Archive for the ‘Coriolis effect’ Tag


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I apologize to those few of you who have been consulting this weblog. My last posting was August 31, 2016. I’m still going strong and my interest has not waned. I’m still in the learning mode and intend to stay there. But it’s been a long time since I retired from teaching full-time college geosciences in 2003 and a lot has changed. I continued adjunct teaching after I retired but then moved away from South Florida in 2005. From 2006 into 2013 I taught 14 short-term courses at the College of Central Florida. Interest in this weblog seems to have diminished since I stopped formal teaching. However, when I checked this site this morning I saw that it has gotten tons of hits over the last few days, probably due to hurricane Irma. Prior to this current event almost all geoscience questions and observations that have come my way have been from a few family members, a few neighbors, and one buddy at church. It is very rare for me to hear from former students.

In-so-far as weather reporting is concerned, the information available to the public has blossomed since I retired and, for the most part, its quality has improved to the point that there is little if anything I can add (beyond basics). Many of my notions concerning tropical weather events fall into the category of hunches or intuition. I don’t believe that my 37 years of teaching meteorology full-time gives me license to clutter minds with my ideas unless I’m honest about them. Instead, in the comments below about Irma, I will share the four tropical weather resources I consult most often.

I am planning a change of theme and/or purpose for this site soon – more in the realm of discovery, opinions, observations, analyses, experiences, and perhaps some attempts at humor. The “About” page for this site was updated earlier today and if you wish to contact me, you will find my address there.





My four primary resources are:

  1. Dr. Jeff Masters’ weblog (blog) at It can be found here:

  2. The Weather Channel on television and on-line – including apps. There are things about the Weather Channel presentations I don’t like. Nevertheless I appreciate the convenience and their efforts.

  3. The National Hurricane Center. I go to this site to get a grip on what is going on in their world. I consider that they might tend to err on the side of caution, subconsciously at the very least. What an awesome responsibility they have. Http://

  4. The ECMWF Model – commonly referred to as the European Model.

I rely upon it heavily because of it’s premier reputation due to its accuracy over the last few years. It has done well for the “Irma type” storms. To be sure, I don’t ignore the other models. The following paragraph is for those who have been trying to understand that model.

You are likely to have heard many references to the European Model. I admit it is confusing. For example, here is a quote from Dr. Jeff Masters. “The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from other major models.” Thus, even though on television or on-line you may see comparisons of the European Model to the myriad other models, you might have noticed that it’s not included in the spaghetti charts that show models from multiple sources. What you will see is either the European “operational” model track or the European Ensemble (a spaghetti graphic). For that spaghetti ensemble the operational model is re-run at a lower resolution (called the control run) and this is then repeated 50 times, each with slightly different starting conditions.

I get my favorite animated European model track from Penn State’s Department of Meteorology at

Please note that this link is time-sensitive.

Of the four charts, I focus upon the one on the upper right as I scroll through f24 through f240 ( which means “24 hours into the future” through “240 hours into the future”).

You might fry your brain with the time signatures on the bottom – depending upon your comfort level with time at the prime meridian (Universal, Greenwich, Zulu) and your knowledge of Victor time.



I’ve been thinking all day long about the Coriolis Effect as it relates to Irma. If you are my former student you might recall that the steering currents at high altitude are, in part, a function of the Coriolis Effect (the Penn State chart on the upper left) and I’ll bet you remember that the counterclockwise circulation of Irma is due to the Coriolis Effect. If you’re still sharp on the subject you might also remember that the outflow at the top of the storm is likely to be clockwise for the same reason – the Coriolis Effect. I know that sounds like a contradiction to those of you who are unfamiliar with this subject. If you are interested in the Coriolis Effect go here:

and here:



Here is my zinger that comes from the “gut level” and is therefore probably not deserving of any classification other than “pure speculation.” (That’s the honesty I referred to in the second paragraph of this blog).

I am expecting (or is it hoping and praying for?) slightly more turning to the right than the experts are indicating. The itty-bitty turn last night was encouraging to me. I keep telling myself that the hurricane is a separate entity of its own and that the Coriolis Effect is influencing it’s path independent of the steering currents and the rotational motion. That path is the consequence of what is referred to as translational motion. Furthermore, the further north the storm gets, the stronger the Coriolis Effect will be. The Coriolis Effect is zero at the equator and increases to 100% at the poles. Maybe I’m just overly excited about last night’s noticeable veering of Irma’s path. Perhaps this is merely a good example of wishful thinking. We’ll see.



