Archive for August, 2008|Monthly archive page

Hanna Expected To Become A Hurricane

The forecast chart below was derived from WeatherUnderground and represents the 2 pm EDT forecast plot.  Currently tropical storm Hanna is expected to become a hurricane if it moves, as expected, over the warm waters of the Bahamas (fed by the warmth of the Florida Current and the Antilles Current).  The current between the Florida Peninsula and the Bahama platform is ofter referred to as the Gulf Stream but most physical oceanographers consider it to be the Florida Current.  It is implied that the Gulf Stream begins where the Florida Current merges with the Antilles Current.  Here is a link to Surface Currents In the Atlantic Ocean:

From the menu box near the top you can individually check out the Loop (mentioned in the previous posting), the Florida Current, the Antilles Current, and the Gulf Stream.

One of the most common misconceptions in physical oceanography is that the current off the eastern coast of Florida is the Gulf Stream.  If you are interested in common misconceptions in science that I have encountered through the years, here is a link to that topic (websites by Davis):

If this does not come up, click on the link at the right margin of this web-log on the Common Misconceptions In Science notation.




Sea surface temperatures can be important variables regarding the likelihood for strengthening or weakening of a tropical system.  Conditions aloft are generally more important.  A tendency toward divergence aloft is likely to increase the strength of a hurricane and a tendency toward convergence aloft is likely to decrease the strength of a hurricane.  Gustav appears to be one of those typical noteworthy storms being influenced significantly by the warm temperature (and higher evaporation rates) at the surface.  The chart below shows heat potential for yesterday (Saturday, August 30).  I added the labeling and I drew the current location (which is nothing more than a guess on my part).

Hurricanes are very unstable.  Vertical motion is imperative to the maintenance of the system.  Though tap water in a sauce pan is stable when you first put in on the stove top, heating it from below tends to make it unstable.  The extreme case of boiling (by heating from below) provides us an example of violent instability.  Clouds are the markers in our otherwise transparent air (I’m ignoring visible pollution here) and the cumuliform varieties show us some of the vertical action occurring – akin to a slow motion rendition of the water boiling in the saucepan.

Conversely, cooling the air (or cooling the water in the saucepan) from below tends toward stability.  No one, wanting to get water up to a boil would put the saucepan on top of a block of ice.

Let us all hope that when Gustav moves over the relatively cooler water that is becomes “less unstable” and weakens significantly.  THE PROBLEM FOR ME IS THIS:  The color coding in such charts is misleading for some because the blues leave one with the impression that the water is cold.  It is not “cold.”  The second chart in this posting gives you a clearer impression of the actual temperatures yesterday.

Hanna May Take A Turn 8-30-08, 11pm EDT Forecast

Left click the image for enlargement.

5-Day with cone of uncertainty

5-Day with cone of uncertainty

GUSTAV AT 11:45 PM EDT, 8-30-08

The image below is a color-enhanced satellite floater of Gustav after the eye completed its buzz saw trip over Western Cuba.  Heating from below coupled with high evaporation rates tend to intensity such storms.  The Gulf temperatures in the storm’s path are high.  It is my opinion that intensification will occur.



Gustav and Hannah On One Map – A Tropical Pair – 8-30-98

The graphic below is a splice of the WeatherUnderground plots showing the 5 pm Eastern Daylight Time 5-day forecast for Gustav and 3-day forecast for Hanna.  Remember that the forecast is not a line; please think in terms of the circles of uncertainty.

This splice is not perfect.  It lines up at the 25th parallel.



Eyewall of Katrina
Eyewall of Katrina

To view an animation of the GFDL model, I recommend going to the following page, . This page is provided by the Penn State University Department of Meteorology. Scroll down to the GFDL horizontal column. Notice that there are three menus on that column and a submit button.

1) Select the most recent (highest) of the system you want to observe from the menu on the left.

2) Make sure the field indicates “sea level pressure.”

3) Select “Animation” on the right side menu if it’s not already there.

4) Click the “submit” button.

5) After the graphics come up, click the forward button on the right to watch the GFDL forecast.

It should loop but if it doesn’t click FWD once again.


Easterly waves are starting to come off Africa as though a youngster is repeatedly firing a pea-shooter toward the west.  The full-disk infrared image below doesn’t even show it all because Gustav is not visible.  So we now have 4 tropical systems causing various degrees of concern over the water, and four that have not yet left Africa.  Busy periods like this are to be expected for this the middle third (August & September) of the official 6 month long hurricane season.

