Choose the appropriate single-word substitute for the phrase below:

A scientist who studies weather patterns and atmospheric conditions

The correct answer is Meteorologist.

Explanation:
Let us analyze the definitions of all the given options to choose the correct single-word substitute:

1. Meteorologist: A scientist who studies the atmosphere, atmospheric phenomena, weather patterns, and climate forecasting. Therefore, this option directly corresponds to the phrase provided.

2. Oceanographer: A scientist who studies the physical, chemical, geological, and biological aspects of oceans and seas.

3. Cartographer: A person who designs, draws, and prepares maps.

4. Astronomer: A scientist who observes and studies celestial objects like stars, planets, comets, and galaxies outside Earth's atmosphere.

Thus, the appropriate single-word substitute for "A scientist who studies weather patterns and atmospheric conditions" is Meteorologist.