Finally, for those of you who live in my county of Florida, Citrus, you might be interested in this August 2014 posting about hurricanes.





Thanks to NOAA's National Hurricane Center for this graphic.

8-30-2010 10:10 pm EDT.

I’ve watched television weather reporters today trying to explain what mechanism will hopefully turn Earl to the right – the sooner the better.  But not one of them mentioned the natural tendency for objects, fluids, and dynamic systems in motion to turn right (in the Northern Hemisphere).  I’m referring to the Coriolis Effect.  At times like this it is unfortunate that the Coriolis Effect cannot strengthened or weakened at will by those of us who would wish to keep these strong storms from plowing into us.

Here are two links for you if you are interested in the Coriolis Effect as it relates to weather:

I remember so well in late August, 1992, as I, my family, my students, and my friends and neighbors were hoping and praying for powerful hurricane Andrew to turn right and stay out over the Atlantic.  It eventually did turn right but not soon enough for us.  Our house was a total loss; the eye of Andrew went right over it.  We stayed in the community and had the house rebuilt; it was exactly one year before we occupied it again even though it wasn’t entirely finished.  I had purchased a 25′ travel trailer which was our palace-in-the-driveway for that year and we spent many Summers thereafter traveling all over the continent with our children.

Bottom line:  Lets hope for a drastic right turn on the part of Earl very soon.  The computer model tracks do not look promising for that.  Things are looking increasingly “ugly” for places like coastal North Carolina and points northward up the coast.  Though weakening is expected to occur before a possible visit to Nova Scotia – the prospect is nevertheless of considerable concern.

NOTE:  Some depictions of the successive forecast mean positions that you might see on television, your computer, or in the print media might be connected with an arcuate line right down the middle of the “cone of uncertainty.”  The National Hurricane Center still provides such a depiction but they favor this one because it has been shown that when people gaze at the midline they tend to either forget or ignore that the storm could fairly easily embark into other parts of the widening cone as it moves along.

Why Is Florida So Humid?

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z fla precip



“Why is it so humid in Florida?”

There is no single, simple answer.  Here are at least eight reasons, most of which are interrelated.  After this listing, amplified explanations are available:

1.  Most of Florida is a peninsula which, by definition is bordered by water on three sides.  The adjacent sea water is the most important source of moisture for the atmosphere.  That part of Florida which is not a peninsula, the panhandle, is also bordered by sea water on the southern boundary.

2.  Sea breeze convergence carries moisture over the land.  Air that has converged near the surface will rise and under the right conditions will form clouds that provide precipitation.  The precipitation is simply distilled sea water.  In some parts of Florida sea breeze convergence provides almost two-thirds of the annual precipitation.

3.  Florida is located along the eastern margin of the continent where warm waters arrive from the North Atlantic Gyre.  Warm waters mean higher evaporation rates and the warmed air is also able to support more water in the vapor state than if it were cooler.

4.  The relatively low latitude location of Florida provides for warmer temperatures which in turn give the air more thermal energy necessary to support large amounts of water in the vapor state.  The warmer temperatures also provide for significant convectional uplift of air which is a key factor in the development of many of Florida’s rain clouds.

5.   Florida’ vegetation transpires large amounts of water vapor (into the air).

6.   Numerous fresh water surfaces within the state provide moisture to the air from evaporation.

7.   Weather systems moving from east to west with the “Trades” provides moisture to the state – especially during the Atlantic hurricane season.

8.   Winds associated with fronts, especially pre-cold frontal winds bring vast amounts of moisture to Florida from components of the south.



1) Most of Florida is a peninsula and by definition it is surrounded by water on three sides.  The rest of the state (the Panhandle) is also coastal.  The surrounding water is the source of a great amount of moisture for processing through the hydrologic cycle.  But, being peninsular is not reason enough for Florida to be humid.  As case in point is the Baja Peninsula of Mexico.  Look at this comparison of the two peninsulas:

zbjfl 75In some instances left clicking twice will be necessary to enlarge.

