## How to Record Wind Direction, Wind Velocity, and Temperature

### 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.

### Tables

 Beaufort Number Wind in: ___________________________ Description Indications Notes Knots ______ mph ______ km/hr ______ m/s ______ 0 0 0 0 0 Calm Smoke rises vertically. Light weight, aerial, insect net held aloft hangs limb. Released thistle down settles vertically. (Tip 1.) (1.) 1 2 2 4 1 light air We usually don't notice the slight air movement. Smoke drifts slowly downwind. Insect net is slightly displaced from vertical by wind but is not inflated. Released thistle down shows a steady drift downwind. (Tips 1 and 2.) 2 5 6 11 3 faint breeze Leaves rustle. Insect net is partially inflated and slopes about 30° towards the ground. (Tip 3.) (2.) 3 10 12 19 5 gentle breeze Leaves are in motion. Most of inflated insect net is roughly parallel to the ground. 4 15 17 28 8 moderate breeze Small branches on trees move. Insect net stands fully parallel to ground. Handle of net pulls noticeably in your grip. 5 20 23 37 10 fresh breeze Small trees sway. (Tip 4.) 6 25 29 46 13 strong breeze Large branches sway. (3.) 7 30 35 56 15 moderate gale Whole trees in motion. (Tip 5.) 8 35 40 65 18 fresh gale Twigs break off trees. (Tip 6.) (4.)

 To convert: into meters per second (m/s) multiply by: knots (kts) 0.51 miles per hour (mph) 0.45 kilometers per hour (km/hr) 0.28

### Notes

1. The normal wind condition for morning and near sunset. As thermal activity dies out at end of day, momentum from the winds in upper layers of the air column are no longer transferred by convective mixing to air near the ground. Friction then causes the air near the ground to slow to a stop (calm conditions). The sky is usually clear because lack of thermal activity means that no moisture is being carried aloft to the dew level. As heat is loss to the dark night sky, the calm layer cools and increases in depth and may reach 300 m by morning. This layer of cool, still, air is called the nocturnal inversion because it is cooler than the air aloft. In the morning, as the sun heat the ground (and the ground heats the air immediately above it), the inversion becomes increasingly unstable, and finally breaks up about mid morning. The break-up is signalled by thermal activity and the the return of the wind to the ground. Thermals that are generated in the morning (usually small in diameter) or late in the day (large in diameter) are often so weak that they can be exploited only by animals with low sink rates when gliding, such as monarch butterflies and vultures.
Calm, and near calm, conditions also occur when the center of high pressure systems pass through. These periods of calm can last days if the High is stationary. Winds are described as light and variable, reflecting thermal activity.

2. If rate of cooling with increase in altitude (lapse rate) is high, there will be abundant thermal activity. If relative humidity is low to moderate, there will also be good viewing conditions for watching butterflies at altitude. If relative humidity is high, the resulting hazy sky and strong background scattering of sunlight can conceal butterflies aloft, giving the impression that no migration is occurring. Monarchs flying in light haze conditions within 200 m of the ground can usually be seen through binocular. If the haze is heavier, the viewing ceiling drops to about 100 m.

3. Thermals either are broken up into strong gusts and useless for soaring, or organised into narrow, powerful, and highly turbulent systems. Birds soaring in these systems are visibly tossed about.

4. Convection is unlikely to organise into thermals unless thunderstorms erupt in the afternoon. Lift is broken up into sharp gusts.

### Tips

1. Monarch butterflies may be seen flying up to the warmer air above the level of the nocturnal inversion (see note 1. ) in the morning. As the nocturnal inversion becomes unstable, sometimes the wind aloft can apparently cause the top layer to form into a series of linear (standing?) rolls. Roll clouds may also be present. The rolls are oriented perpendicular, or nearly perpendicular to the wind. If the alignment of the rolls is approximately in the preferred direction, monarch butterflies may be seen soaring in straight lines at 100 to 200 m in the bands of lift on one side of the rolls. Wayne Wolf, John Westbrook and I were fortunate enough to observe this behaviour visually and with radar several times during our 1988 field trip to Rasaca, Georgia, particularly during the morning of September 28. Four years earlier, while flying an ultralight aircraft in Del Rio, Texas, I had blundered into the same phenomenon at about 60 m without recognising what it was.

2. Monarch butterflies will sometimes soar at 30 m or more above the ground in very light headwind conditions.

3. A faint breeze of 3 m/s is at least 60% of the cruising flight speed (approximately 5 m/s) of monarch butterflies. When they encounter significant headwinds, monarch butterflies either stop migrating and forage among flowers, or avoid the full force of the headwind by flying in the slower moving air close to the ground. Migrants can be observed to make use of wind shadow effects on the lee side of hills, buildings, trees, bushes, etc..

4. Escape flight speed of monarch butterflies. Upwind flight impossible except by avoiding its full force by staying close to ground.

5. Few butterflies migrate when winds reach 30 kts. If you don't see any activity, it is probably time to quit.

6. It is definitely time to quit. If the wind gets any stronger you are in real trouble. A force 9 wind breaks branches and a force 10 wind blows down trees. When you hold your insect net aloft, it's ripped from your hands. Find shelter immediately