Tue., Feb. 21 notes

Tuesday Feb. 21, 2012
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A couple of songs from Alison Krauss to start the new week. You heard "Dimming of the Day" and "Sister Rosetta Goes Before Us".

Quiz #1 has been graded and was returned in class today. Please check carefully to see that your quiz was graded correctly and hang on to this quiz and any other work that is returned to you during the semester. Don't throw anything away until you've received and agree with your final grade at the end of the semester.

A couple of assignments are available for your consideration (neither is required). The due date for the first one, a Surface weather map analysis, has been extended until Tuesday Feb. 28. You can earn up to 5 1S1P pts. The second is an upper level chart Optional Assignment due at or before the start of class on Thursday, Mar. 1. You can earn 0.5 pts of extra credit and a green card (if you answer 85% of the questions correctly) on the second assignment.

To provide some background for the weather map analysis assignment we'll locate the cold front on the following weather map.

Before trying to locate a cold front, we needed to draw in a few isobars and map out the pressure pattern. Fronts are like spokes on a wheel - they rotate counterclockwise around centers of low pressure. It makes sense to first determine the location of the low pressure center.

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 actual 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 need to draw in 996 mb and 1000 mb isobars (you can get a better idea of how to actually draw in the isobars by looking at the 1S1P Bonus Assignment Surface Weather Map Analysis Example.

The isobars have been drawn in on the map below.

The next step was to try to locate the warm air mass in the picture. Temperatures are in the 60s in the lower right portion of the map; this area has been shaded orange. Cool (green), cold (blue) and colder (purple) air to the west of the Low pressure center has also been identified; . Based on just the temperatures just should have a pretty good idea where a cold front would be found.

The cold front has been drawn in at the advancing edge of the cold air. Note how the cold front is positioned at the leading edge of the cold air mass, not necessarily in front of the coldest air in the cold air mass. 3 of the stations from the bottom portion of the map have been redrawn below.

We need to look at some of the other weather changes to see if the placement of the front looks reasonable.

The air behind a cold front is usually drier than the air ahead of the front. This seems to be the case above. Dew points ahead of the front are in the upper 40s/lower 50s. Behind the front dew points are in the upper 20s/lower 30s.

There is a clear change in wind direction. Southwesterly winds found ahead of the front become northwesterly winds behind the front.

Finally pressures ahead of the front are falling, pressure behind the front is rising.

Before we go on and have a brief look at a warm front, here's time lapsed video of last week's cold front. (it may take some time to download the video and you may not see the play, pause, and stop buttons at the bottom of the image)

Warm fronts are found at the boundary between an advancing warm air mass that is overtaking the retreating back edge of a cold air mass. This is show below

The back edge of the cold mass 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 ramp like shape. The warm air rises more slowly and rises over a much larger area out ahead of the warm front. This is an important difference between warm and cold fronts. Much more extensive cloudiness is found out ahead of a warm front that a cold front. With the extensive and varied mix of clouds comes some very different types of precipitation (snow, sleet, freezing rain, rain etc).

And the weather changes found in advance of and following the frontal passage.

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
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 didn't try to locate a warm front on a surface weather, but I promised I would put that on the online notes. Here's the surface map data that we will be working with (p. 149b in the ClassNotes)

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.

Note that the 992 and 996 mb isobars each went through a station with pressures of exactly 992.0 and 996.0 mb (highlighted in yellow). 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?

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). Most of the cloud outlined in green are probably being produced by the warm front. You can see how more extensive cloud coverage is with a warm front. 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.

Here's the picture again with something more drawn in.

There's pretty good evidence of a cold front on the left portion of the map.

One last picture, we go back to the map on p. 39 in the ClassNotes.

One of the questions we had was what might be causing the clouds, rain, and drizzle in the northeastern part of the country, and the rain shower along the Gulf Coast. The cold front is almost certainly the cause of the rain shower and much of the wet weather in the NE is probably being caused by the warm front.

Up to this point we've been 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). We covered some of the basic concepts in the next 1/3 of the period. Some additional supplementary information is available online. This supplementary reading is what you'll need to look at if you are going to try to do the newest Optional Assignment.

Here we'll mostly 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 portion 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). You'll find this basic picture on p. 41 in the ClassNotes.

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 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 western 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 "Upper level charts pt. 2".

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 first section. 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. rember that upper level winds blow parallel to the contour lines from west to east.

