2012-06-14

How to estimate wind speed and direction

Today is Flag Day in the United States of America. Many countries have these "Flag Day" public holidays, during which the adoption of the national flag or a specific event is recognized. Flags can be found, in some form, throughout history, but their usefulness extends beyond identification, patriotism, and decoration. How? Well, the answer, my friend, is blowin' in the wind.


Source: "Palace of the Four Winds" - Wikipedia

It's easy to become so accustomed to our environment that we miss the nuanced information we can collect from it. I previously presented how to predict the weather using observations of your environment in my Survival Guide Series. Today, I would like to detail how you can use similar awareness to discern wind speed and direction, another useful survival skill.

There have been a small collection of measurement systems for estimating wind speed, effects, and direction developed throughout history; one of which is the Beaufort wind force scale [Wikipedia]. Initially used to qualitatively evaluate wind effects, the Beaufort scale was eventually made into a more useful measure of wind conditions.

The Wikipedia article on the Beaufort scale has a wonderfully comprehensive table of the measures and effects of wind at each stage of the scale. I’ve chosen to select specific qualities and quantities from that table to ease memorization. For example, you’ll find that I list specific easy-to-remember speeds approximately within each Beaufort scale, rather than just throw a bunch of data at you. After all, my goal with this article is to make this information easy for you to use without technology or calculation. I want to empower you to look at the world around you and instantly discern the information you desire.

I’ve divided my groupings into three types of observations: flags, land, and sea. The effects described for the flag, are based on a common midsize flag (2:3 ratio, about 1 m ≈ 3 ft on its shortest side). For land, I made sure to include observations of the trees; do keep in mind that other factors can also affect tree movement, such as weather and fauna (e.g. rain may make leaves move, snow may hinder branch motion, squirrels may shake small branches).
CALM
Beaufort 0 - Less than 2 km/h (≈ 1.25 mi/hr)
Flag: Flag is at rest.
Land: Smoke rises gradually straight up, with very little drifting.
Sea: The water’s surface will appear smooth and reflections will be clearly recognizable.
LIGHT AIR
Beaufort 1 - 5 km/h (≈ 3 mi/hr)
Flag: Flag moves slightly in the wind, the bottom corner of the flag will hang.
Land: Smoke will drift with the wind.
Sea: The surface of the water will show some rippling.
LIGHT BREEZE
Beaufort 2 - 10 km/h (≈ 6 mi/hr)
Flag: Flag extends in the wind, the top corner may flap and curl.
Land: Tree leaves begin to rustle.
Sea: Small waves begin forming. No curling or cresting.
GENTLE BREEZE
Beaufort 3 - 20 km/h (≈ 12 mi/hr)
Flag: Waves move across the fabric when the flag is extended in the wind.
Land: Leaves and small twigs will be constantly moving.
Sea: “Wavelets” form, crests begin to form and break.
MODERATE BREEZE
Beaufort 4 - 30 km/h (≈ 20 mi/hr)
Flag: Flag is in constant motion, flapping quickly in the wind.
Land: Small tree branches begin to move. Dust and debris will be carried by the wind.
Sea: Small waves (up to 1 m high) begin to make longer curls. Crests become more numerous and easily visible.
FRESH BREEZE
Beaufort 5 - 40 km/h (≈ 25 mi/hr)
Flag: Ripples move quickly across the fabric.
Land: Small trees with leaves will begin to sway.
Sea: Moderate waves (1-2 m high) form. Many crests are visible, some surface spray is carried by the wind.
STRONG BREEZE
Beaufort 6 - 50 km/h (≈ 30 mi/hr)
Flag: The flag will “snap” and “pop” as it waves, flaps, and ripples. Damage to the flag will begin to occur.
Land: Larger tree branches will begin moving.
Sea: Large waves (2-4 m high) form. More spray is carried in the wind.
NEAR GALE
Beaufort 7 - 60 km/h (≈ 35 mi/hr)
Flag: Flag will begin to tear, possibly detaching from its hoist.
Land: Large trees begin to move and sway. Most people will feel some resistance when walking against the wind.
Sea: The surface of the water begins to heave. Sea heaps up, waves about 5 meters in height will form. Crests will spread down the waves from where the wave breaks.
GALE
Beaufort 8 - 75 km/h (≈ 45 mi/hr)
Flag: What flag? If not already torn from its hoist, there will be little left of the flag to respond to the wind.
Land: Loose leaves and small twigs break free of tree branches. Most people will have difficulty standing and/or staying balanced in the wind.
Sea: Moderately high waves (5-7 m) form. Spray from crests drifts (aka “spindrift”) across the surface with the wind.
STRONG GALE
Beaufort 9 - 88 km/h (≈ 55 mi/hr)
Land: Large tree branches may break. Some damage to buildings.
Sea: High waves (7-10 m) form. The whole surface begins to roll. Foam streaks across the surface. Spray carried in the wind notably fills the air.
STORM
Beaufort 10 - 100 km/h (≈ 65 mi/hr)
Land: Tree trunks may break and/or whole trees will uproot. Significant damage to buildings. (Wind of this speed is a rare occurrence on land.)
Sea: Very high waves (10-12 m) with large crests. The surface is dense with foam and rolls deeply. Visibility is hindered more by spray in the air.
VIOLENT STORM
Beaufort 11 - 115 km/h (≈ 70 mi/hr)
Sea: Waves nearly 15 meters in height. The surface of the water is mostly foam.
HURRICANE
Beaufort 12 - Greater than 115 km/h (≈ 70 mi/hr)
Sea: The air is now filled with foam and spray. Waves are over 15 meters high and the surface of the water is completely white with foam and spray.
I know this list of observations seems long, but it can be fun to focus most on those qualities most commonly relevant to your daily life (Beaufort 2 - 8). In the days writing this article, I have found myself frequently looking out a window to observe, estimate, and check the wind speed.

It’s easy to miss the wonders of the world around us, if we don’t first make an effort to look for them. I hope that the information within these guides inspires your enjoyment of our planet and makes the external environment feel more personally relevant.
Post a Comment