Solar altitude refers to the angle of the sun relative to the Earth's horizon. Because it's an angle, you measure solar altitude in degrees. The value of the solar altitude varies based on the time of day, the time of year and the latitude on Earth. Regions close to the equator have a higher solar altitude than regions near the Earth's poles.
TL;DR (Too Long; Didn't Read)
Solar altitude is the angle of the sun relative to the Earth's horizon, and is measured in degrees. The altitude is zero at sunrise and sunset, and can reach a maximum of 90 degrees (directly overhead) at noon at latitudes near the equator.
Variation by Latitude
The solar altitude varies significantly by your latitudinal position on Earth. If you are at or near the equator, the sun will be high in the sky in the middle of the day. Therefore, the solar altitude will be fairly great. The Earth is tilted at an angle of 23.5 degrees with respect to the plane of the solar system. Hence, the sun is not always directly overhead at the equator. When the sun is directly overhead, the solar altitude is 90 degrees. This occurs at the equator during the vernal and autumnal equinoxes. At the Tropics of Cancer and Capricorn, the sun will have an altitude of 90 degrees during their respective summer solstices.
Variation over the Year
The Earth progresses through its seasons because its north-south axis has a 23.5-degree tilt. During the summer, the solar altitude will be at its maximum. During the winter, the solar altitude will be at its minimum. The change in solar altitude throughout the seasons results in hotter temperatures in the summer and cooler temperatures in the winter. Furthermore, because of the tilt of the Earth, the Southern Hemisphere experiences winter and summer at the opposite times of the year than the Northern Hemisphere.
Variation by Day
Throughout the course of the day, the sun changes its position in the sky. At sunrise, the solar altitude increases from zero degrees. At sunset, the solar altitude decreases toward zero degrees. The instance of the sun's daily maximum altitude is called solar noon, which does not typically coincide with clock noon. Again, this exact measurement of solar altitude varies depending on your latitude and the time of year. If your latitude is 44 degrees north, the solar altitude at solar noon during an equinox will be 90 minus 44, or 46 degrees. During the summer solstice, the solar altitude at solar noon will be 69.5 degrees. During the winter solstice, the solar altitude at solar noon will be 22.5 degrees.
Zenith and Azimuth
The measurements of zenith and azimuth are closely related to the measurement of solar altitude. The solar zenith angle of the sun is relative to the zenith, or directly overhead. This is the complement of solar altitude. Hence, if the solar altitude is 46 degrees, the solar zenith angle will be 44 degrees. Azimuth, on the other hand, measures the sun's angle relative to north, in the eastward direction. If the sun is due north in the sky, the azimuth will be zero. If the sun is due east in the sky, the azimuth angle will be 90 degrees. Solar altitude, zenith and azimuth all change throughout the day and year.