Rains in East Africa primarily fall during the long rains (March through May) and the short rains (October through December). Understanding how climate and climate change influence these rains is particularly important for predicting drought in the region.  Conditions in the tropical Pacific affect weather throughout the world and the tendency for the tropical Pacific to persist in either cool La Niña phases or warm El Niño phases enable forecasters to predict whether seasonal temperatures or precipitation levels will be above or below average. In addition to local Indian Ocean sea surface temperatures, the short rains in East Africa are strongly influenced by conditions in the tropical Pacific, with La Niña phases leading to reduced precipitation. How the tropical Pacific affects the long rain season, on the other hand, is harder to identify. Recent work shows that since 1999, the long rains have been consistently weak and unable to compensate for reduced short rain precipitation during La Niña phases, leading to drought conditions like those experienced in 2011. Also since 1999, there has been a pattern of consistently elevated sea surface temperatures in the western tropical Pacific and consistently high rainfall levels in that region. The tendency of ocean surface temperatures to remain in particularly warm or cool conditions for relatively long periods of time, coupled with what is known about the atmospheric links between the western tropical Pacific and East Africa, enables forecasters to better understand and predict droughts there.

Seasons: Winter, Spring, Summer, Fall

Source: Lynn, B and DeWiltt, DG. “A recent and abrupt decline in the East African long rains.” Geophysical Research Letters 39 (2012): L02702. 

The eruption of Iceland’s Laki volcano from June 8, 1783 to February 1784 had widespread and devastating consequences the world over. The event was one of the biggest natural disasters in British history, with sulfuric acid gas fumes killing over 23,000 Britons. The gas plume initially caused heat waves in Europe before spreading around the globe and cooling the rest of the Northern Hemisphere. Both France and Japan experienced crop failures and famines that year, and Alaskan tree ring records tell of a remarkably cold summer in North America. The Inuit remember it as “the summer that did not come.” The following winter was remarkably cold as well, with Europe being around 3.6 degrees Fahrenheit below normal and eastern North America having temperatures 8.6 degrees Fahrenheit below the 225 year mean. Early Americans skated on the harbor ice of Charleston, South Carolina, the Mississippi River was frozen at its delta, ice floes floated through the Gulf of Mexico and the Chesapeake Bay had its longest freeze-over on record. But was this severely cold winter caused by Laki’s eruption? According to tree ring proxy records used to reconstruct conditions in the tropical Pacific Ocean and North Atlantic over the centuries before instrumental data, at least part of the winter’s severity was due to a strong El Niño in the tropical Pacific and a strongly negative North Atlantic Oscillation (NAO). The strong El Niño event shifted storm tracks to the south, bringing frequent winter storms to the southern United States, while the strongly negative NAO allowed frigid Arctic air masses to easily sweep southward into the mid-latitudes. The coincidence of these conditions, as a result of serendipitous sea surface temperature variability, is similar to what happened during the 2009-2010 winter, which brought record cold to parts of the United States.

Seasons: Winter, Spring, Summer, Fall

Source: D’Arrigo R, et al. “The anomalous winter of 1783-1784: Was the Lake eruption or an analog of the 2009-2010 winter to blame?” Geophysical Research Letters 38 (2011): L05706.

The 2010 Atlantic hurricane season ended on November 30th, but Earth’s climate still “remembers” the tropical storms that formed during the Northern Hemisphere warm season. Far from being just passive products of heat in the tropics, tropical cyclones cool sea surface temperatures while warming sub-surface waters. They transport heat up into the atmosphere and towards the poles, and dry the tropical atmosphere by encouraging rainfall. These effects “linger;” sea surface temperatures can remain anonomalously cool for 50-60 days after the passage of a category three or greater hurricane. Weather conditions and sea surface temperature distributions in the tropics affect the variability of the weather in mid-latitude regions, and the lingering effects of tropical storms can be felt in the United States as late as March. Each year, there are on average 63 tropical cyclones in the Northern Hemisphere. Active tropical cyclone years, or years with 68 or more storms, tend to be years with subsequent Northern Hemisphere winters that feature less of a temperature difference between the tropics and Arctic. The less this temperature difference is, the less potential energy there is to drive the mid-latitude storm tracks. Less active Northern Hemisphere tropical cyclone seasons, or years with 57 or fewer storms, tend to be seasons with more potential energy for the winter storm tracks.

Source: Hart, RE. “An inverse relationship between aggregate northern hemisphere tropical cyclone activity and subsequent winter climate.” Geophysical Research Letters 38 (2011): L01705.

There are close to 5,000 quality-controlled weather stations across the United States and everyday at least some of these locations will set record high or record low temperatures. During periods of warming – even periods of pronounced warming – daily record low temperatures will continue to be set. Daily record high temperatures are also set during periods of cooling. Yet, it is not the existence of record highs or record lows that indicate whether a warming or cooling trend is occurring. Instead, it is the proportion or ratio of record highs to record lows that indicates whether the climate is getting warmer or cooler. During the warmest decade on record, the 2000s, lots of daily record lows were set. Between January 1, 2000 and October 24, 2010, 152,087 record lows were set. 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 October 24, 2010, 310,531 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.

Source: 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.

The chart (left) has been provided courtesy of The Weather Channel and is based on data provided by NOAA’s National Climatic Data Center. The chart is available for further distribution and use on-air provided the embedded “The Weather Channel” and “NOAA: NOAA/NESDIS/NCDC” logos remain in the image.

For more information and updates on daily record highs and lows, visit: http://mapcenter.hamweather.com/records/yesterday/us.html.

To subscribe to weekly updates on United States record temperature events, email Weather Channel Lead Meteorologist Guy Walton: gwalton@weather.com.

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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

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.

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.

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.

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.)

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. <http://www.ars.usda.gov/is/pr/2008/081217.htm>)

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

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 <http://www.sciencedaily.com/releases/2009/09/090908103625.htm>.

• 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.

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.

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.

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

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.

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.

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.