One crazy month in Mon­tana

Popular Science - - CHARTED -

IN MANY PARTS OF THE U.S., IT’S NOT UN­COM­MON FOR A SUMMERY af­ter­noon to trans­form into a be­low-zero night. But no place has felt a greater 24-hour change in the mer­cury than Loma, Mon­tana. Be­tween Jan­uary 14 and 15, 1972, ther­mome­ters surged from -54 to 49 de­grees Fahren­heit in a record-break­ing 103-de­gree swing. In fact, that en­tire month in Loma and its near­est neigh­bor­ing city, Great Falls, serves as an ex­em­plar of the sud­den tem­per­a­ture spikes and drops that can hap­pen any­where. The cause: du­el­ing blasts of air.

1/ Un­der Pres­sure

An Arc­tic blast brought freez­ing conditions as it shoul­dered its way into Great Falls on Jan­uary 11. These high-pres­sure gusts from the poles tend to bring clear skies, and the lack of an in­su­lat­ing cloud blan­ket lets tem­per­a­tures drop even fur­ther at night. Any snow on the ground traps heat inside the earth, keep­ing the air frigid for longer.

4/ Shift Work

At the bound­ary where two air masses meet, the tem­per­a­tures on ei­ther side can dif­fer by dou­ble dig­its. If pres­sure dis­par­i­ties make the bound­ary rip­ple from side to side, a town be­low can rapidly see­saw be­tween warm and cold, as Great Falls did for about seven hours on Jan­uary 22. The tem­per­a­ture tug-of-war can last for sev­eral days.

3/ Front Lines

A cold air mass can quickly dis­place a warm one, as Mon­tanans wit­nessed on the 18th. The denser, cool wall stays close to the ground as the front moves in, forc­ing the warm air to rise, and lit­er­ally makin’ it rain if it holds enough mois­ture. These air masses bring last­ing tem­per­a­ture changes be­cause of their size; they can stretch across thou­sands of square miles.

2/ Swing Time

A Chi­nook wind com­ing off the Rocky Moun­tains caused Loma’s record-set­ting leap. As warm, wet air tum­bles down a moun­tain, the in­creas­ing at­mo­spheric pres­sure heats the air, which is why these winds are some­times called snow eaters; towns near the foothills can go from freez­ing to balmy in a mat­ter of min­utes or hours.

By Sara Cho­dosh / in­fo­graphic by Sto­ryTK

Newspapers in English

Newspapers from USA

© PressReader. All rights reserved.