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January-February 2016

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Weather Queries

I live in Bayside, northern Queens Borough, New York. My backyard measurements of snowfall are usually higher than those at New York City's official measuring site, Central Park. Could this be due to the fact that Bayside is not as urban as Manhattan and perhaps not as much urban heating, or is this a recurring coincidence?

Anthony Gnafakis

Bayside, NY

A map of New York City shows that Bayside is about seven miles east of Central Park in the northeast part of Queens, bordered by Little Neck Bay on the east. Central Park is in lower Manhattan, bordered by the Hudson River on the west and the East River on the east. Because Central Park and Bayside are so close together, at the same elevation (very close to sea level), and close to large bodies of water, I would not expect significant differences in long-term precipitation averages. I suspect that the difference in snowfall totals is related more to methods of measuring snow than to geographical or climatological differences.

Measuring snow depths during and after a storm is complicated business. Central Park snowfall is measured by the New York City office of the NWS. The NWS practice is to measure snowfall every six hours during a storm, preferably at 6:00 a.m., noon, 6:00 p.m., and midnight, but never more than four times in 24 hours. Snowfall should be measured in a flat, open, grassy area, away from trees and buildings to lessen the effects of drifting or scouring by the wind.

Six-hour measurements should be taken on a snow board, for example, a two-foot square piece of plywood painted white. After each measurement, the snow board should be swept clean and placed at a level even with the snow surface. If the snow is fluffy and the snowboard sinks into the snow, the next measurement may contain a mixture of snow that fell in the past six hours as well as snow that drifted into the depression created by the sunken board. The figure shows a snow board being placed on the surface of existing snow. The blue border and the corner flag make the board easier to find if more snow buries it.

The accumulations on the snow board at each of the four measurement times are added to get the 24-hour total snowfall. Note that snow depth on the ground is not the same as snowfall. The reason is that snow settles after it falls because the weight of new snow slowly compresses the layer of snow already on the ground. Moreover, warm ground can melt snow from below, and, if the air temperature is above freezing, some snow melts after it reaches the ground. If snow starts and stops in between the six-hour measurement intervals, the maximum accumulation during the six hours becomes the snowfall reported for that interval. This is important because sometimes new-fallen snow melts before the end of the six-hour period.

Why all the recommendations? Mainly to promote standardization and uniformity of measurement. For example, why measure at six-hour intervals? If you measure and sweep every hour, you will record more snow than someone who measures and sweeps only once every six hours because of the compression and melting effects noted above. Measurements are most reliable in flat, open areas. Trees and buildings intercept snow that might otherwise reach the ground and alter the prevailing wind flow, creating eddies and gusts. In flat, open areas, piling of snow into drifts or scouring of snow from the ground are less likely than elsewhere. Even a wooden fence or a row of shrubs can cause substantial drifting. Measurements with the board in a grassy area are best because grass contains air spaces, and its temperature adjusts very quickly to changing air temperature. Bare soil or pavement retains heat, and this can melt the snow for hours after it begins sticking to a snow board on grass. Finally, open areas get sunshine. Snow could certainly be measured in the shade, where it stays on the ground longer, but the longstanding convention is to measure snow depth where the sun shines. If everyone followed these conventions, snowfall measurements during storms would be directly comparable. Unfortunately, this is not the case.

If wind causes drifting or scouring, it's advisable to measure the depth in several different locations in open areas a few yards apart and take an average. Be aware, however, that what's measured on the snow board may be slightly less than what's measured in the grass because grass contains many air spaces. Once snow covers the grass, the incremental six-hour accumulation on the grass should closely match the six-hour accumulation on the board.

If you have a rain gauge that collects snow, you can measure the water-equivalent depth after each snowfall and compare that with the Central Park precipitation measurement. My guess is that your monthly totals will be about the same, especially in winter, when precipitation tends to fall more uniformly than in summer. However, one caution: wind makes accurate snowfall measurements very difficult. Undercatch of precipitation is common when updrafts above the open end of the gauge carry snowflakes over the opening when they would otherwise fall in. Even if your precipitation gauge and the one in Central Park are similarly exposed to the elements, the size and shape of the gauge and its shielding from the wind (if any, by what are called wind shields) can result in different precipitation measurements even when the same amount of H2O falls from the sky in both places.

Figure 1.  Positioning a snow board on top of existing snow cover. The blue border and the corner flag mark the spot, should additional snow bury the board.

