Monday, September 23 notes

Monday September 23

We'll be using page 39a, page 39b, page 39c, page 40a, page 40b, page 40c, page 40d,
(these are mostly pictures, we'll add some written details in class)

Surface weather map analyses
A bunch of weather data has been plotted (using the station model notation) on the surface weather map in the figure below (page 39a in the ClassNotes). A couple of stormy regions have been circled in green.

Plotting the surface weather data on a map is just the beginning. For example you really can't tell what is causing the cloudy weather with rain (the dot symbols are rain) and drizzle (the comma symbols) in the NE portion of the map above or the rain shower along the Gulf Coast. Some additional analysis is needed.

1st step in surface map analysis: draw in some contour lines to reveal the large scale pressure pattern

Pressure contours = isobars
( note the word bar is in millibar, barometer, and now isobar ,
they all have something to do with pressure)

Temperature contours = isotherms

A meteorologist would usually begin by drawing some contour lines of pressure (isobars) to map out the large scale pressure pattern. We will look first at contour lines of temperature, they are a little easier to understand (the plotted data is easier to decode and temperature varies across the country in a more predictable way).

Isotherms

Isotherms, temperature contour lines, are usually drawn at 10^o F intervals. They do two things:

isotherms (1) connect points on the map with the same temperature
(2) separate regions warmer than a particular temperature
from regions colder than a particular temperature

The 40^o F isotherm above passes through a city which is reporting a temperature of exactly 40^o (Point A). Mostly it goes between pairs of cities: one with a temperature warmer than 40^o (41^oat Point B) and the other colder than 40^o (38^o F at Point C). The temperature pattern is also somewhat more predictable than the pressure pattern: temperatures generally decrease with increasing latitude: warmest temperatures are usually in the south, colder temperatures in the north.

Here's another example starting with just a bunch of temperature numbers (you'll find this figure on the in-class Optional Assignment)

Our "job" is to try to make some sense of this data. To do that we'll draw in 2 or 3 isotherms (40 F, 50 F isotherms and maybe a small segment of a 60 F isotherm). Colors can help you do this.

There is one temperature below 40 it has been colored blue, temperatures between 40 and 50 are green and temperatures in the 50s are colored yellow. It should be pretty clear where the isotherms should go.

The isotherms have been drawn in at right; not how the isotherms separate the colored bands. Note how the 40 F isotherm goes through the 40 on the map. There is one city with a temperature of exactly 60 F so a little piece of a 60 F isotherm is drawn through that city.

Isobars
These are a little harder to draw because you have to be able to decode the pressure data

isobars (1) connect points on the map with equal pressure
(2) separate regions of high pressure from regions with lower pressure
and identify and locate centers of high and low pressure

Here's the same weather map with isobars drawn in. Isobars are generally drawn at 4 mb intervals (above and below a starting value of 1000 mb).

The 1008 mb isobar (highlighted in yellow) passes through a city at Point A where the pressure is exactly 1008.0 mb. Most of the time the isobar will pass between two cities. The 1008 mb isobar passes between cities with pressures of 1009.7 mb at Point B and 1006.8 mb at Point C. You would expect to find 1008 mb somewhere in between those two cites, that is where the 1008 mb isobar goes.

The isobars separate regions of high and low pressure. The pressure pattern is not as predictable as the isotherm map. Low pressure is found on the eastern half of this map and high pressure in the west. The pattern could just as easily have been reversed.

This site (from the American Meteorological Society) first shows surface weather observations by themselves (plotted using the station model notation) and then an analysis of the surface data like what we've just looked at. There are links below each of the maps that will show you current surface weather data.

Here's a little practice

A single isobar is shown. Is it the 1000, 1002, 1004, 1006, or 1008 mb isobar? You'll need to decode the pressure data (add either a 9 or 10 and a decimal point). You'll find the answer at the end of today's notes.

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.

The following figure is on the in-class Optional Assignment.

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

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.

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.

Answers

To a couple questions embedded in the today's notes.

The isobar in the earlier figure is the 1004 mb contour. It separates pressures less than 1004 mb (colored blue and violet) from pressures greater than 1004 (green and orange). The 1002 mb and 1006 mb isobars have also been drawn in (isobars are normally drawn at 4 mb intervals, so the 1002 mb and 1006 mb contours wouldn't normally be included).

First the Low and High pressure centers have been labeled. The brown arrows show the winds blowing counterclockwise and inward around the Low, clockwise and outward around the High. Winds are shown using the station model notation at Points #1, #2, and #3 so that an idea of wind speed could be included. The isobars are most tightly spaced (strong pressure gradient) at Point #3. That's where the fast winds are shown. The wind at Point #2 is coming from the south, that's where the warmest air would most likely be found. Colder winds coming from the NW are found at Points #1 and #3.