How Does Atmospheric Temperature Affect the Water Content of Snow?


Objective

The goal of this project is to investigate the effect of atmospheric temperature on snowfall depth.

Introduction

If you're lucky enough to live in a place that gets snow in winter, you know that the feel of the snow can vary a lot. Sometimes it can be light and fluffy, and other times heavy and wet. The light, fluffy snow has less water content than the heavy snow. What accounts for these differences?

One possibility might be the temperature of atmosphere in the clouds where the snow forms. Another possibility might be the temperature of the atmosphere through which the snow falls on its way to the ground. You may be wondering, "How in the world am I going to measure the temperature of the clouds?" Fortunately, you don't have to make the measurements yourself. The National Oceanic and Atmospheric Administration (NOAA) has already done it for you. Twice a day all over the U.S., weather balloons are used to take atmospheric soundings. The data from these soundings is available online (Unisys Corp., 2005).

Figure 1 shows an example of an upper air sounding plot. This is a standard graph used by meteorologists to analyze data from a balloon sounding. There is a lot of additional information in the graph, but basically it is a plot of temperature (x-axis) vs. height (y-axis). The white data line on the left shows the dewpoint vs. pressure, and the white data line on the right shows the temperature vs. pressure. The pressure (in millibars, mb) is shown on the y-axis in blue lettering, and the height (in m) is shown in white lettering. A sounding plot is also called a "Skew T" plot, because the temperature axis is plotted at an angle (i.e., skew) of 45°. The temperature lines of the Skew T are in blue (at 45°).

example of an upper air sounding plot
Figure 1. Example of an upper air sounding plot from the Unisys Weather webpage. Data shown are from International Falls, MN, March 23, 2007.

Atmospheric pressure decreases with height above the Earth's surface. The higher you go, the less atmosphere remains above you, so the pressure decreases. "Meteorology uses pressure as the vertical coordinate and not height. This works out better for thermodynamic computations that are done on a regular basis. Pressure decreases in the atmosphere exponentially as height increases reaching 0 pressure in space. The standard unit of pressure is millibars (mb or hectopascals-hPa) of which sea level is around 1015 mb. Here is a table of pressure levels and approximate heights (Unisys Corp., 2001):"

Pressure Approximate Height Approximate Temperature
(mb) (m) (ft) (°C) (°F)
1015
(sea level)		 0 0 15 59
1000 100 300 15 59
850 1500 5000 5 41
700 3000 10000 −5 23
500 5000 18000 −20 −4
300 9000 30000 −45 −49
200 12000 40000 −55 −67
100 16000 53000 −56 −69

Figure 2 shows how to read the temperature at a chosen pressure level (height). On the y-axis, find the pressure level (in mb) where you want to know the temperature. Follow the horizontal pressure line over until it intersects with the temperature plot (right-hand data plot, in white). Then follow the 45° temperature line down and to the left to the temperature axis. In the example below, the temperature at 700 mb was about −11°C.

reading temperature of the atmosphere at 700 mb (2962 m) from a sounding plot
Figure 2. Reading the temperature of the atmosphere at 700 mb (3022 m) from the sounding plot. Follow the horizontal pressure line to where it intersects with the temperature plot (right hand data line, in white). Then follow the 45° temperature line down and to the left to the temperature axis. In this example, the temperature at 700 mb was about −11°C.

There is a lot more information in the sounding plot, but it isn't important for this project. If you want to learn more about sounding plots, see the references in the Bibliography section.

In this project you will use atmospheric sounding data combined with your own measurements of the snow depth to liquid ratio to find out if there is a relationship between atmospheric temperature and snow quality.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

  • troposphere,
  • pressure,
  • temperature,
  • snowflake structure.

Questions

  • Where does snow typically form in the atmosphere?

Bibliography

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • tall cylindrical can for taking snow samples,
  • flat cover for the can (could be plastic or sheet metal),
  • a place to collect undisturbed snow from a snowfall event,
  • a ruler,
  • computer with Internet access.

Experimental Procedure

  1. After a snowfall, pick a location to collect snow that will represent the average snow depth for the event.
  2. Place the can upside down and push it down through the snow until it reaches the ground surface.
  3. Cover the opening of the can with your plastic or sheet metal cover.
  4. Bring the can upright, carrying the snow with it. The accumulation of snow in the can should be similar to what is on the ground.
  5. Measure the snow depth on the ground (in cm), close to where the sample was taken.
  6. Bring the can inside and wait for the snow to melt.
  7. Measure the depth of the liquid water in the can (in cm).
  8. Calculate the ratio between the snow depth and the depth of the liquid water. For example, if the snow depth was 30 cm and you measured 2 cm of liquid water in the can after melting, the ratio would be 15:1 (15 cm of snow for every cm of liquid water).
  9. Repeat the measurement for several snow events.
  10. For each snow event, you will also need to examine the upper air sounding that is closest to your location and to the time of the snowfall event.
    1. Examine the temperature profile for the lower troposhpere (surface to 700 millibars pressure).
    2. Upper air sounding plots are available here: http://weather.unisys.com/upper_air/skew/ (Unisys Corp., 2005).
    3. Click on the map location that is closest to where you are.
    4. Sounding data is taken twice a day for each station, at noon and midnight.
    5. At the Unisys Weather site, data is available for a 36-hour time window: the current sounding, plus the last the three soundings. This means that you will have to go online and print out the sounding data within one day of the snowfall event!
    6. See the Introduction for instructions on reading the sounding plot. Further information is available online (Unisys Corp., 1998; Unisys Corp., 2001; Millersville University LEAD Undergraduates, date unknown).
  11. Is there a relationship between the temperature in the lower troposphere at the time nearest to the snowfall and the snow depth:liquid depth ratio? To find out, make graphs of snow:liquid ratio (y-axis) vs. atmospheric temperature for different pressures.

Variations

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on:


Last edit date: 2007-05-11 08:15:00


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