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July-August 2011

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Weather Queries

In late morning on October 14, 2010, I saw a cumulus cloud with a dark base well south of me. I had earlier seen streets of cumulus clouds moving north-northwest, so I hurried to the beach, where I took a picture of a funnel cloud and its associated cumulus. It was windy, slightly onshore. I heard no thunder nor felt a drop of rain. The cloud had a spreading top covering a much larger area than the updraft itself (see photos, to the right). It appeared very turbulent. The funnel formed over Massachusetts Bay and moved onshore before dissipating. Can you explain what was happening?

Tom Adams
Marblehead, Massachusetts

The previous issue (May-June 2011) featured a question and answer about small funnels, sometimes mere wisps, either protruding from a cumulus cloud or even detached from one. The funnel you've photographed is more than a “mere wisp,” but still falls into the class of small funnels not associated with deep convection or thunderstorms. This statement is the result of an examination of weather conditions in Massachusetts on the morning of October 14, 2010.

I checked the rawinsonde (weather balloon) soundings closest in time and location to your sighting: at 8:00 a.m. and 8:00 p.m. EDT at Chatham, Massachusetts (on Cape Cod); Upton, New York (on Long Island); Gray, Maine (on the southeast coast); and Albany, New York. A trough lay to the west of New England, which would not pass until late in the day. The wind was from the southeast at low levels, south-southwest in mid-troposphere, and southwest in the high troposphere. Based on the temperature and dewpoint soundings, I estimate that, at the time of your photo, a moist, well-mixed layer existed from the surface up to perhaps 8,000 feet, then a stable layer above that, and finally a less stable layer with humidity above 60 percent from 20,000 feet to the top of the troposphere.

The convection, which appears robust in your first photo, could hardly have been more than 10,000 feet deep. That you heard no thunder and felt no raindrops is consistent with this because lightning seldom, if ever, occurs in convective clouds fewer than 10,000 feet deep, though drops can reach the ground from such clouds. The air mass was not tropical in origin (surface temperature and dewpoint were too low), but, at low levels, it did have a trajectory over the ocean. The steep lapse rate, brisk wind, and high humidity in the lower troposphere explain the cumulus convection, but the latter was capped. Your second photo shows spreading of some of the cumulus tops like a cottony, quilted blanket, but this spreading would have been at the stable layer in the mid-troposphere, well below 15,000 feet.

This is certainly not a case of an early cold air mass being heated from below by warm land and ocean. In such cases, cold-air funnels can form, but they seldom touch down or do any damage.

By process of elimination, we can conclude that this funnel comes from shallow convection and is in the class described in the previous installment of “Weather Queries” Meteorologists still don't know the specifics about how such funnels form. There has to be prior rotation where the funnel forms, and the rotating column of air must be stretched vertically to make it spin faster. Vertical acceleration of air within the updraft of the parent cumulus cloud probably causes the stretching, but the source of initial rotation is still an open question.

Caption: A small funnel protruding from the dark base of a cumulus cloud over Massachusetts Bay, late morning, 14 October 2010.

Caption: Spreading tops of cumulus clouds in the same area.

Why did the National Weather Service switch from the Fujita Scale to the Enhanced Fujita Scale when assessing the intensity of tornadoes?

Murray Brown
Long Beach, New York

If you have Weatherwise issues from a few years back, check the article by Sean Potter (March-April 2007, pp. 64–71) about the transition from the old scale to the new. The old Fujita scale had four flaws:

  1. It was not based upon a correlation between damage descriptions and wind speeds. For example, damage from an F3 tornado is described thus: “Roofs and some walls torn off well-constructed houses; most trees in forest uprooted; skyscrapers twisted and deformed with massive destruction of exteriors; heavy cars lifted off the ground and thrown.” The wind speeds associated with this destruction supposedly range from 158 to 207 mph, but these were only educated guesses when Fujita first proposed his tornado scale in 1971. Modern engineering studies suggest that winds of 136–165 mph could cause the same damage.

  2. The original scale was difficult to apply consistently. For example, was an overturned car merely rolled or had it become airborne? If just one house in the neighborhood lost some walls but none of the others did, is this F2 or F3 damage?

  3. The original scale failed to account for differences in construction. Many homes in Oklahoma, in the middle of “Tornado Alley,” have hurricane clips or straps that fasten the roofs to the sidewalls. Such a home might survive a tornado, whereas a homeowner who had not taken this precaution during construction might lose his house in the same tornado.

  4. The Fujita scale was unhelpful for types of damage not indicated in the written descriptions. For example, how much wind does it take to destroy a mobile home? A cyclone fence with metal posts buried in concrete? A town water tank? A shopping center? Suppose an F4 tornado hit a cornfield? How would one know how strong the wind was?

The Enhanced Fujita Scale addresses the four flaws just noted, though not completely. The EF-scale is based upon a number of damage indicators (condensed list):

  • Small barns, farm outbuildings

  • One- or two-family residences
  • Single- and double-wide mobile homes
  • Apartments, condos, townhouses (under 4 stories)
  • Motels
  • Small retail buildings (fast food)
  • Small professional buildings (doctor's office, branch bank)
  • Strip mall, large shopping mall, isolated “big box” stores
  • Automobile showroom or service buildings
  • Schools
  • Office buildings from a few to more than 20 stories high
  • Institutional buildings
  • Warehouses or industrial buildings
  • Service station canopies
  • Towers for communication or carrying power lines
  • Free standing poles (for lights, flags)
  • Trees—hardwood or softwood

For each of these structures, there are categories of damage related to wind speed. A single example, for institutional buildings, is given here.

General description:

  • This applies to hospitals, courthouses, university buildings, state and federal buildings, jails.

  • Buildings range in height from one to ten stories.
  • Roofing materials include fully adhered and mechanically fastened single-ply membranes, polyurethane foam, or copper-clad domes.
  • Structure is normally reinforced concrete.
  • Walls are masonry with cut stone or precast panels, often ornate.
  • Buildings often have balconies, porches, and porticos with heavy façade.
  • Windows are relatively small.

Information on damage to 28 different kinds of structures, including the above example, is available at http://www.spc.noaa.gov/faq/tornado/ef-scale.html. The Enhanced Fujita Scale is considered to be a work in progress. More kinds of structures will be added in the future. Notably absent is a scale of damage to automobiles. Still, the EF-Scale is a notable improvement over the original scale for those charged with damage assessment and tornado classification, and the wind speed estimates are more realistic.

Degree of damage

Damage description

1

Threshold of visible damage

2

Loss of roof covering (< 20%)

3

Damage to penthouse roof and walls; loss of rooftop HVAC equipment

4

Broken glass in windows or doors

5

Uplift of lightweight roof deck and insulation; significant loss of roofing material (> 20%)

6

Façade components torn from structure

7

Damage to curtain walls or other wall cladding

8

Uplift of pre-cast concrete roof slabs

9

Uplift of metal deck with concrete fill slab

10

Collapse of some top story exterior walls

11

Significant damage to building envelope

The wind speeds associated with these categories are given in Figure 1.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 325 Chestnut St., Suite 800, Philadelphia, PA 19106.

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