Here's the earlier picture again overlaying surface and upper-level maps.

On the surface map above you see centers of HIGH and LOW pressure. The surface low pressure center, together with the cold and warm fronts, is a middle latitude storm.

Note how the counterclockwise winds spinning around the LOW move warm air northward (behind the warm front on the eastern side of the LOW) and cold air southward (behind the cold front on the western side of the LOW). Clockwise winds spinning around the HIGH also move warm and cold air. The surface winds are shown with thin brown arrows on the surface map.

Note the ridge and trough features on the upper level chart. We learned that warm air is found below an upper level ridge. Now you can begin to see where this warm air comes from. Warm air is found west of the HIGH and to the east of the LOW. This is where the two ridges on the upper level chart are also found. You expect to find cold air below an upper level trough. This cold air is being moved into the middle of the US by the northerly winds that are found between the HIGH and the LOW.

Note the yellow X marked on the upper level chart directly above the surface LOW. This is a good location for a surface LOW to form, develop, and strengthen (strengthening means the pressure in the surface low will get even lower than it is now. This is also called "deepening"). The reason for this is that the yellow X is a location where there is often upper level divergence. Similary the pink X is where you often find upper level convergence. This could cause the pressure in the center of the surface high pressure to get even higher. You can read more about this in Upper level charts pt. 3.

One of the things we have learned about surface LOW pressure is that the converging surface winds create rising air motions. The figure above gives you an idea of what can happen to this rising air (it has to go somewhere). Note the upper level divergence in the figure: two arrows of air coming into the point "DIV" and three arrows of air leaving (more air going out than coming in is what makes this divergence). The rising air can, in effect, supply the extra arrow's worth of air.

Three arrows of air come into the point marked "CONV" on the upper level chart and two leave (more air coming in than going out). What happens to the extra arrow? It sinks, it is the source of the sinking air found above surface high pressure.

During the next couple of weeks we will be moving into a completely different topic and will be concerned with energy, temperature, heat, energy transport, and energy balance between the earth, atmosphere, and space.

It is easy to lose sight of the main concepts because there are so many details. Most of the following figures are found on pps 43&44 in the photocopied ClassNotes.

Types of energy

We will learn the names of several different types or forms of energy.

Kinetic energy is energy of motion. Some examples (both large and microscopic scale) are mentioned and sketched above. This is a relatively easy to visualize and understand form of energy.

Latent heat energy is perhaps the most underappreciated and most confusing type of energy. The word latent refers to energy that is hidden in water and water vapor. The hidden energy emerges when water vapor condenses or water freezes (the energy had been added earlier when ice was melted or water was evaporated).

Radiant energy is a very important form of energy that was for some reason left off the original list in the ClassNotes. Sunlight is an example of radiant energy that we can see and feel (you feel warm when you stand in sunlight). There are many types of radiant energy that are invisible (such as the infrared light that people emit). Electromagnetic radiation is another name for radiant energy.

Energy transport
Four energy transport processes are listed below.

By far the most important process is at the bottom of the list above. Energy transport in the form of electromagnetic radiation (sunlight is a common form of electromagnetic radiation) is the only process that can transport energy through empty space. Electromagnetic radiation travels both to the earth (from the sun) and away from the earth back into space. Electromagnetic radiation is also responsible for about 80% of the energy transported between the ground and atmosphere.

You might be surprised to learn that latent heat is the second most important transport process.

Rising parcels of warm air and sinking parcels of cold air are examples of free convection. Because of convection you feel colder or a cold windy day than on a cold calm day. Ocean currents are also an example of convection. Ocean currents transport energy from the warm tropics to colder polar regions.

Convection is a 3rd way of causing rising air motions in the atmosphere (convergence into centers of low pressure and fronts are other 2 ways we've encountered so far)

Conduction is the least important energy transport at least in the atmosphere. Air is such a poor conductor of energy that it is generally considered to be an insulator.

Energy balance and the atmospheric greenhouse effect

The next picture (the figure in the ClassNotes has been split into three parts for improved clarity) shows energy being transported from the sun to the earth in the form of electromagnetic radiation.

We are aware of this energy because we can see it (sunlight also contains invisible forms of light) and feel it. With all of this energy arriving at and being absorbed by the earth, what keeps the earth from getting hotter and hotter? If you park your car in the sun it will heat up. But there is a limit to how hot it will get. Why is that?

