Lab on Moisture in the Atmosphere and its Importance in Weather Phenomena

Part 1: Water Circulations, States, and Amounts

Objective: This lab is designed to introduce students to the abundance of moisture in the atmosphere and its importance in weather phenomena.  Students will compute various moisture calculations and provide observed results using instruments to measure moisture. 

 Materials needed: Pencil, Calculator, Ruler and Textbook

Part 1: Water Circulations, States, and Amounts

Moisture is the fundamental entity in the atmosphere that allows for production of clouds and precipitation, and the amount of moisture varies widely by location and by height above the ground.  Water can exist in many forms on Earth’s surface and in the atmosphere, and at Earth’s temperatures water easily changes state between the liquid, gas, and water phases. 

1. Use you text book and label every arrow in the following diagram by the name of the process each arrow represents.

Water in 3 Different States

2. Use the lettered arrows from the diagram on the previous page and choose the best letter (process) for each statement.

Which lettered arrow would be most responsible for:

1. Producing clouds
2. Producing fog
3. Creating frost
4. Keeping you cool when you step out of a shower
5. Creating the fog you observe in a scary Halloween movie
6. Producing snow crystals
7. Getting rid of a pond during a drought
8. Losing snow cover on the ground on a warm day
9. Causing snow to disappear on an extremely cold dry day
10. Having precipitation to fall from the clouds but not reach the ground

Part 2: Upper Atmospheric Moisture Concentrations

The amount of moisture in the atmosphere typically tends to decrease with increasing altitude with some exceptions.  To obtain various weather variables including moisture at levels above the ground, meteorologists deploy weather balloons at specific locations around the globe.  These upper air weather balloons are often called “radiosondes” or “rawinsondes”.  Launches are deployed twice daily at specific sites established within the National Weather Service radiosonde network.  An example of the data acquired is often plotted on a Skew-T thermodynamic profile diagram as shown below.

Although these diagrams can be quite involved, they are extremely useful tools for weather forecasters.  To interpret the standard Skew-T diagram:

1. The left vertical axis shows air pressure. The highest pressure is near the surface and decreases with a rise in altitude from 1050 mb to 100 mb. This would represent nearly a 50,000 foot ascension of the balloon!
2. The barbs along the right vertical axis represent wind speed and direction at various altitudes.
3. The red line (right most data line) represents the air temperature encountered by the rising balloon.
4. The blue line (left most data line) represents the dew point temperature encountered by the rising balloon.

An important aspect of these thermodynamic profiles relies on an analysis of the moisture available in the upper atmosphere for precipitation and other related weather phenomena.  Looking at the difference between the observed temperature and dew point temperature at any given level determines the magnitude of the dew point depression.  For example, if the observed temperature at a certain level was 25⁰C and the observed dew point temperature was 15⁰C, then the dew point depression would be 10⁰C.  When these depressions are very small or equal to zero, this implies moisture in the atmosphere and possible clouds and precipitation or foggy situations.  In the example above, most of the sounding is dry as the temperature and dew point are somewhat widely spaced, except at a small location around 680 mb as noted in the diagram.  This sounding would not imply any clouds or precipitation occurring over Miami at this time or in the near future, however with Miami being so close to the ocean, this sounding could quickly change. 

To interpret the wind speed and direction on the diagram use the following information for assistance:

Wind Direction: The wind direction is listed as the symbol denoting the direction from which the wind is blowing.  The Miami sounding on the previous page shows winds blowing predominantly from the northwest above 700 mb.

Wind Speed:  The wind speed is specified as feathers, half-feathers, and / or flags on the wind direction bar.  Each full feather represents 10 knots (12 mph), and each half-feather represents 5 knots (6 mph), and each flag represents 50 knots (55-60 mph). 

Some notable features on the sample diagram from Miami on page 4 include:

1. A low level temperature inversion very near the surface.
2. A very thin moist layer around 680 mb.
3. An extensive dry layer in the upper atmosphere extending from around 600mb to the top of the diagram (100 mb).
4. A peak wind level around 300 mb from the southwest. Note: Although this level has the fastest observed wind speeds, it doesn’t necessarily imply there is a jet stream located over Miami at this time.  Wind speeds within jet streams are typically much stronger than what is currently observed over Miami.

Use the Davenport, Iowa sounding to answer the following questions on the next page. Use the Miami sample diagram located on page 4 of this diagram for assistance along with the profile explanations on page 4-6.

Answer the following questions based on the Davenport, IA sounding from the previous page.

1. True of False? The overall sounding appears more dry than moist.       
2. At what major upper air level would this sounding be experiencing potential clouds?
   1. 850 mb
   2. 700 mb
   3. 500 mb
   4. 300 mb
   5. All of these
   6. None of these

3. Does it appear that precipitation is falling at Davenport at this time?
4. If precipitation were falling, would it most likely be rain or snow? How can you determine this?
5. If precipitation was occurring around 650 mb, would this precipitation most likely be reaching the ground? Explain your answer.
6. What is the predominant direction of the wind in the upper atmosphere over Davenport?
7. The fastest observed wind speed was observed at which level and what was the actual reading?
8. Does it appear that there is a jet stream located over Davenport at this time? Explain your answer.

Complete the following table by determining which sounding(s) above best represents the following weather conditions.  If more than one letter is appropriate, provide each letter in the column below.

Weather Condition	Letter(s)
The station that appears to have the deepest layer of moisture.
The station with the coldest surface temperature.
The station with predominantly southwest winds throughout the entire profile.
The station with the strongest jet stream wind speeds.
Has precipitation falling but is evaporating before it reaches the ground.
The station experiencing overcast skies.
The station experiencing precipitation which is reaching the ground.
The station which has a couple of dry layers throughout the sounding.
The station with changing wind directions (wind shear) near the surface.
The station with the driest layer (lowest dew point).

Part 3: Surface Atmospheric Moisture Distributions

Dew Point Temperatures: The amount of moisture at the surface can often be determined by the dew point temperature.  The dew point temperature can be thought of as the temperature needed to cool a parcel of air enough so that dew will eventually form.  In general, the higher the dew point temperature, the more moisture available for producing clouds and precipitation, hence the air is considered moist.  The lower the dew point temperature, typically the less moisture available for clouds and precipitation and the air is often considered dry.  A more reliable method of looking at moisture in the air relies on looking at dew point depressions which is discussed at a later section of this lab.

1. Which state(s) are experiencing a surge of moisture northward?
2. What is the source region for this surge of moisture? 
3. Explain the sharp drop in dew points observed between Missouri and Kansas.
4. Why are the dew points so low across Nevada?