From AMS Glossary
The elevation angle of that point that a given observer regards as the bisector of the arc from his zenith to his horizon; a measure of the apparent degree of flattening of the dome of the sky.
Because to almost all persons the sky appears not as a hemispheric dome but rather as a more oblate surface, the half-arc angle designated by most observers is substantially less than 45°, usually falling within the interval 20°–35°. The more remote the landmarks on an observer 's horizon, the smaller in general will be his designated value of the half-arc angle. Also, presence of a cloud deck serves to produce an apparent flattening effect. No fully satisfactory explanation exists to account for the apparent flattening of the sky.