going tokyo/sapporo in mid july, heat as bad as everyone says?

for reference, im mostly wondering what clothes to pack.
this will be my 3rd trip to japan but ive gone in march both times, so have no clue what to expect weather wise. im from vegas so im definitely used to high heat, but im also used to 0% humidity with no rain so not quite sure what effect it has during island summers. any advice based on your personal experience will help, thank you!!