Tropical Cyclone Guide

Tropical Depression
0-38 mph
0-33 knots
0-61 km/h
How tropical cyclones start and end.

Tropical Storm
39-73 mph
34-63 knots
63-118 km/h
Examples: Claudette (1979), Alberto (1994), and Allison (2001)

Category 1
74-95 mph
64-82 knots
119-153 km/h
Examples: Jerry (1989), Claudette (2003), and Humberto (2007)

Category 2
96-110 mph
83-95 knots
154-177 km/h
Examples: Erin (1995), Alex (2010), and Ike (2008 at landfall)

Category 3
111-129 mph
96-112 knots
178-208 km/h
Examples: Alicia (1983), Elena (1985), Luis (1995), Jeanne (2004), and Katrina (2005 at landfall)

Category 4
130-155 mph
113-135 knots
209-250 km/h
Examples: Opal (1995), Georges (1998), Bret (1999), and Ike (2008)

Category 5
+156 mph
+136 knots
+251 km/h
Examples: Camille (1969), Gilbert (1988), Mitch (1998), Katrina (2005), and Wilma (2005)

Central Pressure
Is central pressure of the storm related to wind speed? Not so. It is based on the ambient pressure and pressure gradient of the storm. The tighter the gradient, the strong the winds. Examples are Charley and Ike. Both had nearly identical central pressure (Charley-941 millibars and Ike-944 millibars). Charley had winds of 150 mph (130 knots/241 kmh), while Ike had 100 mph winds. Why is this? Charley was a small hurricane with hurricane force winds extending up to 30 miles (26 nautical miles/48 kilometers) from the eye. Ike had hurricane force winds extending up to 125 miles (109 nautical miles) from the eye.

Storm Surge
As mentioned, it is the storm size that produces storm surge. Also, geography plays a role. The larger the area of shallower water, higher the storm surge is. Also, if that area is a shape of a funnel, there will be higher storm surge. That is why Bangladesh and Louisiana and Mississippi are vulnerable to storm surge. Coastal areas with deeper water have less storm surge, but higher waves, like South Florida. If Hurricane Wilma at her peak hit the same area where Katrina hit, the storm surge would be lower due to its size. If Hurricane Ike was to hit that same area, the storm surge would be higher, likely higher than Katrina because Ike was larger than Katrina.

Maximum Sustained Winds
When you hear maximum sustained winds, like Hurricane Gilbert has maximum sustained winds of 185 mph. That means that is the strongest wind that the hurricane is producing based on a 1 minute average. Most areas use a 10 minute average. The maximum sustained winds in a storm is in a small area, usually on the “dirty” side of the storm, whether it is the northeast quadrant (northern hemisphere) or southwest quadrant (southern hemisphere).

When a hurricane makes landfall, friction weakens the wind by 15 percent. Here is how calculate sustained wind on land.
wind speed in mph – (wind speed in mph * 0.15) = wind speed on land.
Example: 100 – (100 * 0.15) = 85 mph

0.15 is a rule of thumb.

However, friction causes turbulence, which means there will be stronger gusts on land. Here is how to calculate gusts.
wind speed in mph * 1.5 = gust
Example: 100 * 1.5 = 150 mph

1.5 is a rule of thumb.

Rainfall is largely based on how fast a tropical cyclone move.

Rule of thumb
The slower it moves, the more rain it will dump.
The faster it moves, the less rain it will dump.

One reason why Allison dumped 40 inches (101.6 centimeters) of rain on Texas. It lingered for a time. Also, same reason why Claudette dumped 45 inches (114.3 centimeters) of rain near Alvin. Also, it depends on the terrain as well. Mountainous terrain draw more moisture, which leads to heavier rain. Hurricane Mitch in 1998 ravaged Central America because it was a mountainous area and they received the heaviest rain, up to 100 inches.

Here is a rough formula to determine how much rain a tropical cyclone will dump.
100 / Forward Speed of Tropical Cyclone = Rainfall Amount
Example: 100 / 10 = 10 inches of rain

It is a rough estimate and as you can see some areas will likely get more than others. Also, a large hurricane, even a fast moving one will dump heavy rain, like Hurricane Floyd in 1999. In addition, tropical cyclones that interact with frontal boundary or troughs will dump more rain despite their speed, like the case with Floyd and Opal.

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