The secrets of noctilucent clouds


Night-shining clouds over a cityscape, with a statue of a huge stone head in the foreground.
Karsten Russ in Nijmegen, Netherlands caught these noctilucent clouds on the night of June 19, 2021. He posted this photo at the Facebook page Noctilucent Clouds around the World and wrote that the statue is: “… a replica of a roman mask which they found in the Waal river in Nijmegen. The original mask can be seen in a museum in Nijmegen, and this statue was placed next to the river.” Thank you, Karsten! 

Noctilucent clouds shine at night

The season for noctilucent clouds at northerly latitudes has begun for 2021! Every year – from about May through August in the Northern Hemisphere, and from November through February in the Southern Hemisphere – people at high latitudes report seeing noctilucent or night-shining clouds. They are, very simply, clouds that shine at night. And they are beautiful.

In recent years, northern summertime noctilucent clouds have set records for low-latitude sightings. In 2019, for example, people observed them as far south as Los Angeles and Las Vegas. Usually, though, they’re seen from higher latitudes. 2021 might be a good year for noctilucent clouds, according to Spaceweather.com, which said as early as May 28, 2021 that:

The reason may be extra water in the mesosphere [about 30 to 50 miles, or 50 to 85 km, above Earth’s surface]. NASA satellite data show that 2021 is one of the wettest years since 2007.

Over the past week (around mid-June 2021), there’ve been many sightings of noctilucent clouds from latitudes like those in the northern U.S., Canada the U.K. and Europe (including France and the Netherlands), as reported by Spaceweather.com and on the excellent Facebook page Noctilucent Clouds Around the World.

Right about now – mid-June to July – is typically when noctilucent clouds are most widespread.

Are you a northern skywatcher looking for noctiluent clouds? If so, here’s a quick tip from Spaceweather.com: Look west about 30 minutes after sunset. If you see luminous blue-white tendrils spreading across the sky, you might have spotted a noctilucent cloud.

Want to see noctilucent clouds, as the photographers on this page did? Keep reading for tips.

A panorama showing bright clouds at night, above the Eiffel Tower.
Noctilucent clouds seen over Paris, France on June 18, 2021. Image via Loic Michel / Notilucent Clouds Around the World.
Bright clouds, shining at night, reflecting in a pool.
View at EarthSky Community Photos. | Harlan Thomas captured this image on June 17, 2021. He was about 20 miles (30 km) north of Calgary, Alberta, Canada. He wrote that this image consists of 7 stitched images and said: “This was simply an amazing morning where the noctilucent clouds poured over the zenith into the southern skies, truly a beautiful sight to behold.” Thank you, Harlan!

How to see noctilucent clouds

If you want to see the clouds, what steps should you take? Remember, you have to be at a relatively high latitude on Earth to see them: typically between about 45 degrees and 60 degrees north or south latitude, although the clouds can sometimes be seen at lower latitudes, particularly following the solstices.

For best results, look for these clouds from about May through August in the Northern Hemisphere, and from November through February in the Southern Hemisphere.

Noctilucent clouds are primarily visible when the sun is just below the horizon, say, from about 90 minutes to about two hours after sunset or before sunrise. At such times, when the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky.

So … how can you see them? They’re not always present. You’ve got to look when a good display is taking place. Your best bet is to hook up with a web page that tracks them. Try Spaceweather.com. Or try the Facebook page Noctilucent Clouds Around the World, where anyone can participate, ask questions, learn, and share photos.

If you take photos of your own, please also share them with us at EarthSky Community Photos. ]

Share your noctilucent cloud photos with EarthSky

What are noctilucent clouds?

Noctilucent clouds form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the Earth’s surface. They’re thought to be made of ice crystals that form on fine dust particles from meteors. They can only form when temperatures are incredibly low and when there’s water available to form ice crystals.

Why do these clouds – which require such cold temperatures – form in the summer? It’s because of the dynamics of the atmosphere. You actually get the coldest temperatures of the year near the poles in summer at that height in the mesosphere.

Here’s how it works: during summer, air close to the ground gets heated and rises. Since atmospheric pressure decreases with altitude, the rising air expands. When the air expands, it also cools down. This, along with other processes in the upper atmosphere, drives the air even higher causing it to cool even more. As a result, temperatures in the mesosphere can plunge to as low as -210 degrees Fahrenheit (-134 degrees Celsius).

In the Northern Hemisphere, the mesosphere often reaches these temperatures by mid-May, in most years.

Diagram of light shining from the sun, bouncing off high clouds to a location over the horizon from the sun.
When the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky. Illustration via NASA.

Since the clouds are so sensitive to the atmospheric temperatures, they can act as a proxy for information about the wind circulation that causes these temperatures. First of all, they can tell scientists that the circulation exists, and also tell us something about the strength of the circulation.

