In September 1993, the FAA adopted the International Civil Aviation Organization (ICAO) definition of airspace segments. At that time, American pilots learned that the ICAO airspace classifications are named A through G. Mysteriously though, most of us were never told just what Class F was supposed to mean, and just why the US decided not to use it. And for those inquiring minds who wanted to know, there wasn’t exactly an avalanche of information on the subject. (There still isn’t.) So…just what in blazes is Class F, anyway?
- Outside the US, it is simply any special use airspace.
- any uncontrolled airspace in which VFR night flight is not allowed
- in most (but not all) countries outside the US, uncontrolled airspace above FL 600
- uncontrolled airspace in which separation is provided to participating IFR traffic
Answer: D (pretty anticlimactic, wouldn’t you say?) First, Class F airspace, in those countries that use it, can differ from the ICAO standard. Amendment No. 33 (there are about 41) to Annex 11 (one of about 18 to the 1944 ICAO Convention, in this case, on Air Traffic Services) explains ICAO airspace classifications. The Preamble to CFR Title 14 FAR Part 71 summarizes them. Anyway…ICAO Class F airspace is uncontrolled airspace in which a separation service (often called an ATS or air traffic advisory service) is provided to participating IFR flights. If IFR flights choose to call the ATS, they will be separated from other IFR flights who are also in contact. There is no requirement to obtain an ATC clearance in class F airspace under IFR, or VFR. The UK implements Class F, pretty much by the book, for standard routes in relatively inactive airspace. In other countries, Class F airspace is uncontrolled airspace in which an advisory and separation service is available for IFR traffic, and in which a Flight Information Service is provided between VFR and IFR as well as among VFR traffic. Canada uses Class F. Some countries, such as New Zealand, do not use Class F except where they cannot, as they say, provide a normal ATC service for natural or human reasons. (No, I have no idea what that might mean, either.)
How Low Can You Go?
The lowest barometric pressure ever recorded in the United States was 26.35 inches. In which state was it measured?
Answer: B, Florida. Not that this is probably a big surprise, but it occurred during a hurricane. It was during the Florida Keys Hurricane of early September, 1935. (This was before they began naming them, which wasn’t until after 1950.) It was also the lowest pressure ever recorded over land in North America. First though, a little background information…
As pilots, we know that the air gets less dense the higher you go, and also as the thermometer climbs higher. Even most non-pilots know that pressure decreases as we increase altitude, which makes jogging up in mile-high Denver Colorado such a challenge for an easterner. And as pilots (being the eager students of the atmosphere that we are), we also know that atmospheric disturbances can provide the equivalent of altitude, right here on earth.
The accepted means of determining how “thin” the air becomes is the use of a barometer to measure the pressure, which is an accurate measure of the weight per unit area of a column of air overlying the instrument. (Of course, we pilots must then correct for temperature, as well.) At sea level, on a normal day (if you’ll allow such unscientific imprecision), atmospheric pressure averages 14.7 pounds per square inch, which is sufficient to support a column of mercury 29.92 inches high in a glass tube. In metric units, this is expressed in terms of millibars. The standard sea-level pressure is 1013.25 millibars, which is the same as a force of 101,325 Newtons per square meter. Of course, on cold days, the density of the air increases, and for a given air mass, pressure rises. It’s also not a big surprise that higher barometric pressures are routinely observed in the Earth’s deepest depressions. The “elevation” of the Dead Sea, along the boundary between Jordan and Israel, is 1286 feet below sea level. The highest pressure ever observed in this area was at Sedom, Israel (-1275 feet) on February 21, 1961: 31.94 inches (1081.8 mb). In comparison, the highest sea-level pressure in the United States was 107.86 kPa, or (since the number of millibars equals the number of kiloPascals times ten) 1078.6 mb, in Northway Alaska, on January 31, 1989. Canada’s record for the highest sea-level pressure is a bit greater, at 107.95 kPa in Dawson YT on February 2, 1989. (So, okay, what’s a Pascal? It’s just one of those honorary units, each one signifying one Newton of force per square meter of area. What’s a Newton? It’s another honorary unit, this one being the force which will accelerate a kilogram of mass one meter per second, per second.) One might think that the South Pole, near the center of the Antarctic plateau, being that it routinely has such unimaginably cold temperatures, might take the cake for high density air. Nope? There’s one problem: it’s already high up. With a physical altitude of about 9,300 feet above sea level, the atmospheric pressure is already about 70% what it would be at sea level. The average pressure there is roughly 681.5 mb, or 20.125 inches of mercury. Ah, but there’s another place where it’s also cold, but not too high up: out in the Gulag of Mother Russia! The accepted world record maximum “sea-level equivalent” pressure was observed at Agata Lake (66 degrees 53 minutes North, 93 degrees 28 minutes East) in Siberia, at 1200 GMT on 31 December 1968. It was 32.01 inches Hg (1083.8 mb). The station elevation is 263 meters, and the temperature was -46 degrees C.
