In Japan during the summer months after the rainy season has ended, we often send each other postcards called shochu omimai. Shochu literally means "during the heat". Omimai means "to visit" or "inquiry".
These cards are usually sent to family and friends whom maybe you haven't talked to or seen in awhile. We usually write these cards in different shades of blue. Why blue? Blue is supposed to give a cooling image similar to water.
After August 8th (which is risshu or the first day of autumn according to the lunar calendar), the greeting changes to zansho omimai or literally visit or inquiry for the remaining heat. The cards are still written in blue.
This photo is of the cards which I sent to Postcrossing members.
I hope you are keeping cool where you are.