If you are landing on a paved runway, especially a blacktop runway, in hot weather, (especially on a sunny day), the surface of the runway will be hotter than the air temperature. Under these conditions, there will often be a layer of hot air just above the surface of the pavement. This layer of hot air will be greater in no wind/low wind conditions. This layer of hot air will tend to be less dense than the cooler air above it. As air density decreases, lift decreases. As your airplane decends into the hotter and less dense air, the wings are suddenly producing less lift and the stall speed increases.
Regards
JC