Several videos from the Community, Collaborative Rain, Hail, and Snow (CoCoRaHS) network give easy-to-follow instructions for accurately measuring snow:

https://www.youtube.com/watch?v=sj37JQnArX4 is short and sweet, about two minutes.

https://www.youtube.com/watch?v=eWbbj57fOcA&feature=youtu.be provides in-depth instruction with copious illustrations. This is a webinar lasting about one hour.

https://www.youtube.com/watch?v=CzWFhbO_NNg is a little older than the first two but intermediate in length, 23 minutes. It's called “Old Classic.”

What makes a cloud appear white versus gray or black?

Rosemary Oellermann

Perryville, Missouri

Permit me to rephrase this question in a more general way: What affects the brightness of clouds? The answer is interesting but not simple. For brevity, I'll consider clouds that contain only liquid water droplets. The discussion will include both sunlit clouds and clouds at night.

Sunlight may be thought of as a beam of incoming quanta of light, called photons, each one carrying a specific energy that defines its wavelength or color on the visible spectrum. The visible spectrum ranges from purple (wavelength near 0.4 micrometers (μm)) through red (wavelength near 0.7 μm), and the mixture of incoming photons comprises what we perceive as white light.

In the vacuum of space, these photons travel undisturbed at the speed of light. When they encounter matter, however, two things can happen. They are either absorbed, in which case the energy associated with them takes another form, or they are scattered, in which case they retain their energy, but their direction of travel is changed.

The diagram in Figure 1 shows a beam of incident light, which may be considered a stream of photons travelling in the same direction, as in direct sunlight. When the photons encounter particles of matter, some are absorbed (not shown) and some are scattered in different directions. Depending upon the wavelength of the incident light and the size and composition of the particles, the scattering may be preferential in direction or not, and more pronounced at some wavelengths than others. Some photons pass through the particle cloud without encountering matter. Those remaining after absorption and scattering are measured in the detector at right, which points in the direction of the incident beam. The energy losses, due to absorption and scattering, are called extinction.

The very molecules of the atmosphere, which are small compared to the wavelengths of visible light, scatter the photons in sunlight selectively by wavelength (shorter-wavelength light is scattered much more readily than longer-wavelength light) and rather indiscriminately in all directions. This accounts for the blue of the sky. Atmospheric particles substantially larger than the wavelengths of visible light, for example, dust, industrial pollutants, or pollen, scatter preferentially in the forward direction (in the same direction as the incident light), and the scattering is essentially independent of wavelength.

Figure 1.  When photons in sunlight, represented by the dark arrows, interact with matter (the dots), some are absorbed (not shown), and some are scattered in different directions (yellow arrows). The remainder (gray arrows) are measured by the detector at right.

Like solid atmospheric particles, cloud droplets, ranging in size from a few to 20 μm, are considerably larger than the wavelengths of visible light, and so they scatter the light preferentially (but not exclusively) in the forward direction.

At this point, a few definitions are helpful. In a cloud, the mean free path of a photon is the average distance it travels before it is scattered by a cloud droplet. The optical thickness of a cloud is a measure of how many times a photon is scattered before it exits the cloud. It is usually measured in the vertical direction. Because cloud droplets scatter preferentially in the forward direction, it may take several scattering events before a photon will deviate very far from its initial direction of travel. After many scatterings, however, say 50 or 100, its final direction is close to random.

The light emitted from an optically thin cloud will still have more intensity along and near the direction of incidence than in other directions. It will have significantly less intensity in the direction opposite the light source. In other words, most of the scattering in an optically thin cloud is forward and much less is backward. In an optically thick cloud, so much internal scattering occurs that the direction of photons emerging from the sides becomes essentially random. As intuition suggests, many small clouds are optically thin, and large clouds are optically thick.

Figure. 2.  Mt. Robson, British Columbia, capped by a large, optically thick cumulus cloud, with optically thin fractostratus clouds in the near foreground. All clouds are sunlit, but the fractostratus clouds scatter comparatively little light toward the camera, whereas the large cumulus cloud scatters much more. The latter cloud thus appears brighter.

Figure 2 shows clouds partially shrouding Mount Robson in Mount Robson Provincial Park, British Columbia. The mountain, the large cumulus cloud over the mountain, and the wisps of low fractostratus clouds (much closer to the camera) are all sunlit, with the sun's rays coming from the left and behind the photographer. The nearer clouds appear gray, not white. Why? The large cumulus cloud over the mountain is optically thick; photons entering the sunlit portion of the cloud, visible in the photo, are scattered so many times that many of them emerge traveling toward the camera. This makes the face of the cloud very bright. In contrast, the fractostratus clouds are optically thin. Forward scattering dominates within these clouds so that little of the light exits from the cloud in the direction of the viewer. Consequently, these clouds appear comparatively dark. Note in passing that the snow on Mount Robson is even brighter than the large cumulus cloud. The reason is that snow cover, even a few inches of it, is optically very thick.

