How the Santa Ana Winds Help Wildfires Spread

The Santa Ana winds are notorious for exacerbating wildfires in southern California.

These strong winds blow warm, dry air across the region at different times of the year, but mainly occur in the late autumn. They form when a large pressure difference builds up between the Great Basin – a desert that covers most of Nevada and parts of Utah – and the coastal region around LA. This pressure gradient funnels air downhill and through the passes of the San Gabriel and San Bernardino mountains toward the Pacific. According to the NWS, the Santa Ana winds can easily exceed 40 mph.

Originating in the high desert, the air starts off cool and dry. But as it travels downslope, the air compresses and warms. In fact, it warms about 5°F for every 1000 feet it descends. This dries out the region’s vegetation, leaving it susceptible to any type of spark. The fast-moving winds then fan the flames of any wildfires that ignite.

The Santa Ana winds are named for Santa Ana Canyon in Orange County, CA.

Credit: NWS

The Supermoon and Other Moon Names

The first and only supermoon of 2017 will rise on Sunday.

Supermoons are the result of the moon’s elliptical orbit around the Earth. They occur when the moon reaches perigee – its closest point to our planet (less than 223,694 miles). As it is so close, a supermoon looks 7% larger and 16% brighter than an average full moon. When seen near the horizon – where buildings or mountains provide a foreground – an illusion is created that makes the super moon look even bigger. They happen about every thirteen months or so.

When the moon is furthest from Earth – at apogee – it is called a micro-moon.

Full moons occur every 29.5 days when the moon is on the side of the Earth directly opposite the Sun. It reflects the sun’s rays and appears as a beautiful silver disk in the sky.

Ancient civilizations used full moons as a guide to schedule important activities, such as hunting and farming. They gave them each a name based on the dominant weather pattern or typical animal and plant activity during a particular month. In North America, according to National Geographic, native tribes used the moon names listed below. Many are still in use today.

  • January: Wolf Moon or Ice Moon
  • February: Snow Moon
  • March: Worm Moon or Sap Moon
  • April: Sprouting Grass Moon
  • May: Flower Moon
  • June: Strawberry Moon
  • July: Buck Moon
  • August: Sturgeon Moon or Grain Moon
  • September: Harvest Moon
  • October: Hunter’s Moon
  • November: Beaver Moon
  • December: Cold Moon or Long Night Moon

A blue moon is when a second full moon occurs in a single month. Given the uneven nature of our calendar system, these happen roughly every 2.5 years.

The apparent size of the moon as seen from Earth. Credit: KQED

Climate Change Indicator: Arctic Sea Ice

There are a number of key indicators, beyond our rising global temperature, that show Earth’s climate is changing. One of these is Arctic sea ice.

Measured via satellite since the late 1970’s, the extent and thickness of sea ice tend to vary from year to year but both have been in an overall decline for decades. According to NASA, the melt season in the Arctic has increased by 37 days since 1979.

Sea ice extent, the area of ocean with at least 15% sea ice, has a strong seasonal cycle. It typically peaks in March as winter ends and then declines during the summer, reaching a minimum in September. In March 2017, it hit a record low maximum for the third year in a row. The record minimum occurred in September 2012.

According to the National Snow and Ice Data Center, the average age of Arctic sea ice is also changing. Thick multi-year ice – the ice that lasts through at least one melt season – has decreased 11% per decade since the satellite era began. That means there is more first-year ice, which tends to be thin and brittle. This is troublesome because it is more vulnerable to warming temperatures and wave action.

Sea ice is frozen ocean water. It forms, grows, and melts in the ocean. In contrast to land ice (glaciers), it does not contribute to sea level rise. However, as it melts it creates a global warming feedback loop. Ice is lighter in color and reflects more sunlight than dark ocean water. So, as more ocean water is exposed, more of the sun’s energy is absorbed. This drives temperatures up even further and causes more ice to melt.

The Arctic is now warming twice as fast as the rest of the planet – a phenomenon known as “Arctic amplification.”  At this rate, scientists expect the region to be ice-free in summer by the 2030s.

Credit: NSIDC

Weather Lingo: ACE

There are a number of different ways to gauge a hurricane season. One of these is the Accumulated Cyclone Energy index, which is widely referred to as ACE.

It expresses the combined intensity and duration of individual cyclones and provides a measure of activity for an entire hurricane season. For a single storm, it is calculated by summing the squares of the maximum sustained wind speeds measured every six hours while they are at least tropical storm strength. This number is then divided by 10,000 to make it more user-friendly. Overall, the stronger and longer-lived a storm is, the higher its ACE value.

