Saint Patrick’s Day and Rainbows

According to Irish folklore, a pot of gold can be found at the end of a rainbow. In reality, however, it is impossible to locate the terminus of this optical phenomenon.

Refraction and reflection inside a raindrop. Credit: Met Office

For a rainbow to form, rain has to be falling in one part of the sky while the sun is out in another. The water droplets in the air act like prisms that refract and reflect the sunlight, revealing the colors of the visible spectrum. Red is refracted the least and is always on the top of a single bow while blue is on the bottom. Since we only see one color from each drop, it takes a countless number to produce a rainbow.

A double rainbow is seen when the light reflects twice inside the raindrops. Since each reflection weakens the intensity of the light, the second bow appears dimmer. The order of the colors is also reversed, with blue on top and red on the bottom.

That said, these colorful arcs are not physical entities that can be approached. No matter how close they appear to be, they are always tantalizingly out of reach. Nevertheless, most people consider seeing one to be a treasure with no gold required.

With a little luck, you can spot a rainbow if you face a moisture source – clearing rain clouds or mist from a waterfall – while the sun is at your back.

Happy Saint Patrick’s Day!

Rainbow and faint second rainbow form after a rainstorm in Bermuda. Credit: Melissa Fleming

2019: Second Wettest Year on Record for US

The defining weather story of 2019 in the contiguous United States was rain. The year was the second-wettest ever recorded in the lower 48 states. Only 1973 was wetter.

The annual precipitation total, according to a report by NOAA’s National Centers of Environmental Information, was 34.78 inches. That is 4.84 inches above the long-term average.

While above-normal precipitation was observed across much of the nation, some places were particularly wet. North Dakota, South Dakota, Minnesota, Wisconsin, and Michigan each posted their wettest year on record. Another 18 states finished in the top ten.

Months-long flooding in the upper Midwest and Mississippi River basin caused a tremendous amount of damage to farms and homes across the region. In fact, the combined cost of the Missouri, Arkansas, and Mississippi River floods was $20 billion. That adds up to about half the total cost of the 14 different billion-dollar weather and climate disasters that occurred in the US during 2019.

Heavy rain events and flooding are nothing new, but experts say climate change is making them worse. As greenhouse gases warm the atmosphere, the air is able to hold more water vapor. With evaporation from oceans, rivers, and lakes increasing, there is more water vapor available to condense and fall as rain.

Weather records for the contiguous United States date back to 1895.

Credit: NOAA

2019: A Year of Weather Extremes in NYC

New York City experienced some noteworthy weather in 2019, especially the dramatic swings between hot and cold temperatures almost every month. In the end, however, these extremes just about balanced each other out. The city’s average temperature for the year in Central Park was 55.6°F, which is 0.7°F above normal.

In the end, only four months produced below-average readings. Of those, November posted the greatest departure from average with a temperature of 3.85°F below normal. This was aided by the two record cold overnight lows that occurred that month.

On the other side of the spectrum, two months posted temperatures among their top ten warmest on record. April 2019 was the third warmest April and July 2019 was the tenth warmest July ever recorded in New York City.

The summer brought NYC a number of oppressively hot and humid days, including 15 days with temperatures in the 90s. The hottest day came during the mid-July heatwave when the mercury soared to 95°F. When humidity was factored in, the heat index or real feel temperature was in the triple digits.

On the precipitation side of things, 53.03 inches of rain was measured in Central Park. That is 3.09 inches above average. Of that soggy total, 7.09 inches came down in December, making it the fifth wettest December on record in NYC.

Snowfall, on the other hand, was somewhat scarce. Only 16.6 inches was reported in Central Park. However, it is interesting to note that the vast majority of that total, 10.4 inches, fell during the first few days of March. The city, on average, gets 25.8 inches for the calendar year.

Records for the Central Park Climate Station date back to 1873.

Credit: The Weather Gamut

December 2019: Fifth Wettest on Record for NYC

December 2019 felt like a weather roller coaster in New York City. Highs ranged from a relatively balmy 58°F to a frigid 25°F. But in the end, these extremes nearly balanced each other out. The city’s mean temperature for the month was 38.3°F, which is only 0.8°F above average.

Credit: The Weather Gamut

In terms of precipitation, December was a month for the record books. The city received 7.09 inches of rain in Central Park, making it the fifth wettest December on record. It is also interesting to note that two different days produced rainfall totals greater than one inch. Snowfall, however, was below average. Only 2.5 inches was reported in Central Park. New York City, on average, sees 4 inches of rain and 4.8 inches of snow in December.

Credit: The Weather Gamut

From Snow to Freezing Rain: Why Winter Precipitation Can Take Several Forms

The winter season can produce various types of precipitation – rain, freezing rain, sleet, or snow. The form we see at the surface depends on the temperature profile of the lower atmosphere.

All precipitation starts out as snow up in the clouds.  But, as it falls toward the Earth, it can pass through one or more layers of air with different temperatures.  When the snow passes through a thick layer of warm air – above 32°F – it melts into rain.  If the warm air layer extends all the way to the ground, rain will fall at the surface.  However, if there is a thin layer of cold air – below 32°F – near the ground, the rain becomes supercooled and freezes upon impact with anything that has a temperature at or below 32°F.  This is known as freezing rain.  It is one of the most dangerous types of winter precipitation, as it forms a glaze of ice on almost everything it encounters, including roads, tree branches, and power lines.

Sleet is a frozen type precipitation that takes the form of ice-pellets. Passing through a thick layer of sub-freezing air near the surface, liquid raindrops are given enough time to re-freeze before reaching the ground. Sleet often bounces when it hits a surface, but does not stick to anything.  It can, however, accumulate.