Scientists studying these clouds have included those from NASA’s Aeronomy of Ice in the Mesosphere (AIM) satellite. This satellite, launched in 2007, has observed noctilucent clouds using several onboard instruments to collect information such as temperature, atmospheric gases, ice crystal size and changes in the clouds, as well as the amount of meteoric space dust that enters the atmosphere. You can find out what they are learning at NASA’s AIM page.

Noctilucent clouds are like regular clouds in that they move in the sky. See the subtle movement of noctilucent clouds in the video below:

And here are a few more recent photos. If you capture a photo of noctilucent clouds, please submit it to EarthSky here.

Night-shining clouds above what looks like a city or town.
View at EarthSky Community Photos. | Rob de Graaf captured this image on June 18, 2021. He wrote: “Outbreak of noctilucent clouds above ‘s-Hertogenbosch, the Netherlands. Visible from about 10:45 p.m. until 12:20 a.m.” Thank you, Rob!
Night-shining clouds over a beach.
View at EarthSky Community Photos. | Kevan Hubbard at Seaton Carew, County Durham, England caught these clouds in the early morning of June 20, 2021. He wrote: “A huge trailing noctilucent cloud and the southern end disturbed by gravity waves.” Thank you, Kevan!
Light clouds streak across a dark blue scene with a river below and mountain silhouettes that divide the water from the sky.
Doris Guevara de Isert caught these noctilucent clouds at the Powell River, BC, Canada, on June 16, 2021. Image via Noctilucent Clouds Around the World.
Electric blue noctilucent clouds shining above a river with a lit-up stadium in the distance.
Katarzyna Kaczmarczyk captured these noctilucent clouds over Warsaw, Poland, at 1:30 a.m. on June 15, 2021.
Black space, shining ripply layer of clouds, dark orange narrow stripe above black silhouette of Earth.
By the way, noctilucent clouds can be seen from space, too. Astronauts in the International Space Station (ISS) took this photo on January 5, 2013, when ISS was over the Pacific Ocean south of French Polynesia. Below the brightly-lit noctilucent clouds, across the center of the image, the pale orange band is the stratosphere. Image via NASA.

Bottom line: You can see noctilucent clouds (aka night-shining clouds or NLCs) during summer at high-latitude. They form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the surface.

Visit SpaceWeather’s RealTime Noctilucent Cloud Gallery

Visit the Facebook page Noctilucent Clouds Around the World

The post The secrets of noctilucent clouds first appeared on EarthSky.



from EarthSky https://ift.tt/3xFC9tT
Night-shining clouds over a cityscape, with a statue of a huge stone head in the foreground.
Karsten Russ in Nijmegen, Netherlands caught these noctilucent clouds on the night of June 19, 2021. He posted this photo at the Facebook page Noctilucent Clouds around the World and wrote that the statue is: “… a replica of a roman mask which they found in the Waal river in Nijmegen. The original mask can be seen in a museum in Nijmegen, and this statue was placed next to the river.” Thank you, Karsten! 

Noctilucent clouds shine at night

The season for noctilucent clouds at northerly latitudes has begun for 2021! Every year – from about May through August in the Northern Hemisphere, and from November through February in the Southern Hemisphere – people at high latitudes report seeing noctilucent or night-shining clouds. They are, very simply, clouds that shine at night. And they are beautiful.

In recent years, northern summertime noctilucent clouds have set records for low-latitude sightings. In 2019, for example, people observed them as far south as Los Angeles and Las Vegas. Usually, though, they’re seen from higher latitudes. 2021 might be a good year for noctilucent clouds, according to Spaceweather.com, which said as early as May 28, 2021 that:

The reason may be extra water in the mesosphere [about 30 to 50 miles, or 50 to 85 km, above Earth’s surface]. NASA satellite data show that 2021 is one of the wettest years since 2007.

Over the past week (around mid-June 2021), there’ve been many sightings of noctilucent clouds from latitudes like those in the northern U.S., Canada the U.K. and Europe (including France and the Netherlands), as reported by Spaceweather.com and on the excellent Facebook page Noctilucent Clouds Around the World.

Right about now – mid-June to July – is typically when noctilucent clouds are most widespread.

Are you a northern skywatcher looking for noctiluent clouds? If so, here’s a quick tip from Spaceweather.com: Look west about 30 minutes after sunset. If you see luminous blue-white tendrils spreading across the sky, you might have spotted a noctilucent cloud.

Want to see noctilucent clouds, as the photographers on this page did? Keep reading for tips.

A panorama showing bright clouds at night, above the Eiffel Tower.
Noctilucent clouds seen over Paris, France on June 18, 2021. Image via Loic Michel / Notilucent Clouds Around the World.
Bright clouds, shining at night, reflecting in a pool.
View at EarthSky Community Photos. | Harlan Thomas captured this image on June 17, 2021. He was about 20 miles (30 km) north of Calgary, Alberta, Canada. He wrote that this image consists of 7 stitched images and said: “This was simply an amazing morning where the noctilucent clouds poured over the zenith into the southern skies, truly a beautiful sight to behold.” Thank you, Harlan!

