ATOC 1070 Weather Lab: Surface Weather Observations

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. Some of the instruments used to make meteorological measurements.

2. Some observing techniques

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

4. How weather data is plotted on weather maps. (This will help you interpret weather maps in future labs.)

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 several different instruments to measure the weather outside the lab:

1. A clock – although it may seem trivial to mention this, the time your observations are made is very important.

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

3. 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 .

4. Wind Speed and Direction.

Now estimate the wind direction and measure the wind speed. Face into the wind, and estimate the direction the wind is blowing FROM to the nearest compass point (N, NE, E, SE, S, SW, W, NW), and enter this into your log. Also visualize yourself as standing at the center of a clock, with due north at the 12 o’clock position. Multiply the "hour" the wind is coming from by 30 to get the direction in degrees. For example, if the wind is from the southeast (SE), it may be from the 4 o’clock direction. 4 times 30 is 120, and you would enter the wind direction as 120 degrees.

To measure the wind speed, set the Turbo-Meter to read in Knots, hold the meter directly into the wind at head level (or slightly higher), and do an "eyeball" average of the speed over 20 or 30 seconds. Enter the speed in knots (to the nearest whole knot). A knot is one nautical mile per hour, or 1.15 statute miles per hour, or about 0.5 meter per second.

This diagram shows a Northeast (NE) wind at 15 knots as it appears on a weather map.

5. Cloud types, amounts, and heights.

For help in identifying the different cloud types, consult the various cloud charts posted in the lab or the sets of cloud photos in the text (inside the front cover and on pages 92-101), or check this cloud chart issued as postage stamps of this cloud diagram . You may also check the "Observing Clouds" background section in the Cloud Observations Lab (in the lab web site). Cloud observations can be rather complicated (or exceedingly simple, if the sky is clear!), so follow closely and pay attention to your log sheet…

First, estimate the total amount of the sky that is covered with clouds. To do this, it may help to visualize the sky as the dome of a planetarium, and to mentally divide the dome into 8 equal size pieces. Estimate how many eighths are covered by clouds. If there are no clouds, the cloud cover is 0 eighths; if it’s completely overcast, the cloud cover is 8 eighths. If the sky is mostly cloudy, it’s usually easier to estimate the amount of blue sky and subtract that number from 8. Enter this number (to the nearest eighth) in the "Estimated TOTAL cloud cover" box.

Next, identify the various cloud types that may be present. Clouds are divided into three basic categories based on their height above the ground (see the chart on page 98 of the text). The categories are, not surprisingly, called Low, Middle, and High clouds. Cloud types belonging to each level, and typical heights above Boulder, are:

Cloud Level
Height of cloud base above Boulder (feet)
Cloud types and their abbreviations
Low
100 to 6,000
Cumulus (Cu), Towering Cumulus (Tcu), Cumulonimbus (Cb), Strato-cumulus (Sc), Stratus (St), Nimbo-stratus (Ns)
Middle
6,000 to 15,000
Altocumulus (Ac), Altostratus (As), Lenticular (AcL)
High
Above 15,000
Cirrus (Ci), Cirrostratus (Cs), Cirrocumulus (Cc)
Obscuring phenomena
0 (on the ground)
Fog, Smoke, Haze, Snow, Dust

The last row in the table, "Obscuring Phenomena", refers to very low phenomena, such as fog or heavy falling snow, that blocks some or all of the clouds or sky that may be above. Also note that Cumulonimbus clouds (thunderstorms) are classified as low clouds even though their tops may reach 30,000 feet or higher.

For more details on observing clouds, click here .

6. Current Weather.

On your log sheet, enter a short description of the current weather. For guidance, the list of "Common Weather Symbols" on the last page shows SOME of the possible types of weather. If nothing is falling from the sky, the weather could be "clear", "partly cloudy", or "cloudy".

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):
Pressure 1000.0 mb or greater: Code = (Sea-level pressure – 1000)*10
Pressure 999.9 mb or less: Code = (Sea-level pressure – 900)*10

7. Station Plot.

You now have enough information to plot your observation using the standard format seen on surface weather maps. On a weather map, a small circle marks the location of the weather station (say, Denver International Airport), and various numbers and symbols are attached to this circle. In the box with the small circle in it, plot your surface weather observation. Include: Cloud cover, Wind speed and Direction, Temperature (degrees F), Dew Point (degrees F), Sea-Level Pressure (3-digit code), and Weather type (if any). The Simplified Surface Station Model tells you almost everything you need to know about doing this. (You will not need the section about "Front Symbols" for this lab).
For a larger variety (99, to be exact) of weather types, see the poster in the lab.

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.

9. Pressure Comparisons.

Do NOT print the data on these links. Just look at them and write them down!

Follow the instructions on the spreadsheet to compare your observations with those at Denver International Airport, 60 km southeast of Boulder and at the same elevation, and with those taken by an automated weather station atop the NCAR Mesa Lab, in the foothills at the southwestern edge of Boulder and 245 meters above the lab (read the current Pressure, which is given in hektoPascals, or hPa, which is the same as millibars).

10. One more thing....

On Netscape (or Explorer, or other internet browser), go to the site www.cloudman.com . Click on the "Cloud Formation Chart". This chart gives weather conditions (moisture, fronts, etc.) associated with common cloud types. Read the chart instructions, and look up "Altostratus" clouds in the table. Then answer question 6. This exercise will acquaint you with the identification and clouds, and will help you get started on your cloud log.

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?

4a. Look at the pressure from the Mesa Lab. Do you think this is a station pressure or a sea-level pressure? Why?

4b. Compare the Mesa Lab pressure with your pressure reading from the digital barometer in class. Which is greater, and by how much? Why does this difference occur ? (For help, read the Background information in the "Pressure" section of this lab).

5. Compare the sea-level pressure at Denver with your sea-level pressure. Which is greater, and by how much? What might this pressure difference imply about the direction of the winds between Denver and Boulder? Again, refer to the Background information.

6. Now that you’ve looked up Altostratus clouds on the Cloudman.com "Cloud formation chart", answer the following:
a. What is the two-letter abbreviation for Altostratus?
b. Are Altostratus clouds caused by a stable or unstable atmosphere?
c. What is the moisture content (Small, Moderate, or Large) for Altostratus clouds?
d. What kind of front can cause Altostratus clouds?

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 %