Earth Gauge » Extreme Weather

Climate Fact: Heat Waves

kraus — Mon, 23 Aug 2010 14:13:53 +0000

In Brief: Nights are warmer than they were in the early 1970s, which exacerbates the urban heat island effect and heat related health problems.

Heat waves – which in the United States kill up to 1,000 people per year – are defined as prolonged periods of abnormally hot weather. They can occur at any time of the year. What “abnormally hot” weather is will vary from place to place and from season to season, making a standard range of dangerous temperatures difficult to determine. What’s more, people become acclimated to seasonal temperatures. This means that sudden onsets of summer weather early in the year are generally much more dangerous than equivalent temperatures later in the summer. Some basic human physiological limits, however, can provide set temperatures that are dangerous. Heat index temperatures of 130 degrees Fahrenheit or higher are considered extremely dangerous, with heat stroke or sunstroke likely. Temperatures between 105 and 129 degrees are in the danger zone, when sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke is possible with prolonged exposure and/or physical activity. When the heat index is between 90 and 105 degrees, extreme caution with the heat is recommended.

Because they are built with heat trapping materials like concrete and asphalt, cities tend to be hotter than surrounding rural areas. This is particularly true at night, when cool temperatures are important for giving the body a break from the heat. Urbanization, in addition to a general warming trend, means that extreme heat events are becoming more common. In North America over the last 50 years, average nighttime low temperatures have risen faster than average daytime high temperatures. There has been a 50 percent increase in the number of unusually warm nights, and nights with temperatures that would have fallen into the top tenth percentile during the 1950s now fall into the top fifteenth percentile. Almost all of this increase has happened since 1975.

Seasons: Spring, Summer, Fall

Sources: Meisner, BN. “Heat Wave.” National Weather Service Southern Region Homepage. NOAA. 15 May 2000. Accessed Online 16 August 2010 and United States Climate Change Science Program. “Weather and Climate Extremes in a Changing Climate.” Synthesis Assessment Product 3.3: GPO. 2008 and American Red Cross. “Talking about Disaster: Guide for Standard Messages.” Available from: and Centers for Disease Control. “Tips for Preventing Heat Related Illness.” Accessed Online 16 August 2010

Climate Trivia: Atlantic Hurricane Frequence and ENSO

kraus — Mon, 21 Jun 2010 14:42:52 +0000

Warm ocean surface temperatures in the North Atlantic provide the warm and moist air that fuels hurricanes, which develop out of random disturbances in the tropics that provide the spark for these storms. Warmer waters in the North Atlantic generally mean more fuel for the storms. But did you know that surface temperature conditions in the tropical Pacific Ocean also influence the Atlantic Hurricane season? The El Niño-Southern Oscillation (ENSO) is the periodic shifting of sea surface temperature distributions in the tropical Pacific. During El Niño phases, water temperatures in the eastern tropical Pacific off the coast of South America are warmer than normal. During La Niña phases, water temperatures there are cooler than normal. During ENSO neutral phases, the temperatures are somewhat in between.

Trivia Question: All other things being equal, during what phase of ENSO does the Atlantic Hurricane season tend to be most active?

a. El Niño
b. La Niña
c. Neutral

The correct answer is b. The amount of vertical wind shear over the ocean can make or break a hurricane season. Vertical wind shear is the change in the speed and direction of wind at different levels of the atmosphere. More vertical wind shear, or lots of variation in wind speed across different altitudes, suppresses hurricane activity. Less vertical wind shear, or more even wind patterns across different altitudes, promote hurricane development. La Niña phases work to reduce the amount of vertical wind shear over the North Atlantic, and thus La Niña years tend to be years with more active Atlantic hurricane seasons. La Niña conditions are now present in the tropical Pacific.

Seasons: Summer, Fall

Sources: Briggs, WM. “On the Changes in the Number and Intensity of North Atlantic Tropical Cyclones.” Journal of Climate 21 (2008): 1387-1402. Donnely, JP and Woodruff, JD. “Intense hurricane activity over the past 5,000 years controlled by El Niño and the West African monsoon.” Nature 447 (2007): 465-468.

Climate Fact: La Niña and the Great Medieval Droughts

kraus — Mon, 22 Mar 2010 14:52:35 +0000

In Brief: Persistently cool conditions in the eastern tropical Pacific during the Middle Ages led to drought in the southwestern United States.

