Thomas Jefferson: Founding Father of Weather Observers

As the main author of the Declaration of Independence and the third President of the United States, Thomas Jefferson is regarded as one of this country’s Founding Fathers. He was also an astute and systematic weather observer.

Portrait of Thomas Jefferson by Rembrandt Peale, 1805. Credit: NYHS

In the summer of 1776, Jefferson was in Philadelphia, PA to sign the Declaration of Independence.  While there, he purchased a thermometer and a barometer – new and expensive weather equipment at that time. For the next 50 years, he kept a meticulous weather journal.  He recorded daily temperature data wherever he was – at home in Virginia or while traveling.

On July 4, 1776, Jefferson noted that the weather conditions in Philadelphia were cloudy with a high temperature of 76°F.

In an effort to understand the bigger picture of climate in America, Jefferson established a small network of fellow observers around Virginia as well as contacts in a few other states. According to records at Monticello, his estate in Virginia, he hoped to establish a national network for weather observations. While this plan did not come to fruition during his lifetime, today’s National Weather Service considers him the “father of weather observers.”

Happy Independence Day!

An excerpt from Thomas Jefferson’s Weather Journal, July 1776. Credit: NCDC

A Look at Why Death Valley is the Hottest, Driest, and Lowest Place in US

Death Valley National Park is famous for being the hottest, driest, and lowest place in the United States. The interesting thing about all these extremes, as I learned during a recent visit, is how they interconnect.

Situated in eastern California near the Nevada border, the park’s topography is known as basin and range. This is where the earth’s crust is rifting apart, creating mountains in some areas and deep basins in others. Death Valley is a long, narrow basin that reaches a depth of 282 feet below sea level. It is also in the rain shadow of four different mountain ranges to the west – the Coastal Range, the Sierra Nevada, the Argus Range, and the Panamint Range.

As storms move inland from the Pacific, they must rise up and over each range. In doing so, they cool and their water vapor condenses into rain or snow that falls on the western side of these mountains. By the time a storm system reaches Death Valley, it has lost most of its moisture. The average annual rainfall in the park, according to NOAA, is just 2.36 inches.

These dry conditions, along with the valley’s below-sea-level elevation, help to produce the park’s famous heat. With cloud free skies and sparse vegetation, a maximum amount of sunlight can reach the ground. The rocks and parched soil absorb the heat and radiate it into the air. The warm air rises but becomes trapped by the steep valley walls. After cooling slightly, it is recycled back toward the valley floor where it is heated even further by atmospheric compression. During the summer months, this process generates hot winds and sizzling temperatures. The average high temperature in the park ranges from 67°F in January to 116°F in July. The hottest temperature ever recorded was 134°F on July 10, 1913 – a world record.

Looking ahead, as the climate changes, the southwestern region of the US is expected to become even hotter and drier. It seems like only a matter of time before Death Valley breaks its own heat record.

At 282 feet below sea level, Basin in Death Valley National Park is the lowest point in the US. Credit: Melissa Fleming

The Wet and Dry Seasons of South Florida

Seasons are a way of dividing up the year based on changes in weather. The traditional four – winter, spring, summer and fall – are familiar to most people.  In the subtropical climate of south Florida, however, there are really only two – wet and dry. While traveling there recently, I had the chance to learn more about them.

The wet season typically runs from mid-May to November. It produces the vast majority of the region’s average annual rainfall, approximately 60 inches. Temperatures often reach into the 90s and humidity levels are high. With sea-breeze fronts developing along both the Atlantic and Gulf Coasts, thunderstorms can occur almost daily during the summer months.

The dry season runs from December to mid-May. Temperatures at that time of year range from the mid-50s to upper-70s and humidity levels are relatively low. On occasion, continental cold fronts dip down into south Florida, bringing near-freezing temperatures to the region. Any rain associated with these frontal systems tends to sweep through the area quickly. Only about 20% of the region’s average annual rain total falls during the winter months.

South Florida’s largest rainfall totals are usually associated with tropical storms and hurricanes, which are not uncommon between June and November.

Data Source: SERCC

Data Source: SERCC

Balmy Weather at the Winter Olympics

Frigid temperatures and abundant snowfall have been dominating this winter season across most of the continental U.S.  In Sochi, Russia, however, the XXII Olympic Winter Games have had a more spring-like feel.

Russia, famous for cold and snowy winters, has a few relative warm spots.  Sochi is one of them. Situated between the Black Sea and the Caucasus Mountains, it has a subtropical climate that supports palm trees.  While there is usually snow in the mountains, the city’s average temperature for this time of year is about 50°F.

This week, temperatures in Sochi have been running above average.  Today, the mercury hit 59°F and highs are forecast to be in the 60°s for the rest of the week.  Even in the higher elevations of the so called “mountain cluster” venue – site of all the skiing, snowboarding, and sledding events – temperatures have been well above freezing during the day.  This mild weather is turning the hard packed competition snow into slush – creating difficult conditions for many of the athletes.

