Airwatch — Measuring winds aloft

Note: The description of this experiment may appear daunting at first. It really is easy to do and can form the basis of an immensely satisfying group activity. As well as providing much insight into the behaviour of the atmosphere, the results can be a useful part of a survey of the air pollution potential of a locality.

Measure winds aloft with a tethered balloon (left) and the equipment (right)

1. hose reel and line
   
2. elevation sighting
   
3. azimuth protractor
   

1. Aims

• To record, at regular intervals with increasing tether length the angle
  of elevation and azimuth of a tethered balloon.
• To calculate the height of the balloon during the experiment.
• To calculate wind speed with height using a computer program
  and the data obtained from the experiment.

2. Background information

The data obtained from this experiment are:

• the length of tether (reading from the scale on the fishing line)
• angle of elevation of the balloon from the horizontal (reading from theodolite)
• azimuth - the angle of the balloon from NORTH in a clockwise direction (reading from compass/protractor)
  

The height of the balloon at each reading is calculated by trigonometry, using the series of measurements of length of tether and the angle of elevation.

The data of azimuth angle and length of tether are used to give the wind speed and direction for each height reading.

When choosing the location the Teacher/Supervisor should take into account the following:

• We recommend that heights be limited to 100 m.
• Particular care should be taken to avoid contact by the balloon or its tethering line with any overhead power lines.
• The tether location should be in a low traffic area.
• The location should be away from buildings/fences.
• Do not attempt to fly the balloon in a wind. Conditions should appear to be practically calm. This is common around the early morning and late afternoon-evening. Otherwise the balloon will not lift - a special balloon is required for windy conditions.

A simplified method gives an approximate height estimate (see Calculating the approximate height of the balloon). This is important because if the balloon is to be flown at heights above 100 m then a permit must be obtained from the Civil Aviation Safety Authority (CASA).

Before an experiment is carried out the Supervisor/Teacher should review the necessary safety procedures:

SAFETY REGULATIONS

SAFETY REGULATIONS
There are strict regulations governing the flying of tethered balloons. It is essential that the following information be adhered to. No balloon can be flown within a 4,000 metre radius1 of an aerodrome. Tethered balloons may be flown to a height of 300 feet without a permit. A permit must be obtained to fly a tethered balloon at a height greater than 300 feet. Permits can be obtained from the local Civil Aviation Safety Authority. They are usually located at the aerodrome. The permit takes seven days to obtain, and at the time of writing there is no fee. CASA requires that the following details to be documented on the permit. Seven days' notice must be given to the Airways Operations Unit of the following: (a) the date and time of proposed flight/s As this experiment can only be carried out in light to moderate winds, it is advisable to nominate 2 or 3 possible flight days of 2-3 hour duration each, to allow for days that may not be suitable. eg. Monday 7/9/95 0900-1200hrs Tuesday 8/9/95 0900-1200hrs Friday 11/9/95 1200-1500hr CASA notifies the flight tower to inform any pilots flying in the area (NOTAM - Notices to Airmen). If any of the dates booked are not required notify CASA. (b) location of experimental site The actual location or area of operation with the street directory map reference or Latitude and Longitude. Key roads at the location should be noted. (c) maximum height to which the balloon will be flown. eg. 1,640 feet (500 metres) Other regulations to be observed The balloon can only be flown in circumstances where the visibility is greater than 5,000 metres and the balloon's distance from cloud is greater than 600 metres horizontally and 500 feet (152 metres) vertically.
1 (Note: CASA refers to vertical height in feet and horizontal distance in metres)

3. Equipment

EQUIPMENT IN KIT:

• 500-600 m fishing line (10 kg breaking strain)
• one hose reel
• fishing swivels (size 4)
• quick release double ended snap hook
• 500 mm length ( 20 mm diameter upvc pipe
• 2 × 360º protractor (150 mm diameter)
• compass
• plumb bob
• 2 × 650 mm red, orange or hot-pink balloons
• 2 × 200 mm tent pegs

EQUIPMENT NOT IN KIT TO BE HIRED OR PURCHASED:

• D size bottle of balloon gas (helium) with regulator (to be hired)
• super glue to assemble simple theodolite
• black and red permanent marker
• string
• 4 hooked tent pegs
• hammer

4. Setting up the equipment

The tethered balloon equipment

• mark a code on the fishing line at 10 metre intervals in permanent ink, to a length of 500 m.
  (black mark = 10 m, red mark = 100 m )

e.g.

At 10 m - one black 1cm length mark on the fishing line
At 20 m - two black 1cm marks on the fishing line
At 30 m - three black 1cm marks on the fishing line, and so on...
At 100 m - one red 1cm mark At 110 m - one red 1cm mark and one black 1cm mark
At 190 m - one red 1cm mark and nine black 1cm marks
At 200 m - two red 1cm marks on the fishing line, and so on to 500 m.

• tie the fishing line to the hose reel, starting at the 500 m mark.

Make sure the fishing line is well secured to the hose reel before winding
it on. Bind the knot with electrical tape to prevent it from coming undone.

