Powder Snow

Powder snow is a trap block obtained from cauldrons.

Obtaining
A cauldron in a snowy biome will fill up with powder snow when it is snowing. Powder snow can then be picked up from this cauldron with a bucket.

Breaking
Powder snow does not drop anything when mined, even with Silk Touch, and tools do not speed up mining.

Powder snow can be picked up with a bucket.

Usage
Entities fall through powder snow without taking fall damage and move much slower in it, similar to cobwebs. Despite being a full block, it does not cause suffocation damage. A player whose head is within the block sees a frosty vignette decorated with snowflakes.

Powder snow can be used to make technical farms, instead of water.

Mobs or players wearing leather boots do not fall through powder snow. In this case the snow behaves as scaffolding when the player sneaks or crouches while on the block and jumps while inside of the block.

Freezing
When an entity is inside a powder snow block, they will begin to freeze, taking damage.

When a player is fully submerged, they will see a frosty vignette slowly fade in at the sides of the screen. When it is fully visible, the player's hearts will turn blue, and the player will begin taking damage. If the player leaves the powder snow block, the vignette will slowly fade away. When a player is frozen, they will move slower than usual until the vignette fully fades away.

Wearing leather armor slows down the rate at which damage is taken. A full set of leather armor will prevent all freezing damage.

Trivia

 * "Snowier Snow" was announced to be part of the Mountains biome in the biome vote at MINECON Live 2019. Powder Snow is referred to as "Snowier Snow" by the community and the Mojang devs.
 * When inside powder snow, your FOV will decrease slowly.