2) The geography and physiography is such that sea breezes of the Atlantic side of the peninsula converge with sea breezes of the Gulf side of the peninsula.  The zone or line of convergence is seldom at the “center” because the sea breezes are seldom of the same strength.  As a general rule Atlantic side sea breezes of the peninsula are stronger than Gulf side sea breezes of the peninsula.  In any case, these sea breezes carry moisture in the vapor form the origin of which is evaporation off the sea surface.  When sea water evaporates the dissolved solids stay behind; therefore the cloud droplets formed when the sea breezes converge and then rise are made of fresh water.  In simplistic terms I have described one of Natures own distilleries of fresh water.  Air that has converged at or near the surface will rise and rising air cools adiabatically.  If that cooling air contains ample moisture the dew point temperature will be reached relatively quickly and further cooling cause by continued ascension of the air causes condensation which releases heat.  That added heat usually causes the air to become buoyant enough to continue rising to form clouds that provide precipitation.  This is akin to a hot air balloon rising through air that is cooler than the air inside the balloon.  Cumulonimbus commonly form when this happens.


In South Florida there are years that two-thirds of the annual precipitation is provided during the warmer 6 months and most of that precipitation is due to sea breeze convergence.  This is a real paradox to me because the huge amount of precipitation from sea breeze convergence (sometimes 40” or more) is the result of weather circulation systems that do not show up in the isobaric configurations of a national weather map!

Of course, statistics vary from year to year depending partly upon the amount of tropical activity due to tropical disturbances (waves), tropical depressions, tropical storms, and hurricanes.  The whole sea breeze convergence process happens most often when the synoptic pressure gradient is weak (synoptic systems are those lows and highs that are seen on the national surface analyses).  Except when tropical synoptic systems are dictating the pressure gradient (e.g. hurricanes) the warmer 6 months of the year are when sea breeze convergence is most likely to occur.  In the cooler 6 months synoptic systems that migrate generally from west to east across the United States dominated the flow patterns at the surface.

3) Florida is located along the eastern margin of the continent.  As in all continents except Antarctica, the eastern margins are generally more humid than the western margins.  The principle reason for this is that the water at the eastern margins is generally warmer than the water at the western margins.  The warm boundary currents belonging to the gyres of the respective oceans are on the western margins of the oceans (which is the same as saying the eastern margins of the continents).  Part of the North Atlantic gyre circulation (sometimes called the Gulf stream gyre) enters the Caribbean and eventually much of it travels through the Yucatan Strait (the gap between western Cuba and the Yucatan peninsula) to flow into the Gulf of Mexico as the Loop Current.  This, then, circulates warmer water and enhances the moisture potential for the air on the Gulf side of Florida.  The Florida Current segment of the gyre provides considerable thermal energy along the east coast of Florida.


The warmer the water the higher the evaporation rates and therefore the more moisture gets into the air in the vapor phase.  NOTE:  Remember, water vapor is invisible so I’m not talking about clouds – but – clouds develop as a result of either the condensation of water vapor (liquid droplets) or the deposition of water vapor (ice crystals).



4) Most of Florida is in the low latitudes defined as that part of the world between the Equator and 30˚ latitude.  Downtown Jacksonville which is at the north end of the state’s Atlantic side is at 30.32 degrees north latitute.  I have shown the location of the 30th parallel on the map of”idealized air circulation on a homogeneous globe” (above) and on the map of the boundary currents before that.

You might know that the planetary circulation is not as simple as shown above because the earth’s surface is far from being homogeneous (the same all over).  The most obvious surface difference is that between land masses and oceans.  Furthermore, there is a hemispherical difference in that category – 39% of the northern hemisphere is land but only 19% of the southern hemisphere is land.  Because of earth surface heterogeneity (differences) the planetary circulation is not nearly as ordered as shown above nor do the hemispheres mirror each other as perfectly as shown.  And – very obvious seasonal differences exist between the continents and the adjacent oceans.  (NOTE:  All of that is “fuel” for another tutorial topic which is likely to be addressed on this site at some time in the future).   There is one aspect of this that I want to mention up front at this time since some of you know about the “Bermuda High.”  It is a warm season phenomenon and in effect, the south half represents the northeast trades (over the Atlantic of our hemisphere) and the north half represents the prevailing westerlies (over the Atlantic of our hemisphere).

Since the lower latitudes have higher sun angles and therefore more intense solar radiation than the higher latitudes, lower latitude surfaces (of both the land and water) get warmer.  This added warmth not only causes higher evaporation rates over water and moist land but also more convection over the heated land than would exist were it colder.

Convectional uplift of air is a key factor in the development of rain clouds, providing there is an adequate supply of moist air.  And – think about this:  When air is heated by the surface and then rises due to it’s positive buoyancy it does not leave a vacuum behind.  Air must flow in to take it’s place and in Florida that is moist air which, in turn, is heated and rises.  Most clouds providing precipitation result from air rising one way or another.

