By: Jeff Beish (04-21-2012)
The Sky Over Florida
Those of us who observe the night sky in South and Central Florida can testify that the "astronomical seeing" is usually excellent; at least most of the time it is. However, periods of less that desirable seeing occur from time to time and this may be cyclical.
During the spring, summer and autumn seasons the sea breezes are common on both the east and west coasts in this state and they tend to cause an inversion layer away from the large bodies of water from a few miles inland, even as far as the center of the state. These inversions stabilize the air from a few yards above the surface to around 1,500 to 2,000 feet altitude. Also, similar conditions can be found further north in central Florida because of the numerous shallow and warm lakes.
Because the warm, humid air above the ocean waters is forced over the landmass areas by winds from the sea the atmosphere is usually very stable near both coastal areas of South and Central Florida. As night fall approaches the air pressure over the oceans becomes higher than over the landmass, so the pressure differential causes a strong sea breeze to occur. This is because heat radiates slower from water than it does from the landmass; therefore the atmospheric pressure over land becomes lower than over water. Also, the air becomes very calm during early morning before sunrise and often a ground fog develops indicating a temperature inversion is present. This condition can produce excellent “seeing.”
In the winter months our prevailing winds are often from the north and northwest, and are less humid and cooler than the air over the oceans. During the winter months the atmosphere becomes turbulent right after a cold front rolls in from the northwest. Most often the seeing is very good a few days before a cold front passes through, but seeing conditions will significantly decrease for at least 48 hours after that. After a cold front passes though our region the upper and lower altitude winds crisscross over us in different directions.
Generally, during cold fronts the upper winds and clouds usually flow from the northwest and the lower air masses flow in a perpendicular direction or from 45 to 90 degrees from the direction of the upper winds. Streams of warm and cold air mixing together will cause atmospheric turbulence. Mixing of the two air systems then will cause turbulence at the point where the two air masses meet, therefore causing a decrease in the quality of “seeing.”
Central Florida Sky Conditions
The activity of the local surface boundary layer is quite interesting to record around my home in Lake Placid. Since moving here records of the outside air temperature (OAT) changes have been made from around 4 a.m. until 9 a.m. There is a definite cycle that occurs an hour before Sunrise when the OAT decreases a few degrees. It remains steady for a few hours then gradually increases until around mid-morning and then begins to increase more rapidly. A ground fog develops after the OAT decreases and the fog rolls over my home that is located on a 20-meter (66 feet) and westward down a slight incline to a shallow valley. This valley is several miles wide and then meets another ridge a few miles away.
In central Florida these conditions are less severe when cold fronts pass over us so seeing doesn’t seem to be effected as much. This may be because Lake Placid is located almost centered of the state where it is widest and the large number of lakes in the region moderate the lower air lasses more so that it does further south of here. Also, when cold fronts pass the higher clouds can be seen to moved form the southwest across the state to the northeast; however, unlike the winds of southeast Florida, the lower wind and cloud systems cross in the exact opposite direction in central part of the state.
The humidity usually runs 50% during spring to 45% to 75% in summer and 45% to 50% in autumn and as low as 30% during the winter. Temperatures can be high during July and August and range from 80 to 100 during the mid-afternoon; however, during 2002 the rainy season and cloud is has been much cooler. The temperature has ranged from 71º F in the early morning to around 85º F during late afternoon. From mid-October through mid-May temperatures may range from 35º F on cold nights to the lower 60’s at night and 75º during the day. An unusually cold winter occurred in 2009-2010 when the temperatures were noticeably colder, especially the first two weeks in January 2010 when early morning temperatures ran between 27 and 32 degrees. Low air temperatures continued into February 2010.
The Jet Stream
When the jet stream flows far south into Florida it will cause a similar effect on the “seeing” that a cold front does. We now know that the advent of the El Niño phenomenon apparently forces the jet stream further south than normal and then we may conclude that it indirectly effects our “astronomical seeing” in south Florida.
During the mid-1970’s several South Florida planetary observers noted that seeing conditions had become mediocre for no apparent reason. Fewer periods of excellent seeing periods were noted and the atmosphere in general was not as stable as "usual." Even in summer, when cold fronts were no problem, we began detailed logs of the climatic conditions at our observing sites.
