when it rains the chlorine, pH and alkalinity will drop.  This will cause algae growth.  Here is a quick tip for you. When it’s raining, have a floater filled with chlorine tabs floating on the pool.  As the rain hits the floater and water and creates movement in the water, the floater is going to be moving around on the water itnis chlorinating the pool.  All of our customers use floating chlorinators as their main chlorination device.   This will chlorinate the pool even with the filter system OFF!   Want to learn more?  Contact us for more information on how to chlorinate a pool without it running.  Save on the electric bill and on chemicals.