Hal Clement. Close to Critical. 1964. It is three times difficult to live on Tenebrae, one of the spherical planets in the Altair system, and not because the gravity on its surface is about three times greater than Earth's gravity. The atmosphere here consists of water vapor, which during the daytime is in a critical state under a pressure of hundreds of atmospheres and a temperature of hundreds of degrees. At night, steam condenses and water, streams, lakes, and oceans are formed. True, lakes and oceans exist on Tenebrae during the day... in the form of concentrated sulfuric acid. There is intelligent life on the planet - scaly creatures like fir cones with four pairs of limbs.