Tag Archives: summer

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

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

NYC Monthly Summary: June 2018

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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

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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

How Hail Forms

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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.

Weather Lingo: June Gloom

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For most people in the US, the month of June is associated with warm temperatures and abundant sunshine. For parts of coastal California, however, it is a month known for cloudy and relatively cool conditions. This regional phenomenon called “June Gloom” is the result of the interaction of several natural elements, including geography, ocean currents, and weather patterns.

With the California Current running south along the coast from the Gulf of Alaska, the water in the area is cold. Ocean temperatures in the region usually hover in the upper 50s to low 60s during the summer, cooling the air that flows over it.

Another significant factor is the temperature inversion aloft created by the North Pacific High, a semi-permanent area of high pressure. This is part of a larger planetary circulation of air known as a Hadley cell, a current of high altitude air traveling poleward from the tropics. As the air cools, it descends around 30N latitude. It compresses and warms as it sinks, making the air aloft warmer than the cold, moist air at the surface. Since air temperatures normally decrease with height, this situation acts like a cap on the cool air below and prevents it from rising any higher.

When the air under the inversion layer, known as the marine layer, is cooled to the point where the moisture condenses, an expansive sheet of low level stratus clouds form.  The region’s prevailing westerly winds, as well as the sea-breeze circulation that often develops during the summer months, carries these clouds inland.  While they create overcast conditions and some light drizzle, the clouds do not produce any significant rain. They also tend to dissipate by the afternoon as the land heats up.

The thickness and inland extent of the marine layer clouds depend on the strength of the high-pressure system. A stronger high will thin the clouds and keep them confined to the coast. A weaker high with allow the clouds to thicken and move further inland. Separated by only a few miles, the cloud-covered coast can be significantly cooler than sunny areas further east.

These conditions are most common in June, but are not necessarily limited to the month. They have been known to develop in May and last on and off through August. The monikers for these events include “May Gray”, “No Sky July”, and “Fogust”.  However, high pressure usually builds over southern California in July, decreasing the impact of the marine layer or eliminating it altogether.

“June Gloom” clouds along west coast. Credit: NWS/UCSD

Summer Preview Brings NYC First 80° of the Year

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It felt more like June than April in New York City on Friday. The temperature in Central Park soared to 82°F, marking the city’s first 80-degree day of the year.

Topping out at 22°F above average, the day was more than unseasonably warm. However, it was not a record breaker. That honor belongs to April 13, 1977, when the mercury soared to 88°F. The low temperature was 60°F, which ironically is the normal high for the date.

After an extended winter that included four nor’easters in March and a snowy start to April, many New Yorkers took full advantage of this summer preview. The parks and outdoor cafes were packed.

This spring heat was the result of a ridge in the jet-stream that allowed warm southern air to move further north than it normally would at this time of year.  While the balmy conditions are forecast to remain in place through Saturday, temperatures are expected to plummet into the 40s on Sunday. So, enjoy it while it lasts, but get ready for weather whiplash!

A summer preview for NYC. Credit: The Weather Gamut

NYC Monthly Summary: August 2017

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August was unusually mild in New York City this year. Of the month’s thirty-one days, nineteen posted below average readings, including an unseasonably cool 68°F on August 29. Furthermore, the month typically produces four days with readings in the 90s, but this year we only had one. In the end, the city’s mean temperature for the month was 74°F, which is 1.2°F below average.

In terms of precipitation, August was mostly dry. Overall, 3.34 inches was recorded in Central Park, marking the fourth month this year and second in a row to deliver below average rainfall. The city usually gets 4.44 inches of rain for the month.

August 2017 was unusually mild in NYC. Credit: The Weather Gamut