Conditions in the tropical Pacific influence weather throughout the world. On a cycle of two to seven years, the eastern tropical Pacific moves from cool (La Niña) conditions to warm (El Niño) conditions. While warm and cool conditions generally balance each other during the cycle, paleoclimate proxy data from coral reefs in the tropical Pacific indicates that this is not always the case. For much of the 14th and 15th centuries, for example, La Niñas were much more common than El Niños. La Niña conditions force storm systems to move farther north than they otherwise would be, leaving much of the northern subtropics and midlatitudes dry. This is especially true in what is today the southwestern United States. During the late Middle Ages, persistent La Niña conditions meant limited wintertime precipitation, and frequent “mega-droughts” covered much of the Southwest region.

Seasons: Winter, Spring, Summer, Fall

Source: Burgman, R et al. “Role of tropical Pacific SSTs in global medieval hydroclimate: A modeling study.” Geophysical Research Letters 37 (2010): L06705.

Climate Number: $16.3 Billion

kraus — Mon, 01 Feb 2010 15:37:37 +0000

When put in 2000 US dollars, freezing rain (ice storm) events in America caused an estimated 16.3 billion dollars in total losses between 1949 and 2000 due to downed power lines, downed trees, agricultural losses, transportation accidents and medical costs from injuries due to slippery conditions. Freezing rain events are most frequent in the Northeast, but are also common across the Midwest and Piedmont regions from North Carolina northward. When freezing rain events hit the Southeast they tend to be accompanied by high dewpoints. This means that while ice storms are rarer in the Southeast, they tend to be heavy and particularly damaging when they do hit. Records kept since the late 1920’s show that ice storms were the least frequent during the 1930’s and rose to a peak in the early 1950’s, showing little or no trend thereafter.

For Comparison: 16.3 billion dollars is around the same amount collectively pledged by all participating parties to fight global poverty following the 2008 U.N. anti-poverty summit. It is also roughly the same amount as NASA’s annual budget.

Seasons: Winter

Source: Houston, TG et al. “Freezing rain events: a major weather hazard in the conterminous United States.” Natural Hazards 40 (2007): 485-494.

U.S. Cold Snap in Context

espinoza — Tue, 19 Jan 2010 20:14:00 +0000

The recent cold snap may be the most severe the eastern United States has experienced in more than 30 years. Do a few weeks of cold temperatures in one region of the world mean that global warming has stopped?

There are a few variables to consider – the behavior of the Arctic Oscillation, the ratio of record highs to record lows and long-term trends.

The Arctic Oscillation
While the eastern United States and central Asia have been in a cold snap, the Arctic has been warmer than normal. This is not coincidental. Weather is controlled by a variety of cycles – daily cycles, seasonal cycles, 11-year solar cycles and even several thousand year cycles that affect Earth’s orbit. One cycle – the Arctic Oscillation – reflects shifts in the difference in atmospheric pressure between the Arctic and the mid-latitude regions of the Northern Hemisphere. This pressure difference affects the strength of the upper-atmospheric westerly winds that drive much of the Northern Hemisphere’s weather, especially during winter. When the westerly winds are strong, it is more difficult for frigid air at the poles to “invade” the mid-latitudes, including the eastern United States. The frigid air is trapped at the pole, keeping the Arctic colder than normal and the mid-latitudes warmer than normal. When the westerly winds are weak, it is easier for Arctic air to invade the mid-latitudes. This leaves the North Pole less frigid than normal, but sends the mid-latitudes into a cold snap like the one experienced over the past few weeks. The state of the Arctic Oscillation is a good predictor of how extreme winter temperatures will be. In Chicago, for example, there are on average three times as many days each year when the temperature drops below zero degrees Fahrenheit during negative phases (weak westerly winds), versus positive phases (strong westerly winds). See the map of recent temperature anomaly distributions below, which is typical of negative phases of the Arctic Oscillation.

BOTTOM LINE: The recent cold snap reflects a change in the distribution of the heat Earth holds – there is more cold air in the mid-latitudes and less cold air in the Arctic. There is no evidence to indicate there is either more or less heat in the system than there was before the cold snap occurred.