How to see noctilucent clouds

If you want to see the clouds, what steps should you take? Remember, you have to be at a relatively high latitude on Earth to see them: typically between about 45 degrees and 60 degrees north or south latitude, although the clouds can sometimes be seen at lower latitudes, particularly following the solstices.

For best results, look for these clouds from about May through August in the Northern Hemisphere, and from November through February in the Southern Hemisphere.

Noctilucent clouds are primarily visible when the sun is just below the horizon, say, from about 90 minutes to about two hours after sunset or before sunrise. At such times, when the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky.

So … how can you see them? They’re not always present. You’ve got to look when a good display is taking place. Your best bet is to hook up with a web page that tracks them. Try Spaceweather.com. Or try the Facebook page Noctilucent Clouds Around the World, where anyone can participate, ask questions, learn, and share photos.

If you take photos of your own, please also share them with us at EarthSky Community Photos. ]

Share your noctilucent cloud photos with EarthSky

What are noctilucent clouds?

Noctilucent clouds form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the Earth’s surface. They’re thought to be made of ice crystals that form on fine dust particles from meteors. They can only form when temperatures are incredibly low and when there’s water available to form ice crystals.

Why do these clouds – which require such cold temperatures – form in the summer? It’s because of the dynamics of the atmosphere. You actually get the coldest temperatures of the year near the poles in summer at that height in the mesosphere.

Here’s how it works: during summer, air close to the ground gets heated and rises. Since atmospheric pressure decreases with altitude, the rising air expands. When the air expands, it also cools down. This, along with other processes in the upper atmosphere, drives the air even higher causing it to cool even more. As a result, temperatures in the mesosphere can plunge to as low as -210 degrees Fahrenheit (-134 degrees Celsius).

In the Northern Hemisphere, the mesosphere often reaches these temperatures by mid-May, in most years.

Diagram of light shining from the sun, bouncing off high clouds to a location over the horizon from the sun.
When the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky. Illustration via NASA.

Since the clouds are so sensitive to the atmospheric temperatures, they can act as a proxy for information about the wind circulation that causes these temperatures. First of all, they can tell scientists that the circulation exists, and also tell us something about the strength of the circulation.

Scientists studying these clouds have included those from NASA’s Aeronomy of Ice in the Mesosphere (AIM) satellite. This satellite, launched in 2007, has observed noctilucent clouds using several onboard instruments to collect information such as temperature, atmospheric gases, ice crystal size and changes in the clouds, as well as the amount of meteoric space dust that enters the atmosphere. You can find out what they are learning at NASA’s AIM page.

Noctilucent clouds are like regular clouds in that they move in the sky. See the subtle movement of noctilucent clouds in the video below:

And here are a few more recent photos. If you capture a photo of noctilucent clouds, please submit it to EarthSky here.

Night-shining clouds above what looks like a city or town.
View at EarthSky Community Photos. | Rob de Graaf captured this image on June 18, 2021. He wrote: “Outbreak of noctilucent clouds above ‘s-Hertogenbosch, the Netherlands. Visible from about 10:45 p.m. until 12:20 a.m.” Thank you, Rob!
Night-shining clouds over a beach.
View at EarthSky Community Photos. | Kevan Hubbard at Seaton Carew, County Durham, England caught these clouds in the early morning of June 20, 2021. He wrote: “A huge trailing noctilucent cloud and the southern end disturbed by gravity waves.” Thank you, Kevan!
Light clouds streak across a dark blue scene with a river below and mountain silhouettes that divide the water from the sky.
Doris Guevara de Isert caught these noctilucent clouds at the Powell River, BC, Canada, on June 16, 2021. Image via Noctilucent Clouds Around the World.
Electric blue noctilucent clouds shining above a river with a lit-up stadium in the distance.
Katarzyna Kaczmarczyk captured these noctilucent clouds over Warsaw, Poland, at 1:30 a.m. on June 15, 2021.
Black space, shining ripply layer of clouds, dark orange narrow stripe above black silhouette of Earth.
By the way, noctilucent clouds can be seen from space, too. Astronauts in the International Space Station (ISS) took this photo on January 5, 2013, when ISS was over the Pacific Ocean south of French Polynesia. Below the brightly-lit noctilucent clouds, across the center of the image, the pale orange band is the stratosphere. Image via NASA.

Bottom line: You can see noctilucent clouds (aka night-shining clouds or NLCs) during summer at high-latitude. They form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the surface.

Visit SpaceWeather’s RealTime Noctilucent Cloud Gallery

Visit the Facebook page Noctilucent Clouds Around the World

The post The secrets of noctilucent clouds first appeared on EarthSky.



from EarthSky https://ift.tt/3xFC9tT
Libellés :

Aucun commentaire:

Enregistrer un commentaire

Inscription à : Publier les commentaires (Atom)