Weather-Wise.org

A. The streaks in Photo 3 are caused by contrails from jet aircraft flying along a major flight route. They appear to be radiating from a common point (hidden by lower clouds in the foreground), but as you suggest, the paths are roughly parallel, distorted by perspective. Judging from their spreading, these contrails must have been in place for quite awhile, probably more than 30 minutes. For those who worry that the aircraft responsible for these marks in the sky were flying too close for comfort, I point out that two commercial jets, one following only a minute behind the other, are still separated by about eight miles. Some of the contrails in the photo have spread much more than others, indicating greater age. In addition to the time separation of the aircraft, there was clearly also lateral separation, probably a mile or more.

Q. I have a rain gauge with a one-inch diameter. When I line up a ruler with the markings on my gauge, the ruler says "two inches" whereas the gauge marking says "one inch." Can you explain? —Stan Myslinski, Johnsonburg, Pennsylvania

Q. My class noticed that on a rain gauge we bought, the marks representing inches were more than one measured inch apart. Why? When we hear on the news that there has been two inches of rain, what does that mean? —Sarah Jordan and classmates, Oxford, England

A. At least two types of rain gauges have an exaggerated scale, that is, the markings of inches on the gauge are farther apart than the markings on a ruler. But the markings on the outside of the gauge still provide the correct rain measurement. The scale is exaggerated due to the shape of the gauge.

One type of gauge is called a "wedge" because of its shape. It is widest at the top and narrows toward the bottom. Therefore, the scale is exaggerated, particularly at the bottom, making it easier to read the rainfall amount—especially smaller amounts.

Another type of gauge employs a funnel. The area open to the sky (that catches the falling rain) is often designed to be ten times greater than the area of a tube into which the rain is funneled. With this design, the markings on the tube are exaggerated by a factor of ten: when one inch of rain falls, the inner tube is filled with rainwater ten inches deep. If more rain falls, a small hole near the top of the inner tube allows the extra rain to empty into the larger tube so that up to ten inches or so of rain can fall before the outer tube overflows. The total rainfall is determined by first noting how much is in the inner tube, emptying it, and then pouring the contents of the outer tube into the inner tube until all the rain has been measured.

When you hear that two inches of rain has fallen, it means that there would be two inches of standing water to splash around in if the rain didn’t soak into the ground or drain away.

Q. I wonder if you could help me identify the cloud in the photo. I photographed this cloud over a salt marsh near Wells, Maine, between 9:09 and 9:25 p.m. on July 10, 2003. There were no thunderstorms in sight. The surface wind was from the west, and the temperature was at least in the 60s. The top of the cloud was moving faster than the bottom, and the sheet-like structure stretched out more and more during the 20 minutes I watched it, until it finally dissipated. It resembled a rainshaft, but there was no substantial cloud structure above it. The bottom of the feature seemed to be only hundreds of feet above ground, and the top perhaps a few thousand feet high. —Dennis Skillman, East Kingston, New Hampshire

A. In your photo (Photo 1), most of the sky is filled with altocumulus clouds, which generally appear at altitudes between 15,000 and 25,000 feet. It appears that higher clouds, perhaps cirrus, are above the altocumulus. They are still brightly lit by the setting sun, whereas the altocumulus are mostly in shadow. The amorphous blob in the center of the picture consists of fall streaks—streamers of ice crystals manufactured within the thicker parts of the altocumulus clouds that then fall out of them.

Many altocumulus clouds consist of supercooled water droplets (liquid droplets at temperatures well below freezing). A few ice crystals introduced within such a cloud can cause the quick growth of more ice crystals at the expense of the supercooled droplets, which evaporate, thereby supplying more vapor for the growth of the ice crystals. Once the ice crystals have grown large enough, they fall from the cloud and drift with the wind below the cloud base.

Your careful documentation of the circumstances pertaining to this photo prompted me to try to find a nearby weather balloon sounding. It turns out that a weather balloon was released at 7:00 p.m. on the evening of July 10, 2003, at Gray, Maine, about 15 miles north of Portland. It is a good match in time and location. Here are a few salient features of the sounding:

1. The atmosphere below 16,000 feet altitude was fairly dry—too dry for clouds.

2. Cloud base, where the measured temperature and dewpoint are the same, is well defined on this sounding (approximately 21,600 feet). Winds just below cloud base were from the west at 28 mph, but a few thousand feet higher they were nearly 60 mph, indicating rather strong shear, which easily explains the long, drawn-out streamers.

3. The cloud-base temperature was +5°F, which supports the formation of a particular kind of crystal called spatial dendrites—large, flat, stellar crystals with intricate branches.

4. The freezing level was at 13,700 feet. Thus, the streaks of ice crystals could have remained frozen through an 8,000-foot fall, which, even given a generous fall speed, would take about 40 minutes. The odds are that the crystals would have evaporated before reaching the melting level.