Record Highs vs. Record Lows
Even during periods when average temperatures rise, variables may come together to cause a given locality to experience a record low temperature for a given day in the calendar year. The same could be said for occurrences of record high temperatures during periods of cooling. During the warmest decade on record (the last ten years), lots of record lows were set – 142,420 record lows between Jan. 1, 2000 and Sept. 30, 2009. There are close to 5,000 quality-controlled weather stations across the United States providing daily temperature data to the National Climatic Data Center. Yet, it is not the existence of record highs or record lows that indicates whether a warming or cooling trend is occurring. Instead, it is the proportion of record highs to record lows that tells you whether things are getting warmer or cooler. All other things being equal – meaning that there is no increase or decrease in average surface temperature – the ratio of record highs to record lows should be around 1:1. But, from January 2000 to September 2009, 291,237 record high temperatures were set, giving a high to low ratio of more than 2:1. The disparity between record highs and record lows reflects the above normal temperatures experienced over the last decade. It will take many more winter cold snaps like this to even out the record high to low ratio.

BOTTOM LINE: A given locality will experience some record warm temperatures during periods of cooling and some record cold temperatures during periods of warming. But, one record event doesn’t create a trend. If you want to know whether things are getting warmer or cooler over the years, you have to look at the ratio of record warm events to record cold events. In the United States over this last decade, the ratio has been around 2:1.

More Cold Snaps to Come?
So, should we be expecting more of these cold snaps in the future? Analyzing past trends might help to answer this question. Patterns of periodic warming and cooling over the North Atlantic in the past – linked to periodic strengthening and weakening of the circulation that brings warm waters into the Atlantic basin from the south – suggest that the Atlantic may cool slightly over the next decade. As this happens, average surface temperatures in North America and Europe may stop their rising temperature trends or even cool slightly. Looking at long-term data (50+ years), which includes periods of both warm and cool North Atlantic temperatures as well as warm and cool periods of other major natural oscillations that help drive our weather, suggests that the extreme cold experienced over the past few weeks is becoming less common for the United States as a whole. Adding up all the cold winter days (when temperatures fall in the lowest ten percent of daily mean surface temperature distribution) and warm winter days (when temperatures fall in the top 10 percent of daily mean surface temperature distribution) over the past 50 years shows that despite significant regional differences, there has been an overall decreasing trend in the number of cold winter days and an increasing trend in the number of warm winter days. In the West, there has been a downward trend in the number of cold winter days, while most of the rest of the country has shown no trend. On the other hand, the West, along with the northern Great Plains and upper Midwest, has witnessed a rise in the number of warm winter days. The only region where warm winter days have become less common is the southeast, particularly the Gulf Coast area. South Florida has experienced a decline in the number of cold days and a rise in the number of warm days.

BOTTOM LINE: Despite significant regional differences, 50-plus year trends in U.S. temperatures show fewer cold and more warm winter days. These trends account for both warm and cold periods of the major natural oscillations that determine year-to-year variability in the severity of America’s winters.

Top image: 50-year trend in the number of “warm” winter days (days falling in the warmest ten percent of daily mean surface temperature distribution). Bottom image: 50-year trend in the number of “cold” winter days (days falling in the coldest ten percent of daily mean surface temperature distribution).

(Sources: Higgins, RW et al. “Relationship between Climate Variability and Winter Temperature Extremes in the United States.” Journal of Climate 15 (2002) : 1555-1572 and Meehl, GA et al. “Relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S.” Geophysical Research Letters 36 (2009): L23701 and Thompson, David W.J. “Regional Climate Impacts of the Northern Hemisphere Annular Mode.” Science 293 (2001): 85-89 and Keenlyside, NS et al. “Advancing decadal-scale climate prediction in the North Atlantic Sector.” Nature 453 (2008): 84-88.)

Climate Fact: Nutrition Change and Extreme Weather

espinoza — Mon, 21 Dec 2009 12:31:32 +0000

In Brief: Soybeans may produce more antioxidants during years of extreme temperature and drought.

A study conducted on Maryland soybeans between 1999 and 2002 found that extreme weather events actually increase the antioxidant levels in the soybean crop. 1999 and 2001 growing season temperature and precipitation levels were normal and the crops exhibited normal levels of the antioxidant alpha-tocopherol. Although there were not any abnormalities in the soybean antioxidant levels between 1999 and 2001, it was noted that when the temperature was slightly warmer, antioxidant levels slightly increased. A similar pattern was noted in 2002, when drought and high temperatures dominated Maryland’s weather. During this year, the antioxidant levels in the soybean crops increased by a factor of 3.5.

