Counterclockwise hurricane forming in Atlantic

Counterclockwise hurricane forming in Atlantic

The Hurricane Atlantic Satellite image shows clouds by their temperature over the Atlantic Ocean. Red and blue areas indicate cold (high) cloud tops.

Atlantic VIS Loop

Coriolis effect

In physics, the Coriolis effect is an apparent deflection of moving objects when they are viewed from a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with anti-clockwise rotation, the deflection is to the right. The mathematical expression for the Coriolis force appeared in an 1835 paper by a French scientist Gaspard-Gustave Coriolis in connection with the theory of water wheels, and also in the tidal equations of Pierre-Simon Laplace in 1778. Early in the 20th century, the term Coriolis force began to be used in connection with meteorology.

The Coriolis effect is caused by the rotation of the Earth and the inertia of the mass experiencing the effect. Newton's laws of motion govern the motion of an object in a (non accelerating) inertial frame of reference. When Newton's laws are transformed to a rotating frame of reference, the Coriolis and centrifugal forces appear. Both forces are proportional to the mass of the object. The Coriolis force is proportional to the rotation rate and the centrifugal force is proportional to its square. The Coriolis force acts in a direction perpendicular to the rotation axis and to the velocity of the body in the rotating frame and is proportional to the object's speed in the rotating frame. The centrifugal force acts outwards in the radial direction and is proportional to the distance of the body from the axis of the rotating frame. These additional forces are termed either inertial forces, fictitious forces or pseudo forces. They allow the application of simple Newtonian laws to a rotating system. They are correction factors that do not exist in a true non-accelerating "inertial" system.

Perhaps the most commonly encountered rotating reference frame is the Earth. Because the Earth completes only one rotation per day, the Coriolis force is quite small, and its effects generally become noticeable only for motions occurring over large distances and long periods of time, such as large-scale movement of air in the atmosphere or water in the ocean. Such motions are constrained by the 2-dimensional surface of the earth, so only the horizontal component of the Coriolis force is generally important. This force causes moving objects on the surface of the Earth to appear to veer to the right in the northern hemisphere, and to the left in the southern. Rather than flowing directly from areas of high pressure to low pressure, as they would on a non-rotating planet, winds and currents tend to flow to the right of this direction north of the equator, and to the left of this direction south of it. This effect is responsible for the rotation of large cyclones. (...)

Polar Cyclone

Cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth.It is usually characterized by inward spiraling winds that rotate counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale cyclonic circulations are centered on areas of low atmospheric pressure. The largest low-pressure systems arecold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale. Subtropical cyclones are of intermediate size.Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the Tropical Upper Tropospheric Trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on other planets outside of the Earth, such as Mars and Neptune.

polarsub-polar, or Arctic cyclone (also known as a polar vortex)is a vast area of low pressure which strengthens in the winter and weakens in the summer.A polar cyclone is a low pressure weather system, usually spanning 1,000 kilometres (620 mi) to 2,000 kilometres (1,200 mi), in which the air circulates in a counterclockwise direction in the northern hemisphere, and a clockwise direction in the southern hemisphere. In the Northern Hemisphere, the polar cyclone has two centers on average. One center lies near Baffin Island and the other over northeast Siberia. In the southern hemisphere, it tends to be located near the edge of the Ross ice shelf near 160 west longitude.When the polar vortex is strong, westerly flow descends to the Earth's surface. When the polar cyclone is weak, significant cold outbreaks occur. (...)


Why hurricanes spin counterclockwise

The air masses that make up a hurricane move towards the low-pressure area, pushed by surrounding higher- pressure air.  But, because Earth's surface spins at different speeds (faster at the equator, slower near the poles), air doesn't move in a straight line from high to low pressure. For example, at about 39 degrees of latitude north and south (about the latitude of Denver in the USA and Hastings in New Zealand), Earth's surface moves about 810 mph (1300 k/h) from west to east.  At the equator it moves about  30 percent faster at about 1040 mph (1670 k/h).

Suppose the hurricane's low-pressure area is off the coast of Florida at the spot marked 'L' in the diagram.  Higher-pressure area from the north, west, east and south will move towards the low pressure L as depicted by the white arrows.  But the air to the north moves eastward slower than the low-pressure area; moreover, the air to the south moves faster than L.  The different speeds cause the air to circulate counter clockwise about the low.  The diagram illustrates the action:
  • The north air mass (north arrow) flows toward L but it moves eastward slower than L. So the north air mass will lag behind (west) the faster-moving low.
  • Likewise the south air mass moves eastward faster than L so it will move ahead of L.
  • The resulting air mass movement is counterclockwise about L.

It is easy to see how different rotational speeds at different latitudes cause a counterclockwise rotation, when we examine the speeds themselves. The diagram shows these two most northerly and most southerly air masses, and some points in between.  I subtracted the speed of the low (908 mph) from each of the W to E speeds, to give the speeds relative to the low.  The winds north of the low blow from east to west, as indicated by the little red arrows.  The winds south of the low blow from west to east, relative to the low.  The result is counterclockwise motion. Thus the air masses circulate counterclockwise in the Northern Hemisphere — because of the direction of Earth's spin. (WeatherQuesting)


Areas of low pressure develop off the East Coast during late fall, winter and early spring.They usually make life hard for forecasters. The storms are called "nor'easters" because they usually bring strong northeast winds over the East as they move north along the Atlantic Coast. Nor'easters often bring heavy rain, heavy snow and severe coastal flooding to the East. As a storm rapidly intensifies, winds blow warm air inland from over the relatively warm Atlantic Ocean water. At the same time, cold air moves south over the East Coast. The combination of warm and cold air can produce snow, sleet, freezing rain and ordinary rain.

The exact track of the storm's center determines the dividing line between rain and snow. If the storm moves over the coast or inland just east of the Appalachian Mountains, it will usually push enough warm air inland to bring rain to the coastal plain, with snow confined to the mountains and points west of the mountains. If the storm moves further east over the Atlantic Ocean, heavy snow can fall along the coastal plain.

Historically, nor'easters that move with an easterly track have brought the East Coast its heaviest snowfalls. If these storms are unusually intense and develop quickly they are known as "bomb cyclones." (USA Today)

Understanding Nor'easters


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