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Chapter 9
Weather

*[See figure ³WEATHER1²]

Weather in Antarctica is characterized by extremes: extreme temperatures, extreme winds, and extremely variable localized conditions. All of these extremes make Antarctica a difficult place to work and live. Temperatures at McMurdo Station can vary from below -20 F to above freezing during the course of a season. The polar plateau experiences even colder temperatures because of its higher altitudes and greater distance from the moderating effect of the ocean.

Winds are common in Antarctica. It's an unusual day when there is not at least a breeze blowing. The winds take their toll on people, making camp chores such as setting up tents more difficult. More importantly, the winds increase the wind chill effect, making people more susceptible to hypothermia and frostbite. The chart on the following page details the effects of wind on temperature.

* [See figure ³WEATHER2²]

Storms arrive quickly and are sometimes fierce enough to halt all outside activity. Storms can also be very localized. Weather in McMurdo can be close to zero visibility with blowing snow (halting flight operations), while the Dry Valleys, which are 50 miles away from McMurdo, might be calm and sunny.

Approaching storms are usually preceded by high thin bands of cirrus clouds (mare's tails) followed by thicker layers of cirrus which may cause a halo-like effect around the sun. The clouds grow progressively thicker and lower over the next 6 to 12 hours until the arrival of low cumulus clouds and the main front. Blizzards, or "Herbies," can happen any time of year and usually last 3 to 6 days.

Storms approaching McMurdo Station usually arrive from the south in the gap between Black Island and White Island, an area known as "Herbie Alley." As the storms approach, they eventually obscure Minna Bluff with blowing snow or low clouds, at which point there's usually less than an hour before the bad weather hits.

Travel will be extremely difficult and dangerous during storms. Blowing snow or whiteouts can be disorienting and can make seeing crevasses or cracks in the sea ice impossible.

Even moderate winds can pick up and move a layer of dense blowing snow that may be as thin as a few feet or as thick as 20 or 30 feet. Sometimes it's possible to see above these layers by standing on a vehicle and taking a bearing for navigation. If the terrain to be travelled over is not known, however, it would be safer to stay put.

Whiteouts are an equally dangerous phenomenon. Heavy, low clouds reduce surface definition, and the horizon is invisible. It's difficult or impossible to know if you are on a flat or sloping surface. It is likewise difficult to judge distances or the size of objects. Travel should be avoided during whiteouts, unless there is an emergency.

9.1 McMurdo Weather

The McMurdo weather office (Mac Weather) issues daily weather forecasts that are updated every four hours and are available by calling the weather office or through McMurdo Operations (Mac Ops). Mac Weather also issues a weather classification for the immediate vicinity of McMurdo that restricts certain activities when the weather deteriorates. These weather conditions are divided into the following three categories:

Condition III Winds up to 48 knots, wind chill down to -75 degrees, and visibility over 1/4th mile. Unrestricted travel and activity are allowed, but severe weather is possible within 12 hours.
Condition II Winds 48 to 55 knots, wind chill -75 to 100 degrees, or visibility 100 feet to 1/4th mile. Restricted pedestrian traffic only between buildings is allowed. Travel is allowed only on marked trails or roads in authorized, radio-equipped vehicles.
Condition I Winds over 55 knots, wind chill lower than 100 degrees, or visibility less than 100 feet. Severe weather is in progress. All personnel must remain in buildings or the neares shelter.

* [See figure ³WEATHER3²]

9.2 Weather Observations from the Field

Prior to deep-field put-ins, at least two members of your field team should attend a briefing at the NSFA Weather Office. At this office, you will be instructed in weather observations and how to relay this information to McMurdo. You'll also be issued a meteorological kit that includes a thermometer, an anemometer, an altimeter, and a cloud identification chart. Refer to the booklet in the meteorological kit for in-depth information on field weather observations.

Taking a weather observation entails viewing the meteorological conditions at your camp and reporting those conditions in such a way that they can be visualized by the forecasters at McMurdo. A typical field weather observation in Antarctica relayed by radio includes the following:

1. Wind direction is expressed in degrees (Grid North) and is rounded off to the nearest whole 10 degrees. Refer to Chapter 20: Antarctic Navigation for information on Grid North.

2. Wind speed is expressed in knots/hour. A wind gust is a sudden change in wind speed characterized by a variation of 10 knots between peak and lull. Both the prevailing wind speed and wind gust (if applicable) are reported. An anemometer is used to determine wind speed and direction.

3. Visibility is given in miles; it is dependent on the geographical features near your camp. Ski-way markers, which are set up at known distances, can be used to determine surface visibility. The maximum visibility on a clear day is seven miles, after which a flat ground horizon will fall away to a point that surface conditions cannot be observed.

4. Cloud height is expressed in feet. At an open field, cloud height is estimated. If you are in an area with geographical features of known elevations, use those features to determine cloud height. Cloud heights are reported "Above Ground Level (AGL)." Be able to convert to the "Mean Sea Level (MSL)" if requested.

5. Cloud type and cloud cell appearance will help determine the height of a cloud layer. The atmosphere over the Antarctic is shallower than it is at the equator; therefore, the heights of cloud layers are lower.

Low clouds (stratus and stratocumulus) are commonly found at the surface up to 6,000 feet (MSL).

Mid-level clouds (altostratus and altocumulus) are generally at levels from 6,000 to 12,00 feet (MSL).

High clouds (cirrostratus and cirrus) are usually 12,000 to 16,000 feet (MSL).

6. Cloud coverage is expressed in eighths of the sky. When reporting cloud layers, start at the ground and proceed upward.

Clear No clouds present.
Scattered Trace to 4/8ths of the sky covered.
Broken More than 4/8ths, but not total sky coverage.
Overcast Total sky coverage.
Partial Obscuration Sky is partially obscured, typically by snow or blowing snow. Some clouds are discernible.
Total Obsuration Sky is totally obscured, typically by snow or blowing snow.
Thin Layer is transparent.
Opaque Layer is dense.

7. Temperature is given in degrees Celsius. Make sure that the thermometer is not directly exposed to sunlight. Protect the thermometer from the wind.

8. Pressure is expressed in millibars.

9. Altimeter setting is expressed in inches of mercury to the hundredths. The altimeter setting is the figure that incoming pilots will want the most, because it allows them to determine the altitude (in reference to mean sea level) at which the aircraft will make contact with the landing field.

10. The following surface definition terms should be used to report observations:

Good Snow surface features such as sastrugi, drifts, and gullies are easily identified by shadow.
Fair Snow surface features can be identified by contrast. No definite shadows exist.
Poor Snow surface features cannot be readily identified except from close up.
Nil Snow surface features cannot be identified. No shadows or contrast. Dark objects appear to float in the air.

11. The following horizon definition terms should be used to report observations:

Good Horizon is sharply defined by shadow or contrast.
Fair Horizon may be identified, but the contrast between sky and snow surface is not sharply defined.
Poor Horizon is barely discernible.
Nil Total loss of horizon, the snow surface merges with the whiteness of the sky.

9.3 Radio Transmission of Weather Observations

The primary frequencies for passing weather observations are 11553 kHz for remote-site field parties and 4770 kHz for Dry Valley and surrounding areas field parties.


On to Section 10: Snow Shelters.

 
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