Tue., Sep. 24 notes

Tue., Sep. 24, 2013

The London Symphony Orchestra playing George Gershwin's Rhapsody in Blue before class today.

The Optional Assignment turned in last Thursday has been graded and was returned. If your paper does not have a grade marked on it you received full credit (0.5 extra credit points). Answers to all the questions are available online.

The Experiment #1 reports were due today (unless you were told otherwise). If you haven't do so already please bring back your experiment materials. The graduated cylinders need to be cleaned so that they can be handed out on Thursday to students doing Experiment #2.

Quiz #1 is Thursday this week. We'll aim to start the quiz about 5 minutes after the start of class. Use the Study Guide to focus your efforts. Reviews are scheduled for Tue. and Wed. afternoon from 2-3:15 pm in Saguaro 225.

We're starting a new topic today - weather maps and some of what you can learn from them.

We began by learning how weather data are entered onto surface weather maps.

Much of our weather is produced by relatively large (synoptic scale) weather systems - systems that might cover several states or a significant fraction of the continental US. To be able to identify and characterize these weather systems you must first collect weather data (temperature, pressure, wind direction and speed, dew point, cloud cover, etc) from stations across the country and plot the data on a map. The large amount of data requires that the information be plotted in a clear and compact way. The station model notation is what meteorologists use.

A small circle is plotted on the map at the location where the weather measurements were made. The circle can be filled in to indicate the amount of cloud cover. Positions are reserved above and below the center circle for special symbols that represent different types of high, middle, and low altitude clouds. The air temperature and dew point temperature are entered to the upper left and lower left of the circle respectively. A symbol indicating the current weather (if any) is plotted to the left of the circle in between the temperature and the dew point; you can choose from close to 100 different weather symbols (on a handout distributed in class). The pressure is plotted to the upper right of the circle and the pressure change (that has occurred in the past 3 hours) is plotted to the right of the circle.

We worked through this material one step at a time (refer to p. 36 in the photocopied ClassNotes). The figures below were borrowed from a previous semester or were redrawn and may differ somewhat from what was drawn in class.

The center circle is filled in to indicate the portion of the sky covered with clouds (estimated to the nearest 1/8th of the sky) using the code at the top of the figure (which you can quickly figure out). 3/8ths of the sky is covered with clouds in the example above.

Then symbols are used to identify the actual types of high, middle, and low altitude clouds observed in the sky. Later in the semester we will learn the names of the 10 basic cloud types. Six of them are sketched above and symbols for them are shown. Purple represents high altitude in this picture. Clouds found at high altitude are composed of ice crystals. Low altitude clouds are green in the figure. They're warmer than freezing are composed of just water droplets. The middle altitude clouds in blue are surprising. They're composed of both ice crystals and water droplets that have been cooled below freezing but haven't frozen.

A copy of the handout passed out in class can be found here. Click here to see a cloud chart with actual photographs of the various cloud types and the symbols used for each.

A straight line extending out from the center circle shows the wind direction. Meteorologists always give the direction the wind is coming from. In the uppermost example the winds are blowing from the NW toward the SE at a speed of 5 knots. A meteorologist would call these northwesterly winds.

Winds in the bottom set of examples are all coming from the south. Small "barbs" at the end of the straight line give the wind speed in knots. Each long barb is worth 10 knots, the short barb is 5 knots. Knots are nautical miles per hour. One nautical mile per hour is 1.15 statute miles per hour. We won't worry about the distinction in this class, we will just consider one knot to be the same as one mile per hour.

Here are four more examples to practice with. Determine the wind direction and wind speed in each case. Click here for the answers.

The air temperature and the dew point temperature are probably the easiest data to decode.

The air temperature in this example was 64^o F (this is plotted above and to the left of the center circle). The dew point temperature was 39^o F and is plotted below and to the left of the center circle. The box at lower left reminds you that dew points range from the mid 20s to the mid 40s during much of the year in Tucson. Dew points rise into the upper 50s and 60s during the summer thunderstorm season (dew points are in the 70s in many parts of the country in the summer). Dew points are in the 20s, 10s, and may even drop below 0 during dry periods in Tucson.

And maybe the most interesting part.

A symbol representing the weather that is currently occurring is plotted to the left of the center circle (in between the temperature and the dew point). Some of the common weather symbols are shown. There are about 100 different weather symbols that you can choose from (here's a nicer version of the list). There's no way I could expect you to remember all of those weather symbols.

The pressure data is usually the most confusing and most difficult data to decode.

The sea level pressure is shown above and to the right of the center circle. Decoding this data is a little "trickier" because some information is missing. We'll look at this in more detail momentarily.

Pressure change data (how the pressure has changed during the preceding 3 hours) is shown to the right of the center circle. We didn't discuss this in class. You must remember to add a decimal point. Pressure changes are usually pretty small.

Here's what you need to know about the pressure data.

