Wind is the variable nobody talks about

Most flying-ant advice starts with warmth. That makes sense: ants are insects, flight muscles need heat, and the biggest British flights tend to arrive on warm summer days. Then the advice wanders off into pressure, rain or a vague promise about mugginess.

Wind is usually left standing outside the conversation.

The largest British dataset says it belongs near the front. Adam Hart and colleagues analysed 13,394 usable reports from the Royal Society of Biology's Flying Ant Survey, gathered across the UK from 2012 to 2014. Winged ants were reported only when mean temperature was above 13 °C (55 °F) and wind speed was below 6.3 m/s (14 mph). As wind rose, the share of days with a report fell.

Temperature was the stronger predictor in the authors' model. Wind still had a clear negative association with observations, while air pressure did not. So the useful question on a warm afternoon isn't only “how hot is it?” It is also “how still is it?”

The 6.3 m/s (14 mph) figure is memorable. It is also easy to misuse.

It came from daily means at weather stations, paired with public sightings. It wasn't measured above each nest at the moment the ants emerged. Of the 436 specimens that participants mailed back for identification in 2013, 88.5 per cent were Lasius niger, the black garden ant. The result therefore tells us a great deal about British garden-ant flights and much less about a desert harvester ant or an Australian bull ant.

The authors called 13 °C (55 °F) and 6.3 m/s (14 mph) estimated thresholds. Those are edges observed in this dataset, not switches inside an ant. A sheltered nest beside a warm wall experiences a different pocket of air from the nearest weather station. The paper itself points to shelter and reduced local wind as possible reasons that some urban nests fly earlier.

That local difference is why standing in the garden can beat staring at one number. A forecast station may say 12 mph while the nest entrance behind a fence is almost still. Or the station may look calm while gusts keep combing the top of the lawn.

Hart's team tested the day's weather and its direction of travel. Reports were more likely when the temperature had risen and the wind had fallen since the previous day. The association with falling wind was statistically significant in their model.

That doesn't mean ants read a 24-hour trend line. The study could not show how a colony detects the change, and its weather came from stations rather than nest sensors. It does say that today's 10 mph isn't necessarily equivalent to yesterday's 10 mph. A settling spell carried more reports than the same conditions reached while the weather was worsening.

There is a sensible biological reason for calm air. The paper's authors note that mating in flight becomes difficult or impossible in stronger wind. A queen also has to land, shed her wings and find a nest site after mating. For a small animal making one consequential flight, uncontrolled drift is expensive.

The survey's 14 mph limit cannot tell you whether a particular nest will fly.

A hot day with brisk, persistent wind is missing one of the conditions linked to British Lasius niger flights. A warm day that grows calmer toward evening deserves more attention, especially around sheltered pavements, walls and gardens. The exact flight still depends on the colony, the species and the air at nest height.

Dealate includes wind in its flight-suitability score alongside temperature, recent rain, season and time of day. The score is uncalibrated, so it is a reason to look up, not a guarantee. The value of wind is simpler than that: it stops a warm day from masquerading as a perfect one.

See the papers and reasoning behind the forecast, or read why ants don't read the barometer for the weather variable that failed the same survey.

Hart, A.G., Hesselberg, T., Nesbit, R. & Goodenough, A.E. (2018). The spatial distribution and environmental triggers of ant mating flights: using citizen-science data to reveal national patterns. Ecography 41(6): 877–888.