There might be one sign spring is around the corner. Birds are taking flight, and that flight is northward.

The National Weather Service radar at Key West, FL showed a lot of returns overnight. Most of the items showing up on the radar weren’t raindrops. There was a large mass of birds migrating from south to north across the Florida Keys last night.

📡 Key West radar has had a busy night, but not because of weather! The most impressive display of migratory birds so far this year occurred overnight. This product shows biological targets in green/yellow flying north over the Keys. Showers/rain are depicted in darker blues. 🐦 pic.twitter.com/V2PJfucxJA — NWS Key West (@NWSKeyWest) February 17, 2020

Click on the arrow above and watch millions of birds fly across the Keys.

The green and yellow areas are birds, while the spotty blue areas after the birds fly by are rain showers.

The birds took advantage of a south-southeast wind to have an easier flight northward. So the birds were flying north on a warm wind.

But why don’t we see this all the time. First off, the atmosphere has to be set-up just right for the birds to be picked up on radar. There has to be what we meteorologists call a “temperature inversion.” A temperature inversion is where temperatures do the opposite of a normal atmospheric set-up. Normally as you go higher in elevation, the temperature gets colder. Occasionally the opposite occurs with temperatures warming as you go higher in the sky. The typical atmospheric temperature profile would have the warmest temperature at the ground and colder at 5,000 to 10,000 feet up.

During a temperature inversion, the radar beam gets trapped in the inverted temperature layer, usually in the lower 10,000 feet of the atmosphere. A radar beam normally goes higher and higher in the sky as the beam gets further from the radar. In this inversion the radar beam stayed in the lower part of the atmosphere and picked up the birds in flight.

Also, with that south wind it’s no surprise Key West, FL tied a record high today at 84 degrees.

The high temperature at Key West International Airport today reached 84°F, tying the daily high temperature record! This record was previously set back in 1927. February temperature records at Key West date back to 1873.#flwx #FloridaKeys #FLKeys #KeyWest #Climate #RecordHeat🌡️ pic.twitter.com/vD1VGkm4oo — NWS Key West (@NWSKeyWest) February 17, 2020

Here’s an animation from Cornell Lab of Ornithology showing the seasonal migration of 118 different types of birds.