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Home Astronauts on board the ISS took this picture of noctilucent clouds near the top of Earth’s atmosphere on July 13th, 2012. (Credit: NASA) Astronauts on board the ISS took this picture of noctilucent clouds near the top of Earth's atmosphere on July 13th, 2012. (Credit: NASA)

Astronauts on board the ISS took this picture of noctilucent clouds near the top of Earth’s atmosphere on July 13th, 2012. (Credit: NASA)

Astronauts on board the ISS took this picture of noctilucent clouds near the top of Earth's atmosphere on July 13th, 2012. (Credit: NASA)

Astronauts on board the ISS took this picture of noctilucent clouds near the top of Earth’s atmosphere on July 13th, 2012. (Credit: NASA)

Comparison of noctilucent cloud coverage in 1880 versus 1990. They have increased over time. (Credit: NASA)
This graphic shows how methane, a greenhouse gas, boosts the abundance of water at the top of Earth’s atmosphere. This water freezes around “meteor smoke” to form icy noctilucent clouds. (Graphic courtesy of Prof. James Russell of Hampton University)