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

Swamped by Barry, Arkansas Sets New State Tropical Rainfall Record

Hurricane Barry made headlines for making landfall in Louisiana over the weekend, but its torrential rain made history Arkansas this week.

According to the NWS, the remnants of the moisture-laden storm dumped 16.59 inches of rain in Dierks, Arkansas, setting a new tropical rainfall record for the state. The previous record of 13.91 was set in Portland, AR during Tropical Storm Allison in 1989.

Arkansas is the fifth state to set a new tropical rainfall record in the last two years. The other four include Texas, Hawaii, North Carolina, and South Carolina.

These types of extreme rain events have been happening more frequently in recent years and most experts see a link to climate change. As the atmosphere and oceans warm, storms are able to carry more moisture, and therefore drop more rain.

Credit: The Weather Gamut

June 2019: A Soggy Start to Summer in NYC

June 2019 was another month of wild temperature swings in New York City. Highs ranged from an unseasonably cool 65°F to a balmy 91°F. In the end, however, the cold and warmth balanced each other out. The city’s mean temperature for the month was 71.7°F, which is only 0.3°F above average.

In terms of precipitation, the city was unusually wet. Overall, 13 out 30 days posted measurable rainfall that added up to 5.46 inches for the month. While that is a soggy statistic, it was not the wettest June the city has seen. That dubious honor belongs to June 2003 when 10.26 inches of rain was measured in Central Park. The city, on average, gets 4.41 inches for the month.

Credit: The Weather Gamut

May 2019: Second Wettest Month on Record for US

When it rains, it pours! This old adage was proven to be true last month. With national weather data dating back to 1895, May 2019 was the second wettest month ever recorded in the contiguous United States. Only May 2015 was wetter.

According to NOAA, an average of 4.41 inches of rain fell across the lower 48 states, which is 1.50 inches above average. Heavy precipitation was reported from the West Coast through the Mid-West, and into parts of the Northeast. Kansas, Nebraska, and Missouri each experienced their wettest May on record.

This relentless rain caused deadly and destructive flooding in several states along the Arkansas, Missouri, and Mississippi Rivers.

While there were a number of factors that contributed to May’s excessive rainfall, including El Nino, climate change also likely played a part.  As the National Climate Assessment points out, heavy precipitation events are increasing in both frequency and intensity across the United States as the atmosphere warms.

As greenhouse gases heat the atmosphere, the air is able to hold more water vapor. More specifically, according to the Clausius–Clapeyron relation, for every increase of 1°F, the saturation level of the atmosphere increases by about 4%. In other words, warmer temperatures lead to more evaporation from oceans, rivers, and lakes, and therefore more water vapor is available to condense and fall as precipitation.

Sadly, with our global temperature continuing to rise, experts say we should expect to see more extreme rain and flooding events in the future.

Credit: NOAA

May 2019: A Soggy Month for NYC

May was another month of wild temperature swings in New York City. Producing several cases of weather whiplash, highs ranged from a chilly 48°F to an unseasonably balmy 86°F. In the end, however, these extremes balanced each other out. The city’s mean temperature for the month was 62.2°F, which is only 0.2°F below average.

On the precipitation side of things, May was unusually wet. The month brought the city a relatively rare spring nor’easter and several impressive thunderstorms. One of which produced golf ball sized hail on Staten Island, one the city’s five boroughs. Overall, 19 out 31 days posted measureable rainfall that added up to 6.82 inches for the month. While that is a soggy statistic, it was not the wettest May the city has seen. That dubious honor belongs to May 1989 when 10.24 inches of rain was measured in Central Park. The city, on average, gets 4.19 inches for the month.

Credit: The Weather Gamut