5)  The warmth of Florida along with its vegetation allows for high transpiration rates.  Transpiration is the process whereby plant leaf surfaces cast water vapor into the air.  A mature oak tree in the Summer will put about 500 gallons of water daily into the air in this way; an acre of mature but still green-leafed corn about 2000 gallons a day; an acre of densely distributed invasive species of the melaleuca tree in the everglades is believed by some to transpire four times as much as a comparable area in native saw grass!

6)  Florida is a state with numerous surface fresh water features within it that provide high evaporation opportunities.

Lake Henderson on the east side of Inverness, Florida
Lake Henderson on the east side of Inverness, Florida

Two left clicks will enlarge this photo nicely.  It is my favorite image of a beautiful lake where I love to sail my little sloop – taken by my photographer son, Colin Toney.

A traveler in the state finds remarkable beauty in glades, lakes, marshes, and rivers.  You might find it interesting (and even sad) that before humans began controlling it, the famous everglades was a 40 mile wide river whose water crept generally southward issuing fresh water into Florida Bay.  I’ve been told that the rate of movement was so slow that strong winds from the south would temporarily but significantly reduce the discharge into the bay and even sometimes cause the water to flow backwards.  Currently, Florida Bay is far more saline than it used to be because so much less fresh water empties into it these days (due to human usage, and interference through water storage and flood control).

7)  Florida is downwind of the North Atlantic segment of the global-scale N.E. Trades.  The Trades, rather than a specific wind, represent a planetary-scale force that causes weather systems to move from the east toward the west across the low-latitude portions of the oceans.  Examples of these weather systems are the array of tropical lows ranging from tropical disturbances on the lower end of the intensity scale to hurricanes on the higher intensity end of the scale.  All of these types of lows bring moisture to Florida.

If you want to get a general idea as to why weather moves across most of the United States from west to east and why hurricanes (and the lesser tropical lows) move generally from east to west across the Atlantic just look at the guiding forces on the “homogeneous globe” diagram – the prevailing westerlies and the northeast trades.  As for the oft-asked question, “Why don’t the synoptic systems move in the same direction as the arrows showing the westerlies and the trades? – it’s the rightward Coriolis effect in the northern hemisphere that is mostly responsible.  Mid-latitude cyclones, air mass anticyclones, and tropical lows act as separate entities which rotate the way they do because of the Coriolis effect but move in translation over the land and the ocean in a direction influenced by the Coriolis Effect.  Here are a couple of links on the Coriolis effect:

There is a link within the one above to take you to Part 2 of the Coriolis effect subject if you wish.

8)  Air ahead of cold fronts moves almost parallel to those fronts.  It responds to the pressure gradient of the middle-latitude systems containing the fronts.  In Florida those pre-frontal winds are generally from some component of the south (typically southeast).  SPECIAL NOTE: Winds are named in accordance with the direction from which they are moving.  In other words, a southeast wind is a wind blowing from the southeast toward the northwest. What that means is that when cold fronts are moving through Florida the pre-frontal winds are carrying relatively warm and humid air from lower latitudes.

z coldfrontIt is this warm and humid air that provides the moisture for condensation making the lines of clouds ahead of the fronts and along the fronts.  The moisture is not being brought down by the cold air but rather, the cold air is forcing the warmer air ahead of it to rise and cool adiabatically just as moisture bearing air does when it is lifted up the windward side of a mountain range.  In fact, I envision cold fronts as moving mountains along whose leading surfaces air is forced to rise, often to a level of free convection where it then “rises freely by convection” forming some very powerful lines of thunderstorms.  Additionally, the dynamics and temperatures aloft help to create squall lines out ahead of and nearly parallel to many of those cold fronts.




It is apparent that there are several reasons for Florida’s sometimes notorious humidity.  It may seem that the explanation you have just read is more detailed than most but the truth is, I have not covered all aspects, particularly those dealing with the winds aloft.  Obviously, a variety of circumstances cause the large amount of moisture in the air over Florida as well as the amount of precipitation, the latter being enough for most of the state to fit within the parameters of the Humid Subtropical climate – though a small part of South Florida is classified as a Tropical Savanna climatic zone.  But – some people are shocked when they learn how little of the water raining upon Florida is available to them.

I will use “ball park” numbers I am comfortable with for the sake of simple illustration.  To keep it simple, I’ll round off numbers used to illustrate a typical annual water budget for South Florida.

Annual precipitation is about 60 inches.

