Monday, November 12, 2012

Why Hurricanes Don't Hit Costa Rica

All Hurricanes tracks 1958 - 2011,
note Costa Rica is not affected
We get asked all the time, does Costa Rica get hurricanes? Well, yes and no, meaning “yes” they do receive the tropical aftermath of one, like heavy rains  of what Tropical Storm Tomas did in November 2010 that caused severe damage and deaths.

In meteorology; a tropical cyclone (or tropical disturbance, tropical depression, tropical storm, typhoon, or hurricane, depending on strength and location) is a type of low-pressure system, which generally forms in the tropics. Hurricane is the term used to describe tropical cyclones that form in the Caribbean where we are located.
Hurricanes need the warm humid air above tropical oceans in order to develop. That’s why they form over ocean waters close to the equator. In addition, that’s why they form only during the summer and early fall, when those waters are about 80 degrees Fahrenheit or above. However, you won’t normally see hurricanes form right at the equator. That’s because at zero degrees latitude there isn’t enough turning of winds in the atmosphere to give tropical cyclones the “spin” they need to get started.
This turning of the winds is known as the Coriolis Force or Effect. Nearly all hurricanes form within 30 degrees of the equator and 87% form within 20 degrees of it.
Tropical Storm Tomas
Hurricane Tomas in November 2010 was downgraded to a Tropical Storm when it hit Costa Rica, but still caused a lot of damages including over 20 killed in a landslide.
However, because the Coriolis effect initiates and maintains tropical hurricane rotation, such hurricanes almost never form or move within about 10 degrees of the equator where the Coriolis effect is weakest. The Coriolis Effect initiates and helps maintain the rotation of a tropical hurricane. This rotational force is zero at the equator and increases as you travel away from the equator, being greatest at the poles.
Hurricanes can’t actually form within 4 degrees of the equator, because the Coriolis effect is just too small. Once a tropical revolving storm is formed though, wind determines its movement. There is very little cross-equatorial flow of wind, as the main winds steer the storm away from the equator.
In addition, hurricanes that form in the Caribbean are not likely to turn toward Costa Rica. Caribbean tropical storms either turn northward or continue westward due to the steering currents of trade winds from the east, then a clockwise flow around a semi-permanent area of high pressure to the north. This has a tendency to turn them northward away from Central America.
Costa Rica is located at 9.55 degrees north of the equator, below the path of most hurricanes. There are seven tropical cyclone zones “basins” where storms occur on a regular basis and Costa Rica is not located in the affected areas.
Above NASA map shows the tracks of all Atlantic hurricanes which formed between
1851 and 2005, so in reality Costa Rica never receives the blunt force,
but they sure can receive the effects when it is downgraded to a major Tropical Storm.
Here are some terms and definitions that relate to hurricanes:
Coriolis Force: 
An artifact of the earth’s rotation. Once air has been set in motion by the pressure gradient force, it undergoes an apparent deflection from its path, as seen by an observer on the earth. This apparent deflection is called the “Coriolis force” and is a result of the earth’s rotation. The Coriolis effect initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds. That force is the heat of condensation. In the northern hemisphere, the earth’s rotation is deflected to the right by the Coriolis force. The amount of deflection the air makes is directly related to both the speed at which the air is moving and its latitude. Therefore, slowly blowing winds will be deflected only a small amount, while stronger winds will be deflected more.
Pressure Gradient Force:
Directed from high to low pressure. The change in pressure measured across a given distance is called a “pressure gradient.” The pressure gradient results in a net force that is directed from high to low pressure and this force is called the “pressure gradient force.”
Geostrophic Wind: 
Winds balanced by the Coriolis and Pressure Gradient forces. An air parcel initially at rest will move from high pressure to low pressure because of the pressure gradient force (PGF). However, as that air parcel begins to move, it is deflected by the Coriolis force to the right in the northern hemisphere (to the left on the southern hemisphere). As the wind gains speed, the deflection increases until the Coriolis force equals the pressure gradient force. At this point, the wind will be blowing parallel to the isobars. When this happens, the wind is referred to as geotrophic.

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