Why the Sky Looks Bluer in Autumn

Autumn is well known as the time of year when leaves change color. Not as well known, however, is the fact that 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 as 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 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

How Climate Change is Impacting Fall Foliage

Colorful foliage is the hallmark of autumn, especially in the northeastern United States. As the season heats up, however, this familiar natural phenomenon is reflecting the impacts of our changing climate.

While decreasing sunlight hours is a key factor that signals the annual color change, the timing and duration of the displays are largely dependent on temperature and precipitation. Dry, sunny days and cool nights are the ideal conditions for beautiful fall foliage. But, as our climate changes, warmer and wetter conditions are becoming more common across the region.

In general, this means autumn colors are expected to peak later and disappear sooner. While there will still be variability from year to year, the fall foliage season, overall, is expected to get shorter. Furthermore, with the increasing probability of extreme weather events, such as heavy rainstorms, leaves could be swept from trees effectively ending the leaf-peeping season in a single day.

More than just an aesthetic detail, these changes are sure to have an impact on the multi-billion-dollar a year ecotourism industry in several states.

Credit: Climate Central

Why Leaves Change Color in the Autumn

Autumn is a season well known for its colorful foliage. The often-celebrated aesthetic displays, however, are actually part of a process that trees use to survive the winter.

As daylight hours decrease in the fall, there is less sunlight available to power photosynthesis – the chemical process that provides nutrients to trees by converting carbon dioxide and water into glucose. This, in combination with falling temperatures, signals the tree to stop producing food and prepare for a period of dormancy, which is similar to hibernation.

To do this, a tree turns off its food producers by slowly corking the connection between leaf-stems and its branches. This blocks the movement of sugars from the leaves to the tree as well as the flow of water from the roots to the leaves.  As a result, the leaves stop producing chlorophyll, the agent of photosynthesis and the reason for the green color of summer foliage. As the green fades, other chemicals that have been present in the leaves all along begin to show. These include xanthophyll and carotene, which produce yellow and orange leaves, respectively. Red to purplish colors are the result of anthocyanin, a chemical produced as a result of any remaining sugars trapped in a leaf.

While leaves change color every year, the timing and duration of the displays are largely dependent on temperature and rainfall. Dry, sunny days and cool nights are the ideal recipe for beautiful fall foliage. Warmer and wetter conditions, on the other hand, tend to dull and delay the color change. Extreme circumstances, such as frost or drought, can be a source of stress for trees and cause the leaves to fall off faster.

It should also be noted that different species of trees react to atmospheric conditions differently. Therefore, the more diverse a forest, the wider the range of colors in autumn.

Tree in Autumn. Credit: Melissa Fleming

October 2019: Unusually Warm and Wet in NYC

October was unusually warm in New York City this year. We had 19 out of 31 days post above-average readings with one day reaching a record-breaking 93°F. This unseasonable heat helped drive the city’s mean temperature for the month up to 59.9°F, which is 3°F above normal.

On the precipitation side of things, October was rather soggy. Overall, 15 days produced measurable rainfall that added up to 6.15 inches in Central Park. Of this total, more than half fell during just two storms. On average, the Big Apple gets 4.4 inches of rain for the month. It is also interesting to note that October marked the first month since July that the city received above-average rainfall.

Credit: The Weather Gamut

Fall Nor’easter Transitions to Sub-Tropical Storm Melissa

The nor’easter that has been battering the northeast coast of the US for several days transitioned to Sub-Tropical Storm Melissa on Friday. It is now the 13th named storm of the Atlantic hurricane season.

Lingering offshore since Wednesday, states from the mid-Atlantic to New England have been feeling its impacts in the form of strong winds, heavy rain, and coastal flooding. On Friday, its sustained winds were measured up to 60 mph.

Classified as subtropical, Melissa is a hybrid between a tropical storm and a regular low-pressure system. A tropical system is fueled by the latent heat released by the evaporation of ocean water while a regular storm is powered by the temperature contrast between air masses. Hybrids are able to access both energy sources.

The National Hurricane Center expects Melissa to become post-tropical and move further out to sea over the weekend.

Subtropical Storm Melissa swirling off the coast of New England. Credit: NOAA

Autumn is Heating Up Across the US

Autumn is a transitional season. It is generally a time when the heat of summer fades away and the chill of winter gradually returns. But, as our climate changes, the season is heating up.

Across the contiguous United States, autumn temperatures have increased an average of 2.5°F over the past fifty years, according to Climate Central. The western part of the country has seen the fastest seasonal increase, with Reno, NV warming 7.7°F.  Las Vegas, NV, and El Paso, TX have each seen a rise of more than 5°F since 1970.

These warmer temperatures may feel like a summer bonus for some, but they also bring a number of negative impacts. Less frost-free days means the allergy season is extended and disease-carrying pests like mosquitos and ticks are able to live and thrive longer. Warmer temperatures also drive up energy bills, as people with air conditioning units use them longer into the season. This in turn, if they are powered by fossil fuels, adds even more heat-trapping greenhouse gases to the atmosphere.

Wildlife is also feeling the effects of a warming fall. The timing of when fruits ripen, for example, is being skewed from its “normal schedule”. In turn, this is impacting the once well-synced patterns of animal behaviors such as bird migration and hibernation.