Tropical waves (or easterly waves) are also called tropical disturbances.  When an African easterly wave gets over the Atlantic it picks up more moisture.  The introduction of water vapor to such a system (through evaporation) has a tendency to lower the pressure.  All other things being equal (like temperature) the more water vapor in the air the less dense it is and therefore the less pressure it exerts. Once rotation near the surface becomes evident the system is then cyclonic and is called a tropical depressions. If the pressure gets low enough it may evolve into a tropical storm (sustained winds = 39 to 73 mph).  The next stage in the possible progression is the hurricane with sustained winds of 74 mph or more.

Here is another sobering thought:  WE ARE NOT YET HALF WAY THROUGH THE OFFICIAL HURRICANE SEASON and we won’t be until the end of Sunday, August 31!


Hanna’s significant forecast change

BELOW – This most recent (REMEMBER – THE CHARTS ARE TIME SENSITIVE) forecast track and cone of uncertainty for Hanna is significantly altered from previous ones.

Gustav’s forecast, on the other hand, has not changed much but one change, if it turns out to be correct, can make a significant difference for New Orleans:  As it now stands the models’ landfall clusters indicates a higher probability than before that the city will experience the right-hand leading quadrant of the storm.  That quadrant is typically the most lethal.  However, it is still a long way off and noteworthy changes can occur between now and then.

5-day Forecast Track

5-day Forecast Track

Cumulonimbus Anvil With An Elbow

The photo below is a scan of a 35mm slide.  I took this photo years ago with my old Argus C-3 camera looking eastward from Haulover Beach, in northern Dade County (now Miami-Dade County).  It looks like some interesting action was going on up there to make the anvil take that sharp turn as it flared out.


Relative humidity figures are deceptive unless one understands that warm air has the energy to possess more water vapor than does cold air.  For example, 77˚F. air at 50% relative humidity will contain more water vapor than 50˚F air at 100% relative humidity.  If you did not know that warm air can contain more water vapor than cold air, you would probably “logically” deduce that the 50° air at 100% relative humidity had the greater amount of moisture because, after all, 100% is greater than 50% – but you would be wrong.


It is a fact that the warmer the air, the greater can be the percentage of water vapor within the whole mixture of gases, (even though the water vapor always represents but a small fraction of the gaseous content of the air).

If the relative humidity of a kilogram of 77˚F. (25˚C) air is 100%, that air will contain 20.00 grams of water vapor.

If the relative humidity of a kilogram of 86˚F. (30˚C) air is 100% that air will contain 26.50 grams of water vapor.

Therefore, if the relative humidity of 77˚F. air is 50%, the air contains 10.00 grams of water vapor in each kilogram (half of 20.00 grams).

Comparatively, if the relative humidity of 86˚F. air is 50%, that air contains 13.25 grams of water vapor in each kilogram (half of 26.50 grams).

So in this example, the warmer of the two is more humid because it contains more water vapor (EVEN THOUGH THE RELATIVE HUMIDITY IS THE SAME IN BOTH EXAMPLES).

Even if the relative humidity of the 86˚F. air was lower, say 45%, it would still contain more water vapor than the 77˚F. air at 50% relative humidity! 45% of 26.50 = 11.925 grams of water vapor per kilogram of air (which is greater than 10 grams).

The 50°F. air discussed in the first paragraph is at 100% relative humidity when it is holding only 7 grams of water vapor per kilogram.  This clearly shows the limitations of cooler air when it comes to the amount of water vapor within it, even if saturated!

So, in conclusion, as is so noticeable in humid climatic zones like where we live in Florida, the hotter the air the more moisture it usually contains. This is true so long as there are available ways for water vapor to enter the air (e.g. evaporation and transpiration). One measure important to climatologists is the “evapotranspiration” rate.

SPECIAL NOTE: This type of explanation that you just read is considered a simplification by some because water is constantly changing phases in the air. When evaporation exceeds condensation within the air, the relative humidity will be below 100% – but when condensation exceeds evaporation, the relative humidity may reach 100%.

A humid afternoon in the Smoky Mountains

A humid afternoon on a LeConte trail in the Smoky Mountains - by Cloudman23©