Tactics and Vectors 98/99
The effect of thermals on the velocity and direction of the wind
Wind direction should be measured: (1) at the beginning and end of an observation period, and (2) after each set of vanishing bearings that you record for a butterfly. This point needs to be emphasised. Every set of directional data for a butterfly must include a measurement of wind direction. The reason is twofold: (1) migrating butterflies respond to changes in wind direction, and (2) much of the time when butterflies are migrating, both wind direction and wind velocity are subject to frequent changes due to the action of passing thermals. Imagine that you are watching butterflies at your site on a day with a light north wind and that a thermal is drifting directly towards your site from the north. The first indication that the near side of the thermal has arrived is steady drop in wind velocity to zero. The brief period of calm is then broken by the appearance of a gentle south wind that initially increases in velocity, then declines with the arrival of the center of the thermal. Surface winds are light and variable while the center of the thermal drifts through the field site. With the arrival of the far side of the thermal, surface wind shift back to north and wind velocity rapidly increases until it exceeds the wind velocity recorded prior to the arrival of the thermal. As the thermal continues to drift downwind, wind velocity begins to decline and eventually settles back to its original value. If the thermal had passed by on the east side of your site, instead of scoring a direct hit, air rushing into the core from all sides (replacing the warmer air rushing aloft) would have caused wind velocity at the field site to slow, then rapidly build, then slow again. Similarly, the direction of the wind would have shifted to NW, then W, then shifted back to NW and N as the thermal passed by on the east. Thermal induced changes in wind conditions usually go cycle through in about 5 - 10 minutes. On days with a large amount of convective activity, fully developed thermals will continuously drift past and through your field site. Some thermals may even lift off from your site, causing air to rush in from all directions. It is easy to see why thermal activity is associated with frequent and unpredictable shifts in both the direction and velocity of the wind. This unavoidable feature of good soaring weather is another reason to locate your study site in a large open area. If your site was on the south side of a wood lot, a ridge, or a line of hills, thermals could boil off from the site continuously, playing havoc with wind measurements.
How to measure wind direction
Wind direction within 3 m of the ground is determined by using either your insect net as a wind sock, or a ribbon on a meter stick, as a wind direction indicator. Hold up the net (or ribbon) to catch the wind at about 3 m, then lay it down on the ground aligned with the wind direction. Use your compass to take the bearing of the direction indicated by the handle of the net (or the stick). In very light winds you may have to resort to the time honored method of releasing a bit of dry grass, thistle down, or dandelion seed (etc.) into the air and noting the direction that it drifts. Take the bearing of the direction of drift (downwind direction) and simply either add, or subtract 180°, as appropriate, to obtain the bearing for the wind. For example, if the thistle down was drifted on a bearing of 125°, the bearing for the wind is 135 + 180 = 315°. If the bearing for direction of drift had been greater than 180°, say 225°, then the bearing for the wind is 225 - 180 = 45°. Wind at altitude can usually be determined by noting the direction of drift of birds and/or butterflies that are circling in thermals. Because thermals, like thistle down, always drift downwind, simply take the bearing of the direction of displacement of the center of the group of birds and/or butterflies, and add or subtract 180°, as appropriate. Other indicators of wind direction include the direction of drift of bits of thistle down taken up by thermals, dissipating fog, and cumulus clouds, particularly low level fractocumulus clouds (dissipating fragments of cumulus cloud).
How to measure wind velocity
UNITS OF MEASUREMENT: Wind velocity data should be reported in meters per second (m/s). If your anemometer is calibrated in kts, mph, or km/hr, record wind velocity in these units and convert to m/s with the aid of the conversion factors in Table I
INSTRUMENTS: Wind velocity can be measured with several simple, hand-held, mechanical, or battery operated wind meters (anemometers) available at stores specialising in equipment for campers, birder's, science teachers, amateur scientists, pilots, boaters, etc. One widely available mechanical model is the Dwyer wind meter®. Wind velocity should be measured at 1.5 m above the ground. An alternate method to using using mechanical or electronic equipment to measure wind velocity is to become proficient in using the Beaufort wind scale shown in Table I. This scale was developed by Admiral Beaufort of the British Navy in 1805 for use at sea and was later modified for use on land. I truncated the scale, added a kilometers per hour (km/hr) column, a meters per second (m/s) column, and a few comments comments, notes, and tips that relate to butterfly watching. Measuring wind velocities that are less than 1 m/s presents a problem. Inexpensive anemometers are not sensitive to air velocities below about 1 m/s. On the other hand, more expensive models can accurately measure very low wind velocities (e.g. 0.1 m/s) but are generally not particularly sturdy when taken into the field. By sturdy, I mean still able to function after being knocked about in a box, rained on, stepped on, dropped in the dirt, in a puddle, or on a rock, and, if battery operated, subjected to frequent cycling between exposure in the field to high humidity and temperatures >25° (much hotter if apparatus left exposed to sun or in a parked car) followed by exposure to temperatures <25° (much cooler if left out in field or in car overnight). Unless you have access to a sturdy anemometer designed for long term exposure to the conditions outlined above, I suggest that you record a low velocity wind as being less than the lower limit for your anemometer (e.g. < 1 m/s for the Dwyer wind meter®).
VARIABLE AND GUSTY WINDS: Wind will usually be variable with spikes in velocity caused by gusts. Thermals are always associated with variable, often gusty, winds. To record wind velocity, ignore gusts and estimate the mid point of the fluctuating readings of your anemometer. For example, if the anemometer indicates a wind velocity fluctuating between 6 m/s and 8 m/s with gusts to 10 m/s, the appropriate wind velocity to record is 7 m/s.
How to Measure Ambient Temperature
Ambient temperature can be measured with any readily available thermometer that has the appropriate range. Outdoor thermometers are preferable because they are mounted in some type of protective enclosure. Avoid bringing standard laboratory glass and mercury thermometers to the field. They are likely to break. Take the ambient temperature at about 1.5 m above the ground. Make sure that the thermometer is in the shade, even if you have to shade it with your hat. Otherwise, the temperature recorded will be the temperature of glass, vinyl, or metal in the sun instead of the ambient temperature. The thermometer will still have to be shaded on overcast days, particularly if the morning haze is dissipating. An easy method to avoid having to wave a thermometer around for each measurement is to mount the instrument on a stake with the sensor set at at 1.5 m above the ground and attach a piece of cardboard for shade.