Looking ahead, as greenhouse gas emissions continue to increase, so too will the temperature and its associated impacts.

Credit: Climate Central

The Science Behind the Autumnal Equinox

Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially began at 7:50 UTC, which is 3:50 AM Eastern Daylight Time.

The astronomical seasons, as opposed to the meteorological seasons, are a product of Earth’s axial tilt – a 23.5° angle – and the movement of the planet around the sun. During the autumn months, the Earth’s axis is tilted neither toward nor away from the sun. This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the summer solstice in June, the arc of the sun’s apparent daily passage across the sky has been sinking and daylight hours have been decreasing. Today, the sun appears directly overhead at the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

Transitioning from summer to winter, autumn is also a season of falling temperatures. According to NOAA, the average high temperature in most US cities drops about 10°F between September and October.

Earth’s solstices and equinoxes. Image Credit: NASA

First Snow of the Season in NYC Breaks Several Records

New York City saw its first snow of the season on Thursday and it was one for the record books.

According to the NWS, 6.4 inches of snow was measured in Central Park, setting a new daily record for the date. The previous record of 1 inch had been in place since 1906. It was also the earliest 6-inch one-day snowfall on record for the city and the largest one-day November snowfall since 1882.

These superlatives came as a bit of a surprise. The forecast originally called for a wintry mix with only a dusting of snow at the onset. However, the temperature was colder than expected and the snow hung on longer. This was largely the result of an area of high pressure to the north being stronger than forecast and therefore able to funnel air toward the city that was colder than anticipated. Closer to home, evaporative cooling also played a part.  The air near the surface was very dry as the storm moved into the area. This allowed some of the snow to evaporate as it fell, cooling the air even further. As result, the change over to rain was delayed by several hours.

While pretty to see, the snow caused a number of problems around the city. Widespread travel delays and falling trees were reported across the five boroughs. As it is only mid-November, many of the trees still had their leaves. The combination of the heavy, wet, snow piling up on the foliage, weighing down the branches, and high winds was too much to bear for many trees, even the healthily ones. Many fell across streets and sidewalks as well as on top of parked cars. The city’s Parks Department has reported receiving over 2000 service requests for downed trees and dangling limbs.

This storm clearly outperformed expectations and caught the city off-guard. On average, New York City sees 0.3 inches of snow for the entire month of November.

Record breaking November snowfall topples trees in NYC. Credit: Melissa Fleming

Fall Foliage and Climate Change

Autumn is a season well known for its colorful foliage. Driven by the combination of sunlight, temperature, and precipitation, local displays vary from year to year. However, as the climate changes, so too will this familiar natural phenomenon.

As daylight hours decrease in the fall, there is less sunlight available to power photosynthesis – the chemical process that provides nutrients to trees by converting carbon dioxide and water into glucose, which is consumed by the tree and oxygen, which is released. This, in combination with falling temperatures, tells a tree to start preparing for winter.

To do this, a tree turns off its food producers by slowly corking the connection between leaf-stems and its branches.  This blocks the movement of sugars from the leaves to the tree as well as the flow of water from the roots to the leaves.  As a result, the leaves stop producing chlorophyll, the agent of photosynthesis and the reason for the green color of summer foliage.  As the green fades, other chemicals that have been present in the leaves all along begin to show.  These include xanthophyll and carotene, which produce yellow and orange leaves, respectively. Red to purplish colors are the result of anthocyanin, a chemical produced as a result any remaining sugars trapped in a leaf.

The change of leaf color happens every year, but the timing and duration of the displays are largely dependent on temperature and rainfall. Dry, sunny days and cool nights are the ideal recipe for beautiful fall foliage. Warmer and wetter conditions, on the other hand tend to delay the color change. However, extreme conditions, such as high heat, frost, excessive rain, or drought, can be a source of stress for trees and cause the colors to change early and the leaves to fall off faster.

As our climate changes, so too will displays of fall foliage. With warmer and wetter conditions forecast for the northeast, autumn colors are expected to peak later and disappear sooner. While there will still be variability from year to year, the fall foliage season in general is expected to get shorter. Furthermore, with the increasing probability of extreme weather events, such as storms with heavy rain, leaves could be swept from trees, effectively ending the season in a single day.

These changes will have more than an aesthetic affect. They are sure to have an impact on the multi-billion-dollar a year leaf-peeping ecotourism industry in several states.

Credit: Climate Central

NYC Monthly Weather Summary: October 2018

October had a split personality in New York City this year. It started off unseasonably warm, but then temperatures plunged dramatically in the middle of the month and remained mostly below average until Halloween. Highs ranged from a balmy 80°F to a brisk 50°F. But in the end, these extremes balanced each other out. The city’s mean temperature for the month was 57.7°F, which is only 0.8°F above average.

In terms of precipitation, the city was fairly dry. Overall, 3.59 inches of rain was measured in Central Park. Of this total, 35% fell in single day during the first nor’easter of the season. On average, the Big Apple gets 4.40 inches of rain for month. This October also marked first month since June that the city received below average rainfall.

Credit: The Weather Gamut