In meteorology

Meteorology

Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

, Buys Ballot's law may be expressed as follows: In the Northern Hemisphere

Northern Hemisphere

The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

, if a person stands with his back to the wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

, the low pressure area will be on his left. This is because wind travels counterclockwise around low pressure zones in the Northern Hemisphere. It is approximately true in the higher latitudes of the Northern Hemisphere, and is reversed in the Southern Hemisphere

Southern Hemisphere

The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

, but the angle between the pressure gradient force

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 and wind is not a right angle in low latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

s.

History

As early as the 16th century extensive weather observations were included as part of a ship's log. These observations as well as other log information, were turned over to national hydrographic institutes in various nations, most notably Germany and England and later the US. The information from many ships about individual voyages was compiled ashore and later became what today is still published by England, a 3 volume set complete with charts titled "Sailing Directions for the World". Additionally the US Defense Mapping Agency publishes a 47 volume set Sailing Directions

Sailing Directions

Sailing Directions is a 42-volume American navigation publication published by the National Geospatial-Intelligence Agency . Sailing Directions consists of 37 Enroute volumes, 4 Planning Guide volumes, and 1 volume combining both types...

which serves much the same purpose. The information is the distillate of empirical observations of thousands of ships masters over thousands of voyages spanning several hundred years.

Buys Ballot's law, which was first deduced by the American meteorologists J.H. Coffin and William Ferrel

William Ferrel

William Ferrel , an American meteorologist, developed theories which explained the mid-latitude atmospheric circulation cell in detail, and it is after him that the Ferrel cell is named. He was born in southern Pennsylvania. His family moved to what would become West Virginia in 1829...

, is a direct consequence of Ferrel's law. The law takes its name from C. H. D. Buys Ballot, a Dutch meteorologist, who published it in the Comptes Rendus, November 1857. While William Ferrel theorized this first in 1856, Buys Ballot was the first to provide an empirical validation

Empirical validation

An empirical validation of a hypothesis is required for it togain acceptance in the scientific community. Normally this validation is achieved by the scientific method of hypothesis commitment, experimental design, peer review, adversarial review, reproduction of results, conference presentation...

.

Buys Ballot's law first appeared in early versions (prior to 1900) of Bowditchs' American Practical Navigator and other publications written to assist in passage planning and the safe conduct of ships at sea and is still included today both in Bowditch and in Sailing Directions (see following reference) as an item of practical reference and information.

Uses

The law outlines general rules of conduct for masters of both sail and steam vessels, to assist them in steering the vessels away from the center and right front (in the Northern Hemisphere and left front in the Southern Hemisphere) quadrants of hurricanes or any other rotating disturbance at sea. Note that prior to radio, satellite observation and the ability to transmit timely weather information over long distances, the only method a ships master had at his disposal to forecast the weather was observation of meteorological conditions (visible cloud formations, wind direction and atmospheric pressure) at his location.

Included in the "Sailing Directions for the World" are Buys Ballot's Law techniques for avoiding the worst part of any rotating storm system at sea using only locally observable phenomena i.e. cloud formations, wind speed and barometric pressure tendencies over a number of hours. These observations and application of the principles of Buys Ballot's Law help to establish the probability of the existence of a storm and the best course to steer to try avoid the worst of it—with the best chance of survival.

From a slightly less esoteric standpoint the underlying principles of Buys Ballot's Law states that for anyone ashore in the Northern Hemisphere and in the path of a hurricane, the most dangerous place to be is in the right front quadrant of the storm. There, the observed wind speed of the storm is the sum of the speed of wind in the storm circulation plus the velocity of the storms forward movement. Buys Ballot's Law calls this the Dangerous Quadrant. Likewise, in the left front quadrant of the storm the observed wind is the difference between the storm's wind velocity and its forward speed. This is somewhat euphemistically called the Safe Quadrant due to the lower observed wind speeds.

To look at it another way, in the Northern Hemisphere if a person is to the right of where a hurricane or tropical storm makes landfall, that is considered the dangerous quadrant. If they are to the left of the point of landfall, that is the safe quadrant. In the dangerous quadrant an observer will experience higher wind speeds and generally a much higher storm surge

Storm surge

A storm surge is an offshore rise of water associated with a low pressure weather system, typically tropical cyclones and strong extratropical cyclones. Storm surges are caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea...

due to the wind direction (onshore). In the Safe quadrant, the observer will experience somewhat lower wind speeds and the possibility of lower than normal water levels due to the direction of the wind being offshore.

These are very general rules that are subject to many other factors, e.g. shapes of the coastline, and topography in any location. Although the principles here to a very limited extent apply to a coastal observer during the approach and passage of a storm in any location, Buys Ballot's Law was primarily formulated from empirical data to assist ships at sea.

External links

• M. Buys-Ballot, "Note sur le rapport de l'intensité et de la direction du vent avec les écarts simultanés du baromètre", Académie des sciences (France). Comptes rendus hebdomadaires, TOME XLV, JUILLET - DÉCEMBRE (1857) pp. 765–768.