(Source: United States. USDA. Extreme Weather Boosts Antioxidant Levels in Soybean Seeds. By Rosalie Bliss. Agricultural research service, 17 Dec. 2008. Web. Nov. 2009. )

Climate Trivia: East Coast Winter Storm Frequency and ENSO

kraus — Mon, 07 Dec 2009 15:59:51 +0000

December is East Coast Winter Storm (ECWS) season. These storms are powered by warm water that flows from the Gulf Stream. The Gulf Stream current flows along the Eastern Seaboard past Florida and the Carolinas before reaching Cape Hatteras, where the warm water heads out into the Atlantic. ECWS’s travel northward along the coast causing high winds and coastal property damage comparable to hurricanes. They also bring heavy snowfall, causing further weather complications. On average, there are 12 ECWS’s during the December to February season, with January being the most active month. One of the best predictors of how intense an ECWS season will be is the ocean temperature along the coast of the southeastern U.S. during the previous summer (Gulf of Mexico temperatures were above average this past summer). The warmer these waters are, the stronger the Gulf Stream generally is and the more active the winter storm season will be. Interestingly, conditions in the eastern tropical Pacific affect ECWS activity as well. What eastern tropical Pacific conditions are most conducive to an active ECWS season?

a)   El Niño conditions (warmer eastern tropical Pacific SSTs)
b)   La Niña conditions (cooler eastern tropical Pacific SSTs)
c)   Neutral Conditions (average eastern tropical Pacific SSTs)

The correct answer is a. More active ECWS seasons tend to coincide with El Niño years. This is in contrast to Atlantic hurricane season trends, as El Niño conditions tend to suppress Atlantic Hurricane formation. Over the second half of the 20th century, the frequency of ECWS events showed little trend, but the storms did become slightly more intense.

Season: Winter

Sources: DeGaetana, AT et al. “Statistical Prediction of Seasonal East Coast Winter Storm Frequency.” Journal of Climate 15 (2002): 1101-1117 and Hirsch, ME et al. “An East Coast Winter Storm Climatology.” Journal of Climate 14 (2001): 882-899 and Eichler, T and Higgins W. “Climatology and ENSO-Related Variability of North American Extratropical Cyclone Activity.” Journal of Climate 19 (2006): 2076-2093 and National Oceanic and Atmospheric Administration: Climate Prediction Center. Accessed Online 7 December 2009 http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/eisdiffobs.meta.gif

Climate Fact: Wind, Rain, Tornadoes, Oh My

kraus — Fri, 13 Nov 2009 15:08:28 +0000

Along with heavy rains and high winds, the impacts of landfalling hurricanes and tropical storms also include more tornado formation. The larger the tropical cyclone and the longer it spends over land, the greater the probability that tornadoes will form as the system moves. Since 1995 in the Gulf of Mexico, hurricane strength has increased 35 percent compared to the 16 year period spanning 1948-1964 (considered to be the last active period for hurricanes). This increase in storm strength has corresponded to twice as many tornadoes produced per cyclone.

Seasons: Summer, Fall

Sources: Belanger, JI et al. “Variability in tornado frequency associated with U.S. landfalling tropical cyclones.” Geophysical Research Letters 36 (2009): L17805 and “Tornado Threat Increases As Gulf Hurricanes Get Larger.” ScienceDaily. 10 Sept. 2009. Web. 14 September 2009 .

Climate Fact: Southern U.S. Drought Occurrence Linked to SST Variability: Causes of Northern Droughts Less Clear

kraus — Thu, 29 Oct 2009 19:23:51 +0000

Better drought prediction systems could potentially save billions of dollars. Current prediction systems largely rely on observations of the circumstances surrounding past droughts to understand the factors that led to drying. New research reveals that:
• Drought in the southern Great Plains states, such as the 1946-1956 drought, can be largely explained by persistently cool conditions in the eastern tropical Pacific.
• Drought in the northern Great Plains states, such as the 1930’s “Dust Bowl,” was likely caused by serendipitous atmospheric variability (i.e. a variety of atmospheric factors came together with the result being the Dust Bowl) and agricultural practices. The initial atmospherically forced drying resulted in dust storms that reinforced these dry conditions.

