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1. ”why are some clouds dark?” – Karen

Cloud particles are excellent and very efficient scatterers of sunlight; only the condensation nuclei in the center (if dark) will absorb light. A solar photon will experience many scattering events while traversing a cloud. (The distance between scattering events in a cloud is approximately 3 meters.) The scattering, which is by the Mie process, is predominately in the forward direction and retains its original color, but with the frequent scattering events the final direction becomes somewhat random. Cloud tops and illuminated sides are white (Sun color) because what we see is being scattered by cloud particles near the edge. Small clouds also have white to light gray bases because the exiting photons have experienced limited scattering. For tall or thick clouds the photons exiting the base are small in number (dark) because most of the Solar photons have been scattered outward higher in the cloud.

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2. Hailstones – Tracing Their Development

Arthur and family members experienced a severe hailstorm while on a Texas Archeological Society Field School in the Texas Panhandle this past June. This storm produced softball size hail; Arthur collected some of the hailstones and has produced a short slide lecture on the development of baseball size hailstones. To view a PDF version of the lecture which includes photographs of hailstones and their structure, go to Arthur Few’s Web Site where you can download the PDF file.

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3. Noctilucent clouds seen in Nebraska. – Ken

This summer there have been documented sightings of noctilucent clouds as far south as Montana(2), Wyoming(1), and Nebraska(1?); the usual region for seeing them is northward into Canada.

NASA photo of a Noctilucent Cloud.

Credit: NASA/Dave Hughes, 7/2/2011, Edmonton, Alberta Canada.

Noctilucent clouds are very special for a number of reasons:
1. They are very high, between 75 and 85 kilometers, in the upper mesosphere just below the region of the aurora and the ionosphere. See figure below.
2.  They form in a region where there should, by ordinary processes, be no water; yet they are composed of ice.
3. They may be a recent phenomenon; there are no reports of sightings prior to 1885, and there is an indication that they are increasing in frequency.
4. They are only visible at night and when the Sun is 6 to 16 degrees below the horizon so that the cloud is still illuminated.
5. They only appear in the summer, and usually between 50º and 70º latitude.

So, how do they exist?

1. It is very unlikely that water from the lower atmosphere can reach the altitude of noctilucent clouds because of (1) the cold trap at the top of the troposphere where the temperature is around -60º C and (2) the lack of convection in the lower stratosphere. [ At -60ºC & 200hP only 1 in 10,000 molecules can be water; whereas, at mean surface conditions 1 in 100 can be water molecules. One might say that a function of clouds is to prevent water from reaching the upper atmosphere. Without convection in the lower stratosphere there is no dynamic vertical motion to transport water upward.]

2. Several possible sources of the water are (1) micrometeoroids which bring water and dust into the upper atmosphere, (2) chemical reactions of methane with ions in the upper atmosphere producing water, and (3) space-bound rockets using hydrogen and oxygen fuels whose product is water. [Micrometeoroids burn up upon entering the upper atmosphere and do produce water and dust needed to form the clouds. But, why no noctilucent clouds seen before 1885? The methane source can add additional water and would be consistent with the human-produced increase in methane and increasing frequency of noctilucent cloud sightings. There have been confirmed instances of small noctilucent clouds associated with space launches; however, the rockets spend so little time passing through the upper mesosphere that they could not by themselves account for the needed quantity of water.]

