- A brown pigment comprised of hydrated ferric and manganese oxides with variable proportions of earthy matter.
- Umbilical Tube
- A tube that connects a growing embryo to the placenta.
- The darkest part of a shadow where no light from the source reaches.
- Umkehr Effect
- An anomaly of the relative zenith intensities of scattered sunlight at certain wavelengths in the ultraviolet as the sun approaches the horizon, due to the presence of the ozone layer.