What is a Monsoon and How Do They Affect the US?

The summer phase of the North American Monsoon is in full swing. But what, you may wonder, is a monsoon and how do they affect 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 land mass 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. In the US, we have the North American Monsoon that impacts states across the southwest. Summer temperatures in the region – 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 U.S. in summer. Its clockwise circulation shifts the winds 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. In fact, summer monsoon rains are reported to supply nearly 50% 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 and hail, as well as frequent lightning.

Monsoon season in the American southwest typically runs from mid-June to the end of September.

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

Aphelion 2018: Earth Farthest from Sun Today

The Earth will reach its farthest point from the Sun today – an event known as the aphelion. It will officially take place at 16:46 UTC, which is 12:46 PM Eastern Daylight Time.

This annual event is a result of the elliptical shape of the Earth’s orbit and the slightly off-centered position of the Sun inside that path. The exact date of the Aphelion differs from year to year, but it’s usually in early July – summer in the northern hemisphere.

While the planet’s distance from the Sun is not responsible for the seasons, it does influence their length. As a function of gravity, the closer the planet is to the Sun, the faster it moves. Today, Earth is about 152 million kilometers (94 million miles) away from the Sun. That is approximately 5 million kilometers (3 million miles) further than during the perihelion in early January. That means the planet will move more slowly along its orbital path than at any other time of the year. As a result, summer is elongated by a few days in the northern hemisphere.

The word, aphelion, is Greek for “away from the sun”.

Earth is farthest from the Sun during summer in the northern hemisphere. Credit: TimeandDate.com

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

Four Day Heat Wave Bakes the Big Apple

New York City is sweltering through its first official heat wave of the summer.

The threshold for what constitutes a heat wave varies by region, but here in the northeastern United States it is defined as three consecutive days with temperatures reaching 90°F or higher. Tuesday marked the city’s fourth day of sweltering conditions.

In Central Park, the temperature reached 93°F on Saturday, 96°F on Sunday, 95°F on Monday, and 92°F on Tuesday. The humidity made it feel even hotter. The heat index – the so-called real feel temperature – reached into the triple digits in some spots.

The main driver of this dramatic heat and humidity is a dominant Bermuda High, a large area of high pressure situated off the east coast. Spinning clockwise, it has been steering hot, humid air from the Gulf of Mexico toward the northeast.

While these conditions are oppressive, they can also be dangerous. The NWS issued both an excessive heat warning and air quality alert for the city.

The normal high for this time of year in the Big Apple is 83°F.

Four day heat wave bakes the Big Apple. Credit: The Weather Gamut

Weather Lingo: Heat Index

Temperature is one of the basic elements of weather.  Our perception of it, however, is often influenced by other factors.  In summer, this is usually humidity.

The heat index, developed in the late 1970’s, is a measure of the apparent or “real feel” temperature when heat and humidity are combined.  Since the human body relies on the evaporation of perspiration to cool itself, the moisture content of the air affects comfort levels. Basically, as humidity levels increase, the rate of evaporation decreases and the body can begin to feel overheated.  For example, an air temperature of 92°F combined with a relative humidity level of 60% will produce a heat index value of 105°F.

The National Weather Service issues heat advisories when the heat index is forecast to be at least 95°F for two consecutive days or 100°F for any length of time.  Extended exposure to high heat index values can lead to serious health hazards.

Heat-Index

Credit: NOAA

NYC Monthly Summary: June 2018

June 2018 was another month of wild temperature swings in New York City. Highs ranged from an unseasonably cool 66°F to a balmy 93°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 dry. Overall, 3.11 inches of rain was measured in Central Park. Of this total, 44% fell in single day during a heavy rain event that produced over an inch of rain. On average, the Big Apple gets 4.41 inches of rain for the entire month of June.

Credit: The Weather Gamut

What the Summer Solstice Means

Today is the June Solstice, the first day of summer in the northern hemisphere. The new season officially began at 10:07 UTC, which is 6:07 AM Eastern Daylight Time.

Our astronomical seasons are a product of the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. During the summer months, the northern half of the Earth is tilted toward the sun. This position allows the northern hemisphere to receive the sun’s energy at a more direct angle and produces our warmest temperatures of the year.

Since the winter solstice in December, the arc of the sun’s daily passage across the sky has been getting higher and daylight hours have been increasing. At noon today, the sun will be directly overhead at the Tropic of Cancer, its northernmost position, marking the “longest day” of the year. This observable stop in the sun’s apparent annual journey is where today’s event takes its name. Solstice is a word derived from Latin and means “the sun stands still”.

While today brings us the greatest number of daylight hours  (15 hours and 5 minutes in NYC), it is not the warmest day of the year.  The hottest part of summer typically lags the solstice by a few weeks. This is because the oceans and continents need time to absorb the sun’s energy and warm up – a phenomenon known as seasonal temperature lag.

