E-liquid is mostly vegetable glycerin (VG) and propylene glycol (PG). Both get runnier when warm. That's physics. Thinner liquid means the wick gets saturated faster, which can cause flooding—that's when you get a mouthful of liquid instead of vapor. It also means more chance of seepage around the mouthpiece or airflow vents.

Pressure is the other factor. Sealed disposables have a little air inside. Heat that air and it expands. In extreme cases, you might hear a faint hiss or notice the mouthpiece feels slightly popped. It's not a safety issue, but it can push liquid out. That's why keeping your device upright matters more in spring than in winter.

Batteries also dislike heat. Lithium-ion cells degrade faster at high temperatures, so a device left on a sunny windowsill may lose capacity quicker than one kept in a cool bag. You might not notice it on a single day, but over a season, it adds up.