Why Does Cold Water Cause More Clumping?

Two separate mechanisms, involving two different components.

Sugar dissolves slowly when cold

Solubility depends on temperature. In cold water, sugar dissolves more slowly and is more likely to remain as undissolved grains.

In sweetened matcha, the sugar portion is the part that genuinely dissolves. Cold water delays it.

Matcha never dissolves at all

The other component is ground leaf. It does not dissolve — the particles disperse and stay suspended.

Pour liquid onto dry powder too quickly and the outer surface of each clump wets, forming a barrier that keeps the interior dry. In cold water, undissolved sugar makes that barrier more stubborn.

Which is why hot water first works

About a tablespoon of hot water, worked into a paste, dissolves the sugar properly and separates the matcha particles. Add ice and cold water afterward and the result is smooth despite being cold.

Alternatively, put powder and water in a sealed container and shake. Mechanical agitation gets you most of the way without any hot water at all.