Earth’s solstices and equinoxes. Image Credit: NASA

May 2018: Earth’s Fourth Warmest May on Record

Our global temperature continued its upward trend last month. May 2018 marked not only the fourth warmest May on record, but also closed out the planet’s fourth warmest March to May season, known as meteorological spring in the northern hemisphere.

According to the State of the Climate report by NOAA’s National Centers for Environmental Information, Earth’s combined average temperature for May – over both land and sea surfaces – was 60.04°F, which is 1.44°F above the 20th-century average. The years 2014-2018 now rank among the five warmest Mays on record.

This May also marked the 401st consecutive month with a global temperature above its long-term norm. That means the last time any month posted a below average reading was December 1984.

While heat dominated most of the planet this May, some places were particularly warm, including much of Europe and North America. Here in the contiguous US, it was our warmest May ever recorded. The previous record had been in place since 1934.

Globally, the three-month period of March, April, and May was also unusually warm. NOAA reports that Earth’s average temperature for the season was 1.48°F above the 20th century average of 56.7°F. That makes it the fourth warmest such period on record.

These soaring temperatures are largely attributed to the long-term trend of human-caused climate change. ENSO-neutral conditions prevailed in May, which means there was neither an El Niño nor a La Niña in the Pacific to influence global weather patterns.

Year to date, the first five months of 2018 tied 2010 as the fourth warmest such period of any year on record. Global temperature records date back to 1880.

May 2018 was the fourth warmest May ever recorded on Earth. Credit: NOAA

Storm King Art Center Exhibition Focuses on Climate Change

Art and science have come together to expand the public conversation on climate change  at Storm King Art Center in Orange County, NY. In an exhibition called Indicators: Artists on Climate Change, the work of seventeen artists and collectives address the diverse impacts of this critical issue.

Curated by Nora Lawrence, the exhibit is a mix of large-scale sculptures and installations spread out across the open air museum’s five-hundred acres as well as smaller pieces displayed in an indoor gallery.

Credit: Mary Mattingly/Storm King

Notable among the various projects is Mary Mattingly’s installation, Along the Lines of Displacement: A Tropical Food Forest. This stand of palm trees imported from Florida offers visitors a glimpse of what the landscape of upstate New York might look by the end of the century when the average temperature is expected to increase by 7.2°F (4°C) if global warming continues unabated.

Credit: Justin Brice Guarigila/Storm King

Another piece that stands out in Storm King’s bucolic setting is Justin Brice Guarigila’s We Are the Asteroid.  The orange colored, solar-powered LED highway message sign – the sort usually seen flashing information about dangerous situations – displays three-line ecological aphorisms by philosopher Timothy Morton. These include the piece’s moniker as well as things like “Danger: Anthropocentrism” and “Warning: Hurricane Human”.

Credit: Hara Woltz/Storm King

Embracing the science of climate change is Hara Woltz’s “Vital Signs”. In this interactive piece, a working weather station is encircled  by nine cylinders (a reference to the shape of ice-core tubes) that present the idea of albedo and melting sea ice. The top of each cylinder gets darker as a viewer moves around the circle. More specifically, the lighter area decreases by 13% per cylinder reflecting the predicted decrease in Arctic sea ice per decade relative to the 1981-2010 average. The cylinders also progressively get taller, referencing sea level rise. Data from the weather station is displayed in real time on Storm King’s website.

Other exhibiting artists include David Brooks, Dear Climate, Mark Dion, Ellie Ga, Allison Janae Hamilton, Jenny Kendler, Maya Lin, Alan Michelson, Mike Nelson, Steve Rowell, Gabriela Salazar, Rebecca Smith, Tavares Strachan, and Meg Webster.

The exhibit is on view through November 11, 2018.

How Hail Forms

The thunderstorms of spring and summer are notorious for their powerful winds and heavy rain. However, when strong enough, they can also produce hail.

Hailstones start off as water vapor that is lifted high into the atmosphere by the updraft of a thunderstorm. Rising into cooler air, it condenses and forms water droplets. Once these liquid droplets reach a level where the temperature is below freezing, they turn into tiny ice crystals. Overtime, they get larger as other water droplets freeze to them on contact, forming layers like an onion.  Once a hailstone gets too heavy for the updraft, it falls to the ground.

The stronger the updraft of a storm, the longer a hailstone remains suspended, and the larger it can grow. For a ball of ice to be considered a hailstone, according to the AMS, it has to measure at least 5mm in diameter.

The largest hailstone ever recorded in the US was found in Vivian, South Dakota on June 23, 2010. It measured 7.9 inches in diameter and weighed 1.94 pounds. The updraft supporting it would have had to exceed 150 mph.

Needless to say, hail can cause serious damage to people and property.