It might be helpful when talking about energy balance to think of a bank account. If you periodically deposit money into your account why doesn't the balance just grow without limit. The answer is that you also take money out of the account and spend it. The same is true of energy and the earth. The earth absorbs incoming sunlight energy but also emits energy back into space (the orange and pink arrows in the figure below)

Energy is emitted in the form of infrared light is an invisible form of energy (it is weak enough that we don't usually feel it either). A balance between incoming and outgoing energy is achieved and the earth's annual average temperature remains constant.

We will also look closely at energy transport between the earth's surface and the atmosphere (see the figure below). This is where latent heat energy transport, convection and conduction operate (they can't transport energy beyond the atmosphere and into outer space).

That is also where the atmospheric greenhouse functions. That will be a important goal - to better understand how the atmospheric greenhouse effect works.

The greenhouse effect is getting a lot of "bad press". If the earth's atmosphere didn't contain greenhouse gases and if there weren't a greenhouse effect, the global annual average surface temperature would be about 0 F (scratch out -4 F and put 0 F, it's easier to remember). Greenhouse gases raise this average to about 60 F and make the earth a much more habitable place. That is the beneficial side of the greenhouse effect.

The detrimental side is that atmospheric greenhouse gas concentrations are increasing. This might enhance or strengthen the greenhouse effect and cause the earth to warm. While that doesn't necessarily sound bad it could have many unpleasant side effects. That's a subject we'll explore briefly later in the semester.

When you add energy to an object, the object will usually warm up (conversely when you take energy from an object the object will cool). It is relatively easy to come up with an equation that allows you to figure out what the temperature change will be (one of those equations I'll probably write on the board during the next quiz if you ask me to - try to understand it, you don't have to memorize it).

The temperature change, ΔT, will first depend on how much energy was added, ΔE. This is a direct proportionality, so ΔE is in the numerator of the equation (ΔE and ΔT are both positive when energy is added, negative when energy is removed)

When you add equal amounts of energy to large and small pans of water, the small pan will heat up more quickly. The temperature change, ΔT, will depend on the amount of water, the mass. A small mass will mean a large ΔT, so mass should go in the denominator of the equation.

Different materials react differently when energy is added to them. A material with a large specific heat will warm more slowly than a material with a small specific heat. Specific heat has the same kind of effect on ΔT as mass. Specific heat is sometimes called "thermal mass" or "thermal capacity." You can think of specific heat as being thermal inertia - a substance with high specific heat, lots of thermal inertia, will be reluctant to change temperature.

Here's an important example that will show the effect of specific heat (middle of p. 45). I've changed the numbers from the example shown in class.

Equal amounts of energy (1000 calories, note that calories are units of energy) are added to equal masses (500 grams) of water and soil. We use water and soil in the example because most of the earth's surface is either ocean or land. Before we do the calculation, try to guess which material will warm up the most. Everything is the same except for the specific heats. Will water with its 5 times larger specific heat warm up more or less than the water?

Here are the details of the calculation.

With its higher specific heat, the water doesn't heat up nearly as much as the soil. If we had been removing energy the soil would have cooled off more than the water also.

These different rates of warming of water and soil have important effects on regional climate.

Oceans moderate the climate. Cities near a large body of water won't warm as much in the summer and won't cool as much during the winter compared to a city that is surrounded by land.
The yearly high and low monthly average temperatures are shown at two locations above. The city on the coast has a 30^o F annual range of temperature (range is the difference between the summer and winter temperatures). The city further inland (assumed to be at the same latitude and altitude) has an annual range of 60^o F. Note that both cities have the same 60^o F annual average temperature. We'll see a much more dramatic example of the moderating effect of water on climate in a couple of weeks.

Here's another situation where you can take advantage of water's high specific heat to moderate "micro climate."

I did plant some of my young tomato plants last weekend (this is a picture from a previous year). It still can get plenty cold enough at night in February or early March to kill tomatoes (the brocolli and lettuce in the background can handle a light frost) so you have to protect them.

Here's one way of doing that. You can surround each plant with a "wall o water" - a teepee like arrangement that surrounds each plant. The cylinders are filled with water and they take advantage of the high specific heat of water and won't cool as much as the air or soil would during a cold night. The walls of water produce a warm moist microclimate that the tomato seedlings love. The plastic is transparent so plenty of sunlight can get through.