ATMO 171 Introduction to Meteorology and Climate

"Climate is what we expect, weather is what we get"

- Robert Heinlein
 

METEOROLOGY :

the study of the atmosphere and the various phenomena occurring in it (i.e., weather!)

Typical questions addressed by meteorology:

  • Why does it rain?
  • Will it rain today?
 
 

CLIMATE :

a description of atmospheric conditions based upon the sum of all weather events affecting a region over an extended period of time (30 years or more)

Typical climate questions:

  • Why is winter cold and summer hot?
  • What is the weather like in Tucson in August? In March?
  • Are summers getting hotter?
 
 

Weather Observations:

What kinds of observations do we need to understand and predict the weather?

 

  1. Current Conditions
  2. Cloud Cover, Type
  3. Visibility
  4. Temperature
  5. Wind Speed, Direction
  6. Humidity (Dewpoint Temp., Rel. Humidity)
  7. Pressure and Pressure Change
  8. Precipitation
 
 

Some sources for current weather info on the Internet:

www.atmo.edu ; click on "Weather Products", or "Related Sites"

www.nimbo.wrh.noaa.gov/tucson/twc.html ; NWS Tucson

www.wunderground.com ; "Weather Underground"
 





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Air pressure is the force exerted by air molecules on a surface.

Mathematically pressure is expressed as force divided by surface area


Atmospheric pressure at the surface of the earth is a measure of the "weight" of the overlying air molecules.

Pressure is typically measured by a mercury barometer or an aneroid barometer



  • Air pressure is commonly expressed in units of millibars (mb) or inches of Mercury (in Hg)
  • Typical sea level pressure is 1013 mb or about 30 in Hg. This is equivalent to about 14 lbs per square inch.
  • Air pressure at Tucson (el. 2,600' above sea level) is somewhere around 925 mb. Pressure decreases very rapidly with increasing elevation - i.e. vertical variations in pressure are very large.
  • Horizontal differences in pressure (i.e. pressure gradient) generate winds; these differences are generally rather small (5-10 mb).
  • To pick out horizontal differences in pressure on a weather map, pressure observations are converted to equivalent sea level pressure. For typical weather conditions, sea level pressures will fall in the range 950-1050 mb.

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Humidity is the measure of atmosphere's water vapor content

                                                                     evaporation
            Solid      <---->       liquid             <---->      gas
            (ice)                                                                        (vapor)
                                                                     condensation
 

Saturation occurs when rate of condensation equals rate of evaporation


[Think of the example discussed in class about the shallow pan of water left out overnight. What factors affect the rate of evaporation/condensation?]




Two ways to describe humidity are :

  1. Dew Point Temperature - this is the temperature at which air becomes saturated - i.e. when water vapor (a gas) will begin to condense and form liquid water (some examples are dew, fog, and clouds). Higher dewpoint temperatures indicate more water vapor.
  2. Relative humidity - describes how close air is to becoming saturated. If RH is high, air temperature and dew point temperature are close, and the atmosphere is close to saturation conditions. If RH is low, air temperature is much higher than dew point, and air is not close to saturation.
 
 



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Since clouds and rain are formed when water vapor condenses, humidity is an important indicator for predicting precipitation


Consider the example in Chapter 1: Humidity in Chicago and Evansville
 
 

                                  Chicago        Evansville

Air temperature            61            90

Dew point                      59             70

Relative humidity        93 %         50 %




Note that the when the difference between the air temperature and the dewpoint temperature is small, the relative humidity is high, meaning the atmosphere is close to becoming saturated. When the difference between the air temperature and the dew point temperature is large, relative humidity will be low. The formation of clouds and precipitation is not likely in this case. However, because the dewpoint temperature is HIGHER at Evansville, there is MORE water vapor in the air there than at Chicago. another way to say this is that the ABSOLUTE humidity is higher at Evansville, while the RELATIVE humidity is higher in Chicago.


 Extra Credit Assignment #1: Read about the different cloud types in pp. 155-159 of Danielson. Then take an observation of the sky and write down what types of clouds you observe, using the classifications from the reading assignment. Hand in your observations on Monday 8/30. Be sure to note the day and time your observation was taken. Keep your description short and concise - you can include a simple sketch if you wish.