The Azure Skies of Autumn

Autumn is well known as the time of year when leaves change color. However, have you ever noticed the sky also changes shades with the season?

In general, we see the sky as blue because of Rayleigh scattering. This is a phenomenon where the molecules of nitrogen and oxygen that make up most of Earth’s atmosphere scatter the incoming light radiation from the sun. More to the point, they are most effective at scattering light with short wavelengths, such as those on the blue end of the visual spectrum. This allows blue light to reach our eyes from all directions and dictates the color we understand the sky to be.

The arc height of the sun’s apparent daily passage across our sky, which varies with the seasons, determines how much of the atmosphere the incoming light must pass through. This, in turn, affects how much scattering takes place. Simply put, the more Rayleigh scattering, the bluer the sky appears.

That said, humidity levels also play a role. Water vapor and water droplets are significantly larger than nitrogen and oxygen molecules, and therefore scatter light differently. Instead of sending light in all directions, they project it forward. This is known as Mie scattering and tends to create a milky white or hazy appearance in the sky.

During the summer months, when the sun is higher in the sky, light does not have to travel that far through the atmosphere to reach our eyes. Consequently, there is less Rayleigh scattering. The warm temperatures of summer also mean the air can hold more moisture, increasing the effect of Mie scattering. As a result, the summer sky tends to be a relatively muted or pale blue.

In autumn, the sun sits lower on the horizon, increasing the amount of Rayleigh scattering. The season’s cooler temperatures also decrease the amount of moisture the air can hold, diminishing the degree of Mie scattering. Taken together, these two factors produce deep blue skies.

When this azure hue is contrasted with the reds and yellows of the season’s famous foliage, all of the colors look even more vibrant.

Photo credit: Azure-Lorica Foundation

Yellow leaves pop against a deep blue autumn sky. Photo credit: Azure-Lorica Foundation

An Autumn Chill in NYC

New York City, along with much of the northeastern US, is suffering from weather whiplash this week.

Last Wednesday, the temperature in Central Park soared to 85°F, setting a new record high for the date. This Wednesday, the mercury only made it to 51°F. That is a difference of 34°F! Our normal high for this time of year is 60°F.

Overnight lows in the city have also seen a dramatic decline. Dropping to 38°F early this morning, it was the coldest reading the Big Apple has seen since last April.

After enjoying summer-like conditions just a few days ago, these brisk temperatures are a reminder, albeit a jarring one, that autumn is a transitional season and winter is not too far off.

Ushered in by a dip in the jet stream, these chilly conditions are not expected to last much longer. Temperatures are forecast to rebound to more seasonable levels by the weekend.

A case of weather whiplash for NYC. Credit: The Weather Gamut.

A case of “weather whiplash” for NYC. Credit: The Weather Gamut.

September 2016: Second Warmest September on Record for Planet Earth

Temperatures around the globe soared last month. In fact, September 2016 was the second warmest September ever recorded for the entire planet.

According to a report by NOAA’s National Centers for Environmental Information, Earth’s combined average temperature for the month – over both land and sea surfaces – was 60.6°F. That is 1.60°F above the 20th-century average and only 0.07°F shy the record that was set last year.

This second place finish effectively ends a 16-month stretch of record warm global monthly temperatures – the longest such streak on NOAA’s books.

While heat dominated most of the planet this September, some places were particularly warm. Several countries in Europe posted readings that were among their top five warmest for the month. These included Germany, the UK, France, Holland, and Austria. Here in the contiguous US, it was our ninth warmest September on record.

These soaring temperatures are attributed to the long-term trend of human-caused climate change. Whereas El Niño gave global temperatures a boost earlier in the year, it dissipated in June. ENSO-neutral conditions, according to NOAA, prevailed in September.

Year to date, the first nine months of 2016 were the warmest of any year on record. This significantly increases the likelihood that 2016 will surpass 2015 as the Earth’s warmest year ever recorded. Global temperature records date back to 1880.

September 2016 was the second warmest September on record for the entire planet. Credit: NOAA

September 2016 was the second warmest September on record for the entire planet. Credit: NOAA

Autumn Heat in NYC

From shorts to sweater weather and back again, Indian summer is in full swing in New York City.

The temperature in Central Park soared to 81°F on both Monday and Tuesday this week. On Tuesday, it was just one degree shy of the daily record set back in 1928. We will get another shot at a record on Wednesday if the temperature climbs to 83°F. (The forecast high is 82°F.) Combining these unusually warm readings with dew points in the 60s, it has felt more like August than mid-October in the Big Apple. Our normal high for this time of year is 63°F.

This unseasonable warmth is the result of a dominant Bermuda High, a large area of high pressure situated over the southeastern part of the country. Spinning clockwise, it has been steering warm, moist air from the Gulf of Mexico toward the northeast.

As pleasant as these summer-like temperatures are, they will not last much longer. Conditions that are more seasonable are expected to return by the end of the week.

How a Bermuda High ushers in hot and humid air to the northeastern US. Credit: Jacksonsweather

How a Bermuda High ushers in hot and humid air to the northeastern US. Credit: Jacksonsweather

What is Indian Summer?

Autumn is a season known for changing leaves and falling temperatures. Every once in a while, however, summer warmth makes a resurgence. When this happens, it is often dubbed an “Indian Summer”.