• carefully wind the fishing line onto the hose reel.

At the 0 m end of the fishing line attach the fishing swivel. This will be the attachment point for the balloon.

• attach a quick release dog catch onto the side of the hose reel.

This acts as a lock on the reel to prevent it from turning when taking readings.

Simple theodolite (to measure angle of elevation) superglue the protractor to the pvc pipe so that the 0° line on the protractor is perpendicular to the pipe axis (see adjacent picture) thread the plumb bob string through the centre hole of the protractor to make a large loop and tie with a 'neat' knot (allow the plum bob to swing freely )
Azimuth angle equipment remove the plastic plate from compass place the compass on the second protractor (shown adjacent). align the NORTH position on the compass scale with the 0o on the outside markings of the protractor secure the compass with small pieces of blue tack.

5. Site procedure

Setting up at the site:

• Align the launching side of the reel downwind and peg the hose reel at each corner.
• Inflate the balloon to a circumference of ~1800 mm or ~575 mm diameter. The easiest way to inflate to the correct size is to cut a 1800 mm piece of string and wrap the string around the middle of the balloon as it inflates. Tie off the balloon when the string ends meet.
• Be careful that you do not let go of the balloon!
• Tie a short piece of string to the balloon, and tie the string to the swivel on the fishing line.

The balloon is now ready to launch.

How to measure the angle of elevation:

The angle of elevation of the balloon is the angle between the balloon tether point and the horizontal. The range of the elevation angle will be between 0º and 90º

Using the simple theodolite The protractor must be on the right hand side of the pvc pipe with the plumb bob hanging freely. When measuring the elevation angle the protractor must be in a vertical position. The outside set of degree numbers on the protractor should be 0º at the bottom of the theodolite.

• Notice that the plum bob string is hanging on the 0º point when the pvc pipe is held in a horizontal position
• Hold the pvc pipe with two hands, one either side of the protractor
• Site the balloon through the pvc tube. The balloon may move around depending on the wind gusts at the time. Take your reading when the balloon is showing least movement.
• Read the elevation angle by counting the number of degrees that the string has moved around from the 0° point (when the pvc pipe was in a horizontal position)

How to measure the azimuth:

The azimuth is the number of degrees from NORTH to the balloon in a clockwise direction.

Using the compass / protractor apparatus Hold the compass/protractor directly over the hose reel. Make sure the compass is in a horizontal position, and the North mark on the compass is aligned with the red needle, ie pointing towards North. Make sure that the cross lines in the middle of the protractor are directly over the bottom of the tether line. Using the scale on the protractor, measure the angle of the balloon from NORTH in a clockwise direction.

6. Conducting the experiment

• Record your data in the table
• Launch 50 m of tether, and lock the reel.
• Record the angle of elevation at 50 m using the simple theodolite.
• Record the azimuth angle at 50 m using the compass /protractor.
• Record time, weather observations, cloud detail.
• Repeat steps 1-5 at 50 m intervals to 300 m or less if you wish.
  
• Keep an eye on the height of the balloon (below 100 m is best).
• Upon winding in the balloon, be sure to relieve the strain on the reel, otherwise it will collapse!

7. Calculating the approximate height of the balloon

The approximate height of the balloon is calculated by using the simple trigonometry formula (assumes the tether is a straight line):

8. Calculating the wind speed with height

A computer program, BALLOON.EXE, can be used to make more accurate calculations of

the balloon height, wind speed at balloon height, wind direction at balloon height.

It uses the wind speed and direction calculated at lower levels to correct for wind drag on the fishing line at higher levels.

Download the program to a convenient directory on a PC and type BALLOON to run. A menu will be displayed with the following options:

Type the option letter (lower or upper case) and then press ENTER.

You first need to use option A to enter your measurements at each level of

• length of tether (in metres)
• elevation angle of the balloon measured from the ground (in degrees between 0° and 90°)
• azimuth angle of the balloon measured from the ground (in degrees between 0° and 360°).

You should check that your data have been entered correctly by using option D and correct if necessary using option R before doing the calculations (option C). The calculations can be done once all three values at a level have been entered or you can wait until you have entered all of your results.

The results are displayed on the screen and also written to a data file called BWINDS.DAT which can be used for graphing vertical profiles of wind speed and direction. The results from each new set of calculations are appended to the end of the BWINDS.DAT file. Each time results are written to the file, a line is included containing the date and time that the BALLOON program was run. Note that the data file (BWINDS.DAT) can be edited to remove old results or simply can be deleted if you have no need of the existing data, since it is not used by the program.

If the program displays an error message or crashes, check that the data have been entered in the correct format and that the measurements are in the correct units. Here is an example of BWINDS.DAT:

We have made a plot of the data on wind speed and wind direction obtained by the BALLOON program. You can see that the data are a little scattered (the points) and a smooth line is drawn to better show what we believe is the true behaviour.

TETHERED BALLOON DATA TABLE

Contact: peter.manins@csiro.au

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Modified: 17 May 2002