Fri., Dec. 3 notes

The rainbow 1S1P reports have all been graded. All the remaining 1S1P reports will be graded by next Monday.

Today's picture of the day is actually a graph. The graph shows the dramatic changes in the daytime high temperatures over the past week. The high last Monday was only 49 F. The high yesterday (Thursday) was 81 F. The dew point is also very low at the present time, the air outside is very dry.

The words of the day are stamina and endurance
(we have a ~20 minute video coming up near the end of the class period)

Some kind of meteorological process that produces low level convergence is needed to initiate a hurricane. One possibility, and the one that fuels most of the strong N. Atlantic hurricanes, is an "easterly wave." This is just a "wiggle" in the wind flow pattern. Easterly waves often form over Africa or just off the African coast and then travel toward the west across the N. Atlantic. Winds converge as they approach the wave and then diverge once they are past it . The convergence will cause air to rise and thunderstorms to begin to develop.

In an average year, in the N. Atlantic, there will be 10 named storms (tropical storms or hurricanes) that develop during hurricane season. 2005 was, if you remember, a very unusual year. There were 28 named storms in the N. Atlantic in 2005. That beat the previous record of 21 names storms that had been set in 1933. Of the 28 named storms, 15 developed into hurricanes.

In some ways winds blowing through an easterly wave resembles traffic on a multi-lane highway. Traffic will back up as it approaches a section of the highway with a closed lane. Once through the "bottleneck" traffic will begin to flow more freely.

Another process that causes surface winds to converge is a "lee side low."

This figure tries to explain how a cluster of thunderstorms can organize and intensify into a hurricane.

1. Converging surface winds pick up heat and moisture from the ocean. These are the two mains sources of energy for the hurricane.

2.   Rising air expands, cools, and thunderstorm clouds form. The release of latent heat during condensation warms the atmosphere. The core of a hurricane is warmer than the air around it.

3.   Pressure decreases more slowly with increasing altitude in the warm core of the hurricane. The result is that pressure at the top center of the hurricane is higher than the pressure at the top edges of the hurricane (pressure at the top center is still lower than the pressure at the bottom center of the hurricane). Upper levels winds diverge and spiral outward from the top center of the hurricane (you can sometimes see this on satellite photographs of hurricanes).

4.   The upper level divergence will cause the surface pressure at the center of the hurricane to decrease. The speed of the converging surface winds increases and the storm intensifies. The converging winds pick up additional heat and moisture which warms the core of the hurricane even more. The upper level high pressure and the upper level divergence increase. The increased divergence lowers the surface pressure even more.

Here's another view of hurricane development and intensification

In the figure at left the upper level divergence is stronger than the surface convergence. Divergence is removing more air than is being added by surface convergence. The surface low pressure will decrease. The decrease in surface pressure will cause the converging surface winds to blow faster.

In the middle picture, the surface low pressure is lower, the surface convergence is stronger. The upper level divergence has also been strengthened a little bit. The upper level divergence is still stronger than the surface convergence so the surface so the surface low pressure will decrease even more.

In the right figure the surface low pressure has decreased enough that the surface convergence now balances the upper level divergence. The storm won't strengthen any more.

Generally speaking the lower the surface pressure at the center of a hurricane the stronger the storm and the faster the surface winds will blow. The following figure (not shown in class) shows this

This figure tries to show the relationship between surface pressure and surface wind speed. The world record low sea level pressure reading, 870 mb, was set by Typooon Tip off the SE Asia coast in 1979. Sustained winds in that storm were 190 MPH. Three 2005 Atlantic hurricanes: Wilma, Rita, and Katrina had pressures in the 880 mb to 900 mb range and winds ranging from 170 to 190 MPH.

The stages of storm development that lead up to a hurricane are shown at the bottom of p. 143a in the photocopied ClassNotes.

A tropical disturbance is just a localized cluster of thunderstorms that a meterologist might see on a satellite photograph. But this would merit observation because of the potential for further development. Signs of rotation would be evidence of organization and the developing storm would be called a tropical depression.

In order to be called a tropical storm the storm must organize a little more, and winds must increase to 35 knots. The storm receives a name at this point. Finally when winds exceed 75 MPH (easier to remember than 65 knots or 74 MPH) the storm becomes a hurricane.

A crossectional view of a mature hurricane (top) and a picture like you might see on a satellite photograph (below).

Sinking air in the very center of a hurricane produces the clear skies of the eye, a hurricane's most distinctive feature. The eye is typically a few 10s of miles across, though it may only be a few miles across in the strongest hurricanes. Generally speaking the smaller the eye, the stronger the storm.

A ring of strong thunderstorms, the eye wall, surrounds the eye. This is where the hurricane's strongest winds are found.

Additional concentric rings of thunderstorms are found as you move outward from the center of the hurricane. These are called rain bands. These usually aren't visible until you get to the outer edge of the hurricane because they are covered by high altitude layer clouds.

That was all the new material we had time to cover in class because a 20 minute segment from a NOVA program (PBS network) on hurricanes was shown. A film crew was on board a NOAA reconnaissance plane as it flew into the narrow eye of hurricane GILBERT. Gilbert set the record low sea level pressure reading for the Atlantic ocean (888 mb). That record stood until the 2005 hurricane season when WILMA set a new record of 882 mb. The world record low sea level pressure, 870 mb, was set in a SE Asian typhoon in 1979.