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Q: I had a student ask me if the Southern Hemisphere has a jet stream similar to the one in the Northern Hemisphere. This is not a field in which I am well versed. Any help?

--John, Greenwood, SC
1999-09-23

A: Dear John,

Indeed, the Southern Hemisphere also has similar jet streams. In this country, we refer to the air mass moving over North America in the upper troposphere as THE jet stream, but among atmospheric scientists a jet stream refers to any strong, narrow current concentrated along a quasi-horizontal axis relative to the planet's surface, with strong vertical and lateral wind shears. At minimum, a jet stream is moving 30 meters per second, although velocities of 500 km per hour have been observed near the upper boundary of the troposphere. Typical jet streams in Earth's atmosphere are thousands of kilometers long, hundreds of km wide, and several km deep.

Jet streams are the product of Earth's coriolis force, as well as temperature and pressure gradients in the atmosphere--mainly horizontal temperature gradients that are particularly strong across cold and warm fronts. The jet streams in the Southern Hemisphere will typically be weaker because the equator-to-South Pole temperature gradient is less in the Southern Hemisphere.


Q: What is meant by the temporal resolution of a satellite?

--Wendy, USA

A: Dear Wendy,

When referring to satellites, one will often ask, "What is its resolution?" By "resolution," folks generally are asking for one of three possible things: (1) temporal resolution, (2) spatial resolution, or (3) spectral resolution.

Temporal resolution refers to the frequency at which a satellite "sees" a given point on Earth."Temporal" means, "pertaining to Earthly time." The MODIS instrument sees the entire surface of the Earth every 1-2 days, whereas ASTER will take 5 years to see the entire surface. So, MODIS has a much higher temporal resolution than ASTER. Yet, because MODIS will "see" the poles much more frequently than it will see a given point on the equator, we say its temporal resolution is higher at the poles.

Spatial resolution refers to the detail at which a satellite sensor "sees" the Earth; or the size of its individual pixels (picture elements) in its viewing "footprint" on the Earth's surface. More precisely, spatial resolution is the area of a single data point on Earth's surface measured by a satellite. For example, a single MODIS pixel on the surface at its highest resolution is 250 square meters (about the size of a city block); whereas ASTER sees the Earth at a much higher resolution of 15-square-meter (the size of your backyard) pixels.

Spectral resolution refers to the number of wavelengths of the Electromagnetic Spectrum in which a given satellite sensor "sees" the Earth. MODIS collects images at 36 different spectral wavelengths over a broader span of the EM spectrum (from visible light through the infrared portion). MISR collects images at only 4 different spectral wavelengths, mostly in the visible region. Therefore, MODIS has a much higher spectral resolution.


Q: Would you please tell me why the reactions occur on the surface of the ice particles that accelerate the Ozone destruction cause by stratospheric Chlorine?

--Cathy, USA

A: Dear Cathy,

The ozone depletion phenomenon over Antarctica is actually fairly complex. It's difficult to explain it briefly and simply, but I'll try: During the Antarctic winter months, air cools dramatically over the South Pole because there is no sunlight to heat the ozone in the air. This air descends, because it is denser, allowing more air to move poleward from middle latitudes to replace it. The Earth's Coriolis force causes the poleward-moving air to be deflected eastward forming a strong wind jet--called the "polar night vortex"--that peaks in wind speed from May to June. This vortex tends to isolate the Antarctic stratospheric air mass from mid-latitude air and it inhibits the transport of warmer air from the middle latitudes into that region. Thus, during the Southern Hemisphere's winter months, when it is always night at the South Pole, the air inside the vortex gets cooler, to about -90 degrees Celsius. Even though the stratosphere is very dry, clouds can form under these very cold conditions. These clouds are mostly made up of nitric acid trihydrate crystals and water ice.

The surfaces of the ice crystals allow chlorine nitrate and hydrochloric acid (HCl)--what we call chlorine "reservoir" gases--to chemically combine to form nitric acid (HNO3) and chlorine gas (Cl2). This reaction also takes place while the molecules are in their gaseousphase, but very slowly. Surface reactions are usually faster than gas-phase reactions because bond strengths are lowered for materials that adhere to a surface. So, the nitric acid stays bound to the ice crystal while the chlorine gas escapes. This is important because if the nitrogen compounds were also released, they would begin to tie up the chlorine again. Things change quickly with the onset of spring and the return of sunlight to the region. The chlorine gas "photolizes," and the chlorine atoms react with ozone to form chlorine monoxide. If enough chlorine monoxide is formed, it reacts with itself to form the dimmer Cl2O2, which itself photolizes into chlorine atoms and molecular oxygen, leaving the chlorine again free to attack ozone.

This catalytic ozone destruction cycle continues until all the ozone is gone. What stops the process is that after the ozone is gone, the chlorine begins to attack methane (you need a lot of chlorine atoms to do this, so all the ClO has to be gone, which means all the ozone has to be gone.) The methane- chlorine reaction ties up chlorine into HCl, which is fairly inert. Later in the spring, the vortex breaks up and the chemically perturbed Antarctic air is dispersed to middle latitudes. I might add that about 7/8 of the chlorine in the stratosphere is man-made from CFC's.

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