I believe she described the summer as sweaty
In July, Susan Hartley Swett basically describe how july provided us with the perfect environment to calm down and relax but also creates an uncomfortable heat for our body.
We could see it from these lines
When the heat like a mist veil floats,
And poppies flame in the rye,
And the silver note in the streamlet's throat
Has softened almost to a sigh,