Sound operators often have wind problems when filming outside. Not that kind of wind — the sky kind of wind! When filming outside, you may pick up noise from the wind through the microphone, which can sometimes make your actors’ dialogue hard to hear. The noise is caused by the wind hitting the microphone.
If you’re not sure what this sounds like, try gently blowing on the microphone on your camera and listen through the headphones — it’s not a nice sound at all.
Wind noise can really only be detected by monitoring the sound using headphones during filming.
If you can hear wind noise when filming, then you need a windshield or a dead cat. (Not a real dead cat; it’s just another name for a windshield.) A windshield is a furry cover placed over the microphone to protect it from the wind. Windshields are also available for onboard microphones on video cameras.
Here’s an example of two types of windshield available for external microphones. (Now you can see why they’re sometimes called “dead cats.”)

This is what both microphones look like without the windshields.

If you still have wind noise when filming, try booming from underneath or forming a barrier between the wind and the microphone to shelter the microphone from wind noise.