Tue., Feb. 13 notes

Tuesday Feb., 13, 2018

Some Cajun music for Mardi Gras: "Emilie Vidrine & Tee Franglais" (6:58), "Marais Bouleur" (10:32), "Belisaire" (3:48), "Bal de Maison" (6:55),
"Pain de Mais" (10:47), "Steve Riley & the Mamou Playboys" (17:56), "Savoy Family Cajun Band" (24:52)

What can you begin to learn about the weather once you've draw isobars on a surface weather map and revealed the pressure pattern?

1a. Surface centers of low pressure

We'll start with the large nearly circular centers of High and Low pressure. Low pressure is drawn below. These figures are more neatly drawn versions of what we did in class.

Air will start moving toward low pressure (like a rock sitting on a hillside that starts to roll downhill), then something called the Coriolis force will cause the wind to start to spin (don't worry about the Coriolis force at this point, we'll learn more about it later in the semester).

In the northern hemisphere winds spin in a counterclockwise (CCW) direction around surface low pressure centers. The winds also spiral inward toward the center of the low, this is called convergence. [winds spin clockwise around low pressure centers in the southern hemisphere but still spiral inward, we won't worry about the southern hemisphere until later in the semester]

When the converging air reaches the center of the low it starts to rise.

Convergence causes air to rise (1 of 4 ways)
rising air e-x-p-a-n-d-s (it moves into lower pressure surroundings at higher altitude)
The expansion causes the air to cool
If you cool moist air enough (to or below its dew point temperature) clouds can form

Convergence is 1 of 4 ways of causing air to rise (we'll learn what the rest are soon, and, actually, you already know what one of them is - warm air rises, that's called convection). You often see cloudy skies and stormy weather associated with surface low pressure.

1b. Surface centers of high pressure
Everything is pretty much the exact opposite in the case of surface high pressure.

Winds spin clockwise (counterclockwise in the southern hemisphere) and spiral outward. The outward motion is called divergence.

Air sinks in the center of surface high pressure to replace the diverging air. The sinking air is compressed and warms. This keeps clouds from forming so clear skies are normally found with high pressure.

Clear skies doesn't necessarily mean warm weather, strong surface high pressure often forms when the air is very cold.

Divergence causes air to sink
sinking air is compressed and warms
warming air keeps clouds from forming - clear skies

Here's a picture summarizing what we've learned so far. It's a slightly different view of wind motions around surface highs and low that tries to combine all the key features in as simple a sketch as possible.

2. Strong and weak pressure gradients - fast or slow winds
The pressure pattern will also tell you something about where you might expect to find fast or slow winds. In this case we look for regions where the isobars are either closely spaced together or widely spaced. I handed out a replacement for p. 40c in the ClassNotes (don't throw p. 40c away).

A picture of a hill is shown above at left. The map at upper right is a topographic map that depicts the hill (the numbers on the contour lines are altitudes). A center of high pressure on a weather map, the figure at the bottom, has the same overall appearance. The numbers on the contours are different. These are contours (isobars) of pressure values in millibars.

Closely spaced contours on a topographic map indicate a steep slope. More widely spaced contours mean the slope is more gradual. If you roll a rock downhill on a steep slope it will roll more quickly than if it is on a gradual slope. A rock will always roll downhill, away from the summit in this case toward the outer edge of the topographic map. Air will always start to move toward low pressure

On a weather map, closely spaced contours (isobars) means pressure is changing rapidly with distance. This is known as a strong pressure gradient and produces fast winds (a 30 knot wind blowing from the SE is shown in the orange shaded region above). Widely spaced isobars indicate a weaker pressure gradient and the winds would be slower (the 10 knot wind blowing from the NW in the figure).

Winds spin counterclockwise and spiral inward around low pressure centers. The fastest winds are again found where the contour lines are close together and the pressure gradient is strongest.

Contour spacing
closely spaced isobars = strong pressure gradient (big change in pressure with distance) - fast winds
widely spaced isobars = weak pressure gradient (small change in pressure with distance) - slow winds

This figure used to be found at the bottom of p. 40 c in the photocopied ClassNotes.

You should be able to sketch in the direction of the wind at each of the three points and determine where the fastest and slowest winds would be found. (you'll find the answer at the end of today's notes). Once you know which directions the winds are blowing you should be able to say whether the air at each of the points would be warmer or colder than normal.

3. Temperature patterns and fronts
The pressure pattern causes the wind to start to blow; the wind then can 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 from south to north but not from west to east.

This picture gets a little more interesting if you put centers 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.

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.

The 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 (there are probably additional meteorological processes that help to create 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.

This is the dividing line between material you should be familiar with for this week's quiz (above)
and material that won't be covered on the quiz (below).

3-dimensional structure of cold fronts

A 3-dimensional cross-sectional view of a cold front is shown below (we've jumped to p. 148a in the photocopied 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 them.

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.

This is as far as we got in class today. I'll leave the following section here to finish this material on cold fronts. The material on warm fronts will be moved to the Tue., Feb. 20 notes.

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.

Here is the answer to a question embedded in today's notes.

Winds from the NW at 20 knots at Point #1, SE winds at 30 knots at Point #2, and NW winds at 10 knots at Point #3.
The southerly winds in the middle of the picture at Point #2 would probably be the warmest.
You would find colder air coming from the north at Points #2 and #3.