During heavy rains water, molecules in the rainwater react with the carbon dioxide in the atmosphere to form carbonic acid, which makes the rainwater slightly acidic.
The enzyme carbonic anhydrase catalyzes this reaction in the red blood cells, where carbon dioxide and water are converted into bicarbonate and hydrogen ion.
The water molecules and carbon dioxide molecules can then react to each other, to form carbonic acid, which lowers the pH of the water, making it slightly acidic.