马尼亚河的卫星图像，马达加斯加 Satellite image of the Mania River in Madagascar (© NASA Earth Observatory image by Joshua Stevens, using Landsat data from the US Geological Survey)

马尼亚河的卫星图像，马达加斯加 Satellite image of the Mania River in Madagascar (© NASA Earth Observatory image by Joshua Stevens, using Landsat data from the US Geological Survey)

Uncommon clouds are gathering

A satellite view of the Mania River in Madagascar allows us to see a curious cloud pattern. It's common for cool, moist marine air to rise and form dense clouds over bodies of water, then for the clouds to evaporate as they drift over warmer, drier land. The opposite is happening here: Puffs of clouds are forming over land, but not over water. That's because Madagascar's tropical rainforests are warm and wet enough that evaporating moisture rises as the day heats up. When it rises high enough, the moisture encounters cooler air, which condenses the water into clouds. Generally speaking, clouds will form where the air is rising, which in this case is only over the land. Above the river, the air is cooler and descending, so no clouds are forming there.

If learning about an unusual weather pattern puts you on cloud nine, then join the United Nations in celebrating World Meteorological Day today. The UN created the annual observance back in 1950 as a day to recognize the contributions of meteorologists to our safety and well-being. We may not all know our cirrus from our cumulus, but we can at least thank meteorologists for letting us know if we should pack an umbrella or sunscreen.

不寻常的云彩正在聚集

马达加斯加马尼亚河的卫星图像让我们看到了一个奇怪的云层模式。很常见的情况是，凉爽潮湿的海洋空气上升，在水体上方形成浓密的云层，然后云层在温暖干燥的陆地上飘浮时蒸发。相反的情况正在这里发生：云层在陆地上形成，但在水面上没有。这是因为马达加斯加的热带雨林足够温暖和潮湿，随着天气变暖，蒸发的水分会上升。当它上升到足够高的位置时，湿气会遇到较冷的空气，从而将水凝结成云。一般来说，在空气上升的地方会形成云，在这种情况下，云只在陆地上形成。在河的上方，空气是凉爽的和下降的，所以那里没有云层形成。

如果你了解到一个不寻常的天气模式，你会感到非常沮丧，那么今天就加入联合国庆祝世界气象日吧。早在1950年，联合国就设立了一年一度的纪念日，以此来表彰气象学家对我们安全和福祉的贡献。我们可能不都知道我们的卷云来自我们的积云，但我们至少可以感谢气象学家让我们知道我们是否应该带伞或防晒霜。