THANKSGIVING WEEK: 19-23 November 2001

• Floods in Texas claim at least five lives -- Record breaking rains accompanying severe thunderstorms across central Texas at the end of last week caused flooding in Austin.  At least five people lost their lives in the storms that also spawned tornadoes.  [Dallas Morning News]
• Collaboration makes global ocean color data set available -- In an agreement between National Space Development Agency (NASDA) of Japan and  the National Aeronautics and Space Administration  (NASA) in the United States , a consistent, long-term ocean color data set is being assembled which encompasses measurements from several satellite instruments and missions.  The resulting program entitled, Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Program, is intended to assist those attempting to assess the maritime biological response to climatic change. [EurekAlert!]
• Another C-MAN Station on the Lakes - The National Weather Service (NWS) announced the deployment of the fifty-seventh C-MAN station on the Lake St. Clair Lighthouse near Detroit, MI.  A C-MAN (Coastal-Marine Automated Network) station is an automatic weather station deployed by the National Data Buoy Center (NDBC)  to monitor air temperature, humidity, winds, barometric pressure and in some cases water temperature and tides. The data collected from these stations are transmitted by satellite to NDBC at Stennis Space Center, MS and to NWS Offices.  These C-MAN stations, located on piers, lighthouses, beaches or fixed offshore platforms complement the numerous moored weather buoys that NDBC has deployed in the last 20 years.  [NWS Weather News]
• Water sale criticized - Some Oklahoma residents were critical of a recently announced plan developed by the state of Oklahoma and several Native American tribes that would sell several million gallons of water from southeast Oklahoma to drought-stricken portions of  west Texas.  [ENN]
• Study of Arctic thaw lakes used to track climatic change -- A research team led by a geographer from the University of Cincinnati is studying Arctic thaw lakes and their surrounding drainage basins that are perched upon the tundra. They are attempting to identify, date and monitor these lakes in an attempt to understand how they have changed in light of recent climatic change, as well as how the changes in the lake could affect the climate.   [Univ. of Cincinnati] 
• A warm globe more common than previously thought - A scientist at the University of North Carolina at Chapel Hill has announced that analysis of deep sea cores collected from the Pacific Ocean off Japan indicates that more warm episodes have occurred during the last 135 million years than previously thought. During these relatively warm intervals, the Pacific Ocean had reduced dissolved oxygen levels.  [ENN]
• Earth diary - A summary of the week's major environmental events on Planet Earth. [CNN]

Note: This Concept of the Week is repeated from last week.

Concept of the Week: Water in Microclimates

Water is an important component of the planetary-scale climate system, playing a key role in the global radiation balance (e.g., the greenhouse effect) and large-scale transport of heat (e.g., latent heat transfer). The global climate system exhibits considerable spatial variability so that Earth is a mosaic of numerous climate types and water is often an important factor in distinguishing among individual climates. Hence, for example, the climate of a desert differs from that of a rainforest primarily because of differences in precipitation and potential evaporation rate.

Climate (average atmospheric conditions) also varies on smaller spatial scales. Here, we consider examples of how the local water and heat energy budget vary on the scale of microclimates, having horizontal dimensions from less than 1 m to 100 m (3.3 ft to 330 ft) and vertical dimension from Earth’s surface to an altitude of 100 m (330 ft).

All other factors being equal, afternoon air temperatures tend to be lower over an irrigated farm field than over a non-irrigated field. Where soils are wet, more of the available heat is used to evaporate water (latent heating) leaving less to raise the temperature of the soil surface and overlying air (sensible heating). In the Northern Hemisphere, snow tends to persist longer and the growing season is shorter on the shaded north- and east-facing hill slopes (facing away from the sun) than on south- and west-facing slopes (facing the sun). In portions of the Rocky Mountains, the sunnier, drier, and warmer south-facing slopes are sparsely vegetated by grasses, ponderosa pine, and juniper. Meanwhile, dense stands of fir and spruce grow on the shaded, moist, and cooler north-facing slopes.