Predicting drought may be more difficult in the northern Great Plains than southern Great Plains, as the causes of drought in the former region are much more complex.

Seasons: Winter, Spring, Summer, Fall

Source: Hoerling, M et al. “Distinct causes for two principal U.S. droughts of the 20th century.” Geophysical Research Letters 36 (2009): L19708.

Climate Fact: North American Extremes

kraus — Mon, 05 Oct 2009 15:42:29 +0000

The concepts of weather extremes and thresholds are tightly coupled and important to remember when planning effective and reliable infrastructure. For example, just one day of extreme heat, even if it falls during a particularly cool summer, can cause railroad tracks to buckle and transportation systems to shut down. Extreme rainfall can have similar effects; highways are designed to function during moderate rainfall events, but underpasses may flood during extreme events. Two inches of rain in a 24-hour period is considered to be a threshold that, when exceeded, forces the average municipal water treatment facility to discharge untreated sewage into local surface waters, where it may poison wildlife and ultimately be hazardous to people in the municipality it serves. Over the past 40 years in North America (including Hawaii, Puerto Rico and the U.S. Virgin Islands), the average highest summertime maximum and minimum temperatures at weather stations have increased by 1.6 degrees Fahrenheit, while the lowest winter maximum and minimum temperatures have increased by 6.3 degrees Fahrenheit. The average amount of precipitation falling during the highest one-day and five-day precipitation event periods has also increased.

Seasons: Winter, Spring, Summer, Fall

Source: Peterson, TC et al. “Changes in North American extremes derived from daily weather data.” Journal of Geophysical Research: Atmospheres 113 (2008): D07113.

Climate Fact: Climate and North Atlantic Hurricanes

kraus — Fri, 04 Sep 2009 17:50:51 +0000

The torrential rainfall and storm surges associated with hurricane landfall events can cause what are known as “overwash deposits” that leave definitive marks in the sediment layers that accumulate in coastal areas. Analyses of sediment cores from various locations along the Eastern Seaboard and Puerto Rico show that for the last 1500 years, Atlantic Hurricanes are more frequent when a) the El Niño-Southern Oscillation is in a La Niña phase, which reduces the amount of vertical wind shear over the North Atlantic and b) the North Atlantic is relatively warm, which usually coincides with positive phases of the Atlantic Multidecadal Oscillation. Over the 20th century, ocean temperatures in the North Atlantic main development region warmed during peak hurricane season, with the most pronounced warming occurring over the last four decades.

To view the chart of 20th century North Atlantic main development region peak hurricane season SST’s visit: http://www.earthgauge.net/climate-facts-image-library#6. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image is in the public domain.

Seasons: Summer, Fall

Sources: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson, (eds.). Cambridge University Press, 2009 and Mann, ME et al. “Atlantic hurricanes and climate over the past 1,500 years.” Nature 460 (2009): 880-883.

Climate Fact: Temperatures and Crop Yields

kraus — Wed, 26 Aug 2009 16:45:02 +0000

In North America over the last 50 years, average nighttime low temperatures have risen faster than average daytime high temperatures. There has been a 50 percent increase in the number of unusually warm nights and nights that fell into the top tenth percentile in terms of temperature for the climate of the 1950’s now fall into the top 15th percentile. Increases in daily minimum temperatures mean that crops are using more water for evaporation in order to keep cool, which decreases the amount of water the plants use producing edible parts. Snap peas, for example, show marked reductions in production once the nighttime temperature rises above 80 degrees Fahrenheit. When daily maximum temperatures become too hot, plants like corn and soybean will not grow as well. Corn will not reproduce when temperatures are above 95 degrees and soybeans will not reproduce when temperatures exceed 102 degrees.

To view how corn and soybeans respond to given temperatures, visit http://www.earthgauge.net/climate-facts-image-library#7. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image is in the public domain.

Seasons: Spring, Summer

Source: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009

Climate Fact: Extreme Heat in Phoenix

administrator — Tue, 25 Aug 2009 19:11:03 +0000

While Phoenix has always been hot for U.S. standards, over the last 50 years the city has been getting even hotter. The average number of days per year when the temperature is over 100 degrees has doubled over this time period. Part of this is likely due to the Urban Heat Island effect, which has grown as the area’s concrete cover has expanded. Arizona leads the Nation in heat related deaths, at three to seven times the national average.

