Wednesday Feb. 9, 2011
click here to download today's notes in a more printer friendly format

Some old Beatles songs ("I Don't Want to Spoil the Party", "I'll Follow the Sun", "Mr. Moonlight", "No Reply", "What You're Doing" and part of "Honey Don't") to accompany the distribution of Experiment #2 materials before class this morning.

I'll bring the remaining materials again on Friday, so don't worry if you weren't able to pick up your materials today.

The Quiz #1 Study Guide is now online.  Quiz #1 will cover material on both the Practice Quiz and Quiz #1 Study Guides.  A total of 3 reviews have been scheduled for early next week (2-3 pm and 4-5 pm on Monday, 4-5 pm on Tuesday).



I've been neglecting the Picture of the Day.  I found this photograph (taken from the Space Shuttle) in last semester's notes - it shows thunderstorms forming over Brazil.  The flat cloud tops are about 15 km high and mark the top of the troposphere/bottom of the stratosphere.


Study this figure carefully.  It contains everything you need to know about coding and decoding pressure data on surface weather maps and also the altitude correction made to measured station pressure values. 

A station pressure of 931.0 mb was measured in Tucson on Tuesday afternoon at 3 pm.  At the same time a station pressure of 854.6 mb was measured in Albuquerque.  A 75 mb altitude correction was added to the Tucson value to obtain the 1006.0 mb sea level pressure estimate (75 mb because Tucson is 750 meters above sea level and pressure drops about 1 mb for every 10 of altitude).  The leading 10 and the decimal point were removed from 1006.0 and the remaining three figures 060 were plotted on the surface map. 

A larger, 150 mb, altitude correction was added to the Albuquerque station pressure (854.6 mb + 150 mb = 1004.6 mb) because Albuquerque is found at higher altitude (1500 meters above sea level).  The 10 and the decimal point were removed from 1004.6 and 046 was plotted on the map.

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 (Tucson stays on Mountain Standard Time year round) and Universal Time.  You must add 7 hours to the time in Tucson to obtain Universal Time.

Here are several examples of conversions between MST and UT (not done in class)

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)

2:30 pm MST:
first convert to the 24 hour clock by adding 12 hours   2:30 pm MST + 12:00 = 14:30 MST
add the 7 hour time zone correction --->  14:30 + 7:00 = 21:30 UT (7:30 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. 
We will 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

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 to map out the large scale pressure pattern.  We will look first at contour lines of temperature, they are a little easier to understand (easier to decode the plotted data and temperature varies across the country in a fairly predictable way).


Isotherms, temperature contour lines, are usually drawn at 10 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 40o F isotherm highlighted in yellow above passes through a city which is reporting a temperature of exactly 40o.  Mostly it goes between pairs of cities: one with a temperature warmer than 40o and the other colder than 40o.  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.    Isobars are generally drawn at 4 mb intervals.  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 pattern on this map is very different from the pattern of isotherms.  On this map the main features are the circular low and high pressure centers. 


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 at the end of today's notes)

Just locating closed centers of high and low pressure will already tell you a lot about the weather that is occurring in their vicinity.
1.
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 (we'll learn more about the Coriolis force 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, don'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.  Rising air expands (because it is moving into lower pressure surroundings at higher altitude), the expansion causes it to cool.  If the air is moist and it is cooled enough (to or below the dew point temperature) clouds will form and may then begin to rain or snow.  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).  You often see cloudy skies and stormy weather associated with surface low 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 but not necessarily warm weather, strong surface high pressure often forms when the air is very cold). 

2.
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. 


Closely spaced contours means pressure is changing rapidly with distance.  This is known as a strong pressure gradient and produces fast winds.  It is analogous to a steep slope on a hillside.  If you trip walking on a hill, you will roll rapidly down a steep hillside, more slowly down a gradual slope.

The winds around a high pressure center are shown above using both the station model notation and arrows. The winds are spinning clockwise and spiraling outward slightly.  Note the different wind speeds (25 knots and 10 knots plotted using the station model notation)



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



This figure is 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 below because this is about as far as we got in class on Wednesday).
















Answer to the isobar question: 1004 mb isobar

Answer to the wind directions and wind speeds question above

The winds are blowing from the NNW at Points 1 and 3.  The winds are blowing from the SSE at Point 2.  The fastest winds (30 knots) are found at Point 2 because that is where the isobars are closest together (strongest pressure gradient).  The slowest winds (10 knots) are at Point 3.  Notice also how the wind direction can affect the temperature pattern.  The winds at Point 2 are coming from the south and are probably warmer than the winds coming from the north at Points 1 & 3.  We'll be looking at this in more detail on Friday.