Trees, sand dunes, buildings and other obstacles slow the wind and reduce the wind’s ability to transport material (including soil particles) in suspension. Farmers take advantage of this effect by constructing shelterbelts (or "windbreaks") that reduce wind erosion of soil and modify the microclimate. A shelterbelt may consist of a fence or multiple rows of trees or bushes planted upwind of an agricultural field—usually perpendicular to the prevailing wind direction. In the winter, snow-bearing horizontal winds slow as they encounter a shelterbelt. The wind’s ability to transport snow diminishes and snow accumulates on the field immediately downwind of the shelterbelt. The air spaces within a snow cover make it a relatively poor conductor of heat so that the snow cover protects the underlying soil from deep penetration of subfreezing temperatures. In areas where winter wheat is grown, the snow cover protects the dormant plants from potentially damaging fluctuations in soil temperature. Snow trapped by shelterbelts also increases the local supply of soil moisture that becomes available during the spring thaw. Furthermore, slowing the wind reduces the evaporation of soil moisture. Significant increases in crop yields can be attributed to the microclimatic effects of shelterbelts.

Climatic contrasts are also evident between forested and non-forested areas. Wind speed diminishes considerably within a forest—as much as 60% to 80% at a distance of only 30 m (100 ft) into a deciduous forest. Weaker winds coupled with reduced solar radiation reaching the forest floor (due to the forest canopy) decreases the amount of evaporation from the forest floor. But the relatively high rate of transpiration by forest vegetation more than compensates for the reduction in evaporation from the forest floor. Overall, the air in a forest tends to be somewhat more humid than the air over a nearby non-forested area. Even tall field crops such as corn have a similar effect on the microclimate.

Concept of the Week: Questions

1. In the Northern Hemisphere, snow is likely to persist longer on the shaded [(north-) (south-)] facing slope of a hill.
2. The relatively high humidity in forested areas is primarily the result of relatively high [(precipitation) (evaporation) (transpiration)] occurring in forested areas.

• 19 November 1921...The Columbia Gorge ice storm finally came to an end. In Oregon, 54 inches of snow, sleet and glaze blocked the Columbia River Highway at The Dalles. Apart from traffic on the river itself, all transportation between Walla Walla, WA and Portland, OR came to a halt. Nine trains were stopped as railroads were blocked for several days. (David Ludlum)
• 19 November 1978...A waterspout came onshore to become a tornado near Muhio Wharf in Hilo Harbor on Hawaii's Big Island. Some industrial buildings lost their roofs. The proximity of the waterspout-tornado caused an airliner to change its landing approach to Hilo's airport. (Accord Weather Calendar)
• 21 November 1967...Excessive rains in southern California caused the most severe flooding and the most damaging mudslides in 33 years. Downtown Los Angeles received 7.96 inches of rain, and 14 inches fell in the mountains. (David Ludlum)
• 21 November 1987...Truk Island (Federated States of Micronesia at 7.4 degrees North latitude and 151.7 degrees East longitude) was struck a rapidly intensifying Tropical Storm Nina, as winds gusted to 95 mph. Five died, and most buildings destroyed. A storm of such intensity so close to the equator is somewhat unusual. (Accord Weather Calendar)
• 22 November 1641...An observer at Boston, MA recorded a "great tempest of wind and rain from the southeast all night, as fierce as a hurricane, and thereupon followed the highest tide which we have seen since our arrival here". (David Ludlum)
• 22 November 1992...Supertyphoon Gay generated gusts up to 120 mph on Guam in the western Pacific. Only one injury was sustained. Earlier when at its peak approximately 1000 miles southeast of Guam, Supertyphoon Gay had sustained surface winds estimated to 185 mph with gusts to 225 mph. (Accord Weather Calendar)
• 23-25 November 1940...Said to be the worst ice storm in the U.S. through 1940, freezing rain and freezing drizzle fell in the northern Texas Panhandle. Ice on power lines accumulated to 6 inches in circumference. The weight of 13 pounds per linear foot downed thousands of power poles and trees. All power was lost in Amarillo and Canyon for three days. (Accord Weather Calendar)
• 23-24 November 1981...Typhoon Irma, the worst in 10 years, struck north central Philippines (mostly Luzon) with winds to 139 mph and a storm surge of 16 feet. More than 236 people died, while 600,000 were made homeless. Entire provinces were left without power or communication. (Accord Weather Calendar)
• 24 November 1982...Hurricane Iwa lashed the Hawaiian Islands of Niihau, Kauai, and Oahu with high winds and surf. Winds gusting to 120 mph caused extensive shoreline damage. Winds at Honolulu gusted to 81 mph. Damage totaled 150 million dollars on Kauai, and fifty million dollars on Oahu. The peak storm surge on the south shore was six to eight feet. It marked the first time in 25 years that Hawaii had been affected by a hurricane. (The Weather Channel)

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Prepared by AMS WES Central Staff and Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.