When you view a large, cauliflower-like cumulus cloud, many thousands of feet tall and equally wide, you see gradations in brightness throughout the entire surface of the cloud. Because cloud droplets are very poor absorbers of sunlight, gradations in brightness are caused almost entirely by variations in the intensity of light scattered toward the eye from different parts of the cloud. Most of the light scattered out of the cloud comes from the sunlit side. Since there is no internal source of light within the cloud (forget about lightning), light scattered out on the sunlit side is not available for scattering deeper inside the cloud, and so the deeper one penetrates into the cloud the darker it gets. That's why the bottom or the shadowed side of a thick cloud appears gray. Much of the incident light has been scattered out through the top and sunlit side of the cloud before it can reach the bottom or the shadowed side.

You can experience this firsthand by observing what happens when your commercial flight ascends through a thick cloud layer. Note first that the light intensity is uniform in all directions because of the intense scattering. In cumulus clouds, it is not unusual to have 200 droplets per cubic centimeter (cm−3). This may seem like a lot, but the volume occupied by 200 droplets with a nominal radius of 5 μm is only about 10−7 cm−3. To a photon entering this volume, the space is essentially empty, but the path a photon takes through an optically thick cloud is long, ensuring that it will be scattered so many times that its direction of travel becomes essentially random. This ensures uniform internal brightness.

As your altitude increases, the intensity of light increases because there are more photons to be scattered and hence more scattering events. Before you reach the top of the cloud layer, the light may become too dazzling to look at without sunglasses.

Figure 3.  Ship tracks are obvious in thin, shallow, low-lying stratus clouds off the California, Oregon, and Washington coasts. The text explains how they form.

The above effect would occur even if droplet concentration (the number per cubic centimeter) were constant throughout the cloud, but this is seldom the case. Here is a striking example. For many years, satellite observations have shown bright streaks in low-lying clouds over the ocean (Figure 3). These streaks mark ship tracks. Sean Twomey first explained these streaks in 1974. Cloud droplets form on cloud condensation nuclei (CCN), minute particles upon which water vapor readily condenses when the relative humidity is close to 100%. CCN are always present in the air but usually in smaller concentrations over the ocean, where the air is cleaner, than over land. Passing ships release CCN through their stacks, increasing their concentration by orders of magnitude. The effect is to produce large numbers of small droplets within the exhaust plume, whereas the natural low-lying cloud before the ship's passage contained smaller numbers of larger droplets. This marks a change in the distribution of drop sizes and perhaps an increase in the water mass per unit volume (the extra mass coming from vapor in the ship's effluent). Higher concentrations of droplets means more scattering, which, in turn, brightens the cloud forming within the exhaust plume. In general, if two clouds contain the same amounts of liquid water per unit volume, the one with more droplets and smaller drop sizes will be brighter than the one with fewer droplets and larger drop sizes.

Figure 4.  Clouds illuminated from below by the city lights of Berlin, Germany.

What about the brightness of clouds in the night sky? The sources of ground light are widely distributed. It comes from streets, buildings, ballparks, and parking lots. This light enters the base of clouds, and some of it is scattered back toward the ground, creating a dim, often orange glow, corresponding to the mixture of wavelengths emitted. The lower the cloud, the brighter the glow. You have perhaps experienced an extreme case of this when ground fog scatters light from street lamps, creating an eerie glow. Figure 4 is a striking image taken by Christopher Kyba, who works at the German Research Center for Geoscience and studies the ecological effects of artificial lights at night.

Figure 5.  Clouds appear black against a starlit sky in Glacier National Park, Montana, far from any significant source of ground light.

I was once driving eastbound late at night on I-70 through the empty desert of eastern Utah. I could see the glow of lights on mid-level clouds above Grand Junction, Colorado, from more than 40 miles away.

On moonless nights, far from any sources of ground light, clouds appear black against the starlit sky (Figure 5). If the sky becomes overcast with thick clouds, your surroundings become inky black—so black that you can't discern your hand in front of your face, even after your eyes have adjusted to the darkness.

To summarize, the brightness of a cloud strongly depends upon the intensity and spectrum of the light that shines upon it and the scattering of that light by droplets within the cloud. Extinction of light within the cloud is controlled almost entirely by scattering. Absorption plays an insignificant role.

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, 530 Walnut Street, Suite 850, Philadelphia, PA 19106.

I want to acknowledge Craig F. Bohren, whose book Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Dover Publications, 1987), was the primary reference. This book includes material that appeared in Bohren's Weatherwise column from 1981 through 1987. His writing style is lucid, engaging, and tailored for the nonspecialist.       

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