An impenetrable fog rolled into London Wednesday morning (Dec. 11), which caused some travel woes, and also produced rare views of the city's skyline from above, with only the tallest buildings poking above the mist.
While many planes at London's major airports were grounded, a team of officers with the city's Metropolitan Police Service (MPS) flew above the fog in a helicopter. One of the members of this Air Support Unit snapped this amazing photo with an iPhone and posted it to Twitter.
Fog collects under very humid conditions when water vapor condenses into very small droplets of water, which are suspended in air.
A type of fog known as radiation fog most commonly forms in the winter, under calm conditions and clear skies, when the sun's absence overnight cools air close to the ground. Radiation fogs often dissipate in the morning, when the sun comes out again and warms the ground, according to the Met Office, the United Kingdom's national weather service. [See Photos of the Weirdest Clouds]
London's fog enshrouded the city overnight Wednesday and lasted through mid-morning Thursday. According to AccuWeather.com, weather conditions in London late Tuesday evening were ripe for fog formation.
"Clear skies and calm winds are the main causes of fog," said AccuWeather meteorologist Dan DePodwin, adding that winds out of the southeast on Tuesday had helped bring moisture to the area.
The Met Office had issued warnings that visibility would be lower than 165 feet (50 meters), and the fog, which has since lifted, caused many cancelations and delays at Heathrow and London City airports, The Guardian reported.
"Fog is a challenging condition to fly around, but once above it we were able to carry out our normal taskings," Captain John Roberts (call sign India 99), who was piloting the MPS helicopter, said in a statement.
Though London police officials said their @MPSintheSky Twitter account was initially set up to field complaints about helicopter noise, they've seized the opportunity to share photos from their unique perspective of the city and its landmarks.
"We're proud to be able to provide great images of our iconic city," Roberts said in a statement.
London may be better known for its history of smog than fog. What is sometimes referred to as the great fog of 1952 in London was actually the great smog, as the thick veil covering the city was the result of smoke pouring from chimneys during a cold spell that December. Typically, that smoke would rise and disperse, but a weird weather pattern called an anticyclone was hanging over the region, according to the Met Office. These conditions, combined with calm winds, caused what's known as a temperature inversion, trapping cool air close to the ground underneath warmer air. Chimney smoke and other particles got trapped in the resulting fog, creating a serious bout of air pollution. About 4,000 people died due to the great fog, according to the Met Office.