I believe that this question is asking you to come up with your own words to describe the weather in this chapter rather than using a word that the chapter already provides to describe the weather.
The chapter itself does not give readers that much detail regarding the weather. Readers are specifically told that the children longed to leave the school's classroom in order to go out and enjoy the sunshine. This makes me think that the weather is clear rather than occluded or mostly cloudy. We are also told that the grass felt cool to sit in, and that tells readers that the air temperatures are hot.
The first sentence of the chapter actually tells readers that midsummer heat "lay heavily" upon the area. If you are personally experienced with "heavy" heat, then this weather description gets easier. Heavy heat is heavy feeling due to the amount of moisture in the air, so I would describe the weather as humid.
Personally, I do not like humid weather, so I would also describe the summer weather as oppressive feeling. This is because in high humidity situations, sweat does not evaporate off of the body because the air is saturated or nearly saturated. The result is that you experience little to no cooling effect. The heat is constant and oppressive.