We now know that the advent of the El Niño and La Niña phenomena apparently forces the southern jet stream to flow drop down south and indirectly effect our “astronomical seeing.” Following the peak period of El Niño in 1977 and 1978, and again 1982 - 1983, our seeing became very good -- ranging from excellent to perfect for long periods of time. Then mediocre seeing appeared to return to again throughout the late 1980’s, and early 1990’s. A weak El Niño appeared in the autumn of 1986 and continued until late winter of 1988 and strengthened during mid-1991 and late 1992, causing an apparent shift of our weather patterns back to that of the 1970’s.
By 1992 the sky was just not as steady as usual, and with several of the volcanoes irrupting around the world the sky also became brighter than usual. The sky in the daytime was not that usual deep blue, but was hazy and bright -- less blue. The night sky-glow had increased noticeably and seeing seemed to diminish accordingly.
This author feels that the weather in Florida is affected by the two ocean-atmosphere systems in the tropical Pacific called El Niño and La Niña. El Niño apparently began to strengthen during mid-2004 and weakened in early 2005, followed by a period of “normal” ocean temperatures in the equatorial Pacific. Then La Niña was noticed in mid-2007 and began to weaken during May 2008. We also experienced a fairly long drought beginning after the 2004-2005 El Niño and the 2007 La Niña periods ended and now seems to be gradually ending with the onslaught of summer thunderstorms in 2008. South Florida observers have long noted that “astronomical seeing” appears to vary from as a result of the El Niño/La Niña cycles and maybe will improve now that the El Niño/La Niña periods have ended.
The southern jet stream has been south of Florida much of the winter of 2009-2010 causing variable seeing conditions from fair to bad. For much of February 2010 the jet stream was centered over Florida: (see The ARCHIVE of Jet Stream Maps for North America).
As time went by we heard more and more reports of the effects of El Niño and La Niña the we soon reasoned that the jet stream was running further south and started watching for signs of high level clouds streams that may be associated with the jet stream. Recording the direction and movements of jet aircraft contrails that crossed over the area proved to be a more reliable indicator of where the jet stream was.
Following the peak period of El Niño in 1977 and 1978, and again 1982 - 1983, our seeing became very good -- ranging from excellent to perfect for long periods of time. Then mediocre seeing appeared to return to again throughout the late 1980’s, and early 1990’s. A weak El Niño appeared in the autumn of 1986 and continued until late winter of 1988 and strengthened during mid-1991 and late 1992, causing an apparent shift of our weather patterns back to that of the 1970’s.
El Niño is an exaggeration of the usual seasonal cycle. During the El Niño in 1986-1987, you can see the warm water (red) penetrating eastward in the Spring of 1987. There is another El Niño in 1991-1992, and you can see the warm water penetrating towards the east in the northern hemisphere spring of 1992. The El Niño in 1997-1998 is a very strong El Niño.
The weather in Florida apparently is affected by the two ocean-atmosphere systems in the tropical Pacific called El Niño and La Niña. El Niño apparently began to strengthen during mid-2004 and weakened in early 2005. After El Niño weakened in early 2005 there was a period of “normal” ocean temperatures in the equatorial Pacific then La Niña was noticed in mid-2007. By the end of May 2008 La Niña began to weaken and the Pacific Ocean returned to “normal” and remained so until the summer of 2009 when El Nino began to develop and continues through February 2010 [See: NOAA El Niño Page].
It seems like the these two conditions affects our weather here in Florida and after we experienced a fairly long drought beginning after the 2004-2005 El Niño and the 2007 La Niña periods ended, the drought seemed to be gradually ending with the onslaught of summer thunderstorms in 2008; however, “mother nature” threw us a curve ball and the drought continued until May 2009 when the rains returned in full fury. The winter of 2009-2010 started off with average temperatures lower than “normal” then turned unseasonably colder than normal in January 2010 and with above average rain fall and continued through February 2010.
