Ask a Scientist

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Q: I am a freshman at Dr. Michael M. Krop High School. There is a thin, halo-like, rainbow around the sun. The sun itself, is giving off a very light, white light. This rainbow-halo was spotted between 12:00 and 1:00 o'clock, when the sun was very high in the sky. I, as well as the entire school of students, would like to know what this is.

--Krishton, Miami, FL U.S.A

A: Dear Krishton,

You hit it on the head - it was a halo. It results from the refraction of sun or moon light by hexagonal plate-like ice crystals that are approximately 30 microns in size. Larger-sized particles give a greater separation of colors than smaller-sized particles. Clouds that contain such ice particles are less common in subtropical locations such as Miami than in areas further north, so it's possible that most of the students have never seen such a phenomenon. Note the related answer on this site on "moon dogs." The halo results from randomly oriented crystals, whereas the sun dogs and moon dogs are specifically the result of horizontally oriented crystals. Near local noontime the viewing angles strongly favor the halo.

Q: I was reading the web article about utilising satellite imagery of glaciers to monitor changes in climate. It is a very good set of pages. I was wondering what is done in such research to account for rock fall on to glaciers covering up the ice. It would seem to me that this would give a inaccurate reading of the actual amount of ice that is changing, especially because of the fact the as a glacier melts it would be expected that more rocky material would be exposed at the surface. As rock that had previously been trapped in the ice would get exposed as ice melts. How is such a discrepancy accounted for or is just assumed minimal and ignored?

--Nicholas, Unknown

A: Dear Nicholas,

You are correct that debris on a glacier affects the accuracy of measuring changes in a glacier terminus from space. Rocks and other material that are included in the ice tend to accumulate on the surface as the glacier melts down. This, as you've suggested, is a greater problem on a retreating or stagnant glacier, and is not generally a concern on an advancing glacier. Sometimes, even on the ground, it is difficult to distinguish the glacier tongue from the surrounding morainal material. The accuracy improves when we have multiple years of satellite images to study. In spite of such problems, the accuracy of the measurements have been validated by field measurements and can be as high as plus or minus 30 meters (the size of a Landsat pixel).

Q: I have a question concerning phytoplankton. What happens when they die, in that how does carbon get recycled?

--Wajeeha, Unknown

A: Dear Wajeeha,

Phytoplankton are grazed by zooplankton which package their waste products into fecal pellets that sink through the water column. Most of these particles decompose or are consumed by other orgamisms in the surface layer, hence recycling the carbon and nutrients. Only about 1% of sinking organic material reaches the seafloor.

Q: I've seen recent news reports suggesting that global warming could "switch off" the warm waters of the Gulf Stream - resulting in a colder climate for North west Europe. Is there any evidence that this is happening now, and what are the best predictions for this effect in future years.

--Phil, Manchester, UK

A: Dear Phil,

There is no adequate computer model to address this question. The Gulf Stream is unlikely to stop, but its course could be altered. In the far North Atlantic Ocean one of the complicating factors is as follows:

The thermohaline circulation carries water from the surface in polar regions to the deep ocean, where it circulates as bottom water before being recycled to the surface thousands of miles away. The name indicates that the circulation is driven by temperature (thermo) and salinity (haline) differences between different masses of ocean water. Geological evidence suggests that this circulation has fluctuated significantly in the past, perhaps on timescales of less than a century. It is currently a matter of research as to the conditions that might provoke such a transition. A warming trend should favor a reduction in the thermohaline circulation, but it is very hard to assess its strength and the necessary computer models for forecasting are still relatively primitive.

Q: How long does it take for light from the sun to reach the earth?

--Byron, Dothan, Alabama

A: Dear Byron,

Working in round numbers and English units, the Sun is about 93 million miles away, and the speed of light is roughly 186 thousand miles per second, so the travel time of light from the Sun to the Earth is about 8.3 minutes.

Q: What is the sensitivity, (percipitation density) required for rain/moisture to be "seen" by Doppler radar?

