Q About 20 years ago I read an article concerning a weather phenomenon called a neutercane, which was smaller than a hurricane and larger than a tornado. Is there really such a thing?
Greg Anderson
Omaha, Nebraska
A
Robert Simpson, former director of the National Hurricane Center, is credited with the introduction of the termneutercane. During several reconnaissance flights in the early 1970s, Simpson and a colleague, Robert Bundgaard, discovered small, circular, hurricane-like cyclones, which lacked many of the characteristics associated with hurricanes. These circulations originated over land and appeared to be associated with an upper level cold low. When they left the East Coast and moved over Atlantic water, these cyclones acquired distinct eyes, spiral bands of precipitation, and even an eye wall. One flight documented a warm core, which is typical of tropical storms.
Bundgaard suggested a name for this curious cyclone: neutercane, as an acknowledgment that it belonged to neither of two opposing classes; that is, it could not be considered a pure tropical or a pure extratropical entity. Simpson agreed with the nomenclature and introduced the term in 1973 at the Eighth Technical Conference on Hurricanes and Tropical Meteorology in Key Biscayne, Florida. His main point was that a new type of storm had been discovered, one that deserved further study.
The suggestion was not well received. Up until this time, Atlantic hurricanes had received only women’s names. To some, the word neutercane had the connotation of gender, the hurri- in hurricane suggesting female (her), partly because of the hurricane naming convention, and the neuter- in neutercane implying that hurricane is a compound word, which it is not. The scientific interest in Simpson’s paper was lost in controversy over the name, and Simpson was directed not to pursue the research any further.
The National Hurricane Center used the term neutercane for only one hurricane season, 1973. It has not been used officially since that time.
Q
While serving in the U.S. Navy, I was told by a senior forecaster that the dewpoint in the afternoon would closely correlate with the minimum temperature the following night, provided that the air mass remained the same. Is this a good rule of thumb?
Joe Brulotte
White River Junction, Vermont
A
This rule of thumb is valid under the restrictive conditions of a clear sky and light or calm wind. A clear sky ensures that surface radiative cooling will be maximal. Clouds radiate more energy toward the ground than a clear sky; thus overcast skies will minimize surface radiative cooling and keep the surface temperature up. As surface cooling proceeds, air in contact with the ground is also cooled, forming a surface-based inversion that is initially just a few feet thick but can grow as thick as hundreds of feet. Night breezes destroy this inversion and thus keep the surface temperature elevated, which is one reason for requiring light wind or calm conditions. Light wind also ensures that a different air mass, bringing changes in temperature or moisture content, does not move in overnight.
Why should this rule of thumb work? The afternoon dewpoint is usually close to the day’s minimum value because any moisture evaporated from the ground is mixed through a deeper layer of air extending up from the ground than at any other time of day. During the night, if the ground temperature reaches the dewpoint temperature of the air, condensation begins, and dew or frost forms on the surface. The loss of water vapor from the air decreases its dewpoint near the ground, and release of the latent heat of condensation slows the drop in temperature. Should shallow ground fog form, the effective radiating surface becomes the top of the fog layer rather than the ground and the drop in surface temperature ceases. To some extent, the surface cooling is self-limiting once the dewpoint temperature is reached.
It would be interesting to learn which is a better predictor of tomorrow morning’s minimum temperature, given clear skies, light winds, and no change in airmass: this morning’s minimum temperature or this afternoon’s dewpoint.
Q
In the May/June issue (p. 60), one important feature of my photo was not addressed: that the two parallel lightning paths diverge in the lower part of the photo and take different paths to the ground. Why does the lightning jump from one place to another? Can wind play a role? And is this less common than lightning that strikes the ground at multiple locations simultaneously?
Tom Adams
Marblehead, Massachusetts
A
Before addressing this question, I would like to correct some misinformation I conveyed in my May/June column. I said that lightning with downward branching indicates a “negative” stroke, one that transfers negative charge to the ground. Not so. Downward branching merely indicates that the lightning was initiated within the cloud.
The process works as follows: An electrically charged channel propagates rapidly from the cloud toward the ground in a series of discrete steps. The path traveled by this stepped leader branches downward and is conductive. One branch of the stepped leader will be the first to approach to within a few tens of meters of the ground or some exposed object. When this happens, another leader from the ground moves upward to connect with it. Once connection is made, a conducting path is established between ground and cloud. A luminous return stroke ensues as charge stored in the leader channel comes to the ground. All the pathways taken by the stepped leader light up, not just the path that connected with the ground. In summary, when the branches of the return stroke point downward, this indicates that the stepped leader originated in the cloud.
