Basic ASHRAE standards have a recommended range of 18C to 27C, but a maximum allowable range of 15C to 32C. If you specify an A2 or A3 ASHRAE compliance when buying your hardware you can stretch that allowable range all the way up to 35C (A2) or even 40C (A3),
Most datacentres these days are looking closely at the ASHRAE limits and at monitoring to raise the average cold aisle temperature and make major savings. There are a lot of steps on this path if you're bringing an older datacentre up to the modern ways of thinking, including strict hot/cold aisle separation, re-alignment of hot aisles to match CRAC / CRAH units, implementation of live temperature, pressure and humidity monitoring, all the way up to a fluid dynamics analysis of airflow. You only have one chance to get it right and a huge number of ways to get it wrong so it's a very conservative approach. On the other hand being able to make a $500K annual saving by raising the overall temperature by 2C in the cold aisle and still deliver the same service is the sort of numbers that make a lot of sense.
In addition the point I was making is that it's only during daylight that the external air temp will mean you need additional cooling, at night the temperature drops - so you only need to run your cooling plant for a percentage of the day, and the temperature at night is absolutely fine.
Disclaimer: I'm a Certified DC Designer and a Certified DC Management Professional with 8 years experience running a blue-chip datacentre, so I live this stuff every day