Weather Systems Introduction Air Masses Frontal Systems Mid-latitude Cyclones Thunderstorms Tornadoes Hurricanes Summary A great thunderstorm; an extensive flood; a desolating hurricane; a sudden and intense frost; an overwhelming snowstorm; a sultry day, - each of these different scenes exhibits singular beauties even in spite of the damage they cause. Often whilst the heart laments the loss to the citizen, the enlightened mind, seeking for the natural causes, and astonished at the effects, awakes itself to surprise and wonder. St. John de Crévecoeur Introduction • Weather is the state of the atmosphere at a given time and place. • Extreme weather events threaten lives, disrupt transportation systems, and cause destruction. • The National Weather Service makes over a million daily weather observations that will be used in forecasts by media sources nationwide. • Advanced technology such as weather satellites and Doppler radar allow the accurate prediction of dangerous weather systems like tornadoes and hurricanes. No scientific phenomena concern us as much as the daily evolution of weather systems. We live in a culture where weather, the state of the atmosphere at a given time and place, helps us define regional cultural variations. States such as California and Florida are defined in part by their warm, sunny weather. Seattle, Washington, is known for its rain, North Dakota and Minnesota for cold winter temperatures, and Oklahoma for tornadoes. Superimposed on the regular rhythms of the atmosphere are more extreme events that threaten lives, disrupt transportation systems, and cause destruction (Figs. 1, 2). Approximately 90% of presidential disaster declarations are weather related. In the dozen years between 1988-1999, there were 38 U.S. weather disasters that generated at least $1 billion in damages. In 1998 alone there were seven billion- dollar weather disasters (Fig. 2). Figure 1. Extreme weather events in the lower 48 states. Note hurricanes are found along Atlantic and Gulf coasts, and tornadoes are most common over the Great Plains and upper Midwest. 2 The first half of the chapter is divided into three sections that describe how common weather systems develop across much of the nation. Weather in any region is influenced by the atmospheric changes that occur when masses of air with contrasting properties interact. The characteristics of air masses vary with location ranging from dry and cold to warm and humid. The daily clash of air masses over North America generates our common weather patterns characterized by high- and low-pressure systems bounded by warm and cold fronts. These frontal systems are relatively narrow, curvilinear zones that mark a transition from one air mass to another. Weather experienced over much of the central and eastern U.S. is the result of the west-to-east migration of regional-scale low- pressure systems, termed mid-latitude cyclones, and their associated warm and cold fronts. Mid-latitude cyclones affect much of the continental landmass for up to a week at a time. Meteorologists attempt to predict the path of these mid-latitude cyclones and their frontal systems by monitoring their associated atmospheric conditions such as moisture, temperature, pressure, and wind direction. Using these characteristics they can predict the potential weather for two to five days in the future. However, these dynamic systems are subject to change, and the short-term, relatively accurate forecast becomes a long-term calculated guess as the forecast extends beyond two or three days. Figure 2. Top: Cost of damages associated with weather events, 1998. Total cost was over $16 billion. Bottom: Proportion of fatalities associated with specific weather- related phenomena. 3 Such is our devotion to understanding how future weather patterns will affect us that millions daily tune in to the cable weather news station, the Weather Channel. Regardless of where we get our information on the weather forecast, almost all of it comes from the same place, the National Weather Service (NWS). The NWS processes over one million weather observations per day. These basic observations may be reprocessed by commercial weather companies (e.g., Accuweather) to generate maps and graphics for public distribution to a variety of media sources. Figure 3. Early national weather map, created September 1, 1872, shows a high-pressure system over the Northeast. Image courtesy of NOAA photolibrary. The NWS began life on February 9, 1870, as part of the Signal Service Corps in the Department of War. It initially had the unwieldy title of The Division of Telegrams and Reports for the Benefits of Commerce and was given the charge “to provide for taking meteorological observations at the military stations in the interior of the continent and at other points in the States and Territories . and for giving notice . of the approach and force of storms.” The fledgling service made its first simultaneous observations at just 24 sites on