ATOC 1070 Weather Lab: Moisture in the Atmosphere

Follow the instructions in the lab manual, not this web page.
This is only a sketch outline of the lab.

A. Introduction:

At thousands of locations across the planet, from airports to research outposts to ships at sea, weather observers brave the elements to make hourly observations of the weather. Despite advances in satellite imaging, radar detection, and automated weather stations, these human observations still provide the observational backbone of meteorology. The data collected by these observers is the basis for regional, national, and global weather maps upon which our daily weather forecasts are based. Over the years, the international meteorological community has developed a set of standard guidelines for taking and reporting weather observations; in this way, an observation from Mongolia can be plotted on a map the same way an observation from Denver can.

B. Objectives:

This laboratory exercise will introduce you to the art and science of observing the atmosphere. You will become familiar with:

1. Two of the instruments used to make meteorological measurements.

2. Some observing techniques

3. How "raw" observations are reduced into more useful forms.

5. In addition, you will do a statistical analysis of your observations in order to gain insights into the accuracy and limitations of atmospheric measurements.

C. Equipment:

You will use two instruments to measure the temperature and humidity (dew point) inside and outside the lab:

1. A barometer – we will use either a standard mercury barometer or a digital barometer to measure the pressure.

2. A sling psychrometer – this instrument consists of two thermometers, one of which has a wet cloth wick placed over the bulb, mounted on a handle. With this instrument you will measure air temperature and the dew point temperature (a measure of moisture content of the atmosphere). Don’t forget water!

4. A "Turbo-meter" anemometer, or wind speed indicator. This is the same digital wind instrument used in the Wind Tunnel lab.

5. A cloud chart.

6. Your eyes – to describe the current weather conditions, and to estimate the type, amount, and height of different cloud layers.

7. The Log Sheet (included with these instructions).

D. Procedure (Part I: Surface Weather Observations):

Parts 1 and 2 will be done inside. Then you will wander out to Duane Field, south of the Stadium, for the rest of your observations. Then you will return to the lab and enter your observations on an active Excel spreadsheet. Things will go more smoothly and quickly if you read all the instructions below before venturing outside into the weather. A complete weather observation should be made as quickly as possible so that the weather doesn’t change much while you’re gathering your data – the idea is to make the observation as much a "snapshot in time" as possible. Professional observers can usually complete a full observation in 5 or 10 minutes; first-time observers usually take a bit longer.

1. Time.

Use any reasonably accurate clock, including your own watch, to note the time you begin your sequence of observations. Depending on the season, local time will be either Mountain Standard or Mountain Daylight Time. Enter the date and time on your log sheet. Add 7 hours (6 hours for Daylight Time) to obtain the Universal Time, and convert to a 24 hour clock. Example: 11:00 a.m. + 7 hours = 18:00 UT.

2. Pressure.

Read the barometric pressure (in millibars, which is the same as hectoPascals [hPa] ) from the digital barometer in the lab and enter on your log sheet. When you return to the lab after making your other observations, you will enter this pressure on an active Excel spreadsheet, which will compute a "Sea-level pressure" and "Altimeter setting" for you. Click here for more about measuring pressure.

3. Temperature, Dew Point, and Humidity.

Next, you will measure the temperature and moisture content of the local atmosphere using an instrument called a SLING PSYCHROMETER, which consists of two thermometers mounted on a swivel.

Each pair of lab partners will take a sling psychrometer out to Duane Field. Don’t forget to bring a small amount of cold water! Each person will take their own psychrometer readings. To do this, stand with your back to the sun (to shade the thermometers), wet the cotton wick, and whirl the thermometers for 10 or 15 seconds. Note the readings on both thermometers, then whirl them again for 10 or 15 seconds. Check the two thermometer readings again, and mentally note any changes from the previous readings. Repeat this procedure until you obtain the SAME dry- and wet-bulb readings (to the nearest whole degree C) on successive whirls. Log these numbers on your spreadsheet.

Make sure the wick is still wet when you take your final readings! If it isn’t, wet the wick and try again. If the air is very dry and the wick dries too rapidly, take the lowest wet bulb reading. On cold days, the wet wick may cool below 0C before freezing; if this happens, log the lowest wet-bulb reading. For more information about the Sling Psychrometer, click here .

NOW – return to the lab and bring up the Excel spreadsheet !

Bring up the Excel spreadsheet (called "SFC WX OBS.xls" in the "1070 Labs" folder on WXPAOS09, under "Surface Obs."). Bring up "Sheet 1" of the spreadsheet. Enter your observations on this spreadsheet – but ONLY in the boxes with the double-line borders! The boxes with single-line borders contain formulas that will compute values for you – making any entry in these boxes will erase the formulas. The spreadsheet will calculate sea-level pressure, dew point, etc. for you. After you have entered ALL your observations, print a copy of Sheet 1.

Convert your sea-level pressure into a 3-digit weather map code (be sure to include any leading zeroes):

Procedure (Part II: Comparison of your Data):

8. Temperature and Dew Point

It’s time to do some statistical analysis of your (and your classmates’) observations. Bring up "Sheet 2" of the spreadsheet. Each person in the class will call out their Temperature and Dew Point (NOT wet bulb!) readings (in degrees F), and everyone will enter these numbers in the "Temp." and "Dew Point" columns. Be sure to include your own readings. The spreadsheet will kindly compute the average and standard deviation of temperature and dew point. The standard deviation is a measure of the spread of the individual values. Roughly, about 2/3 of the values will fall in the range of the average plus/minus the standard deviation.

You now have all the data you need to ponder the following questions:

E. Questions.

1. The thermometers used in the psychrometers have an accuracy (i.e., error) of plus or minus 1 degrees F. What is the standard deviation of the outdoor temperatures (Dry Bulb readings) measured by your lab class? Is it greater than 1 degree? If so, explain, speculate, or guess why this might be the case – in other words, why might the outdoor temperatures recorded by you and your fellow students vary more than just the errors of the thermometers?

2. What if you took another psychrometer reading a few minutes later, and your wet-bulb reading was 1 degree lower than before, while your dry-bulb temperature remained the same. Would your Dew Point and Relative Humidity be higher, lower, or the same? Explain why, based on your understanding of how a psychrometer works.

3. Which has a greater standard deviation: Temperature or Dew Point? Explain why you think this might be the case (consider how Dew Point is calculated, from dry and wet bulb readings, and consider some of the lessons of error analysis from the Gas Law lab). What does this say about the relative accuracy of measurements of temperature vs. moisture in the atmosphere?

F. Lab Report and Grading

All parts of the lab report (see syllabus) should be turned in.
Objective and Procedure: 15 %
Activity (your observations properly taken and logged on the spreadsheet): 20 %
Questions 1 through 5: 10 % each.
Question 6: 5 %
Conclusion: 10 %