As we make the shift from 梅雨 (tsuyu, rainy season) to 夏本番 (natsu honban, summertime in earnest), you'll find that the weather will take center stage in many Japanese conversations. In fact, at this time of year people are more likely to swap out the usual こんにちは (konnichiwa, hello) for a more straightforward 暑いですね (atsui desu ne, it's hot, isn't it) or 蒸暑いね (mushiatsui ne, it's humid) as a bit of an icebreaker (or an ice-melter in this case).
On the other hand, if you really want to complain about summer — which most Japanese will tell you gets hotter, wetter and longer with each passing year — try sticking the term かなわない (kanawanai, unbearable/unbeatable) on adjectives like 暑い (atsui, hot): 暑くてかなわない (atsukute kanawanai, I can't take the heat) is a phrase you'll hear often once we hit 梅雨明け (tsuyuake, the end of the rainy season) and the thermometer spikes to well over 30度 (sanjū do, 30 degrees Celsius).
With kanji, かなわない can be written as 敵わない and has the meaning of being bested by an opponent. Another kanji used for the same word is 叶わない, and it is used when wishes and hopes go unanswered, as in 願いが叶わなかった (negai ga kanawanakatta, my wishes went unanswered). When it comes to complaining about the heat you can use both kanji, but most people stick to using hiragana.