國家地理:不再凍人的五大湖(7)

So, although one year may be warmer than the one before, some recent winters were icy cold. In 2013-14, the polar vortex carried frigid air from the Arctic into the continental U.S., and the cold stretched well south of the Great Lakes. Total ice cover on the Great Lakes spanned more than 90 percent, growing so thick in some places that the augers ice anglers used couldn't reach the water.

因此，盡管一年比一年的溫度要高，但最近這些年的冬天還是很冰冷的。在2013-2014年，極地渦旋將寒冷的空氣從北極帶到了美國大陸，這種寒冷延伸到了五大湖以南。五大湖的總冰層覆蓋面積超過90%，有些地方的冰層越來越厚，以至于冰釣人使用的螺絲鉆都無法接觸到水面。

The extra-tricky part is that the presence and growth of lake ice each winter create a complicated sequence of events.

特別棘手的是，每年冬天湖冰的存在和增加，會(huì)造成一系列復(fù)雜的情況。

Maybe it gets cold enough for ice to form early in the winter -- but if a stretch of bitter wind keeps the water's surface churning, ice will form later. Maybe the summer before was extra warm, zapping enough extra heat into the water that it takes it extra long to cool off and get to the point when it can start to freeze. Maybe a bunch of snow falls early in the season, insulating the ice from the top and, counterintuitively, keeping it from growing quickly through the cold temperatures.

比如說，在初冬時(shí)節(jié)，天氣會(huì)變得很冷，從而形成冰，但是如果一股刺骨的寒風(fēng)使水面保持翻騰，那么結(jié)冰就會(huì)晚一些。比如說，之前的夏天特別暖和，把足夠多的熱量注入水中，這樣水就需要很長時(shí)間才能冷卻下來、達(dá)到可以開始結(jié)冰的程度。再比如說，在初冬時(shí)節(jié)下得很大的雪，會(huì)使冰與頂部隔絕，而且，與我們的直覺相反的是，這會(huì)阻止冰在低溫中迅速增加。

Some factors aren't so complicated. The air is getting warmer. So is the water, in many places faster than the air. In the Northern Hemisphere, nearly 15,000 lakes that used to freeze consistently now ice over intermittently, if at all.

有些因素并不那么復(fù)雜?？諝庠絹碓脚土恕Ｋ惨粯?，在很多地區(qū)，其變暖的速度比空氣還要快。在北半球，以前持續(xù)結(jié)冰的近15000個(gè)湖泊，現(xiàn)在只是間歇性地結(jié)冰了。