You can easily check wind speed via installed forecast app on your iPhone or Android phone, but if you want to know real-time wind speed, the following Vaavud wireless wind meter may be more suitable for you.
This is an innovative and practical wind meter designed to offer you an easy way to measure the real-time wind speed using your smartphone. The wind meter contains no electronics, but using two small magnets in the rotor your smartphone’s magnetic field sensor can detect when the wind meter rotate, and the rotations can be converted to wind speed by using algorithms normally used for sound processing, moreover, the custom designed detection app shows you the real-time wind speed info on the screen of your smartphone. Apart from that, the wind meter works with iPhone 4/4S, iPhone 5, Samsung Galaxy S3 and Galaxy S2.
At present the team of Vaavud wireless wind meter is raising fund at Kickstarter. Pledging £20 (approx $30 USD) will let you own the wind meter. If you’re interested, jump to Kickstarter official site for more details.