For the accompanying visual, visit http://www.earthgauge.net/climate-facts-image-library#3. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image is in the public domain.

Seasons: Summer

Source: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009.

Climate Fact: Drought Trends

administrator — Wed, 19 Aug 2009 15:04:08 +0000

As Earth warms, the water cycle intensifies. While this means more total rainfall for some regions, it also means that more rainfall comes in the form of heavy and extreme events and periods between rainfall events become longer. Longer periods without rain and higher temperatures lead to losses of soil moisture; if drying of the soil continues for long enough, droughts occur. Since the 1950’s, some areas of the United States, including most of the Northeast and Midwest, have been experiencing drought conditions less frequently. Other areas, including most of the West and Southeast, have been experiencing drought conditions more often.

To see how drought frequency has changed in your local area since the 1950’s, visit http://www.earthgauge.net/climate-facts-image-library#3. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image was originally created as part of an ongoing analysis of drought trends by Guttman and Quayle (see citation below). The image is in the public domain.

Seasons: Summer, Fall

Sources: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009 and Guttman, NB and Quayle, RG. “A historical perspective of U.S. climate divisions.” Bulletin of the American Meteorological Society 77 (1996): 293-303. Operational practices described in this paper continue.

Climate Fact: Stagnant Air and Heat Waves

administrator — Mon, 17 Aug 2009 14:32:35 +0000

Heat waves, defined as three or more consecutive days when temperatures exceed 90 degrees Fahrenheit, can create public health hazards. In the United States, heat and drought account for the biggest share of hazard-related deaths at 19.6 percent. Death rates rise an average of six percent during heat waves and over the 20th century, the average number of heat waves doubled across the United States. Lack of rainfall and the prevalence of stagnant air conditions when there is little or no wind both work to compound the problem. Over the second half of the 20th century throughout most of the West, the southern Great Plains and the Southeast, stagnant air conditions were prevalent more the 25 percent of the time during heat waves.

To see how frequent stagnant air conditions are in your local area, visit http://www.earthgauge.net/climate-facts-image-library#1. This is a public domain image from the recently published “Global Climate Change Impacts in the United States” report published by the U.S. Global Change Research Program. The data used came from the NOAA’s National Climatic Data Center.

Season: Summer

Sources: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009 and Patz, JA. et al. “Impact of regional climate change on human health.” Nature 438 (2005): 310-317.

Climate Fact: Heaviest One Percent Now Even Heavier (Great Plains)

administrator — Tue, 14 Jul 2009 22:55:23 +0000

In terms of total annual rainfall, most of the United States became “wetter” over the 20th century. Most of this increase, however, is being expressed in “extreme” rainfall events, which are now more frequent and even more extreme than they were in the 1950s. In the Great Plains, for example, the amount of rain that falls during the heaviest one percent of rainy days has grown by 15 percent over the last 50 years. While more moisture is generally good for the region’s agriculture and wildlife, the general trend of more rainfall happening during extreme events has corresponded to more frequent and longer periods when no rain falls. The longer soil goes without moisture, the less absorptive it is when the rains finally return. Heavy rainfall also causes much greater amounts of soil erosion (soil loss to streams and ultimately the ocean) than moderate events do. In some locations, increases in rainfall severity can cause the amount of soil erosion to grow exponentially.

To see the region-by-region changes in the frequency of heavy rainfall events since the 1950’s, visit http://www.earthgauge.net/climate-facts-image-library#8. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image is in the public domain.

Seasons: Spring, Summer, Fall

Sources: Groisman, PY and Knight RW. “Prolonged Dry Episodes over the Conterminous United States: New Tendencies Emerging during the Last 40 Years.” Journal of Climate 21 (2008): 1850-1862 and National Weather Service: Climate Prediction Center. U.S. Temperature and Precipitation Trends: Annual. Accessed Online 3 July 2007 and Trenberth, K et al. “The Changing Character of Precipitation.” Bulletin of the American Meteorological Society, September 2003: 1205-1217 and Easterling, D et al. “Observed climate variability and change of relevance to the biosphere.” Journal of Geophysical Research 105 (2000): 101-120 and Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009.

