Tue., Feb. 13 notes

Monday, Feb. 12, 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)

Temperature patterns and fronts
Drawing isobars on a surface weather map reveals the pressure pattern. Differences in pressure cause winds 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 that you should be familiar with for this week's quiz (above)
and material that won't be covered on this week's 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.

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.

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's the 3-dimensional view again that's in the ClassNotes.

and the map view

Here are the kinds of weather changes that usually precede and follow passage of a warm front. We'll probably go through this fairly quickly in class.

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

We need to go back to the figure where this section on surface weather maps all began.

After learning how weather data are plotted on a map using the station model notation we found that the data, by themselves, were not enough to really be able to say what was causing the cloudy, rainy weather in the NE and along the Gulf Coast.

We added some isobars to reveal the pressure pattern and to locate large centers of high and low pressure. Winds converging into the center of low pressure cause air to rise and might be part of the explanation for the unsettled weather in the NE. That would explain the rain shower along the Gulf Coast however.

Now we've added cold and warm fronts to the picture. The approaching cold front is almost certainly the cause of the shower along the Gulf Coast. The clouds in the NE are probably mostly being produced by the warm front.