Staying well-hydrated is key for keeping your body functioning properly, not to mention keeping you at the top of your running game.
In Summer this is especially important; we lose most our water through sweat, which as it evaporates from our skin, takes the heat with it helping us to cool down.
When the humidity rises, it reduces our body's evaporation rate because of the water already in the air, leaving you soon feeling hot, sweaty, and needing to slow down.
The more dehydrated you become, the harder your heart needs to work to pump your blood around your body. This is calleIf the air is dry, the sweat evaporates much more quickly, almost as you're producing it, so you're more likely to become dehydrated cardiac draft; your heart rate increases over the course of the run, even when your intensity is the same.
The heat and the sun also increase your body temperature, as soon as you start feeling too warm, your running will feel difficult.
Even if you're properly hydrated, it's normal for your pace to slow down as your internal temperature rises. The general rule is for every 5°C, above 12°C, the temperature rises, your marathon pace can increase by 1.5-3%.
Add to that the effects of dehydration and that PB you're chasing could suddenly feel a long way away.