20” evaporate

20” transpire

18” discharge into the sea by surface runoff and groundwater transport

2” remain for all other usage!

People move to Florida and clearly see “water, water everywhere.”  But the truth is that very little of that water, probably less than 3% is captured and exploited by humans.  It is always a good idea to conserve water in Florida, even during the rainy season.



The word “humid” is used in a variety of ways by the general population and humidity is expressed in more than one way by meteorologists.  “Relative humidity” is a percentage expression of the amount of water vapor in the air compared to the amount that could exist within it at that particular energy level (based upon its temperature).    The left column below shows air temperature in both Celsius and Fahrenheit and the right column shows how much vapor, in grams, a kilogram of that air can support in the vapor (gaseous) state at those temperatures.   Consider the following illustrations using the chart provided.

Left column is the Temperature in degrees Celsius and (Fahreheit)

Right column is grams of water vapor per kilogram of air representing specific humidity at saturation.  Saturation indicates 100% relative humidity.

-40 (-40)                                             0.1

-30 (-22)                                             0.3

-20 (-4)                                               0.75

-10 (14)                                              2

0 (32)                                                 3.5

5 (41)                                                  5

10 (50)                                               7

15 (59)                                               10

20 (68)                                              14

25 (77)                                              20

30 (86)                                              26.5

35 (95)                                               35

40 (104)                                             47

Here are four usages of the chart above

to give you a “feel” for humidity.

1.  If the specific humidity of 68 degree Fahrenheit air is 3.5 grams per kilogram, the relative humidity is 25%.  (3.5 is 25% of 14).

2.  If  there are 10 grams of water vapor in a kilogram of 77˚F. air near the surface that air has a relative humidity of 50%.  Why?  The chart tells us the air has the thermal energy to keep 20 grams of water in the vapor phase so if there are only 10 grams in the vapor state that represents one-half (50%).  On the other hand, if the temperature where then to drop down to 59˚F. the relative humidity would be 100%!  This is because 10 grams of water vapor in 59 degree air represents the saturation level for air at that temperature.  In this case, the meteorologist would say that the 77 degree air (at 50% relative humidity) had cooled down to its “dew point” (59 degrees) – the point or temperature where condensation would occur if there were any further cooling.  If that cooling occurred on the ground or on your windows overnight dew would form; if it occurred near the surface fog droplets would form; if it occurred further up, cloud droplets would form.  NOTE:  Actually, fog is no more than cloud close to the surface.

3.   If air over the Arctic at -10 degrees Fahrenheit had a relative humidity of 100% a kilogram would have 2 grams of water (per kilogram) in the vapor phase.  Yet if the relative humidity of 104˚F. air over Yuma, Arizona was a very low 15%, that air would contain more water in the vapor phase than the 100% relative humidity Arctic air.  Why?  Because 15% of 47 grams is 7.05.  More than 7 grams of water vapor in a kilogram of air is a lot more than 2 grams within a kilogram.  Therefore, even though the relative humidity of the Yuma air is a low 15% compared to 100% for the Arctic air, the hot Yuma air has more than 3 ½ times the amount of water vapor in it than the colder Arctic air.  So – the Yuma air at 15% relative humidity has more water vapor in it than the 100% relative humidity air of the Arctic location!  A meteorologist might say (if he/she is being careful), “The specific humidity of the Yuma air is much higher, more than 3 ½ times higher, than the air at the Arctic site.”  Yes – this is a paradox.  What I hope you learn from this is that the warmer the air, the more energy it has for keeping water in the vapor state and as the temperature increases the ability to hold water in the vapor state does not increase linearly, but exponentially.

You can see that on the chart.  For example, 20˚Celsius air has the ability to keep 14 grams of water per kilogram of air in the vapor state.  But double the temperature to 40˚Celsius and the water vapor “capacity” does not double; in fact, it more than triples!  The calculator in my computer tells me that it increases 3.3571428571428571428571428571429 times.  Please memorize that number.  There will be a test question on the midterm!  LOL

4.   If a kilogram of 77˚F. air was keeping 18 grams of water vapor within it, the relative humidity would be 90%.  Why?  Because 18/20ths (reduces to 9/10ths) translates to 90%.  You may find this hard to believe but many of my college students (particularly in the last 20 of my 41 year teaching career) in pre-testing could not successfully change a fraction into a percent.  Divide the numerator by the denominator and then multiply by 100 to get percent.  Percent means “parts per 100.”  So, 18 divided by 20 (or 9÷10) = 0.90.  0.90 X 100 = 90.  Most people instantly recognize that 0.90 is 90 one-hundredths and therefore do not need to multiply by 100.

