Monday, October 29, 2012

Sandy’s record low pressure for a Northeast hurricane.

I thought I would post a quick post on Sandy. This will cover air pressure and why the winds are stronger on the right side of a hurricane. But first I will talk about Sandy.   

How do you calculate a potential disaster?

 First, add a strong wintery cold front coming in from the West. Second, include a high-pressure ridge off the coast of Greenland is keeping the storm from the usual course away from the U.S. mainland. Third, Sandy is an extremely low-pressure storm giving it extra form and punch than would normally be found with a 90 mph storm. Finally, add a nearly full harvest moon to exacerbate tidal effects In low lying areas.        

Hurricane Sandy’s affects are being felt along the entire east coast and by everyone east of the Mississippi river. As expected once Sandy got back over the warm Gulf Stream that flows parallel to the U.S. East Coast last night and this morning , She exploded in size and power. On satellite she is looking increasingly big and scary. Her central pressure is 940 mb, this is something you would normally see in a Category 3 or 4 hurricane. She is a few miles from Cape May, NJ where she will make landfall. We will see tropical force winds and gust to hurricane force, especially for those over 1500 feet in elevation. We will be feeling the effects of Sandy for the next 30 to 48 hours.    

Why worry about Sandy’s pressure?    

The lowest atmospheric pressure ever recorded at sea level was an astounding 870 mb, That's around 140 mb below what is considered normal. This pressure was measured at the center of Typhoon Tip in the North Pacific in 1979. Why is that astounding? Who cares about air pressure measurements anyway?     To answer these questions, we must understand a couple of ideas. Every place on Earth has a measurable, atmospheric pressure. The pressure at that location is caused by the weight of the column of air above that location. Typically, meteorologists measure atmospheric pressure using barometer. The weight of the air above the Earth is equal to the weight that is shown on the barometer. This column's height can be precisely measured. Standard, sea-level atmospheric pressure is 1013 mb. This amount of air pressure on the barometer is 29 59 / 64 .inches high. Air pressure decreases as we rise above the Earth because there is less air above us the higher up we go. But atmospheric pressures can change at the surface of the Earth as well. They go up and down by small amounts and cause the "highs" and "lows" you hear about on weather reports.       Under normal conditions and over a normal period of time, the atmospheric pressure usually changes no more than 20 mb The mercury in the barometer usually rises or falls only a little bit in a period of 12 to 24 hours. But even small differences in atmospheric pressure cause the air to be pushed around. This air movement is called wind. In this way air pressure and wind are closely related. Wind moves out of areas of high air pressure and sweeps into areas of low air pressure. The greater the difference in air pressure from one place on Earth to another, the stronger the wind.    

How does a hurricane work?

  hurricanediagram The atmospheric pressure in the eyes of hurricanes is lower than in the surrounding atmosphere, so air spirals inward in the form of strong surface winds. If the center of low pressure is over warm ocean waters, the spiraling winds whip up waves and froth. This adds to the already high levels of water evaporation. The warm, wet air spirals in toward the center of the storm and then cools as it rises The water vapor the air is carrying condenses and forms clouds and rain (grey areas in the illustration). As the condensation evaporates, it again heats the rising air, causing the air to rise upward even faster. The expanding air rises to altitudes of 6 to 9 miles. Then, as the air cools, it flows outward over the top of the storm. This cool air flow outwards, lowers the weight of the air above the storm's center even more. These winds, drawn in from hot air spiral up the wall of the eye. These hot rising winds circulate at speeds of 30 mph. The strongest winds, with gusts of up to 225 mph are found beneath the eye wall, immediately outside the eye.

As long as the hurricane is over warm water, this cycle continues and tends to get stronger. The pressure in the storm's center gets lower and lower, and the winds blow faster and faster. The air in the eye of the hurricane is at low pressure, and is calm. As the eye passes over, the winds may drop altogether, and a small circle of clear sky may be visible overhead for a short period of time.  

Why are the strongest winds in a hurricane typically on the right side of the storm ?

The right side of the storm is defined with respect to the storm's motion: Take Sandy, she is moving to the west, the right side would be to the north of the storm; if the hurricane is moving to the north, the right side would be to the east of the storm. In general, the strongest winds in a hurricane are found on the right side of the storm, especially the forward right quadrant because the motion of the hurricane also contributes to its swirling winds. Hurricane sandy with a 90 mph while stationary however you have to add the forward motion of 28 mph in to the calculation. For Sandy the forward right quadrant could have Surface winds of 118 mph. Conversely, in the forward left quadrant of the storm,, the storm motion subtracts from the surface wind speed, which would be in Sandy's case 62 mph.

I will close on this, listen for any warnings given, and if you are told to evacuate, do so right away. Please stay safe!



  1. thanx for the explanation. I have a question. Why if the pressure is so strong does this particular hurricane not match up to wind speed expectations?

  2. The size of the storm has a lot to do with it. So that when the pressue dropped instead of having stronger winds...she went for larger size. Also, she had to fight dry air and shear on her way up the coast.


Thank you for taking the time to comment, I will answer as soon as I can.