3. Extremely cold temperatures are required to produce ice clouds with the scarce water available at these altitudes. [The absolute coldest region of the atmosphere (-100ºC) is in the high latitude mesosphere in the summer. One would expect that noctilucent clouds would be visible throughout this region, but inside the arctic circle the Sun never sets so there is no darkness. The usual observable latitudes are from 50º to 70º.

ref.http://en.wikipedia.org/wiki/Noctilucent_cloud

Graphic above from Meteorology Today by Ahrens

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4. Explain the Moon’s Major and Minor Standstills. – Karen

The Moon’s orbit is inclined ~5.1º to the ecliptic (the plane containing the Earth’s orbit). The Earth’s rotational axis has a angle of 23.5º to the ecliptic. Owing to gravitational influences the Moon’s orbital tilt axis rotates with respect to the Earth’s spin axis with a period of 18.6 years. When aligned parallel the two axes add to produce a Moon declination of 28.6º; this is the Major Standstill. 9.3 years later (half of 18.6 years) the Moon’s orbital axis is anti-parallel to the Earth’s spin axis, and the Moon’s declination is 18.4º; this is the Minor Standstill. The last Major Standstill was centered on 2006 and the next will be centered on 2025. The next Minor Standstill will be centered on 2015. The period is long, and I use the term “centered on” because the standstills do not change much  in a couple of years and vary with the observer’s latitude. To view a slide lecture (PDF) on the Standstills, which includes a photograph of a Moon set in Gold Hill near the last Major Standstill, go to Arthur Few’s Web Site where you can download the PDF file.

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5. Spectacular lightning photo from Gold Hill. – Gary Siemer

The photograph above was taken by Gary Siemer around 10P on Thursday August 6, 2009, from in front of the Gold Hill Inn. This is a time exposure (~30 s) using a tripod. In the photo we see five lightning flashes all from the same thundercloud and occurring within the 30 second exposure. Photograph used with the permission of Gary Siemer.
Take note of the bright spot on the cloud base where the flash exits the cloud.

Flash 1. This is the brightest of the flashes and probably the first. In technical terms this is a negative, multi-stroke, forked, cloud-to-ground flash; it is fairly common. Negative means that negative charge is transported to the ground. Multi-stroke means there is more than one current surge from the ground. Forked means that at least two of the branches contact ground; some of the dimmer branches may or may not have made ground contact.

Flash 2. The channel of this flash passes in front of the bright exit of Flash 1 making it difficult to trace the channel, but it appears that the channel branches in this region with one branch going to the right and the other proceeding in the original direction. The channel to the right branches again with the upper branch going toward the cloud base. The lower two branches terminate in the air.

Flash 3. Is highly tortuous as it proceeds downward at ~ 45º angle. About half way to the ground it branches; the lower branch executes an exotic dance, going first down then back up appearing to wrap around itself. (We are viewing a two dimensional projection of a three dimensional channel.)  The upper branch of channel #3 proceeds upward passing in front of (?) channel #4 and into the cloud base.Technically this is an Intra-cloud discharge. There is a positive charge on the base of the cloud called a screening layer.

Flash 4. This flash is called an air discharge for obvious reasons; it does not terminate.

Flash 5. This is a winner and somewhat unusual. It is an upward discharge; we know this from the way that channel branches upward and into the cloud base. It probably originated from a distant mountain peak or power-line tower. Tall structures (e.g. the Empire State Building) frequently eject these upward flashes. This flash was probably triggered by the large electric field surge from nearby Flash 1 and occurred immediately following #1.

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6. What makes tsunamis so different and destructive?
 
A tsunami is a special type of gravity wave. Gravity waves can occur in any fluid in which density decreases with height. In the atmosphere density decreases with altitude; thus we have atmospheric gravity waves in the stable layers of the atmosphere. Oceans have constant density; however, at the surface (ocean – air interface) the density decreases by approximately a factor of 1000. This provides an excellent condition for gravity waves. All ocean surface waves are gravity waves. The wavelength of an ocean surface wave is the length between adjacent wave peaks. When the water depth is greater than the wavelength the waves are ordinary or deep-water waves; when the water depth is smaller than the wave length then they are shallow-water waves, and you can get tsunamis. The average depth of the oceans is 3.8 km; thus the wavelength of an ocean tsunami is many km. Over the open ocean the height of a tsunami will be less than 1 m making them difficult to detect. An important property of tsunamis is that the deeper the water the faster they travel. As the tsunami approaches land the leading part of the wave slows down while the following part catches up forming a very large and destructive flooding wave.
 