If the stepped leader carries negative charge, the return stroke lowers negative charge to the ground. This is called a negative discharge. If the stepped leader carries positive charge, the return stroke lowers positive charge to the ground. This is called a positive discharge. Negative discharges are usually associated with multiple strokes. Following the first return stroke, a so-called dart leader comes from cloud to ground in a single step, usually but not always along the same path established by the stepped leader, and initiates another luminous return stroke. This happens an average of three to four times in negative discharges but seldom more than once in positive discharges, which are much less common than negative discharges. Since the photo points to at least two return strokes (we see them because they were separated by the wind), the evidence points to a negative discharge.
According to lighting expert Ron Holle of Vaisala, Inc., in Tucson, Arizona, in 30 to 50 percent of negative lightning flashes, the discharge takes multiple paths to the ground. In most cases, such as illustrated in your photograph, the first return stroke takes a different path to the ground than the second or subsequent return strokes. It is often assumed that the wind is involved with the change in path, but the mechanisms and processes are not well understood. In the photo, the parallel paths of the two return strokes are made visible by the wind, but there is a branch point near the bottom of the photo at the apex of the inverted “V.” One stroke, following the other by tens to hundreds of milliseconds, chose to take a different path to the ground. It is not unusualfor the attachment points at the ground to be a kilometer or more apart. Multiple strokes taking different paths tothe ground are much more commonthan a single stroke with multiple (simultaneous) attachment points at the ground, though the latter has been observed and photographed.
Q
Each fall meteorologists offer leaf foliage forecasts for the New England region that are unreliable. Has there been any serious research on foliage forecasting? This may seem unimportant, but in New England, many millions in tourist dollars are at stake.
J. Gerald Phillips
Fitchburg, Massachusetts
A
Predicting the date of peak fall color remains an inexact science. The average time of peak colors varies by nearly a month in New England, earlier in the north and at higher elevations, especially in the Green and White Mountains, and later in the south near sea level.
The color of most leaves are determined by three kinds of pigments: chlorophylls, carotenoids, and anthocyanins. Chlorophylls, which create the green color found in growing plants, participate in photosynthesis, a process plants use to manufacture carbohydrates. Carotenoids impart a yellow color to leaves after the chlorophyll breaks down in autumn. Anthocyanins are red pigments; their production speeds up under intense light and low (but above freezing) temperatures. Photosynthesis declines at lower temperatures, and scientists now believe that the production of anthocyanins helps maintain the efficiency of photosynthesis within leaves, late in their life cycle.
The retention or production of pigments determines the color of leaves before they fall to the ground. As the days shorten in early fall, the loss of chlorophyll accelerates, allowing the carotenoid pigments to show their yellow color. If anthocyanins are present as well, the leaf color may be various shades of orange (a combination of yellow and red). Depending upon the uniformity in the mix of pigments, whole trees may have the same color, or parts of a tree and even a single leaf may be multi-colored.
Summer drought stresses trees and may cause an earlier than normal fall of leaves and less brilliant coloration. Anthocyanin production is maximized by cool, sunny days in the fall. If the weather is cloudy and rainy, the reds are not as striking. In addition, strong winds or an early hard freeze can end the show prematurely by stripping leaves from the trees.
Scientists have tracked budding, leaf development, flowering, fruit development, fall coloration, and leaf drop for representative tree species at the Harvard Forest, in north central Massachusetts, for over a decade. Many more years of record keeping will be necessary before antecedent weather conditions can be used to reliably predict the dates of peak autumn colors. Further information on fall colors can be found at: http://harvardforest.fas.harvard.edu/research/leaves/autumn_leaves.html.
Contributing editor THOMAS SCHLATTER is a meteorologist at NOAA’s Earth System Research Laboratory in Boulder, Colorado. He is also affiliated with the Cooperative Institute for Research in Environmental Sciences, University of Colorado. Readers are encouraged to submit queries to the author in care of Weatherwise; 1319 18th St. NW; Washington, D.C. 20036; or by email to ww@heldref.org. Submissions without full names and addresses will not be answered. Due to the volume of questions received, personal replies should not be expected.