Given this quantitative evidence, it’s safe to say you photographed a dense curtain of ice crystals falling from an altocumulus cloud, where the cloud particles were being rapidly converted from droplets to ice crystals. This altocumulus cloud was probably originally thicker than its neighbors. It’s quite possible that the ice crystals that "seeded" this cloud and caused its rapid glaciation (conversion from a supercooled liquid-droplet cloud to an ice-crystal cloud) came from the overlying cirrus clouds.

Despite appearances, these fall streaks were not close to the ground. In all likelihood, most of the crystals evaporated in the dry air below cloud base before even reaching the freezing level.

If you would like to learn how ice crystals grow at the expense of supercooled water droplets in a cloud that contains both, consult the "Weather Queries" column in the January/February 2003 issue of Weatherwise, page 38.

Q. In January, I observed an unusual phenomenon. The sun had just set behind some low mountains about 15 miles to the west near Medford, Oregon. The only way I could estimate the subtended angle was by the width of my knuckles; it was wider than one knuckle and less than two knuckles at arm’s length. I didn’t see a sun pillar or sun dogs. The sky was clear with a few widely scattered cirrus, but not in the direction of the phenomenon, and there was some atmospheric haze. Can you explain what I saw? —Rick Thowless, Medford, Oregon

A. Your description fits that of a corona—rings of colored light that appear frequently around the sun or the moon. Often they are not noticed around the sun because the light is too dazzling, but when the sun is just below the horizon, coronal displays can be quite striking.

You viewed only the upper portion of a corona because distant hills masked the lower half.

The colored rings are usually blue or indigo on the inside and red on the outside, sometimes with other colors in between. Colors are usually pastel, not intense. The phenomenon is caused by the diffraction of sunlight (or moonlight) by a cloud of small, usually spherical particles of fairly uniform size in the range from 10 to 20 micrometers.

The best coronas occur in thin, lens-shaped mountain wave clouds, through which light easily passes. The smooth flow of air through such clouds helps to ensure uniform particle size. It used to be thought that coronas form only in water droplet clouds, but measurements have shown that clouds of nearly spherical ice particles of uniform size can also produce coronas.

You mentioned that there were no clouds near this corona. It is possible that thin, sheet-like clouds near the horizon escaped your attention. It is also possible that the corona was not caused by clouds at all. Pollen has been known to cause coronas, but this would be unlikely in January. Perhaps haze or smoke could do the trick, but I am not aware of any documented reports of this. The requirement of a narrow range of particle sizes might be hard to meet.

For further information and photos of coronas and a related phenomenon—iridescence—see the October 2003 issue of the Bulletin of the American Meteorological Society, pages 1373–1386.

Q. What is the significance of “standard atmospheric pressure at sea level”? I have seen this expressed in many different units: 29.921 inches of mercury, 760 millimeters of mercury, 33.899 feet of water at 4°C, 1.000 atmosphere, 14.696 pounds per square inch, and 1013.25 millibars or equivalent metric units. Is this pressure actually observed at a certain gauge in the world and at a temperature of 0°C (32°F)? Finally, is there a formula that one can use to estimate the pressure at various altitudes using some type of regular decrease in temperature? —J. Richard Harris, Calgary, Alberta, Canada.

A. This question was partially answered in the July/August 2000 “Weather Queries” column. Standard sea-level atmospheric pressure was defined in 1947 by what is now the World Meteorological Organization as the pressure exerted by a column of mercury 760 millimeters high, having a density of 1.35951 x 10⁴ kilograms per cubic meter, and subject to an acceleration due to gravity of 9.80665 meters per second per second. It may at first seem strange that the acceleration due to gravity should enter into the definition until one recalls that pressure is a force per unit area, and force, as discovered by Newton and expressed in his equation of motion, is the equivalent of mass undergoing acceleration. The precise values of the density of mercury and the acceleration due to gravity together prescribe the force per unit area that will be exerted by a column of mercury 760 millimeters high. The same force per unit area is exerted by a column of pure water 33.899 feet high at the temperature of its maximum density (4°C). At this temperature, the density of water is 1.0000 x 10³ kilograms per cubic meter.

Expressed in physical units instead of the height of a column of a specific liquid, the standard sea-level pressure is 1013.25 millibars (metric units) or 14.696 pounds per square inch (English units). This pressure is actually observed at sea level on many occasions and throughout a large range of temperatures. The estimated global annual mean sea level pressure is not the standard pressure but slightly less, about 1011 millibars.

Standard sea-level conditions are defined by a pressure of 1013.25 millibars and a temperature of 15°C (59°F). There is also a Standard U.S. Atmosphere, defined in 1976, which can be calculated fairly easily from the sea-level values. This atmosphere is idealized in that it contains no water vapor. It is not approximated from statistically averaged observations, but it is sufficiently realistic that engineers use it almost universally in their calculations. The troposphere, where clouds and weather occur, is assumed to be 11 km deep. The lapse rate (decrease in temperature with altitude) is a constant 6.5°C per kilometer. Here are the formulas for computing the standard pressure at a given altitude in the troposphere.

1.) Start with geometric height z expressed in meters. Enter the following formula, designed to take into account the slight variation in the acceleration due to the earth’s gravity as one ascends above sea level. R_o is the mean radius of the earth: 6,356,766 meters.

