Fall Migration
Cold Fronts and Winds

Contributed by Dr. William Calvert

It's easy to see why monarchs come in on cold fronts - "northers" they are called in Texas and probably other southern states. It's like catching a bus going your way - in this case the ride is even free!

Monarchs probably ride the layer of uplifted air associated with the advancing edge of these cold fronts. (For more help in visualizing of a cold front, see USA Today's cold front graphic.)

But that is not all the butterflies ride. They generally move with any wind that has a northerly component, and may still be seen traveling days after the front has past. They may also migrate in not-so-strong southerly winds! In these cases, they are often found flying low to the ground and are very noticeable.

The curious thing is that times with strong north winds are NOT the times that the migrating monarchs are most noticeable. In many cases they fly high above everyone and may even be beyond the vision of the naked eye. People used to the appearance of monarchs during a certain season often complain that the monarch population is way down during a particular year when what has really happened is that the monarchs have over flown them on favorable winds and have landed somewhere to the south.

It is counter intuitive, but the times when we get the most spectacular reports to the Texas Monarch Watch Hotline is when the winds are blowing fairly strong from the south. Winds with a strong southerly component cause the monarchs to behave differently. They spend this time looking for nectar to refuel. This means that they are searching and accumulating in low (riparian) areas near rivers and streams where most nectar is found in Texas during the fall. In these low areas they are out of the wind which might damage their wings and in the flowers where they can feed. Sometime they accumulated by the tens of thousands and dazzle those who are lucky enough to stumble across them.

North winds after cold frontCold Fronts and the Wind

1) Wind direction BEFORE
Winds blow from the south before a cold front arrives.

2) Wind direction AFTER
What happens after the front passes?

Put your mouse on the map and see!


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