Monarchs come in on cold fronts -  or "northers" as these cold fronts are called in Texas and 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. 

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. Monarchs 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, monarchs fly high above everyone and may remain unseen without the use of binoculars. When monarchs are not seen as expected, people may take this as evidence that the monarch population is down. Actually, what may be really happening is that the monarchs have over flown an area on favorable winds and have landed somewhere further to the south.

The most spectacular observations of monarch migration sightings are reported more often when strong winds blow in from the south. Winds with a strong southerly component cause 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. Sometimes they accumulate by the tens of thousands and dazzle those who are lucky enough to stumble across them.

(contributor: Dr. William Calvert)