Climate Fact: El Niño and North Atlantic Cyclones

administrator — Wed, 08 Jul 2009 15:18:43 +0000

The El Niño-Southern Oscillation, the periodic “sloshing” back and forth of warm water between the eastern and western tropical Pacific, is believed to affect the frequency of tropical cyclone development in the North Atlantic. Specifically, during La Niña phases (when the waters off the west coast of South America are unusually cool) circulation in the upper atmosphere reduces the amount of wind shear, favoring tropical cyclone development. El Niño phases (when the waters off the west coast of South America are unusually warm) increase the amount of vertical wind shear over the Atlantic, which suppresses tropical cyclone development. While this two-phase model has been useful for making weather predictions, a closer analysis of tropical Pacific sea-surface temperature records reveals that not all events that have been classified as El Niño events were the same. During some El Niño events, the warmest waters were indeed directly off the west coast of South America (an eastern Pacific warming or EPW), while during others, the warmest waters were instead concentrated in the central Pacific (a central Pacific warming or CPW ). CPW events have the same basic impact on North Atlantic tropical cyclone development frequency as La Niña phases do and CPW events tend to steer storms on more southerly tracks towards the U.S. Gulf Coast and Central America. Eighty percent of the CPW events that have occurred since 1950 have happened in the last 20 years.

Seasons: Spring, Summer, Fall

Sources: Holland, G. “Predicting El Niño’s Impacts.” Science 325 (2009): 47 and Kim, HM et al. “Impact of Shifting Patterns of Pacific Ocean Warming on North Atlantic Tropical Cyclones.” Science 325 (2009): 77-80.

Climate Fact: Heaviest One Percent Now Even Heavier (Southeast)

administrator — Mon, 06 Jul 2009 16:03:19 +0000

In terms of total annual rainfall, most of the United States became “wetter” over the 20th century. Most of this increase, however, is being expressed in “extreme” rainfall events, which are now more frequent and even more extreme than they were in the 1950s. In the Southeast, for example, the amount of rain that falls during the heaviest one percent of rainy days has grown by 20 percent over the last 50 years. While more moisture is generally good for the region’s agriculture and wildlife, the general trend of more rainfall happening during extreme events has corresponded to more frequent and longer periods when no rain falls. The longer soil goes without moisture, the less absorptive it is when the rains finally return. Heavy rainfall also causes much greater amounts of soil erosion (soil loss to streams and ultimately the ocean) than moderate events do. In some locations, increases in rainfall severity can cause the amount of soil erosion to grow exponentially.

Seasons: Spring, Summer, Fall

Climate Fact: Heaviest One Percent Now Even Heavier (upper-Midwest)

administrator — Mon, 06 Jul 2009 16:01:48 +0000

In terms of total annual rainfall, most of the United States became “wetter” over the 20th century. Most of this increase, however, is being expressed in “extreme” rainfall events, which are now more frequent and even more extreme than they were in the 1950s. In the upper-Midwest, for example, the amount of rain that falls during the heaviest one percent of rainy days has grown by 31 percent over the last 50 years. While more moisture is generally good for the region’s agriculture and wildlife, the general trend of more rainfall happening during extreme events has corresponded to more frequent and longer periods when no rain falls. The longer soil goes without moisture, the less absorptive it is when the rains finally return. Heavy rainfall also causes much greater amounts of soil erosion (soil loss to streams and ultimately the ocean) than moderate events do. In some locations, increases in rainfall severity can cause the amount of soil erosion to grow exponentially.

Seasons: Spring, Summer, Fall

Climate Fact: Heaviest One Percent Now Even Heavier (Northeast)

administrator — Mon, 06 Jul 2009 15:59:24 +0000

In terms of total annual rainfall, most of the United States became “wetter” over the 20th century. Most of this increase, however, is being expressed in “extreme” rainfall events, which are now more frequent and even more extreme than they were in the 1950s. In the Northeast, for example, the amount of rain that falls during the heaviest one percent of rainy days has grown by 67 percent over the last 50 years. While more moisture is generally good for the region’s agriculture and wildlife, the general trend of more rainfall happening during extreme events has corresponded to more frequent and longer periods when no rain falls. The longer soil goes without moisture, the less absorptive it is when the rains finally return. Heavy rainfall also causes much greater amounts of soil erosion (soil loss to streams and ultimately the ocean) than moderate events do. In some locations, increases in rainfall severity can cause the amount of soil erosion to grow exponentially.

Seasons: Spring, Summer, Fall