David Feldman - Science Imponderables

T he expression partly sunny was brought to you by the same folks who brought you comfort station and sanitary engineer . As a technical meteorological term, partly sunny doesnt exist. So while you might assume that a partly sunny sky should be clearer than a partly cloudy one, the two terms signify the same condition. You have merely encountered a weathercaster who prefers to see the glass as half full rather than half empty.

Actually, most of the meteorological terms that seem vague and arbitrary have precise meanings. The degree of cloudiness is measured by the National Weather Service and described according to the following scales:

Where does fair weather fit into this spectrum? Fair weather generally refers to any day with less than a 50 percent cloud cover (thus even some partly cloudy days could also be fair). But even a cloudy day can be termed fair if the cover consists largely of transparent clouds. On days when a profusion of thin cirrus clouds hangs high in the sky but does not block the sun, it is more descriptive to call it a fair day than a partly cloudy one, since one thick cloud formation can screen more sunshine than many willowy cirrus formations.

You might also have heard the aviation descriptions of cloud cover used in weather forecasts. Heres what they mean:

Not many people know what the weather service means when it forecasts that there is a chance of rain. Precipitation probabilities expressed in vague adjectives also have precise meaning:

How does the National Weather Service determine the daily cloud cover in the space age? Do they send up weather balloons? Satellites? Not quite. They send a meteorologist to the roof of a building in a relatively isolated area (airports are usually used in big cities) and have him or her look up at the sky and make a well-informed but very human guess.

T he movement of the straw depends upon the liquid in the glass and the composition of the straw itself. The rapidly rising straw phenomenon is usually seen in glasses containing carbonated soft drinks. Reader Richard Williams, a meteorologist at the National Weather Service, explains the phenomenon:

the rise occurs as carbon dioxide bubbles form on both the outside and inside of the straw. This increases the buoyancy of the straw and it gradually rises out of the liquid.

The gas is under considerable pressure when the drink is first drawn or poured. When that pressure is released the gas forms small bubbles on the sides of the glass and on the straw. As the bubbles grow the straw becomes buoyant enough to float higher and higher in the container.

Occasionally, though, a straw will rise in a noncarbonated beverage, and we didnt get a good explanation for this phenomenon until we heard from Roger W. Cappello, president of strawmaker Clear Shield National. We often get asked how our sources react to being confronted with strange questions. The only answer we can give isit varies. Sure, we like authoritative sources who fawn over us and smother us in data. But we must confess we have a special place in our hearts for folks like Cappello, who make us sweat a little before divulging their secrets. Here is his letter to Imponderables , verbatim, skipping only the obvious pleasantries:

After pondering your question for a while, I decided to toss your letter as I was too busy for this. I later retrieved the letter and decided I would attempt to give you an answer that is slightly technical, mixed with some common sense and some B.S.

First off, I know the action you were referring to had something to do with specific gravity. Specific gravity, as defined by Webster, is the rate of the density of a substance to the density of a substance (as pure water) taken as a standard when both densities are obtained by weighing in air.

Straws today are formed from polypropylene, whereas many years ago they were made of polystyrene, before that paper, and before that, wheat shafts.

Assuming water has a specific gravity of 1, polypropylene is .9, and polystyrene is 1.04. A polypropylene straw will float upward in a glass of water, whereas a polystyrene straw will sink. However, a polystyrene straw will float upward in a carbonated drink as the carbonation bubbles attach themselves to the side of the straw, which will help offset the slight specific gravity difference between water and polystyrene. A polypropylene straw will float higher in a carbonated drink for the same reason. If you put a polypropylene straw in gasoline, and please dont try this, it will sink because the specific gravity of gas is lighter than water.