November 1, 1870. Within two years it was creating national weather maps (Fig. 3), and by 1878 daily observations were being collected at 284 sites and relayed cross-country by telegraph. The current NWS mission is to provide “weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national 4 Figure 4. Geostationary satellites generate thousands of images per day. Image courtesy of NOAA photolibrary. economy.” Today the NWS uses sophisticated satellite technology to keep tabs on developing weather systems worldwide. The Geostationary Operational Environmental Satellite (GOES) Program began in 1968 and today has two satellites in synchronous orbit above Earth that provide weather coverage for 60% of the planet's surface (Fig. 4). The NWS has over one hundred Doppler radar sites nationwide that are used to track rapid changes in regional storms. The nationwide expansion of Doppler radar installations resulted in an increase in the warning times given for sudden weather phenomena such as tornadoes and flash floods that claim hundreds of lives annually. The latter half of the chapter is divided into three sections that review extreme weather events in the U.S., thunderstorms, tornadoes, and hurricanes. Thunderstorms form as warm, humid air is forced aloft, either in advance of cold fronts that are migrating toward the east or as a result of differential warming of air near Earth's surface. The high winds, hail, heavy rains, and lightning associated with these storms claim approximately a hundred lives a year in the U.S. Furthermore, over the much of central and eastern U.S., thunderstorms produce even more violent tornadoes (Fig. 5), the highest velocity winds on Earth. The use of new technology, such as Doppler radar, has increased the average lead time for tornado warnings in the U.S. from 5 minutes (1986) to 12 minutes in 1998. Figure 5. A tornado near Dimmit, west Texas, 1995. Image courtesy of NSSL's photo album. 5 Dangerous weather phenomena such as tornadoes and hurricanes cannot be stopped but with detailed observations meteorologists can provide timely warnings to protect people from the onslaught of these hazardous winds. The sheer sizes of hurricanes, hundreds of kilometers across and bigger than most states, mean that they will have significant impact on people and property when they come on shore. The most expensive natural disaster in U.S. history occurred in 1992 when Hurricane Andrew wrecked havoc across southern Florida, causing $30 billion in damages (Fig. 6). Damages could easily have been doubled if the storm had made landfall in the highly developed areas further north. Continued coastal development makes a future $50 to $100 billion disaster inevitable. Figure 6. Property damage in southern Florida resulting from Hurricane Andrew, 1992. Image courtesy of NOAA. Think about it . Examine the map at the end of the chapter that illustrates the distribution of extreme weather events for the conterminous U.S. during 2000. What patterns can you identify in the weather characteristics displayed on the map? Air Masses • Air masses are large regions of the lower atmosphere with uniform characteristics that are originally defined by a source area. • Air masses are identified by temperature (polar vs. tropical) and the nature of the source area (continental vs. maritime) • North American weather patterns are dominated by continental polar and maritime tropical air masses. 6 • Air masses are modified as they move over areas with different temperatures or topography than the source area. Air masses represent large regions (1,000s km2) of the lower troposphere with relatively uniform properties (temperature, moisture content). The characteristics of individual air masses are dependent upon the attributes of a source area and the modification of the air mass that occurs as a result of movement from the source region. Weather in any region is influenced by the changes that occur in the air mass over time and the interactions that occur at fronts, the boundaries between contrasting air masses. Source Areas An air mass develops when the atmosphere is located above a relatively uniform land or water surface for several days. The lower atmosphere assimilates some of the properties of the underlying surface. Air masses are identified by their temperature (polar/tropical) and the character of the underlying surface (continental/maritime). The latter property is a proxy for moisture content. Air masses that develop above oceans contain much more moisture than those formed over land. The distribution of air masses is relatively intuitive. Arctic and Figure 7. polar air masses are located at high latitudes (+50o) in the Approximate Northern Hemisphere and tropical air masses are located closer locations of air masses developed to the equator (Fig. 7). Continental air masses are found over over the Northern land, maritime air masses over ocean. The boundaries between Hemisphere (left) individual air masses vary with seasons.