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Why Deserts Get So Cold at Night

昼は40度、夜は氷点下。砂漠のジェットコースター気温の理由。

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If you have only ever seen deserts in movies, you probably picture them as hot. They are — during the day. The Sahara can reach forty-five degrees Celsius under direct sun. But come back at three in the morning, and the same desert can drop close to zero. People who travel there learn quickly to bring blankets. The reason has to do with what is missing. Most places on Earth are warmed at night by an invisible blanket: water vapor in the atmosphere. Even on a clear summer night in Tokyo, the air holds a great deal of moisture, and that moisture traps heat that would otherwise escape into space. Deserts are dry by definition. The air above them carries very little water, so once the sun sets, there is almost nothing to slow the loss of heat. Within a few hours, the ground releases everything it absorbed during the day. The same sand that burned your feet at noon will feel cold by midnight. It is a useful reminder that what surrounds something invisible can matter more than what we can see.

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  • If you have only ever seen deserts in movies文法

    映画でしか砂漠を見たことがない人は

    〈have only ever + 過去分詞〉= 『今までに〜しか〜したことがない』、現在完了 + 限定。

  • The reason has to do with what is missing.表現

    理由は『そこに無いもの』に関係している。

    〈have to do with〉= 『〜と関係がある』、慣用句。

  • warmed at night by an invisible blanket表現

    夜、見えないブランケットによって温められる

    受動態 + 比喩。科学エッセイの定番メタファー。

  • that moisture traps heat that would otherwise escape into space文法

    その湿気が、そうでなければ宇宙に逃げていく熱をつかまえる

    関係詞 that + 仮定法 would。〈otherwise〉= 『そうでなければ』、仮定の代替を示す副詞。

  • Deserts are dry by definition.表現

    砂漠は定義上、乾燥している。

    〈by definition〉= 『定義上』、論述の前提を強調する副詞句。

  • what surrounds something invisible can matter more than what we can see文法

    目に見えないものを取り巻く何かが、目に見えるものよりも重要であり得る

    〈what + 動詞〉構文を3つ重ねた論理対比。エッセイの締めで一般化する詩的構造。

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