When it is hot outside you shouldn't be doing any hard physical activity. Dogs can quickly suffer from heat stroke and other heat related illness as they cannot regulate their body temperature like we can.
A word of warning on the frisbee; it is not a risk I would take. Labradors are not the type of dogs suited to a high impact exercise like frisbee. The jumping and awkward landing is not good for the joints. Frisbee is better suited to lighter framed dogs such as Border Collies.