What Causes the Autumnal Equinox

Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially begins at 9:54 PM Eastern Daylight Time.

The astronomical seasons, as opposed to the meteorological seasons,  are a product of Earth’s axial tilt – a 23.5° angle – and the movement of the planet around the sun. During the autumn months, the Earth’s axis is tilted neither toward nor away from the sun. This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the summer solstice in June, the arc of the sun’s apparent daily passage across the sky has been sinking and daylight hours have been decreasing. Today, the sun appears directly overhead at the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

Transitioning from summer to winter, autumn is also a season of falling temperatures. According to NOAA, the average high temperature in most US cities drops about 10°F between September and October.

Earth’s solstices and equinoxes. Image Credit: NASA

How Hurricanes are Classified

Hurricanes are one of nature’s most powerful storms. When formed in the Atlantic Ocean or North-Eastern Pacific, they are rated according to the Saffir-Simpson Scale.

Developed in the early 1970’s by Herbert Saffir, a civil engineer, and Dr. Robert Simpson of the National Hurricane Center, the scale classifies hurricanes into five categories based on the strength of their sustained winds. Each category is considered an estimate of the potential damage that a storm will cause if it makes landfall.  As conditions change within a storm, its category is re-assessed.

The different categories, 1 through 5, represent increasing wind speeds and escalating degrees of damage. Storms rated category 3 or higher are considered major hurricanes. The last category 5 storm to make landfall in the US was Hurricane Andrew in 1992.

While a useful tool, the Saffir-Simpson scale does not tell the whole story of the dangers to life and property posed by a hurricane. Regardless of category, these storms can produce dangerous storm surges in coastal areas and flooding rains further inland. Recent examples of these types of impacts were seen during Sandy and Harvey, respectively.

Wind Cave National Park and the Science Behind What Makes the Wind Blow

I recently visited Wind Cave National Park in South Dakota, which protects a beautiful expanse of the Northern Great Plains as well as one of the largest and most complex cave systems in the world. While well known for its geology, the park’s namesake feature is also an excellent example of the science behind a basic weather phenomenon – wind.

Wind, which is air in motion, is the result of differences in atmospheric pressure. These pressure differences are caused by the temperature differences created by the uneven heating of the Earth’s surface by the Sun.  Several factors contribute to this unbalanced process, including cloud cover, large bodies of water, topography, and vegetation.

As the surface warms, air heats and rises, creating an area of low pressure. To fill that void, air from an area of relatively higher-pressure rushes in, creating a flow of air that we recognize as wind. The greater the pressure differences between these two areas, the stronger the breeze.

Atmospheric pressure conditions at the cave entrance during my visit. Credit: Melissa Fleming

At Wind Cave, given its vast size, the air pressure inside the cave is constantly working to equalize with that above ground. Therefore, when there is an area of high pressure at the surface, the wind will blow into the cave. If there is an area of low pressure on the surface, the wind will blow out of the cave. For this reason, the cave is described in the oral histories of the Lakota – a Native American tribe who consider it scared – as “the hole that breathes cool air”.

Park Ranger demonstrates the flow of air coming out of the small cave entrance with a ribbon. Credit:RVDreamLife

While other large cave systems can generate barometric winds, those at Wind Cave are more noticeable because of the small size of its entrance. As the Venturi Effect shows, when space is constricted, air will flow faster. Legend says that the first non-native settlers to discover the cave – two brother named Jesse and Tom Bingham – did so by accident when the wind from its entrance blew the hat off one of their heads in 1881.

According to the NPS, winds at the cave’s natural entrance have reached up to 25-mph.

Wind Cave National Park, SD. Credit: Melissa Fleming

The Continental Divide Determines Where Rain Goes After it Hits the Ground

Most people are familiar with the various types of precipitation that falls from the sky. However, have you ever wondered where all that water goes after it falls or melts? The answer is largely dependent on what side of the continental divide it landed.

A continental divide is a natural boundary that separates the river systems of a continent. They are usually tall mountain ranges that direct the flow of rivers and streams to different oceans. Essentially, any precipitation that falls or melts on one side will flow into one ocean basin and anything that falls or melts on the other side of the divide will flow into another basin.

Sign in Rocky Mountain National Park marks the location of the Continental Divide in CO. Credit: Melissa Fleming

Every continent has at least one and some have multiple. In the United States, the main divide is the Rocky Mountains. It is part of the Great Continental Divide of the Americas, which runs from western Alaska through the Andes Mountains in South America. It separates water that runs into the Atlantic Ocean from water that flows into the Arctic or Pacific Oceans.

In some cases, water finds its way into an endorheic basin with no outlet to an ocean.  Utah’s Great Salt Lake and Oregon’s Crater Lake are well known examples.  Here, the water re-enters the water cycle via evaporation. A small percentage of precipitating water also seeps into the ground where it replenishes soil moisture and underground aquifers. That said, the vast majority of water returns to the world’s oceans where it will eventually be evaporated and fall as precipitation again somewhere on the planet.

North America has several drainage divides, but the Great Divide (red) is the largest. Credit: ContinentalDivide.net


Hurricanes, Typhoons, and Cyclones: What’s the Difference?

As Hurricane Lane makes its way toward Hawaii, many people have been asking me why the storm is not being called a typhoon given that it is taking place in the Pacific. The answer is all about location.

Hurricanes, typhoons, and cyclones are all the same type of storm – tropical cyclones. They are just called different things in different parts of the world. It’s like the way people in certain parts of the US say “soda” when referring to a cold fizzy drink, while people in other parts of the country use the word “pop”.

The term hurricane is used for tropical cyclones in the northern hemisphere from the Greenwich Meridian (0°) westward to the International Date Line (the 180° line of longitude). That includes the Atlantic basin as well as the eastern and central Pacific. The eastern Pacific is defined as everything north of the equator from the west coast of the North American continent to 140°W. The central Pacific, where Hawaii is located, extends from 140°W to 180°W.

Typhoon is the word used for storms west of that line, any area known as the western Pacific. If a hurricane crosses the International Date Line and maintains its strength, it will be renamed as a typhoon. In 2014, for example, Hurricane Genevieve became Typhoon Genevieve when it crossed into the western Pacific.

Across the southern hemisphere, all tropical cyclones are simply called cyclones.

These powerful storms, regardless of what we call them, can pose a threat to life and property. All warnings should be taken seriously.

Credit: American Red Cross

Weather Lingo: Humidity

“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

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

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

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.