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November-December 2011

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Weather Queries

I live outside New York City in Westchester County, New York. I remember snow on the ground lasting long after a snowstorm, but over the years, the snow seems to disappear more quickly, and not because of snow plows. So here is my hypothesis: Acid rain is known in the Northeast, so there must be acid snow as well, and the acidity is one reason why it melts more quickly.

Paul Grimaldi
Yonkers, New York

A clarification up front will be helpful. “Acid snow” really refers to the acidity of the melt water, because pH, discussed below, refers to liquid water solutions.

You are correct that acid snow is just as likely as acid rain. Over populated areas, there are always numerous microscopic particles in the air. Some occur naturally, but most are derived from industrial emissions and other human activity. Many of these particles are hygroscopic, that is, they attract water vapor. When water vapor condenses on these particles, they dissolve into a solution that can be either acidic or alkaline, but is most often acidic. That is the origin of acid rain. Snow crystals form on microscopic particles as well, but the particle doesn't dissolve until the crystal melts, often not until after the snow has accumulated on the ground. On their way from cloud to ground, raindrops and snow crystals may collide with other particles and collect them, thereby becoming more acidic, but raindrops are more efficient at this than snowflakes. Raindrops also absorb atmospheric carbon dioxide, whereas snowflakes do not unless they are partially melted. For these reasons, rain is usually more acidic than melted snow.

The pH is a measure of the acidity or alkalinity of a solution on a logarithmic scale from 0 to 14. A pH of 7 is neutral; lower values are more acidic, higher values more alkaline. Distilled water has a pH of 7.0, which is neither acidic nor alkaline. As examples of increasing acidity, carrot juice has a pH of about 6.0, watermelon is 5.4, tomato juice is 4.3, orange juice is 3.7, and lemon juice is 2.3.

Melted snow in the eastern United States normally has a pH between 4.6 and 6.7. Does snow with this degree of potential acidity melt at a lower temperature than snow with a pH of 7.0? No. The depression of the freezing temperature of an acidic solution is negligible for pH values down to 4.6 and considerably lower, so one must look elsewhere to explain why snow on the ground might not linger as long as it once did. Several alternative explanations come to mind. Winters are warmer in the Northeast than they were long ago, though the last two winters are a temporary reversal of this trend. Perhaps more airborne dust or soot settles on the snow now than in the past, making the surface darker, thus absorbing more energy from the sun than a pristine white surface would and melting the snow more quickly. The length of time snow remains on the ground also depends upon the duration of the cold spell that accompanies the snow, which can range from just a day to several weeks.

Can you explain the “Norlun trough”? I live on the coast of Maine, where it is not uncommon to fall under the influence of this system in the wintertime.

Jim Klick
Freeport, Maine

I confess that I had not heard of a Norlun trough until you submitted your question. Like many other meteorological phenomena (Walker circulation, Rossby wave, Ferrel cell, Bergeron-Findeisen precipitation process, Showalter stability index), this one is named for the meteorologists who first discovered and described it. The “Nor” comes from Steve NOguieRa, and the “lun” comes from Weir LUNdstedt. These two wrote an apparently unpublished report in 1993 on the phenomenon, which can cause brief (less than 12 hours), sometimes unanticipated, intense snowfall in coastal New England.

A trough is an elongated area of relatively low atmospheric pressure. A trough line defines the axis of the trough, in that pressure increases to either side of the line. Pressure along the trough line normally decreases in the poleward direction, but occasionally an inverted trough will form, in which pressure along the trough line increases in the poleward direction. As illustrated in the figure, the Norlun Trough is an inverted trough. The trough line (dashed) is oriented northwest to southeast, extending into New England from a center of low pressure (red “L”) in the Atlantic Ocean. The solid arrows indicate onshore flow north of the trough line and offshore flow south of it.

When forecasters consider the possibility of snow with a Norlun trough, they look for these things:

Caption: Surface pressure pattern associated with a Norlun Trough in New England. Constant pressure contours are shown at two-millibar intervals. The center of the surface low (red “L”) is well offshore. The dashed line is the trough line. Bold arrows show onshore flow north of the trough line and offshore flow south of it.

Caption: Surface pressure pattern associated with a Norlun Trough in New England. Constant pressure contours are shown at two-millibar intervals. The center of the surface low (red “L”) is well offshore. The dashed line is the trough line. Bold arrows show onshore flow north of the trough line and offshore flow south of it.

  • A rapid decrease in temperature with altitude from the ground to mid-troposphere. The lapse rate is another name for the change in temperature with altitude. A practical limit to a temperature decrease with altitude is 1°C per 100 meters of altitude. This is called the dry adiabatic lapse rate. The closer the actual lapse rate is to the dry adiabatic lapse rate, the greater the potential instability in the atmosphere, and greater the likelihood of convection. In winter, if the entire atmospheric column is below freezing, this leads to snow showers and squalls.

  • Relative humidity close to the ground of at least 50% with an onshore wind on one side of the trough. In the figure, note the southeast wind along the Maine coastline. Norlun troughs usually form on the colder, drier side of coastal storms, and so the onshore flow is essential for moving moist, oceanic air inland.

  • Upward motion at 700 mb and positive vorticity advection at 500 mb. For nonmeteorologists: the upward motion cannot be measured, but it is computed in prediction models. If the model indicates upward motion at 700 mb (about 3,000 meters above sea level), that leads to cloud formation. Positive vorticity advection at 500 mb (about 5,200 meters) usually means that a disturbance (a trough) is approaching in the upper-air wind flow. It goes hand in hand with upward motion.

