Hi Rachel,
Here is very very hot in summer. Maximum temperatures in July are betwen 35ºC and 40ºC at 3pm or 4pm. Is not strange to reach 30ºC at 11pm.
Minimum are betwen 15ºC and 25ºC.
At least the relative humidity is very low so you can take the heat better.
It is very strange to have rain in July, may be a summer storm. It is not windy in July in Madrid.
I cannot give you much advice about young girls fashion , sorry. I just can say that they wear jeans, skirts, mini skirts and shorts. They don't wear shorts when they go out at night.
Here you have the links for the most popular inexpensive shops among Spanish young girls.
http://www.zara.es/v07/index.html http://www.mango.es/paises.htm http://www.bershka.com/ http://www.massimodutti.com/eng/index1.htm?pais=11