Um is defined in the lower three layers of the model.

It fires if every node in the lower layer does not fire.

Normally, it takes about a month for new skin cells to move up from the lower layers to the surface.

There was no sound from that low layer of darkness.

I looked down where they had disappeared through the lowest layers of the mist.

It can be anything from a low layer of grasses to a plastic material.

You have to soak the lower layers of earth first.

The lower layers were of small stones that allowed good drainage.

The trees are so close together that very little light penetrates through to the lower layers.

Those in the lower layer were completely free from anything harmful.