国家地理:不再冻人的五大湖(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，尽管一年比一年的温度要高，但最近这些年的冬天还是很冰冷的。在2013-2014年，极地涡旋将寒冷的空气从北极带到了美国大陆，这种寒冷延伸到了五大湖以南。五大湖的总冰层覆盖面积超过90%，有些地方的冰层越来越厚，以至于冰钓人使用的螺丝钻都无法接触到水面。

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

特别棘手的是，每年冬天湖冰的存在和增加，会造成一系列复杂的情况。

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.

比如说，在初冬时节，天气会变得很冷，从而形成冰，但是如果一股刺骨的寒风使水面保持翻腾，那么结冰就会晚一些。比如说，之前的夏天特别暖和，把足够多的热量注入水中，这样水就需要很长时间才能冷却下来、达到可以开始结冰的程度。再比如说，在初冬时节下得很大的雪，会使冰与顶部隔绝，而且，与我们的直觉相反的是，这会阻止冰在低温中迅速增加。

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.

有些因素并不那么复杂。空气越来越暖和了。水也一样，在很多地区，其变暖的速度比空气还要快。在北半球，以前持续结冰的近15000个湖泊，现在只是间歇性地结冰了。