This weather phenomenon, according to the NWS glossary, is defined as “an unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.”  In the northeastern US, it is generally associated with an area of high pressure to the south that ushers warm air northward.

In popular use since the 18th century, the exact origins of the term “Indian Summer” are a bit foggy. One of the more reasonable explanations behind this unique phrase suggests a connection to when Native Americans began their hunting season, but no one knows for sure.

In other parts of the world, this summer-like weather goes by a variety of different names. In Europe, a number of countries associate the unusual warmth with the nearest saint’s day. It is known as “St. Luke’s Little Summer” if it develops in October or a “St. Martin’s Summer” if it occurs in November. In temperate parts of South America, it is simply known as “Veranico” (little summer).

Regardless of its name, the timing and intensity of these autumn warm spells vary from year to year. Nevertheless, when they do occur, they usually only last a few days. So, as we inevitably move toward winter, enjoy them while they last.

Hurricane Matthew Slams the Southeastern US

Hurricane Matthew, the 13th named storm of the Atlantic Hurricane Season, hammered the southeastern United States this weekend from Florida to Virginia.

Tearing up the coast as it trekked northward, Matthew made landfall near McClellanville, SC on Saturday as a category-1 hurricane with 75mph winds. It had reached category-5 status in the Caribbean – the first storm to do so since Hurricane Felix in 2007- but weakened as it moved toward the US.

Despite this downgrade, Matthew still packed a powerful punch. Its strong winds, flooding rains, and storm surge caused significant property damage and widespread power outages throughout the region. The death toll from this storm currently stands at 26 people from across five states and is expected to increase in the coming days.

With successive bands of heavy rain, Matthew also caused catastrophic inland flooding. In Fayetteville, NC – 100 miles from the coast – 14.82 inches of rain was reported. As a result, several rivers in the region rose to record or near-record levels and overflowed their banks, inundating communities.

All told, Matthew dumped 13.6 trillion gallons of water on Florida, Georgia, South Carolina, North Carolina, and Virginia before heading out to sea as a post-tropical storm. That is enough water to fill over 20 million Olympic-size swimming pools. The highest rainfall total, 17.49 inches, was reported near Savannah, GA.

The damage caused by Matthew is currently estimated at $6 billion.

Hurricane Matthew batters the south eastern US. Credit: NOAA/NASA

Hurricane Matthew batters the southeastern US. Credit: NOAA/NASA

Paris Climate Change Agreement Enters into Force

The Paris Climate Agreement is signed, sealed, and delivered. Approved domestically by the requisite number of signatories with unusual speed, it will enter into force in 30 days.

The historic deal negotiated at the UN Climate Change Conference in Paris last December required ratification by 55 countries, representing 55% of global greenhouse gas emissions before it could go into effect. This double threshold was passed this week when the EU, Canada, and a number of smaller states officially ratified the agreement. To date, according to the UN, 72 countries representing more than 56% of emissions have signed on to the deal including the US, China, and India – the world’s three largest carbon polluters.

The aim of this international agreement is to limit global warming to 2°C (3.6°F) above pre-industrial levels. To achieve this goal, it employs a mix of voluntary and legally binding actions. While every country submitted their own emissions reduction plan known as nationally determined contributions (NDCs), there was no requirement quantifying the amount of greenhouse gases they had to cut or how it had to be done. Additionally, there are no penalties for those who do not live up to their promises. Instead, the accord depends heavily on global peer pressure and the hope that no country wants to be seen as a slacker in the eyes of the world.

Based on the current collection of national plans, which vary widely in ambition, this agreement will only cut greenhouse gas emissions by about half of what is necessary to reach the 2°C (3.6°F) goal. The accord, however, does legally obligate countries to publically report how much emissions they have actually eliminated and to reassess their plans every five years.

While flawed, the Paris Accord is the world’s first truly global climate agreement. It marks the culmination of a very long, and often tumultuous, international political process that began at the Rio Earth Summit 24 years ago. That said, the hard part still lies ahead.

Individual nations need to stay the course and implement the commitments in their NDCs. The framework needed to monitor and report on these independent undertakings will be negotiated at the next UN Climate Change Conference (COP22) in Marrakesh, Morocco this November. The Paris Agreement will come into force on November 4th.

Paris Climate Change Agreement has met the double threshold required to enter into force. Credit: UN

The Paris Climate Change Agreement met the double threshold required to enter into force on October 5, 2016. Credit: UN

NYC Monthly Summary: September 2016

Summer had an extended stay in NYC this September. Overall, 21 out of 30 days posted above average temperatures. These included three days with readings in the 90s, which is two more than what we typically see in September. With overnight lows also running mostly above normal, the city’s mean temperature for the month was 71.8°F, which is 3.8°F above average. That creates a three-way tie with September 1983 and 1884 for NYC’s 8th warmest September on record.

On the precipitation side of things, September was unusually dry and marked the sixth month this year that NYC received below average rainfall. All told, the city received a mere 2.79 inches of rain in Central Park, which is 1.49 inches below normal. As a result, according to the latest report (9/27) from the US Drought Monitor, the city remains in a moderate drought.

September was unusually warm in NYC this year. Credit: The Weather Gamut

September was unusually warm in NYC this year. Credit: The Weather Gamut.