H = R_o z / (R_o + z)

H is called the geopotential height (meters). The difference between z and H increases slowly from zero at sea level to only 19 meters at an altitude of 11,000 meters (just over 36,000 ft). If precision is not too important, one can make the approximation H = z.

2) Use H (in meters) in the following formula to calculate the absolute temperature in degrees Kelvin (K), bearing in mind that T(K) = T(°C) + 273.15

T = 288.15 K – (0.0065 K/m) H for H #11 km (in the troposphere)

Note that the constant temperature in this equation is just 15°C, the standard for sea level. For each 1,000 meters of ascent, this equation subtracts 6.5°C from the standard sea level temperature.

3) Finally, use T (in absolute degrees K) in the following equation for standard pressure.

P = (1013.25 mb) (288.15 K / T) ^-5.255877 for H # 11 km.

The answer comes out in millibars (mb).

If I wanted to find the pressure in the U.S. Standard Atmosphere at an altitude of 5,000 meters, I would use, in succession, the first equation to calculate the value of H (4996.07 meters), the second equation to calculate T (255.676 K), and the third equation to calculate P (540.48 mb).

Q: I am used to aneroid barometers scaled in inches, but I came across a friend’s German barometer, which is quite old. It has different numbers, incremented 27, 28, and 29 instead of the more usual 28, 29, and 30. And there are three sets of eight tick marks between each number on the dial totaling 24 increments. What kind of measurement system is this? The manufacturer is Naudet and Cie; “phnb” is the hallmark. —Michael Paulus, Portland, Oregon

Q: Why does mist form on the glass inside the car when we close the windows? —Sarah Jordan, Oxford, England

A: The human body loses vapor through the pores of the skin and through exhalation. Thus, when passengers sit in an enclosed car, they are adding moisture to the air, and the dew point rises. If the temperature on the inside of the window is lower than the dew point of the air inside the car, condensation occurs. This is most likely to occur on winter days when the outside air temperature is low. If it’s really cold, frost can form. If many passengers are in the car, it takes a good defroster to keep the inside of the windshield clear of frost. The defroster clears the windows of tiny droplets of frost by heating them to a temperature above the dew point.

Q: I’ve enclosed an image of an unusual icicle that formed after the New England snowstorm of February 16, 2003. The portion of the icicle shown is three to four centimeters in length (less than two inches). The icicle has a prominent spike near its lower tip. The icicle spikes don’t appear to be caused by wind since it was calm when I took these photos, and new spikes formed during the photo session. Has anything been written about these strange appendages? —Russell D. Sampson, Willimantic, Connecticut

A: This question was answered by Charles Knight, a specialist in the physics of ice at the National Center for Atmospheric Research, Boulder, Colorado.

Let’s consider the growing tip, where the icicle is lengthening. Water descends from above and drips off, leaving part of itself behind as ice. The hanging drop at the tip, before it separates, is losing heat to the air (partly by conduction, but mostly by evaporation), so it freezes some on the outside. This leaves a shell of ice on the outside and water inside. In fact, if you probe upward in this cavity with a straw or a toothpick, you will find that the water-filled cavity may extend a long way up into it.

The water may get to the tip by flowing down the outside. In this case, the icicle thickens because the water freezes to the outside. This process leads to the common tapering form of icicles. Early in an icicle’s growth, however, the delivery may be mostly down the central tube when there is an opening at the top where the icicle is attached. The icicle in the photo is about five millimeters in diameter and does not taper, so I expect it grew as a hollow tube with much of the water coming down the inside.

What happens when the icicle stops growing? The water supply is cut off when the active melting stops. The water within the icicle freezes slowly from the outside in. Since ice occupies more volume than water, the interior pressure increases. The expansion upon freezing is relentless. One possible result is fracturing of the icicle; another is the squeezing of water slowly out through a weak spot in the ice shell. The slow expulsion of water can form its own tube, which grows outward while water is squeezed out to its tip. It forms in much the same way as the icicle formed, except at a smaller size.

Consider your photo of the icicle. Note that most of the large air bubbles in the icicle are concentrated toward its center, a result of freezing from the outside in. Almost no air is in the ice so that, as the ice casing grows from the outside in along this straw-shaped icicle, the volume of remaining liquid shrinks, and more and more of the dissolved air shows up as bubbles at the boundary between liquid and solid. Because the center is often the last to freeze, the larger bubbles are concentrated there.

Near the center of the icicle’s tip, just opposite the point where the spike began to grow, there is a small gray patch. This is a cloud of very tiny air bubbles, too small to be seen individually. Directly above, below, and to the right of the patch, and near the root of the spike, are what appear to be cracks in the ice. The area near the root of the spike is relatively free of air bubbles. These features suggest that, as the tip of the icicle froze from the outside in, water was squeezed out to form the spike. The growing spike apparently froze solid before all the liquid within the tip of the main icicle froze, leaving a cavity of liquid encased in ice. Pressure built as freezing progressed, which was eventually relieved when the ice cracked in three different places. At that instant, the dissolved air in the water formed a cloud of tiny bubbles.