On a hot July day, a single mature tree in your yard pumps somewhere around 100 gallons of water up from its roots and out through its leaves as water vapor. A mature elm with 150,000 leaves can clear that in a day. As that water evaporates off the leaf surface, it carries heat away with it, the same basic physics that makes sweat work. The air around the canopy drops measurably. The cooling effect of a single large tree transpiring 100 gallons of water is roughly equivalent to two household air conditioning units running all day. Except it runs on sunlight and groundwater, costs nothing, and has been doing it since before your house was built. A yard with mature canopy runs 5 to 10 degrees cooler than a paved or treeless yard next door. That's not a feeling. It's a physics difference you can measure with a thermometer. The urban heat island is real, and it gets worse one removed tree at a time. The best time to plant a tree was twenty years ago. The second best time is this weekend, before another hot ass July like this one comes around.