The bottommost layer of the atmosphere is called the Troposphere.

The troposphere is the closest layer to the Earth's surface and extends from the surface up to an average height of about 7 to 20 kilometers (4 to 12 miles) depending on the location. It is the layer where weather occurs and where we live and breathe.

Characteristics of the Troposphere:
- Temperature: The temperature in the troposphere decreases as you go higher up. On average, the temperature drops by about 6.5 degrees Celsius per kilometer of altitude. This is known as the lapse rate.
- Weather: The troposphere is the layer where weather phenomena, such as clouds, rain, snow, and storms, occur. This is because it is the layer where most of the Earth's atmospheric moisture is found.
- Composition: The troposphere is made up of a mixture of gases, including nitrogen, oxygen, carbon dioxide, and trace amounts of other gases. It also contains small particles, such as dust and pollutants.
- Pressure: The pressure in the troposphere decreases with altitude. At the surface, the air pressure is the highest, and it decreases as you go higher up in the troposphere.
- Life: The troposphere is the layer where most living organisms, including humans, plants, and animals, are found. It provides us with the air we breathe and protects us from harmful radiation from the Sun.

Importance of the Troposphere:
- The troposphere plays a crucial role in regulating the Earth's climate and weather patterns. It helps distribute heat across the planet and influences the movement of air masses.
- It is where most of the Earth's weather occurs, including the formation of clouds, precipitation, and the development of storms.
- The troposphere is also important for human activities, as it is the layer where we live and carry out various activities. It provides us with the air we breathe and protects us from harmful radiation.

In conclusion, the troposphere is the bottommost layer of the atmosphere. It is where weather occurs and where we live and breathe. It plays a crucial role in regulating the Earth's climate and weather patterns and is important for human activities.