Monthly Archives: July 2018

Weather Lingo: Humidity

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“It’s not the heat, it’s the humidity.” This old adage heard throughout much of the summer in the eastern US, refers to how the amount of water vapor in the air affects human comfort. Since the body’s main source of cooling is evaporation of perspiration, the more moisture there is in the air, the less evaporation takes place and the warmer we feel. Two ways to indicate atmospheric moisture content are relative humidity and the dew point temperature.

Relative humidity (RH) measures the actual amount of moisture in the air compared to the total amount of moisture that the air can hold. It is expressed as a percentage and is commonly used in generic weather reports and apps. A high RH can produce fog and a low RH can cause rapid dehydration in both people and plants – important information for some sectors such farmers and crews fighting wildfires. But, since warm air can hold more moisture than cool air, the relative humidity changes as the air temperature changes.

The dew point temperature, on the other hand, is an absolute measurement and is often the preferred metric of meteorologists. It is the temperature to which air must be cooled in order to reach saturation. In other words, when the air temperature and the dew point temperature are same, the air is saturated and the relative humidity is 100%. If the air were to cool further, the water vapor would condense into liquid water, such as dew or precipitation.

The classic example of this phenomenon is a glass of cold liquid sitting on a table outside on a warm, muggy day. The beverage cools the air around it and beads of water form on the outside of the glass. The temperature at which the beads of water form is the dew point.

Simply put, the closer the dew point temperature is to the air temperature, the more humid it feels. In summer, when the air is warm and can hold a lot of moisture, a dew point temperature in the 50s is generally considered comfortable. Dew points in the 60s are thought of as muggy and once they reach the 70s or higher, the air can feel oppressive. On the opposite end of the spectrum, dew points in the 40s or lower are considered dry, and dry air has its own set of comfort issues. 

June 2018: Earth’s Fifth Warmest June on Record

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Our global temperature continued its upward trend last month with June 2018 marking the fifth warmest June ever recorded on this planet. The ten warmest Junes have all occurred since 2005, with 2016 earning the top spot.

According to the State of the Climate report by NOAA’s National Centers for Environmental Information, Earth’s combined average temperature for June – over both land and sea surfaces – was 61.25°F, which is 1.35°F above the 20th-century average. June also marked the 402nd 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 June, some places were particularly warm, including much of Europe, central Asia, and parts of the Middle East. Here in the contiguous US, it was our third warmest June on record.

These soaring temperatures are largely attributed to the long-term trend of human-caused climate change. ENSO-neutral conditions prevailed in June, 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 six months of 2018 were the fourth warmest such period of any year on record. Global temperature records date back to 1880.

June 2018 was the planet’s 5th warmest June on record. Credit: NOAA

Powerful Thunderstorm Brings Funnel Cloud and Flooding to NYC

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An intense thunderstorm swept through New York City on Tuesday afternoon. With bands of torrential downpours, it unleashed nearly half a month’s worth of rain in just a few hours.

According to the NWS, 2.24 inches of rain was measured in Central Park. While that is an impressive total, it did not break the daily rainfall record for the date. That honor belongs to June 17, 1995 when 3.13 inches of rain was reported. New York City, on average, gets 4.60 inches of rain for the entire month of July.

Funnel cloud forms over NY Harbor. Credit: Michael Uturn/Twitter

The powerful storm also produced a funnel cloud over New York Harbor. However, according to the NWS, it did not reach down to the surface and therefore was not a tornado.

The heavy rain, on the other hand, produced a number of problems on its own. It caused flash floods and disrupted travel across the city. Torrents of water poured into several subway stations creating underground waterfalls and parts of the FDR Drive, a major highway on the east side of Manhattan, were closed due to flooding. Significant delays and cancellations were also reported at the area’s airports.

This type of heavy rain event, according to NOAA, is expected to become more common in the northeast as global temperatures rise and precipitation patterns change.

Heavy rain causes flooding on FDR Drive in NYC. Credit: Patch

Wildfires Are Scorching the American West

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Summer is wildfire season in the American West and it is off to a blazing start.

As of Monday, according to the National Interagency Fire Center, fifty-nine large wildfires – defined as greater than 100 acres – are currently burning in nearly a dozen states. These include Alaska, California, Colorado, Idaho, New Mexico, Nevada, Oregon, Texas, Utah, Washington, and Wyoming.

One of the newest conflagrations, the Ferguson Fire, is raging just outside of Yosemite National Park in California’s Sierra Nevada mountains. At the height of tourist season, the fire has closed down Highway 140, one of the main entrances to the Park. It has forced the evacuation of several communities along Yosemite’s western edge as well as some hotels inside the Park. Ignited on Friday, the fire has burned more than 9,000 acres and is only 2% contained. Sadly, it has also claimed the life of one firefighter who was battling the flames.

Another hard hit state is Colorado, where seven large fires are burning. The largest is the Spring Creek Fire, which stared at the end of June and has burned more than 108,000 acres in Costilla and Huerfano counties.

These huge fires are being fueled by extremely hot and dry conditions that have left the region’s vegetation susceptible to any type of spark. Just a few days ago, excessive heat advisories were in effect for a large swath of the west as temperatures soared well above average.

Year to date, 3.3 million acres in the US have been charred, which is above average for this point in the season. The country’s worst wildfire year on record was 2015 when more than ten million acres burned.

Ferguson Fire in the Sierra National Forest, outside of Yosemite National Park in California. Credit: InciWeb/BlakeScott

What is a Rip Current and Why is it Dangerous?

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Summer vacation season is in full swing across the US. As millions of people head to beaches to have fun and beat the heat, it is important to remember that the ocean is a dynamic environment that can pose a number of hazards for swimmers. Chief among these are rip currents.

Rip currents are fast, localized channels of water moving away from the shoreline. According to NOAA, they are a result of “complex interactions between waves, currents, water levels and nearshore bathymetry.” They can form in several different ways on any beach with breaking waves. That said, they are typically found at breaks in sandbars and along permanent structures that extend out into the water such as jetties or piers.

Moving at speeds up to 8 feet per second – which is faster than an Olympic swimmer – rip currents can easily drag unsuspecting swimmers hundreds of yards out to sea.  While they will not pull anyone underwater, they can cause fatigue and panic. According to the U.S. Lifesaving Association, rip currents are responsible for 80% of all surf zone rescues. Nationally, they cause more than one hundred deaths every year.

To spot a rip current, look for a gap in the breaking waves.  This is where the water is forcing its way back out to sea. The area also usually appears murky and darker than the surrounding water.  On guarded beaches, red flags indicate hazardous conditions for swimmers.

If caught in a rip current, the Red Cross recommends not trying to swim against it.  Instead, they say to swim parallel to the shoreline until you are out of the current. Once free, you can start swimming back toward the beach.

For more information on beach safety, visit: http://www.redcross.org/prepare/disaster/water-safety/beach-safety

Credit: NOAA

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

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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

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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

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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

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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

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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