Measuring The Wind
Wind is the movement of air, caused by the uneven heating of the Earth by the sun and Earth’s own rotation. It can range from light breezes to natural hazards, such as tornadoes and hurricanes. In most countries, official wind speed measurements at airports and other locations are made in knots, where 1 knot equals 1.15 mph. In the U.S., this wind speed is then converted to miles per hour (mph) for the general public. In countries that use the metric system, wind speed is reported to the public in either kilometers-per-hour (kph) or meters-per-second (m/s).
Wind speed normally increases with height and can be affected by factors such as the roughness of the ground and the presence of buildings, trees and other obstacles in the vicinity.
Let’s examine how to measure wind.
Windsock
A windsock, or wind cone, is a conical textile tube that resembles a giant sock. It alternates stripes of high visibility orange and white.
The windsock can be used as basic guide to wind direction and speed. They are typically used at airports to show the direction and strength of the wind to pilots, and at chemical plants where there is risk of gaseous leakage. They can also be sometime located alongside highways at windy locations. Windsocks are lit at night, either by floodlights on top surrounding it or with one mounted on a pole with lights shining inside it.
Wind direction is the direction in which the windsock is pointing. For example, a windsock pointing due north indicated a southerly wind. Wind speed is indicated by the windsock’s angle relative to the mounting pole. Therefore, in low winds, it droops; in high winds, it flies horizontally. A fully extended windsock suggests a wind speed of 15 knots (17 mph) or greater.
Anemometer
An anemometer is an instrument that measures wind speed and direction. It is the only exact, objective way of measuring wind, which also makes it the most used method of measuring wind. There are several different types of anemometers, including the cup, vane, hot-wire, Laser Doppler, ultrasonic and ping-pong ball anemometers.
- Cup anemometer: This type of anemometer has a spinning wheel. The stronger the wind blows, the faster the wheel rotates. The anemometer counts the number of rotations, which is then used to calculate wind speed.
- Vane anemometer: It may be described as a windmill or a propeller anemometer. Like the cup anemometer, the stronger the wind blows, the faster the windmill or propeller spins.
- Hot-wire anemometer: This type of anemometer uses a fine wire that is electrically heated to some temperature above the ambient. Air flowing past the wire then cools the wire. The stronger the winds, the faster the cooling occurs.
- Laser Doppler anemometer: This uses a beam of light from a laser to measure the speed of the air molecules around the anemometer. Laser Doppler can measure even the slightest changes in airflow, but it is very expensive.
- Ultrasonic anemometer: This anemometer uses ultrasonic waves to measure wind velocity. It has several “arms” on this anemometer and wind speed is measured based on the time of flight of sonic pulses between these “arms.”
- Ping-pong ball anemometers: This is constructed from a ping-pong ball attached to a string. When the wind blows, it moves the ball. Measuring the angle between the string-ball apparatus and the vertical gives an estimate of the wind speed.
The Beaufort Wind Force Scale
The Beaufort Wind Force Scale is a measure that relates wind speeds to observed conditions at sea or on land. It was devised in 1805 by Britain’s Admiral Sir Francis Beaufort. He developed the scale to help sailors estimate the winds via visual observations. The Beaufort scale is not an exact nor an objective scale as it is based on visual and subjective observation of the sea and land.
The scale starts with 0 and goes to a force of 12. It is as follows:
- Beaufort Number: 0; meaning Calm
- Beaufort Number: 1; meaning Light Air
- Beaufort Number: 2; meaning Light Breeze
- Beaufort Number: 3; meaning Gentle Breeze
- Beaufort Number: 4; meaning Moderate Breeze
- Beaufort Number: 5; meaning Fresh Breeze
- Beaufort Number: 6; meaning Strong Breeze
- Beaufort Number: 7; meaning High Wind, Moderate Gale or Near Gale
- Beaufort Number: 8; meaning Gale or Fresh Gale
- Beaufort Number: 9; meaning Strong/Severe Gale
- Beaufort Number: 10; meaning Storm or Whole Gale
- Beaufort Number: 11; meaning Violent Storm
- Beaufort Number: 12; meaning Hurricane-Force
The Enhanced Fujita Scale
The Enhanced Fujita Scale is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation. The Fujita scale was first introduced in 1971 by Ted Fujita, which was later updated in 1973 to account for account path length and width. On February 1, 2007, the Fujita scale was decommissioned, and the Enhanced Fujita Scales was introduced to the U.S. The new scale more accurately matches wind speeds to the severity of damage caused by the tornado.
The Enhanced Fujita Scale has six ratings as follows:
- Rating: EF0; 3-second wind gusts of 65 to 85 mph
- Rating: EF1; 3-second wind gusts of 86 to 110 mph
- Rating: EF2; 3-second wind gusts of 111 to 135 mph
- Rating: EF3; 3-second wind gusts of 136 to 165 mph
- Rating: EF4; 3-second wind gusts of 166 to 200 mph
- Rating: EF5; 3-second wind gusts over 200 mph
The Saffir-Simpson Scale
The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based only on a hurricane's maximum sustained wind speed. This scale does not account for other potentially deadly hazards such as storm surge, rainfall flooding, and tornadoes. It was developed by civil engineer Herbert Saffir and meteorologist Robert Simpson in 1971, with the scale being introduced to the general public in 1973.
The five ratings are as follows:
- Category 1: Sustained winds of 74 to 95 mph
- Category 2: Sustained winds of 96 to 110 mph
- Category 3: Sustained winds of 111 to 129 mph
- Category 4: Sustained winds of 130 to 156 mph
- Category 5: Sustained winds of 157 mph or higher
Source: NWS, National Geographic Society, Wikipedia
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Image: Windsock in a storm via Shutterstock