An earlier posting on the subject of humidity can be found here:

More related topics will appear in the near future, among them, a fundamental presentation on adiabatic processes that form clouds with a high potential as precipitation providers.  Adiabatic processes are so important to us that without them, almost all of the land of the world would be desert.  If you are interested, click on the following link to a November, 2008 post for a starter:

The Coriolis Effect In the Real World – A Tutorial (Part 2) – Cyclones & Anticyclones

Left Click To Enlarge Image

I suggest that, though there may be some repetition, please read Part 1 first.  To go to it quickly, either scroll down or click on this link:

The set of weather maps, provided by NOAA, shows the remains of Ike after it began to head toward the northeast. At this latitude there is a tendency for weather systems in the middle latitudes to travel generally from west to east.  Notice the cold fronts which indicate that Ike had changed from tropical to extratropical.  The cold fronts represent the leading edge of cooler air being thrown out of the anticyclone (high with rotation) centered over the Eastern Dakotas. That air is coming “down” from some component of the north whereas the air on the “warm” side of the cold fronts is coming up from some component of the south and is being thrown out of the anticyclone centered off Florida.  So, we have a cyclone (low with rotation), Ike, between two anticyclones.

The only alterations I have made to the first map are 1) cropping of the original, 2) labeling of the fronts 3) placement of the red L and the two blue H’s, and 4) darkening of two of the isobar values making it easier for you to read.

Isobars are imaginary lines, of course, and plot equal pressure.  For example, every point on the 1020 isobar was believed to have had a pressure of 1020 millibars at the time of observation. In many ways isobars are analogous to contour lines on topographic maps.  In fact, the two “highs” on a topographic map would be hills and the “low” would be a trough-shaped valley between the hills.  Sticking with that analogy, surface runoff water would tend to flow down the hillsides along a stream gradient (or gravity gradient) toward the lower valleys.  In the simplest of topographic and geological settings the water would flow down the hills in a radial pattern, just as air would flow out of the highs were it not for the Coriolis effect.  Though water flowing down hillsides in stream channels does not respond to the Coriolis effect, air flowing on the scale depicted here does by deflecting to the right of the pressure gradient direction.  So, in the second version of the weather map I have drawn blue lines of which two are comparatively long, showing the direction that the air would flow if the earth did not rotate on it’s axis.  But, remember, rotation of the globe causes the Coriolis deflection to the right in the northern hemisphere and to the left in the southern hemisphere.

It’s important to note that the deflection is with reference to the object or fluid in motion.  For example, if someone driving directly toward you turns right, he/she will have turned to your left.  Though that person’s turn would be to your left, it is still a right turn.  So, in northern Texas the green line shows that the air is moving to the right of the pressure gradient direction (light blue) – even though that green arrow points toward the left side of the map.  Just put yourself in the position of the air in motion and you should not have difficulties with this.

This, then, shows why air in the northern hemisphere moves clockwise around anticyclones and counterclockwise around cyclones.

Near the end of my first tutorial on the Coriolis effect I revealed that the following question had come up often during my teaching career: “If the Coriolis effect is an important influence in large scale weather systems, and since hurricanes are synoptic scale (a type of macroscale) system, why do hurricane winds turn left in the northern hemisphere and right in the southern hemisphere?”

The “obvious” left turning of air within hurricanes causes confusion among many people who are trying to understand air circulation – particularly if they are starting from scratch without knowledge of the Coriolis effect or the pressure gradient force and how the two engage in a tug of war.  I understand the confusion because seeing the shape of hurricane rain bands on radar and arcuate cloud band alignment clearly shows how the air turns left as it gets closer and closer to the hurricane’s eye wall.

Not too many years ago I heard a person who should know better, during a television weather report, explain to the viewing audience that hurricanes were so powerful that they did not respond to the Coriolis effect – referring to the “left turns” that she was showing on the satellite loop that was being projected.  I don’t know whether or not in some previous weather report she had mentioned the Coriolis effect but it seemed to me that might have been the case.  In her honest attempt to educate some of her audience, she gave them information which was entirely incorrect – perhaps because of misinformation given her or maybe some general assumptions she had made.  You see, it is the Coriolis effect that forces the counterclockwise rotation in the first place!