Meteorologists hope to map out small horizontal pressure changes on surface weather maps (that produce wind and storms). Pressure changes much more quickly when moving in a vertical direction. The pressure measurements are all corrected to sea level altitude to remove the effects of altitude. If this were not done large differences in pressure at different cities at different altitudes would completely hide the smaller horizontal changes.

In the example above, a station pressure value of 927.3 mb was measured in Tucson. Since Tucson is about 750 meters above sea level, a 75 mb correction is added to the station pressure (1 mb for every 10 meters of altitude). The sea level pressure estimate for Tucson is 927.3 + 75 = 1002.3 mb. This sea level pressure estimate is the number that gets plotted on the surface weather map.

Do you need to remember all the details above and be able to calculate the exact correction needed? No. You should remember that a correction for altitude is needed. And the correction needs to be added to the station pressure. I.e. the sea-level pressure is higher than the station pressure.

The calculation above is shown in a picture below

The full 1002.3 mb value wouldn't be plotted on a surface map. Here are some examples of coding and decoding the pressure data.

First of all we'll take some sea level pressure values and show what needs to be done before the data is plotted on the surface weather map. These should be the same numbers that we used in class.

Sea level pressures generally fall between 950 mb and 1050 mb. The values always start with a 9 or a 10. To save room, the leading 9 or 10 on the sea level pressure value and the decimal point are removed before plotting the data on the map. For example the 10 and the decimal pt in 1002.3 mb would be removed; 023 would be plotted on the weather map (to the upper right of the center circle). Some additional examples are shown above.

You'll mostly have to go the other way - read data off a map and figure out what the sea level pressure is. This is illustrated below.

When reading pressure values off a map you must remember to add a 9 or 10 and a decimal point. For example
118 could be either 911.8 or 1011.8 mb. You pick the value that falls closest to 1000 mb average sea level pressure. (so 1011.8 mb would be the correct value, 911.8 mb would be too low).

Another important piece of information on a surface map is the time the observations were collected. Time on a surface map is converted to a universally agreed upon time zone called Universal Time (or Greenwich Mean Time, or Zulu time). That is the time at 0 degrees longitude, the Prime Meridian. There is a 7 hour time zone difference between Tucson and Universal Time (this never changes because Tucson stays on Mountain Standard Time year round). You must add 7 hours to the time in Tucson to obtain Universal Time.

Here are several examples of conversions between MST and UT

to convert from MST (Mountain Standard Time) to UT (Universal Time)
10:20 am MST:

add the 7 hour time zone correction ---> 10:20 + 7:00 = 17:20 UT (5:20 pm in Greenwich)

2:45 pm MST :

first convert to the 24 hour clock by adding 12 hours 2:45 pm MST + 12:00 = 14:45 MST

add the 7 hour time zone correction ---> 14:45 + 7:00 = 21:45 UT (7:45 pm in England)

7:45 pm MST:

convert to the 24 hour clock by adding 12 hours 7:45 pm MST + 12:00 = 19:45 MST
add the 7 hour time zone correction ---> 19:45 + 7:00 = 26:45 UT
since this is greater than 24:00 (past midnight) we'll subtract 24 hours 26:45 UT - 24:00 = 02:45 am the next day

to convert from UT to MST
18Z:

subtract the 7 hour time zone correction ---> 18:00 - 7:00 = 11:00 am MST

02Z:

if we subtract the 7 hour time zone correction we will get a negative number.
So we will first add 24:00 to 02:00 UT then subtract 7 hours 02:00 + 24:00 = 26:00
26:00 - 7:00 = 19:00 MST on the previous day
2 hours past midnight in Greenwich is 7 pm the previous day in Tucson

Next we'll start to see what analysis of that data can start to tell you about the weather.

A bunch of weather data has been plotted (using the station model notation) on a surface weather map in the figure below (p. 38 in the ClassNotes).

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. 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, temperature contour lines, are usually drawn at 10^o F intervals. They do two things: (1) connect points on the map that all have the same temperature, and (2) separate regions that are warmer than a particular temperature from regions that are colder. 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). Temperatures generally decrease with increasing latitude: warmest temperatures are usually in the south, colder temperatures in the north.

Now the same data with isobars drawn in. Again they separate regions with pressure higher than a particular value from regions with pressures lower than that value. The isobars also enclose areas of high pressure and low pressure. Isobars are generally drawn at 4 mb intervals (starting with a base value of 1000 mb). Isobars also connect points on the map with the same pressure. 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 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 (this figure wasn't shown in class).

Is this the 1000, 1002, 1004, 1006, or 1008 mb isobar? (you'll find the answer below)

Pressures lower than 1002 mb are colored purple. Pressures between 1002 and 1004 mb are blue. Pressures between 1004 and 1006 mb are green and pressures greater than 1006 mb are red. The isobar appearing in the question is highlighted yellow and is the 1004 mb isobar. The 1002 mb and 1006 mb isobars have also been drawn in (because isobars are drawn at 4 mb intervals starting at 1000 mb, 1002 mb and 1006 mb isobars wouldn't normally be drawn on a map)