South Florida observers have long noted
that “astronomical seeing” appears to vary from very good to
not so good and some equate this to the El Niño/La Niña
cycles. Current thinking is that “seeing’ improves after
an El Niño/La Niña period has ended. Often during El Nino
cycles the southern jet stream will move southward over central
and south Florida causing our “seeing” to be less than
LEFT: El Niño, with water warmer than usual (red) in the eastern Pacific, during in 1986-1987, 1991-1992, 1993, 1994 and 1997-1998. Notice the very cool water (blue), in the Eastern Pacific, in 1988-1989. RIGHT: El Niño, with water warmer than usual (red) in the eastern Pacific, during in 1986-1987, 1991-1992, 1993, 1994 and 1997-1998. It is unusual for El Niño to occur in such rapid succession, as was the case during 1990-1994. Chart by NOAA’s National Weather Service, Weather Prediction Center. NOTE: a table of El Nino/La Nina periods: http://tinyurl.com/4332hsDuring my years working at the U.S. Naval Observatory Time Service Alternate Station in Dade County, Florida I had the opportunity to work with several experienced professional astronomers that had been stationed there for several decades. A few years before my time they would regularly use several of the 8-inch f/22 Photographic Zenith Tube (PZT) systems to record on glass photographic plates certain star crossings and drifts to determine the exact time these events took place. This data was used in comparing the Cesium Standard time (Atomic Clock Time) with actual celestial times. Each experiment was accompanied with a simple star drift plate to determine the seeing conditions before the brief observing period began.
A Scientific Approach to Measuring Astronomical Seeing
Using Star Trails on PZT Test Plates
One of the astronomers spoke to me about his study to determine "astronomical seeing" at the station and suggesting that it might be fun for me to replicate his study. Since the computer version of the measuring engine was inoperative I found an older manual precision measuring engine and set out on this adventure. In 1990, I began to measure hundreds of star trails from glass plates to determine the steadiness of the South Florida air over a period from the early 1960’s throughout 1970’s. I selected plates to represent a good percentage of each seasonal period and in a random fashion to measure plates taken in good and bad atmospheric conditions. A precision measuring engine was used and plates taken by the U.S. Naval Observatory’s 8-inch f/22 Photographic Zenith Tube (PZT). I measured each plate for of star trails and based the seeing conditions on the amplitude and frequency of the variations in the trails. This method was developed on my on volition and from my experience as an engineer, but it turned out to be almost exactly the same metrology as previous studies were conducted by professional staff.
With the measuring engine one can record the amplitude and frequency of the variations in the star trails; then plot the sections of the plate relative to time base for each second of the exposure period. The amplitude is compared to the plate scale to determine the amount of movement the star experiences and translated to the seconds of arc or to some “seeing” scale, and the frequency can be interpreted to mean the type of “seeing cells” or type of scintillation in the air mass above the telescope.
While sticking around after work or during our monthly all-night observing periods at the radio telescope it took me several years to complete. It was tedious work using the manual device, but as my friend had suggested – it turned out to be fun. I wrote several programs for my PC and entered all the data for analysis. I concluded that the typical seeing for all seasons at this observing site was less than one second of arc (< 1").An image processing rendition of typical PZT test plate with star trails. The TOP line is a star trail during perfect astronomical seeing. In the next line down indicates slow moving turbulent air masses causing stars to wiggle erratically. Third line shows high frequency turbulence interferes with the stars and exposures a fuzzy, high speed oscillation.
Unfortunately the detailed notes and computer data from this study were destroyed by hurricane Andrew in August 1992 and are not available for a more detailed report. Such is life. However, from general notes taken and remembering the general results seems good enough for this discussion. One has only to ask old timer observers who have lived in south Florida to confirm this conclusion.
This study of the “astronomical seeing” conditions in Florida is far from over and is based primarily on my observing logs of more than 40 years of personal experience while observing here. So, much of my study is subjective and should be seen in that light. The effects of our atmosphere is highly complex and without a staff of trained scientists and an endless amount of funding this research will obviously be slow and not perfect; however, as more and more amateur observers take note of their variable seeing conditions they may wonder what is causing it and join in this study I started decades ago.