  • The trough axis at the surface is expected to move little for at least six hours.

  • The wind at 850 mb is either very weak or parallel to the surface trough. This helps to ensure that the surface trough will remain nearly stationary.

The Norlun trough can produce snowfall rates of three to four inches per hour for several hours. An obvious prerequisite is that the entire air mass must be below freezing. The prediction of these snowfalls is tricky because (1) they usually occur in narrow bands and are not widespread, (2) they don't last long (less than 12 hours), and (3) the main area of low pressure is usually well offshore, and its extensive precipitation shield is no longer affecting New England.

Thanks to Mike Cempa of the NWS forecast office in Gray, Maine, for help in answering this question.

With the proliferation of electronic weather instruments, an algorithm uses the station's elevation to correct ambient air pressure to equivalent sea level pressure. When I compare my equivalent sea level pressure readings to those from a nearby NWS station, there are discrepancies. By setting the reference altitude to zero, I can determine the actual station air pressure. How can this reading be mathematically corrected to sea level in order to compare with the sea level pressure reported by the NWS?

Bill Rebuck
Elwood, Indiana

There are several reasons why your barometer readings don't agree perfectly with those from the NWS.

  1. Horizontal pressure gradient. Your home and the NWS office are in different locations. Standard sea level pressure is 29.92 inches of mercury. For two points at the same elevation just 50 miles apart, differences in pressure of one hundredth of an inch or so are very common. This is called the horizontal pressure gradient. Sometimes the pressure gradient can be much greater.

  2. Elevation differences. Near sea level, the pressure decreases at roughly 0.01 inch per 10 feet of altitude. You can carry your aneroid barometer up a flight of stairs and see the pressure drop. Your electronic barometer can correct for the difference in elevation between your home and sea level, but it doesn't know anything about the horizontal pressure gradient.

  3. Correction algorithm. There are two common ways, discussed below, to correct the actual pressure at your home to sea level. They give different answers.

  4. Different sea level pressures. The official surface observation from many NWS offices gives sea level pressure in millibars. This is probably a good estimate of what the pressure would be if the office were located at sea level, but this value is seldom reported. By far, the most commonly reported “sea level pressure” is the one that pilots use: the altimeter setting. It is always reported in inches of mercury, and is, almost invariably, the pressure you hear on NOAA Weather Radio, The Weather Channel, and local radio and TV outlets. It's important to note that the altimeter setting is not the real sea-level pressure. It's the pressure used by pilots to set their altimeters so that it reports the altitude of the runway when t0he plane lands.

If there were an atmosphere between the elevation of your home and sea level, an accurate calculation of sea level pressure from the pressure at your home would take into account the profile of temperature and humidity between your elevation and sea level. The effect of humidity on this computation is usually quite small and is neglected here. Since there is only solid ground between your home and sea level, we make an additional assumption that the lapse rate in this fictitious atmosphere is constant; that is, the temperature increases at a constant rate from your home down to sea level. The lapse rate commonly assumed is 0.0065 K m−1 (degrees Kelvin per meter), which is the same as 0.0036°F per foot. The equation that gives the approximate sea level pressure p(0), given the pressure p(z) and temperature T(z) at the elevation z of your home is

The exponent contains three constants:

g = 9.8 m s−2 is the acceleration due to the Earth's gravity.

γ = 0.0065 K m−1 is the lapse rate just mentioned.

Rd = 287 J kg−1 K−1 (joules per kilogram per degree Kelvin) is the gas constant for dry air.

It is essential that the outside air temperature at your home is expressed in degrees Kelvin and your home elevation in meters so that the units are consistently in the MKS system (meter-kilogram-second). The conversion from Fahrenheit to Kelvin degrees is

Also, whatever units you use for the pressure at your home will be the units of the estimated sea level pressure. Because this equation extrapolates the pressure to sea level and assumes a constant but not always realistic lapse rate, it stands to reason that the greater the elevation of your home, the less accurate this extrapolation will be.

Less accurate still is the following extrapolation. It allows a direct comparison between your barometer reading p(z) and the altimeter setting palstg commonly reported by the NWS. This extrapolation assumes a Standard Atmosphere. In a Standard Atmosphere, the sea level temperature T0 is 288.15 K, the sea level pressure p0 is 29.92 inches, and the lapse rate is γ (temperature decreasing with altitude throughout the troposphere).

This formula is more complicated than the one above, and you certainly would not want to compute the altimeter setting with a hand calculator. On the other hand, the formula has the considerable advantage of not requiring you to measure the outside air temperature, and the calculated result is directly comparable to the sea level pressure commonly reported. Thus, I prefer it to the formula previously given. Be sure to use MKS units everywhere except for pressure. Because the pressures always appear as ratios, it does not matter what units are used, so long as all pressures are expressed in the same units.

Again, the computed pressure will not be the true sea level pressure because the atmosphere is seldom “standard.” This may be the formula programmed into your electronic barometer, but is it hard to know. If it is, and you are not so far away from an airport that the horizontal pressure gradient is a problem, you should find agreement (to within a few hundredths of an inch) between the airport's altimeter setting and your “station” pressure reduced to sea level using the Standard Atmosphere.

Did you know that it's now easier than ever to submit questions to this column? Just use our new email address: weatherqueries@gmail.com. If you attach photos, please be sure that they have a resolution of at least 300 dots per inch when they appear on the printed page. Photos taken with 5-megapixel or higher digital cameras will normally satisfy this requirement. Also be sure to include your name and the city or town where you live (or the nearest town if you live in a rural area). Weatherwise  cannot answer questions in print without this information.

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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