Snow is another type of frozen precipitation.  It takes the shape of six-sided ice crystals, often called flakes.  Snow will fall at the surface when the air temperature is below freezing all the way from the cloud-level down to the ground.  In order for the snow to stick and accumulate, surface temperatures must also be at or below freezing.

When two or more of these precipitation types fall during a single storm, it is called a wintry mix.

Precipitation type depends on the temperature profile of the atmosphere. Credit: NOAA

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

What Causes the Smell of Rain?

Rain is often associated with particular smells. But, rain itself is odorless. So, where do these aromas come from?

The distinctive scent that lingers in the air after a rainstorm is known as petrichor. It is the product of two reactions that occur when rainwater hits the ground. Its main driver is a soil-dwelling bacteria called actinomycetes. These microorganisms thrive in moist conditions, but as the soil dries out, they produce spores. These are then released into the air by the moisture and force with which the rain hits the ground. This happens at the same time the rainwater is mixing with oils that were secreted by plants onto nearby rocks and soil during times of dryness. Together these reactions produce the musky petrichor smell, which is particularly strong after a long dry spell. The term was coined in 1964 by two Australian scientists, Isabel Joy Bear and RG Thomson. They derived it from the Greek words, “petra” meaning stone and “ichor”, the term used to describe the blood of the gods in ancient mythology.

A different, and often more pungent, rain smell is associated with thunderstorms. After the powerful electric charge of a lightning bolt splits the oxygen and nitrogen molecules in the atmosphere, they often recombine as nitric oxide. This, in turn, interacts with other atmospheric chemicals to form ozone (O3). When people say they can “smell the rain coming”, this is the scent they detect as it often arrives in the wind ahead of an approaching storm.

Credit: freepik

September 2019: 8th Driest September on Record for NYC

September 2019 felt like a weather roller coaster in New York City. Highs ranged from a balmy 89°F to a chilly 67°F. But, with 18 out of 30 days posting above-average readings, the warmth won out in the end. The city’s mean temperature for the month was 70.4°F, which is 2.4°F above average.

In terms of precipitation, September was a month for the record books. The city only received 0.95 inches of rain in Central Park, marking its eighth driest September on record. It was also the second month in a row to deliver below-average rainfall in NYC. On average, the Big Apple gets 4.28 inches of rain for the month.

Weather and Art: Louisa McElwain’s “Desert Rain God”

The weather has long been a muse for artists. For Louisa McElwain (1953-2013), an American painter based in the southwest, summer storms were particularly special.

According to Evoke Contemporary, the gallery that represents her estate, McElwain was drawn to “the way the light changes and the clouds dance across the horizon ahead of the rain.” While visiting the Phoenix Art Museum recently, I came across her 2009 piece, Desert Rain God.

This large-scale oil on canvas painting captures a storm in the New Mexico desert just as a dramatic downpour is starting. These storms, largely associated with Monsoon Season in the southwest, are a vital source of moisture in the arid region.

Working outdoors, using the bed of her pick-up truck as sort of mobile studio, McElwain’s paintings aimed to capture her experience of nature. In a statement, she described her approach to painting as a “dance made to the tempo of the evolving day”.  Working quickly to depict the changing effects of the atmosphere, she often used palette knives and masonry trowels to apply paint to the canvas. These tools allowed her to avoid being overly descriptive and to create an impasto effect. Most of her paintings, regardless of size, were completed in less than four hours.

Louisa McElwain’s “Desert Rain God” on display in the Phoenix Art Museum. Photo Credit: Melissa Fleming

What is the North American Monsoon?

The summer phase of the North American Monsoon is underway. But what, you may wonder, are monsoons and how do they impact the United States?

While most people associate a monsoon with rain, that is only half the story. It is actually a wind system. More specifically, according to NOAA, a monsoon is “a thermally driven wind arising from differential heating between a landmass and the adjacent ocean that reverses its direction seasonally.” In fact, the word monsoon is derived from the Arabic word “mausim”, meaning seasons or wind shift.

In general, a monsoon is like a large-scale sea breeze.  During the summer months, the sun heats both the land and sea, but the surface temperature of the land rises more quickly. As a result, an area of low pressure develops over the land and an area of relatively higher pressure sits over the ocean. This causes moisture-laden sea air to flow inland. As it rises and cools, it releases precipitation. In winter, this situation reverses and a dry season takes hold.

Monsoon wind systems exist in many different parts of the world, with the most famous one setting up over India and Bangladesh. In the US, we have the North American Monsoon that impacts states across the southwest. Summer temperatures in the region, which is mostly desert, can be extremely hot. Readings in the triple digits are not uncommon. This intense heat generates a thermal low near the surface and draws in moist air from the nearby Gulf of California. In addition, an area of high pressure aloft, known as the subtropical ridge, typically moves northward over the southern Plains in summer. Spinning clockwise, this shifts the winds in the area from a southwesterly to a southeasterly direction and ushers in moisture from the Gulf of Mexico. This combination of heat and moisture-rich air produces thunderstorms and heavy rainfall across the region. Monsoon rains reportedly supply 50-70% of the area’s annual precipitation.

Replenishing reservoirs and nourishing agriculture, these seasonal rains are a vital source of water in the typically arid southwest. Conversely, they can also cause a number of hazards such as flash flooding, damaging winds, dust storms, hail, and frequent lightning.

The wet phase of North American Monsoon typically runs from mid-June to the end of September.

The North American Monsoon pulls most air (green arrows) inland over the typically arid southwest region of the US. Source: NOAA/NWS