Reading Weather Conditions

Before heading out on a planned survey, it is important to check the weather conditions for any signs of excess wind. However before looking at a forecast, it is important to understand the relationship between altitude and wind speed.

At ground level, wind is inside what is known as a ‘boundary layer’ meaning that friction with objects on the ground such as terrain, trees, etc. causes the wind speed to be lower on the ground than at altitude. Therefore, the further away from the ground, the less of an effect this friction will have, and the higher the wind speed. A visual example of this is shown.

Essentially this graph shows that depending on local conditions, the wind speed at your mapping altitude can be as much as double the wind speed at ground level, but is usually in the region of 20% higher if mapping at 100-120m.

Referencing Forecasts

When reading a forecast, it is important to realise that most apps like Windfinder or Ventusky are measuring the wind at a height of 10m or so, thus a margin must be included for drones to get a conservative approximation of the wind speed at your transition altitude, typically 25% on top of the maximum wind gust is sufficient.

A screenshot from Windfinder is shown. This forecast for February 24th shows a decreasing wind throughout the day, so assuming a 25% margin for the maximum wind gust, it is not until 1300 that the wind will be below the maximum 45km/h limit (35 x 1.25 = 44km/h).

Backup Anemometer Measurement

Even after verifying that the forecast is within Marlyn’s limits, it is important to note that these forecasts are approximate and are applied over large areas. Therefore, when the forecasted wind is near the maximum tolerance, Atmos always recommends taking a backup measurement in the field with an Anemometer like the one shown below.