Noctilucent Clouds: A Glimpse into Earth's Changing Atmosphere
Noctilucent clouds, also known as night shining clouds, are delicate cloud-like phenomena found in the upper atmosphere of Earth. These clouds consist of ice crystals and are visible only during astronomical twilight when viewed from the ground. The term "noctilucent" originates from Latin and roughly translates to "night shining." They are most commonly seen during the summer months from latitudes between ±50° and ±70°.
Spotting the elusive beauty
With altitudes ranging from around 80 km noctilucent clouds claim the title of the highest clouds found in Earth's atmosphere. These clouds are too faint to be seen during the day and become visible only when both the observer and the lower atmospheric layers are in Earth's shadow, while the high clouds are still illuminated by sunlight. To spot them, venture outdoors after sunset on a clear night and find a vantage point where you can see to north (or south if youre on southern hemisphere). If you are lucky you might be greeted with a display to remember for the rest of your life!
How they are formed
Noctilucent clouds primarily develop near the polar regions where the mesosphere is at its coldest. These unique clouds are able to form due to the mesosphere's minimal moisture content, approximately one hundred millionth that of air found in the Sahara, and its extremely thin nature. For ice crystals to materialize, temperatures must drop below approximately −120 °C (−184 °F). Despite the counterintuitive nature of the mesosphere being coldest during summer, this is a result of seasonally varying vertical winds causing upwelling and adiabatic cooling in the upper mesosphere, leading to cold conditions.
Recent research suggests that increased methane emissions in the atmosphere lead to additional water vapor, contributing to the formation or strengthening of existing noctilucent clouds.
Noctilucent clouds on Mars:
In 2006, scientists from the Mars Express mission announced the discovery of carbon dioxide-crystal clouds on Mars, which extended up to 100 km above the planet's surface. These clouds represent the highest clouds ever detected on our rocky red neighbour. Similar to Earth's noctilucent clouds, they can only be observed when the Sun is below the Martian horizon.
Micrometeorites might play a crucial role:
It is also worth noting noctilucent clouds display notable radar reflectivity in the frequency range of 50 MHz to 1.3 GHz. The exact cause of this behavior is not fully understood, but a possible explanation suggests that the ice grains within the clouds acquire a thin metal film consisting of sodium and iron. This metallic coating significantly enhances the cloud's radar reflectivity. However, this explanation remains a subject of controversy. When micrometeoroids enter the atmosphere, sodium and iron atoms are released and settle into a layer just above the altitude where noctilucent clouds form. Observations have revealed a significant depletion of these elements when the clouds are present. Additional experiments have shown that sodium vapor can rapidly condense onto an icy surface at the extremely cold temperatures found within noctilucent clouds.
The summer months offer a fantastic opportunity to observe the mesmerizing beauty of noctilucent clouds in detail. These ethereal clouds, composed of ice crystals, grace the upper atmosphere as astronomical twilight sets in. So, venture outside and prepare to be mesmerized by their pale blue shine!