The ACE for a season is the sum of the ACE values from individual storms that occurred that year. NOAA considers a season with an ACE of 111 or higher to be above average, while an ACE of 66 or lower is regarded as below average.

The Atlantic hurricane season runs from June 1 through November 30.

Source: CSU

(Note: Year to date the Atlantic Basin has had 13 storms with a combined ACE of 202 and there are still two months left in the 2017 hurricane season.)

Today is the Autumnal Equinox – Here’s What That Means

Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially begins at 20:02 UTC, which is 4:02 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 moving southward 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”.

With the sun sitting lower in the sky and daylight hours continuing to shorten, autumn is 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

Weather Lingo: The Brown Ocean Effect

Tropical cyclones are fueled by warm ocean water and typically peter out over land. Sometimes, however, their lives are extended by something called the “brown ocean effect”.

This is a phenomenon where a storm derives energy from the evaporation of abundant soil moisture deposited by previous rainfall. Essentially, the saturated soil mimics the role of the ocean allowing a tropical cyclone to maintain its strength or even intensify after making landfall.

For the brown ocean effect to occur, according to a NASA funded study by Theresa Andersen and Marshall Shepherd of the University of Georgia, three criteria need to be met:

  • The soil needs to contain copious amounts of moisture.
  • Atmospheric conditions near the ground must have tropical characteristics with minimal variation in temperature.
  • Evaporation rates must be high enough to provide the storm with sufficient latent heat that it uses for fuel, at least 70 watts averaged per square meter.

Although this process supplies less energy than the ocean, it is enough to sustain a storm for a longer period than normal over land. It was first noticed in 2007 after Tropical Storm Erin made landfall in Texas and then intensified as it traveled inland. It formed an eye over Oklahoma and unleashed a massive amount of rainfall.

Storms that are impacted by the brown ocean effect maintain a warm-core and are known as Inland Tropical Cyclone Maintenance and Intensification events (TCMIs). While rare, they are most common in the US, China, and Australia.

Credit: NBC/TWC

Weather Lingo: Invest

The Hurricane Season officially began in June, but August is when things typically start to ramp up. It is also when the word “invest” (short for an investigative area) becomes more prevalent in weather forecasts.

When the National Hurricane Center (NHC) wants to take a closer look at an area of disturbed weather that could possibly develop into a tropical cyclone, it designates it as an invest. This opens up specialized resources, such as computer models and satellites that provide forecasters with additional data on the area in question. That said, an invest does not always become a tropical system.

More than one invest can exist at any given time, so the NHC has a special way to identify them. They are numbered from 90 to 99, followed by a letter. If the invest is in the Atlantic, the letter will be “L” and if it is in the Eastern Pacific, it will be an “E”. The numbers can be reused throughout a season, as necessary.

The Central Pacific Hurricane Center and the Joint Typhoon Warning Center follow a similar system. Letters for the basins they cover include “C” for the Central Pacific, “W” for the Western Pacific, “A” for the Arabian Sea, and “B” for the Bay of Bengal.

If an invest in any basin develops into a tropical storm, it is reclassified and given a name from that season’s pre-determined list.

An example of how the National Hurricane Center monitors and forecasts the development track of an Invest. Red is the current Invest 99L and Yellow is 90L Credit: NHC

What are the Dog Days of Summer?

The “Dog Days” of summer have arrived. This popular saying refers to what are traditionally the hottest and most oppressive days of the season.

Rooted in astronomy, the phrase is linked to Sirius, the brightest star seen from Earth. As part of the constellation Canis Major, it is known as the Dog Star.  During most of July and August, Sirius rises and sets with our Sun. Ancient Greeks and Romans believed it acted like a second Sun, adding extra heat to summer days. Today, we know that light from this distant star does not affect our weather, but the name has endured.

Varying by latitude around the globe, the so-called “Dog Days” of summer typically run from July 3 to August 11 in the United States.

Sirius, the “Dog Star”.  Credit: EarthSky/Tom Wildoner

Aphelion 2017: 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 20:11 UTC, which is 4:11 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:

Summer Solstice 2017

Today is the June Solstice, the first day of summer in the northern hemisphere. The new season officially began at 04:24 UTC, which is 12:24 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 apparent daily passage across the sky has been moving northward and daylight hours have been increasing. Today, it reached its northernmost position at the Tropic of Cancer (23.5° north latitude) marking the “longest day” of the year. This observable stop 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 all year (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