To view a PDF version of a lecture on tsunamis, go to Arthur Few’s Web Site where you can download the PDF file.

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7. Observations of Equinox sunrises on the Gold Hill Town Meadow?


 

There are two photos above; the lower photo is of equinox sunrise on 9/22/08 at 7:28, and the Sun is rising directly behind the equinox pole. Clouds obscured the sunrise September 22 and 23, 2009, so no photos were obtained of the sunrise. However, broken clouds on 9/24/09, equinox + 2 days, allowed sunrise photos between gaps in the clouds. The upper photo was taken 9/24/09 at 7:36; the sunrise this day was at 7:34, but the Sun was behind a cloud at that time. Had our sunrise photo 2009 been taken on the Equinox, the Sun would have been behind the pole as it was in 2008; note how far to the south the sunrise has shifted in two days. Computations show that in two days, the time of sunrise is delayed by two minutes, and the angle to the sunrise position shifts 1º southward.

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8. Is there a special “Blue Moon” on New Year’s Eve?

On New Year’s eve, 2009, we will have a Blue Moon. The astronomical Blue Moon has been defined in various ways over time, but mostly (since 1946) it relates to the occurrence of a second full moon in a calendar month. The last time that we had a Blue Moon on December 31 was in 1990, and the next time will be 2028. Months with 31 days are more likely to have a Blue Moon, and poor February can never have a Blue Moon.

The time between full Moons is 29.53 days. The mean year (including the leap-year effect) is 365.25 days. Thus the number of full Moons per mean year is 12.37. So, each mean year we have 12 full Moons, and we gain an extra 0.37 full Moons left over per year. The inverse of 0.37 is 2.7 or roughly 3 years. Approximately every 3 years we will have 13 full Moons, so in one of the 12 months (except February) we will have two full Moons and the second one is called a Blue Moon.

A previous method of defining a Blue Moon employed a solar-based calendar. The first day of the year was winter solstice and there were four quarters: Winter solstice to spring equinox, spring equinox to summer solstice, summer solstice to fall equinox, and fall equinox to winter solstice. In a normal year each quarter had three full Moons, but on the approximate 3-year cycle one of the quarters would have four full Moons. The Blue Moon was designated as the third full Moon in the quarter having four full Moons. Why the third? The church, using the ecclesiastical calendar determines the date of Lent and Easter using full moons; the Lenten Moon is the last full Moon of winter, and the Easter Moon is the first full Moon of spring. Easter is then the Sunday following the Easter Moon, and Lent starts on Ash Wednesday 46 days before Easter. The Lenten Moon occurs during this 46 day period.  Since the dates of Lent and Easter are determined by full Moons and the equinox, having a fourth full Moon in the winter quarter would have to be the Lenten Moon, hence the Blue Moon would need to be the third. If the extra full Moon was the third then the ecclesiastical calendar would remain in the designated bounds. It is unclear when or why the term blue became attache to the third full moon in a quarter. Thankfully this usage has been trashed in favor of the second full moon in a calendar month.

See the photo below. As the Blue Moon was setting in the west on January 1, 2010, the Sun was rising in the east; in the photo the tops of the trees in the west are just being illuminated by the rising sun.

My thanks are extended to daughter Alice Few (web site) for providing Blue Moon websites (NASA, Sky and Telescope)
 
On December 31, 2009, we had a rather rare event; a Blue Moon occurring on New Year’s Eve. A Blue Moon is a second full Moon in a calendar month. This event will occur on New Year’s Eve only every 19 years; if you’re lucky and healthy you can experience this event again in 2028.

This photo was taken from our Gold Hill home as our Gold Hill Blue Moon was setting on 1/1/10 at 7:23 MST.

