Why do cirrus clouds only form at higher altitudes?

Question

Why do cirrus clouds only form at higher altitudes?

Answer 1

in comparison to lower altitude clouds, cirrus clouds are made of ice crystals, giving them that wispy, "mare's tail" look in comparison to the fluffier-looking clouds at lower altitudes, which are made of water vapor.  higher altitudes = colder air  --> ice crystals instead of water vapor.

Answer 2

they are composed entirely of ice, and at that altitude its very cold

Answer 3

in the earth's lower atmosphere there are many different kinds of cloud which are formed in essentially the same manner. warm air can hold more moisture than cold air. therefore when a region of warm air is forced upwards, and is cooled, the temperature falls below the dewpoint, and some of the water vapour must condense out. at a height of 20,000 feet or above, it takes the form of ice crystals. cirrus clouds are made of ice crystals and often mark the beginning of a warm front. they are a frequent indication of approaching bad weather.