According to the Guinness Book of World Records, the lowest sea-level pressure ever recorded was 25.69 inches (87.00 kPa, or 870 mb) in the eye of Typhoon Tip. It was recorded 300 miles west of the island of Guam in the Pacific Ocean at latitude 16 degrees 44 minutes North, longitude 137 degrees 46 minutes East, on October 12, 1979. The U.S. record is 26.35 inches (89.23 kPa), produced by the 1935 Labor Day hurricane as it passed Long Key, Florida, at 10:00 p.m. on September 2, 1935. Other records were during Hurricane Camille, on August 17, 1969 (905 mb, or 26.72 inches Hg), during Hurricane Gilbert on September 14, 1988 (888 mb, or 26.22 inches), and Hurricane Andrew, on August 23, 1992, when the storm was still east of the Bahamas (922 mb, or 27.23 inches). In case you noticed that the low pressure for Gilbert was even less than that of the 1935 hurricane, and wondered why it wasn’t listed as the all-time US record low pressure, that was because this occurred as it passed about 200 miles south of the western end of Cuba in the northwest Caribbean, and the worst of it missed the US altogether. (It made landfall in Cozumel, Mexico.) According to the Saffir/Simpson hurricane scale, the worst “Category 5” hurricanes (where winds are greater than 135 knots) have central pressures below 27.17 inches, or 920 mb. It is also possible that even lower pressures occur (however briefly) during the passage of a strong tornado, but so far, no direct barometric pressure measurement has ever recorded the lowest pressure at ground level in the center of a tornado. (A recent commercial venture involving “hardened in situ tornado pressure recorders” has recently begun operations, although meteorologists involved with tornado research remain sceptical. Due to the extreme winds, most measuring instruments are usually destroyed or badly damaged. ) Pressures could be estimated with doppler radar, but the drawback is that this isn’t at ground level. In addition, the very lowest pressure inside a tornado is at the top of the vortex, rather than near the ground.)
Oh, Just Great…More Tornado Paranoia
True or False: Tornadoes are always accompanied by visible funnel clouds.
Answer: False. Tornadoes can occur without funnel clouds, as “shown” in this image. Here, the dust cloud and cloud base above it were rotating, indicating a continuous cloud-to-ground vortex. The lack of a visible funnel can be related to several processes. One aspect is that before the funnel cloud touches down, it hasn’t yet had a chance to ingest dirt and debris, and so it would not yet be as dark. Another very relevant factor is that the rotation and high tangential velocity of a funnel cloud are also accompanied by a significant drop in pressure, which often results in the air being cooled to its dew point, where moisture condenses. But if the air that feeds into the tornado is relatively dry, there will not be enough moisture to make the funnel cloud visible. In this case, it was likely that either the pressure drop and lift in the tornado vortex was too weak to cool and condense a visible funnel, or the air below the cloud base was just too dry (or both). All that said, generally, tornadoes without any funnel cloud are, thankfully, in the minority.
It is still most important to remember however that in the vortex of a tornado, the pressure is relatively uniform along its entire height, but the temperature profile is usually much steeper, being higher near the ground than near the top of the vortex. This is the main reason that the funnel clouds that are the most easily recognizable parts of a tornado usually seem to descend from the cloud base. However, in fact a vortex may already extend from inside the cloud, all the way down to the ground. When the air pressure inside the vortex drops, due to increased wind speed, it causes the ambient moisture in the air to condense. As the pressure inside the vortex drops, the moisture condenses, often beginning at the top of the vortex, where the temperature is lowest, and continuing downwards, which could give an observer the impression that the funnel cloud is descending. However, if the temperature is warmer near the top of the vortex, surprise! The funnel cloud suddenly appears and begins to rise to join the cloud base.