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Category Archives: Meteorology
Did weather forecasting play a role in D-Day?
Last Friday was the 81st anniversary of the Allied invasion of Europe that began with the landings on the beaches at Normandy. The combined land, air, and sea assault of June 6, 1944 remains the largest such event in history. The success of the invasion was extraordinarily dependent of weather conditions. More than three months before the invasion, a combined British and American forecasting team began rigorous forecast exercises designed to iron out the physical and logistical kinks of such a coordinated effort. As June drew near, the nature of this collaboration was still problematic as the two groups employed vastly different methods in fashioning the requisite 3-5 days forecasts – at the time, absolutely primitive in the underlying science as compared to what is possible at such ranges today. The British were attempting to make such forecasts based upon the understanding of atmospheric dynamics that had grown substantially during the war. The Americans were employing a method based on a statistically- based search through old weather data for historical analogues that could be used to guide the forecast. Continue reading
Category: History, Meteorology, Severe Weather
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What causes tornados and do they have a lifecycle?
A tornado is a powerful column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph. We don’t know if a particular storm will produce a tornado but we do know the necessary conditions needed for tornado formation.
The required conditions for a thunderstorm to produce a tornado are warm humid air near the surface with cold dry air above. These conditions make the atmosphere very unstable, in the sense that once air near the ground is forced upward, it moves upward quickly and forms a storm. Severe thunderstorm conditions also include a layer of hot dry air between the warm humid air near the ground and the cool dry air aloft. This hot layer acts as a lid that allows the sun to further heat the warm humid air, making the atmosphere even more unstable. Continue reading
Category: Meteorology, Phenomena, Severe Weather
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How do satellites help forecast the weather?
Satellite data help forecast the weather in two ways: expert forecasters interpret the satellite images and numerical weather-prediction models assimilate the data they collect. Image analysis plays an important role in short-term forecasts, those that predict the weather 1 to 3 hours into the future, while numerical weather predictions are more useful in 12-hour to 3-day forecasts.
While weather forecasters routinely analyze current satellite observations, most data never reach forecasters’ eyes. Most satellite observations are assimilated into numerical weather-prediction models. Today’s weather forecast models rely on satellite data to make accurate weather predictions. These satellite observations include the vertical distribution of temperature and humidity, cloud distributions, land and sea surface temperatures, location of volcanic ash, fires, and wind speeds and directions. Continue reading
What is vapor pressure?
Weather reports often include the dew point temperature and the relative humidity. These are just two of several ways to express the amount of water vapor in the atmosphere. Vapor pressure is another way. Each method has advantages and disadvantages.
Gas molecules exert a pressure when they collide with objects. The atmosphere is a mixture of gas molecules and each type of gas makes up a part of the total atmospheric pressure. The pressure the water molecules exert is another useful method of representing the amount of water vapor in the atmosphere. The pressure caused by these water vapor molecules is called the vapor pressure. Atmospheric vapor pressure is expressed in millibars (mb). Continue reading
What are radiosondes?
Radiosondes are instrument packages that measure the vertical profiles of air temperature, relative humidity, and pressure from the ground all the way up to about 19 miles. These radio-equipped meteorological instrument packages are carried aloft by a helium-filled “weather balloon.” … Continue reading