In this last illustration (below) you are looking at a satellite image of hurricane Fran (1996).  I have drawn blue pressure gradient lines and red air flow lines which clearly show the rightward deflection (in spite of the fact that the air does turn left as it approaches the eye wall.  Notice, however, that no matter where pressure gradient lines are placed along the air flow lines, the deflection of the “real wind” is always to the right of the pressure gradient line.

Once again, as in Part 1, I have not truly explained the Coriolis effect; I have merely described it and illustrated it.  I have not explored the nitty-gritty.  I have implied that it is only an apparent force.  You might want to explore other attempts to describe the Coriolis effect – perhaps via an Internet search.

Finally, in the interest of accuracy, I must admit that I have simplified to the point of leaving out some important forces that play a roll in determining the actual direction that air moves (from high toward low) in its quest to reach pressure equilibrium.  Among those are friction, centripetal force, and centrifugal force.  The conservation of angular momentum is an important consideration and accounts for the increase in wind velocity as the air gets closer to the storm’s center.

Left Click To Enlarge Image
Left Click To Enlarge Image


Some physicists prefer to use the term “Coriolis Effect” over Coriolis Force claiming that it is only an apparent force.  I tend to agree with that.

The Coriolis force is something that just about everyone in school learns about at one time or another.  To be sure, it is a topic in secondary school earth science and physics courses.  A low percentage of students enroll in the latter but a very large number are exposed to the former partly because in many school systems earth and/or environmental science is required.  Non-science majors in college enroll in earth and/or environmental sciences partly because it is perceived to be far easier than some of the other options – e.g. physics or chemistry.

The crux of the Coriolis force with regard to earth is that because our planet is rotating – objects and fluids in motion tend to deflect to the right in the northern hemisphere and to the left in the southern hemisphere.  The larger the circulation system the more there is likely to be an obvious response to the force.  Physicists, by the way, tell us that it is but an “apparent” force and that it is more accurate to call it the Coriolis “effect” which I intend to do from here on. There is no need to debate the term here but if you want to learn more about the Coriolis effect I suggest you use both terms in your search.

Earth’s period of rotation is once per day.  The rotational direction is from west to east.  If you looked at earth from “above” the north pole you would discover that the rotation is counterclockwise, and if you looked at the earth from “below” the south pole you would find a clockwise rotation.  If you have difficulty envisioning that “reversal” I recommend that you pick up an item and rotate it watching the rotation from one end of the axis.  Then continue rotating it in the same direction – don’t stop – but view it from the other end of the axis.  You should observe the reversal; from one end it will be counterclockwise and from the other end it will be clockwise.

SPECIAL NOTE: One of the greatest myths or misconceptions in physics is that the Coriolis effect determines the direction of rotation of water down a toilet or other drain.  That is absolutely untrue.  If you live in the United States and observe the direction the water moves down a toilet in your dwelling, then, crate it up and ship it to New Zealand and have someone install it there, upon flushing the water would go down the same way.

Next, look at the demonstrations shown on Quick Time at the following site.  Before you go there take note of this.  The first boy, wearing the blue headgear is rotating clockwise when the playground device is viewed from atop the axis of rotation so his setup is analogous to the southern hemisphere.  The other two boys (one with red headgear and the other bare-headed) are rotating counterclockwise so their setup is analogous to the northern hemisphere.

Hopefully you saw that the first boy’s ball went to HIS left as would be expected in the southern hemisphere (clockwise) and the other two experienced the opposite (to THEIR right) as would be expected for the northern hemisphere (counterclockwise).  You might want to scroll down a little further on that page and you will find a Quick Time animation of a ball deflecting to the right on a rotating table.  The rotation will not be apparent because the camera was fixed above the table and rotating at exactly the same period.  Since the ball deflects to the right you should correctly deduce that the rotation of the table was counterclockwise like the rotation of earth from the northern hemisphere point of view.

There are many examples of the Coriolis effect here on earth.  Cold air masses in the northern hemisphere rotate clockwise because of the right turn of the air which, after sinking toward the surface flows outward from the domal system’s high pressure core; this is a great example of an anticyclone.  But my favorite example of the Coriolis phenomenon, surprisingly, is not an atmospheric example.  It is the manner in which most of the water being carried by the oceanic gyres turns right in the northern hemisphere, especially when it reaches a continental margin and left in the southern hemisphere especially when it reaches a continental margin.  Observe the image below where I have removed all but the gyre components of oceanic surface circulation.

