A powermeter won't tell you anything about the wind, just the power you're putting into the bike. This is actually a benefit in a way. If you're doing power-based training, it doesn't matter how fast you go, just how much effort you give for the defined time interval. So on windy days, I'll do my work intervals into the wind. Psychologically it feels better to be pushing into the wind, and because you go slower (and therefore a shorter distance for a given time) it shortens the amount of road you need to complete the interval. It's changed my attitude towards windy days--now instead of ruining a ride, it feels like a good day to work.