The Beaufort Wind Force Scale is an extremely easy to use tool to assess weather conditions especially under field conditions. It is a well researched, scientifically based system that is endorsed by the National Weather Service for use by its Weather Spotters due to its extreme versatility under almost any conditions. Likewise many other agencies use it as well. I was trained in this by State of Maine for my Guide training, in multiple Search and Rescue classes, as well as in my US Forest Service Forest Fire Fighter training. US military uses it extensively. Likewise it is still used routinely in international shipping and aviation.
It is extremely useful due to its ease of operations even when your batteries died, your wifi is down &/or your anemometer is frozen up. So useful that U.S. Air Force Combat Air Controllers (CAC), still use it to call in Air Traffic Control for complex air support operations for multiple planes and multiple drops of paratroopers and cargo from behind enemy lines. (Sorry guys, but Marine motto of "First In" usually isn't true. CAC's jump solo behind enemy lines up to two weeks before a large operation to call in weather. They make the Go/No Go decision based off field observations from the ground)
Granted the Beaufort Scale is not as specific as what new fangled fancy tech can give you, but it is quite versatile and adaptable, and with field ops such as Search and Rescue (SAR) or Forest Fire Fighting rarely does a 2-3 MPH difference matter. Overall patterns and trends make a difference not an individual reading. In field conditions you can get constant "read outs" of conditions as the operation is ongoing just by maintain your normal situational awareness.
However as with ANY device it should be calibrated before use. You want to make sure the numbers you are calling in are accurate. In this case calibration means training your eyes to see and your brain to recall what those movements mean. You need to learn how to make the observations accurately. Then you can do wind speed any time anywhere under almost any conditions, even at night!
The best way to do this is pick some regular views with a variety of trees and start observing their movements. Yes tress actually move. A LOT! You just probably never had reason to take notice of it before.
There are set and predicable ways the tress move depending on the wind speed (i.e. pressure) that is applied to them. Of note different trees will give different readings. Soft woods vs Hard woods obviously have different flexibility and foliage so will respond to wind slightly differently. Thus you need to observe a variety of trees and get used to their responses. It is quite easy, just takes a little time and focused effort.
So, first things first, forget the scale and just look at the trees. Look at the smallest branches and twigs, can you see that they are actually swaying back and forth? If not keep watching the same spot for a few minutes, you'll be surprised at the choreography been shown to you!
Once you see the very tips move, move up the branch a bit and watch how that moves. It's actually quite different than how the tips do. This is due to a complex combination of biology and physics that we need not go into, just have fun and watch the dance. Next move back farther towards the trunk and see how the bigger branches move in a waltz compared to the line-dance of the branch versus the break-dancing of the twigs.
Once you see that trees move in predictable ways, then you can look at the chart below and start to correlate each dance step of the tree to the tempo of the music the wind is playing.
Last step of the calibration process is once you can consistently correlate your observations to the Beauford Scale then compare your Beauford Scale readings to your own high tech wind speed gauge and tweak your assessments to make sure they line up. Again tress dance differently. Hardwood trees are usually best to observe, but when you can correlate an evergreen's dance against a hardwood's dance now you are adding a layer of sophistication to can rival the high tech gadgets! And no internet required!