For most of our lives we have little choice in selecting an observing site. We must provide for our family and go where the jobs are, find available of housing, shopping, transportation, etc., and this usually means living in large metropolitan areas. Even if our choices are limited we can usually make our home a better place to observe from by following a few rules in studying micrometeorology. This deals with the atmosphere a few yards above the ground. A study of our neighborhood is important in determining what obstructions are close by that will cause turbulence about our telescopes is also helpful.
Having retired and fortunate to have an observing site that offers excellent telescopic observing conditions this period of time has given me the opportunity to reflect on the reasons why Florida is such a great place to observe in. Hopefully this paper will guide you to a better understanding of the conditions you may find in this great state.
I have found the conditions for telescopic observing here in central Florida, around Highlands County, first rate year round. During the 2001 apparition of Mars I began to observe the planet nearly every afternoon and evening beginning in mid-October through out April 2002. On average the seeing ranged from 5 to 7 during less than excellent conditions and 8 to10 during most of the observing sessions (using the ALPO Mars Section seeing scale form 0 to 10, where 0 is the worst and 10 is perfect). After a few years away from telescopic observing I returned to observing Mars in February 2012 and found the prevailing winds most often came from the southeast passing over Lake Okeechobee and Lake Istokpoga rendering the sky very stable.
The sky at night is generally very dark and the visible stars on clear nights can range from magnitudes from 6.5 to 7.5, depending on the relative humidity of course. The humidity usually runs 60% during spring to 75% in summer and 45% to 50% in autumn and winter. At times it will fall below 40% during cooler weather in the wintertime.
Temperatures can be high during July and August and range from 80 to 100 during the mid-afternoon; however, during 2002 the rainy season and cloud is has been much cooler. The temperature has ranged from 71° F in the early morning to around 85° F during late afternoon. The year 2002 is working out to be a welcome interlude from the typical hot and steamy climate of Florida. From mid-October through mid-May temperatures may range from 35° F on cold nights to the lower 60’s at night and 75° during the day.
With the many lakes in the area the weather is moderate and is less tropical that Southeast Florida or close to the two coasts. The topography in the area where I live is well suited for astronomical observing and the easterly breezes from large lakes form a stable atmosphere for excellent astronomical seeing. The rolling hills and ridges that run north and south through the center of Florida provide much area for building higher elevation observing sites. The sparsely populated forest of pine trees and large orange groves in the region has little effect on the atmosphere.
Hopefully the city dwellers will not move
in to light up the sky and cause the air quality to be
unstable. More automobiles and other exhaust belching machines
and the dust raised by construction causes the air quality to
decrease and so the astronomical seeing must suffer. Of course,
as people from the populated northeastern cities move in here
they also bring their security lights and their own burglars
Advanced Reading List for Theorists
Amateur Astronomer’s Handbook , by: J.B. Sidgwick, Dover Publications, Inc., New York ISBN 0-486-24034-7, 1971, p445 - 470.
Descriptive Micrometeorology , by R.E. Munn, Advanced in Geophysics, supplement 1, 1966. LCCCN 65-26406, Academic Press, 111 Fifth Ave., New York 10003.
Elements of Meteorology , By: Miller and Thompson, Charles E. Merrill Publishing Company, Columbus, OH. ISBN 0-675-09554-9.
Handbook for Planet Observers , Gunter D. Roth, Van Norsrand Reinhold Company, 420 West 33rd St., New York, NY, 1970.
Manual for Advanced Celestial Photography , by: Brad D. Wallis and Robert W. Provin, "Chapter 12, High Resolution Photography: Seeing," Cambridge University Press, New York, ISBN 0-521-255553 8, pp 257-266. 1988
Observing the Moon, Planets, and Comets , Clark Chapman and Dale Cruikshank, Association of Lunar and Planetary Observers (A.L.P.O.).
Introduction to Observing and Photographing the Solar System , Dobbins, Parker, and Capen, Willman-Bell.
The Saturn Handbook , Julius Benton, Association of Lunar and Planetary Observers (A.L.P.O.).
The Solar System, Volume III: Planets and Satellites , Audouin Dollfus (Observatoire de Paris), Chapter 15 - Visual and Photographic Studies of Planets at the Pic du Midi, University of Chicago, 1961.
Through the Telescope ,
Michael R. Porcellino, Tab Books, Inc., ISBN