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9. Why does the ash plume coming from the Icelandic volcano sometimes create lightning? – Ken Fernalld

Arthur’s response to the volcano lightning question has been moved to his web page. You can download the PDF response with the photographs at: http://www.ruf.rice.edu/~few/

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10. Which day is the fall equinox, the 22nd or the 23rd? The TV news and the newspaper disagree. – Cherry
 

From an Earth-based perspective fall equinox corresponds to the passage of the Sun from the northern to the southern hemisphere. There are a couple of ways of visualizing this process. The subsolar point on the Earth’s surface is the point directly below the Sun. At that point the Sun would be directly overhead, at your zenith. At that moment no other place on Earth would see the Sun directly overhead. Subsolar points exist only in the tropics, between 23.5º north and 23.5º south. At the fall equinox the Sun’s subsolar point crosses the equator from north to south. We can also consider the Sun at local noon; at the equator on the fall equinox the noon sun moves from north to south. However, if you live at 40º north latitude, as we do, the noon Sun is never north of overhead. It moves from a little south to way south. Only if you are equator-ward of 23.5º (the tropics) will the noon Sun ever appear north of overhead. The sunrises and sunsets are more complicated and much more interesting for those of us not living in the tropics. The simplified version of this is that in northern hemisphere summer the sunrise and sunset points along the horizon are north of due east and due west; in winter the sunrise and sunset points are south of due east and due west. This movement of the sunrise and sunset positions is something that we (and all human cultures in the past) can easily observe. Stonehenge is a good example, and it is only one of many similar observatories.

The astronomical definition of equinox employs the ecliptic coordinate system defined by the Earth’s orbit about the Sun. In this system the Earth’s spin axis is directed 23.5º with respect to the ecliptic plane, but the direction of the spin axis is always pointed to a fixed direction in space near the North Star. As the Earth rotates around the Sun the relationship between the direction from the Sun to Earth and the Earth’s spin axes changes from  -23.5º to +23.5º; the equinoxes (fall and spring) are defined as the moments in time when this angle passes through 0º. Equinoxes are moments in time not a designated day.

The common definition that day and night are equal on the equinox is not exactly true; the equal day and night date depends upon the time of the equinox and your latitude and longitude. This year, 2010, the fall equinox occurred at 3:09 UT on September 23; this corresponds to 9:09 p.m. MST on  September 22. To illustrate that the length of night and day are not necessarily equal on the equinox we can calculate the length of the day (sunrise to sunset) in Gold Hill (latitude 40.06º, longitude 105.41º). On September 22 it is 12 hours and 10 minutes; on September 23 it is 12 hours and 7 minutes. The date that is closest to the equinox having equal day and night is September 26; on which the day is 11 hours and 59 minutes, one minute off equal day and night.

In Gold Hill we have traditionally celebrated equinox sunrise as opposed to sunset. The main reason being our topography lends itself best to observing sunrises. Which date, September 22 or 23, is most appropriate? Sunrise on the 22nd occurred 14 hours and 20 minutes from the equinox at 9:09 p.m.; whereas, on the 23rd it was 9 hours and 41 minutes. Sunrise on the 23rd is closer to the equinox than sunrise on the 22nd. That said; I made equinox observations on both dates as well a several days bracketing the equinox.

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11. Fall equinox, Part 2, Special Days. Arthur


Some special days are special because of connections with the past such a birthdays, anniversaries, national holidays, etc. Other special days are special in their own right just because of the confluence of events of the day.  The two fall equinox days of 2010, September 22 and 23, that were the subject of my previous “Ask Arthur” response on “Equinox” (Part 1) are special days of this second kind.
 





On the evening of September 23, the Gold Hill Inn reopened following the evacuations and the 4-Mile Canyon Fire. We joined the packed house and had dinner. Gold Hill was slowly returning to some semblance of normalcy.

During the entire month of September Gold Hill received 0.06” of measurable precipitation. This rainfall occurred on September 22.

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12. The Belt of Venus. Arthur