check the front location using some of the other weather changes (wind shift, dew point, pressure change etc.) that precede and follow a cold front.

The air ahead of the front (Pts. B & C) is warm, moist, has winds blowing from the S or SW, and the pressure is falling. These are all things you would expect to find ahead of a cold front.

Overcast skies are found at Pt. B. very near the front.

The air behind the front at Pt. A is colder, drier, winds are blowing from the NW, and the pressure is rising. That is just what you would expect behind a cold front. So our location of the front looks pretty good.

Locating a warm front on a weather map

We need to finish our study of surface weather maps by trying to located a warm front.

This is the map we will be working with (see p. 148d in the ClassNotes). It's worth pausing and noting that you really can't make any sense out of this jumble of weather data at this point.

Step #1 - draw in some isobars and locate the low pressure center
We'll start by drawing some isobars to map out the pressure pattern. A partial list of allowed isobars is shown at the right side of the map above (increments of 4 mb starting at 1000 mb).

We've located located the highest and lowest pressure values on the map. Then we choose allowed isobar values that fall between these limits. In this case we'll need to draw 992 mb and 996 mb isobars.

Here's the map with color coded pressures. Pressures less than 992 mb are purple, pressures between 992 and 996 mb are blue, and pressures greater than 996 mb are green. Note that station B has a pressure of exactly 992.0 mb, the 992 mb isobar will go through that station. The 996 mb isobar will go through station A because it has a pressure of exactly 996.0 mb.

Here's the map with the isobars drawn in. On the map below we use colors to locate the warm and cooler air masses.

Step #2 - locate the warm air mass

The warm air mass has been colored in orange. Cooler air east of the low pressure center is blue. Can you see where the warm front should go?

Step #3 - draw in a tentative warm front location
Here's the map with a warm front drawn in
(the map was redrawn so that the edge of the warm (orange) air mass would coincide with the warm front).

The change in wind directions was probably more pronounced than the temperature change. Most of the clouds outlined in green are probably being produced by the warm front. You can see how more extensive cloud coverage is with a warm front.

Step #4 - double check the front location
Two of the stations near the right edge of the picture and on opposite sides of the front are redrawn below.

The station north of the front has cooler and drier air, winds are from the east, skies are overcast and light rain is falling. The pressure is falling as the warm front approaches. These are all things you'd expect to find ahead of a warm front. Behind the front at the southern station pressure is rising, the air is warmer and moister, winds have shifted to the south and the skies are starting to clear.

In this case there is a Step #5 - have a look at the rest of the surface map

Have a look at the left, western, side of the map. There's pretty good evidence of a cold front.

There's a big temperature change (low 60s to low 40s and 30s) and a very noticeable wind shift (SW ahead of the cold front and NW behind).

Lets go back to the surface map we began this section with

We were trying to figure out what was causing the cloudy & rainy weather in the NE part of the map and why a rain shower was being reported along the Gulf Coast. There's pretty clear evidence of both a warm and cold front on this map. They're shown in the figure below

The stormy weather in the NE appears to be largely caused by the warm front. The rain shower along the Gulf Coast was associated with a cold front.

Upper Level Charts - Basic Features

This section is undergoing some figure improvement work.

We've already spent some time learning about surface weather maps. Maps showing conditions at various altitudes above the ground are also drawn. Upper level conditions can affect the development and movement of surface features (and vice versa).

In this first section we'll just learn 3 basic facts about upper level charts. First the overall appearance is somewhat different from a surface weather map. The pattern on a surface map can be complex and you generally find circular (more or less) centers of high and low pressure (see the bottom part of the figure below). You can also find closed high and low pressure centers at upper levels, but mostly you find a relatively simple wavy pattern like is shown on the upper portion of the figure below (sort of a 3-dimensional view). The figure below is on page 42 in the Class Notes.

A simple upper level chart pattern is sketched below (a map view). There are two basic features: wavy lines that dip southward and have a "u-shape" and lines that bend northward and have an "n-shape". The following figure is on page 41 in the ClassNotes.

The u-shaped portion of the pattern is called a trough. The n-shaped portion is called a ridge.

Troughs are produced by large volumes of cool or cold air (the cold air is found between the ground and the upper level that the map depicts). The eastern half of the country in the map above would probably be experiencing colder than average temperatures. Large volumes of warm or hot air produce ridges. We'll see why this is true in the 2nd part of the supplementary reading material.

The 10 am MST 500 mb upper level chart for Monday, Oct. 15, 2018. Note the trough positioned over the western states.

This feature is partly responsible for the wetter- and cooler-than-average weather we have been experiencing.

11 am surface temperatures for Monday, Oct. 15, 2018. The western 2/3rds of the US are under an upper level trough. Temperatures in Arizona, New Mexico, and western Texas are in the 30s and 40s. The eastern 1/3rd of the country is under the the edge of a ridge and is considerably warmer. Temperatures in Mississippi, Alabama, and Georgia are in the 80s.

We can begin to see how surface features can influence the upper level pattern in the map below.

A surface center of low pressure is often found east of an upper level trough. Surface high pressure is often east of an upper level ridge. Note how the surface winds in between the surface high and surface low are moving colder air southward. This is directly below the upper level trough axis. Warmer air is moving northward to the east of the surface low and to the west of the surface high. This warmer air is directly below upper level ridges.

There is one last feature to be aware of.

The winds on upper level charts blow parallel to the contour lines generally from west to east. This is a little different from surface winds which blow across the isobars toward low pressure. An example of surface winds is shown below.

That's it for this section on the basic features of upper level charts. Really all you need to be able to do is
1. identify troughs and ridges,
2. remember that troughs are associated with cold air & ridges with warm air, and
3. remember that upper level winds blow parallel to the contour lines from west to east.