When earth was first formed, its atmosphere was likely composed of hydrogen, helium, and other gases that contained hydrogen. ... Over a vast amount of time, millions of years, the earth gradually cooled. When the temperature dropped enough, water vapor condensed and went from a gas to liquid form. This created clouds.
Before life began on the planet, Earth's atmosphere was largely made up of nitrogen and carbon dioxide gases. After photosynthesizing organisms multiplied on Earth's surface and in the oceans, much of the carbon dioxide was replaced with oxygen.
<em>When earth was first formed, its atmosphere was likely composed of hydrogen, helium, and other gases that contained hydrogen. Over a vast amount of time, millions of years, the earth gradually cooled. When the temperature dropped enough, water vapor condensed and went from a gas to liquid form. This created clouds.</em>