--R., Ontario, CA

A: Dear R.,

There are many different models of weather radar that have Doppler capabilities. That is, in addition to sensing the reflected power from a volume, it can also use the Doppler effect to interpret the phase shift contained in the returned energy, providing information about velocities along the radar beam. The most common Doppler radar in the United States is the National Weather Service's Weather Service Radar - 1988, Doppler (WSR-88D). These systems blanket the country. Sometimes still referred to by its development name (Next Generation Radar, or NEXRAD), the WSR-88D is a key tool for real-time analysis of weather situations. It has two sensitivities, "clear air" and "precipitation." As the name implies, clear air mode is able to pick up echoes from dense clouds, before precipitation ever starts. This capability is used directly to track the clouds, and also to gain Doppler velocity information over a wider area than would be possible from precipitation alone. Under the right conditions clear air mode will pick up echoes from other airborne objects, most notably large concentrations of dust and/or insects, and large flocks of birds. However, the Doppler signal from self-mobile objects (insects, birds, etc.) is not useful for studying wind conditions!

Q: Why leaving Earth's atmosphere, does it appear to be smooth sailing, passing from blue sky, to space, when re-entering the earth's atmosphere, you could essentially burn up? Where does the heat and fire come from, when it is not there in the beginning?

--Ashlee, Austin, TX

A: Dear Ashlee,

The heat in the reentry phase is due to friction between the spacecraft and the air. In the case of the space shuttle, the de-orbit burn reduces the forward speed by about 200 mi/hr (300 km/hr) to 17,100 mi/hr (27,400 km/hr). Some 30 minutes later the orbit has decayed from an altitude of about 250 mi (400 km) to 400,000 ft ( 122,000 m) and the atmosphere becomes dense enough to exert significant drag. About 18 minutes later the shuttle is descending through 180,000 ft (55,000 m), having slowed to 8300 mi/hr (13,000) km/hr). All that extra kinetic energy has been converted to heat. The same concept applies to the meteors you see streaking across the night sky.

When a space vehicle is launched, enormous amounts of energy are required to lift it away from the surface. Thus, lift-off is a somewhat slower process, and the boost phase can be designed so that the really high speeds needed to achieve orbit are not attained until the vehicle is above the level at which friction drains away energy as wasted heating. Even so, the ride isn't exactly smooth sailing! The space shuttle exceeds the speed of sound about 44 seconds after launch, and supersonic flight up through the remaining layers of the atmosphere can be pretty bumpy. Also, the standard shuttle acceleration is about three times normal gravity. Needless to say, *everyone* stays in their seats with their seat belts fastened until they reach orbit!

Presumably the fiery reentry could be avoided by using enough retrorocket power to slow the spacecraft. However, the cost of lifting the fuel and rockets into orbit is so great that the use of atmospheric friction to achieve the reentry deceleration is absolutely standard.

Q: How does the earth spin on its axis and does the speed affect weather?

--David, Birmingham

A: Dear David,

The Earth's spin is mostly an accumulation of the spin that was contained in the huge number of chunks that went into building up the Earth at the beginning of its existence. Major effects since then have been tidal forces caused by the Sun and Moon and a slow transfer of energy to the Moon's orbital speed.

The effect of the Earth's rotation on the weather is two-fold. First, the daily cycle of the Sun drives local weather circulations, most notably seabreeze/landbreeze circulations and the afternoon maximum in thunderstorms over land. The speed of rotation governs how long these circulations have to become established and evolve. The second effect of the Earth's rotation is that the rotation causes a Coriolis acceleration, causing things moving horizontally to curve to the right. The strength of the Coriolis acceleration depends on the speed of the Earth's rotation, and a stronger Coriolis acceleration makes large-scale weather systems more important compared to local or region circulations (seabreezes and monsoons).

One standard question on Ph.D. exams in Meteorology is to ask what would happen to the weather and climate if the solar input or rotation rate were changed by some specific large amount!

Q: How did earth get its name?

--Clive, Unknown

A: Dear Clive,

A little dictionary work shows that "earth" comes from the Ino-European root that means, well, "earth." Lots of early civilizations had the view that the universe consisted of the sky, the air, and the dirt and rock surface on which they lived. The same word sufficed as a collective term for the entire surface and for what we would also call dirt or soil. The name stuck as a more complete picture of our home planet emerged.

Q: What is the temperature of the exosphere?

--David, Piedmont

A: Dear David,

The exosphere is the outermost layer of the earth's atmosphere, starting at 500-1000 km above the surface. Unlike the ocean's surface, the "edge" of the atmosphere is fuzzy, gradually thinning to the vacuum of space. At these altitudes the air is so tenuous that temperature has to be defined in terms of the energy of individual molecules. With each molecule on its own, so to speak, daytime energies reach well over the equivalent of a thousand degrees as the Sun's rays energize the molecule. Then at night the molecule radiates its energy away and its energy level plunges to within a few degrees of absolute zero.