While the current balmy weather has caused some delays, it has not hindered the Games. With hundreds of snowmaking machines and a stockpile of natural snow stored from previous years, officials say they are prepared to supply as much snow as necessary.  That said, these Winter Olympics are on track to be the warmest in history.

Average minimum temperatures for January and February from 1911-2011 for all the locations that have hosted the Olympic Winter Games. Image Credit: NOAA

Average minimum temperatures for January and February from 1911-2011 for all the locations that have hosted the Olympic Winter Games.    Image Credit: NOAA.

San Francisco’s Microclimates

San Francisco, a city known for its fog, is actually a composite of microclimates. Last week, I was visiting the Bay Area and was reminded of its unique climate situation.

The area, in general, has a Mediterranean climate with mild, wet winters and dry summers. It is rare for the city to get warmer than 70°F in summer or cooler than 45°F in winter. Situated on a peninsula along the California coastline, San Francisco is kept mild by the Pacific Ocean’s chilly currents and local coastal upwelling.

Famous for its hills, the city’s complex topography is another major influence on its climate. The Golden Gate, a break in the mountainous Coast Range, funnels Pacific air into the Bay Area. The hills and basins of the peninsula then capture and divert the circulating marine air in intricate ways, forming a variety of  microclimates. As a result, weather conditions can vary widely across short distances, such as 10°F between neighborhoods.  In San Francisco, the forty hills that form the center of the city create a general weather divide. The western side of the city usually bears the brunt of the incoming Pacific air, with cool temperatures, strong winds, and fog.  The more sheltered eastern side generally sees more sun and warmer temperatures.

On a larger regional scale, temperature differences have an even wider scope.  For example, the average high temperature in July in the city is 68°F, while temperatures can reach 100°F in the Sacramento Valley, just 50 miles inland.

La Nina Returns

La Nina has returned for a second year in a row.  According to NOAA’s winter climate outlook, this oceanic-atmospheric phenomenon will strongly shape our upcoming winter season.

La Nina is a climatic episode associated with the larger El Nino-Southern Oscillation (ENSO) climate pattern in the Pacific Ocean.   During a La Nina event, sea surface temperatures in the eastern Pacific are cooler than normal.  This ocean temperature anomaly influences weather around the globe.

In the US,  La Nina will impact both the temperature and precipitation in many parts of the country. The southern states are likely to see conditions that are both warmer and dryer than normal.  This is not good news for the drought-stricken state of Texas.  The northern tier is expected to experience  below average temperatures with the northwest getting above average precipitation.  The northeast and mid-Atlantic states have a 50/50 chance of seeing irregular conditions from La Nina.  In this region, the Arctic Oscillation (AO), a different oceanic-atmospheric pattern, has a stronger influence on winter weather.

Less predictable than La Nina, the AO continually transitions between positive and negative phases. A negative phase will bring cold arctic air and snowy conditions to the eastern US.  The cold snaps and heavy snow we saw last winter in the northeast were influenced by a very strong negative phase of the AO.  These strong phases can last anywhere from a few days to a few weeks and can be difficult to anticipate in long term forecasts.

The current La Nina event is forecast to last through February.

Photo Credit: NOAA

Weather v. Climate

The daily high temperatures this month seem like they are riding a roller-coaster.   While these variations are par for the course in a transitional month like October, they often get people talking about the weather.  In listening to some of these conversations, however, I frequently hear the terms “weather” and “climate” confused.  To set the record straight, weather and climate are related, but different.  The main difference between the two is time.

Weather is the short-term state of the atmosphere at a particular time and place.  It references current local conditions like temperature, wind, and precipitation.  Driven mainly by the relative differences in temperature and air pressure from one location to the next, weather is constantly changing.

Climate is the long-term, ”normal” weather conditions of a given region.  It refers to the prevailing weather that we expect for particular seasons based on past experience and tends to be stable for the course of centuries.  Over extended periods of geologic time, however, climate does change. For example, our planet has seen the extremes of both ice ages with massive glaciers and warmer periods with higher sea levels.

While the Earth’s climate has changed a number of times throughout its long history, the changes happening today are occurring more quickly than they have in the past.  Experts say rising global temperatures – the result of increasing greenhouse gases in the atmosphere – are causing weather patterns to shift and, in turn, regional climates to change.


What is “Normal” Weather?

As seasons change, months end, and years come to a close, we often hear about how that particular period’s weather compares to normal. You may wonder, what is “normal”?

“Normal”  refers to the 30-year average of a particular meteorolgical variable, such as temperature or precipitation, in a specific location.  This average is a statistical number that gives us an idea of what to expect as well as a point of reference for historical comparisons.  These figures are useful to scientists who study climate change and to industries, such as power and construction, who utilize the data for planning purposes.

The “normals” are calculated every decade for the previous 30-year period by the National Climatic Data Center (NCDC),  a division of the National Oceanic and Atmospheric Administration.  They have a mandate from Congress dating back to 1890 to “… to establish and record the climatic conditions of the United States.”   In addition, the United States is a member of the U.N.’s World Meteorological Organization (WMO).  As a member nation, we follow their guidelines for computing 30-year averages.

The NCDC recently released their new “normals” based on the years 1981 to 2010.