SPECIAL NOTE:  Though not discussed here, it is the general circulation of the atmosphere at or near the surface that creates these gyres and general circulation is guided by the Coriolis effect.  If you wish to learn more about the “general circulation” of the atmosphere, other terms are global circulation, planetary circulation, and large macroscale circulation.

Left click image to enlarge.

Have you noticed – I have not explained the earth’s Coriolis effect!  I have described it, I have linked you to visual evidence, I have described a meteorological example and shown you an oceanic example via a very generalized map of the 5 oceanic gyres.  But I have not provided an explanation other than indicating that it is caused by rotation of the earth. If you have stuck with me to this point, I want to entice you with an “issue” that has often remained unaddressed/overlooked by some teachers and learners of meteorology.  That is this:  If the Coriolis effect is an important influence in large scale weather systems, and since hurricanes are synoptic scale (a type of macroscale) system, why do hurricane winds turn left in the northern hemisphere and right in the southern hemisphere?  THAT WILL BE THE TOPIC OF MY NEXT TUTORIAL POST AND IT WILL BE COMING SOON.  Now, let’s look at a hurricane.

If you want a head start on understanding a hurricane’s circulation, read this repeat of a 9-9-2008 post using hurricane Ike as an example


Image source of Ike radar loop =

Image source of Ike radar loop = Weather Underground


(see item 13 below).


©* Tonie Ansel Toney (see conditions for copying at the end)

I have learned of these misconceptions by communicating through the years with my students, friends, neighbors, attendees of some of the hurricane seminars that I have conducted and visitors to hurricane expos where I have given presentations.  Most of this occurred in Florida.  I learned that these items have been relatively “common” misconceptions through informal pre-tests I have given to college students at the beginning of certain semesters, answers to questions I have asked in classes during the course of myriad semesters, through conversations with people of all walks of life (and a broad range of ages and experience), and by listening carefully.


1. IF THE SPEED OF WIND BLOWING DIRECTLY INTO THE SIDE OF A DWELLING CHANGES FROM 40 MPH TO 80 MPH, THE FORCE THAT IT EXERTS INTO THE STRUCTURE WILL INCREASE TO TWICE WHAT IT WAS. THE TRUTH: A doubling of the velocity will cause a four-fold increase of the force upon a surface being struck at right angles.  The relationship is “exponential,” not “linear.”

2. IF, DURING A HURRICANE, YOUR TRUE WIND DIRECTION IS FROM THE SOUTH, THE HURRICANE’S EYE IS TO THE NORTH OF YOU. THE TRUTH:  It is generally west of you.  Hurricane winds move approximately parallel to (or concentric with) the nearly circular eye-wall.  A good rule-of-thumb for eye location (in the Northern Hemisphere) is: Imagine standing with the wind at your back.  Extend your left arm out from your side and your hand will be pointing toward the eye.

3. IF AN APPROACHING HURRICANE IS ABOUT ONE DAY AWAY, PRUNING OF TREES IS ADVISABLE. THE TRUTH:  It is too late to prune at that time – it should have been done much sooner, preferably prior to the hurricane season.  Pruned material must be disposed of properly – if lying around the items can become a dangerous airborne hazards. Please read on by clicking here; there are 20 more which might interest you. And, don’t miss viewing the animated image of Ike at the beginning of this post.



Inflow consists of the harder-edged clouds with sharp contrast – Outflow consists of the more diffuse cirrus and cirrostratus of the upper layer.


With this post you can either simply enjoy the high resolution image of Ike and leave it at that point or explore deeper into the dynamics of storms such as this.

I have provided a large image (above) of Ike completed earlier today followed by smaller images contrasting the circulation below with the circulation above.  Thirdly, you will find below an image of what might seem like a very odd looking hurricane compared to what you have been looking at this season.  To see it, you must ask for more detail when the invitation appears at the end of the next paragraph.

Because of only a small amount of sheer and other factors, Ike is a well-formed system.  And – if you can get past its destructive character you might marvel at its beauty.  I speak of it as though it were a living thing.  In many respects, it is a separate entity with a life of its own.  We even talk about the life cycle of such a storm.  We personify it with a name, in this case a male name.  Its winds spiral because of the Coriolis effect and the whole storm’s path responds to the Coriolis effect – sometimes that is evident, sometimes it is not.  If you find yourself confused about the Coriolis effect, please be patient because I intend to post an item soon, with an explanation of certain aspects of hurricanes which might seem to be contradictions when they are not at all.  Believe me, misunderstandings about the Coriolis effect does cause considerable confusion. If you are interested in more detail about the movement and the energy within this storm, please read on