The sun's path

Understanding the following points will help you plan your shade:

Shadows

Shadows are always moving, shifting in response to the height and angle of the sun's path. There are three basic shade patterns every day:

  • Morning, during which shadows will fall in a westerly direction away from the object casting the shadow
  • Midday, when the shadow will be close beneath the object
  • Afternoon, when the shadow will be in an easterly direction away from the object.

Knowledge of the sun's path is essential when designing barriers to UVR, as you need to accurately predict where trees and structures will cast their shade. Too often, shade canopies are erected or trees are planted on the assumption they will do a good job of providing shade, when the actual shadow pattern is entirely different.

Azimuth

The azimuth is the angle in a horizontal plane between true north and the direction of the sun, measured clockwise from true north. It can have any value from 0° to 360°. The sun’s azimuth at noon is always 0°. The azimuth determines the direction in which the shadow will fall.

Diagram showing solar azimuth angles
Image from Queensland Health (1997) Shade for Public Pools - Planning Sun-safe Outdoor Environments in Queensland, pg 92. (Available from Preventive Health Unit, Queensland Health)

Solar altitude

Solar altitude is the angle between the sun and the horizon on a given latitude. It varies according to the time of the day and season. The angle of altitude is used to determine the length of the shadow cast by a solid object. UVR levels increase in intensity as the angle of altitude of the sun increases throughout the year. This is because:

  • The higher the solar altitude angle, the longer the daylight period
  • The higher the solar altitude angle, the shorter the path of radiation through the atmosphere. More radiation, including UVR, therefore reaches the earth’s surface since less radiation is absorbed by ozone, vapours and dust particles in the atmosphere
  • The higher the solar altitude angle, the higher the intensity of radiation falling on any particular area.

Diagram showing solar altitude angles diagram
Image from Queensland Health (1997) Shade for Public Pools - Planning Sun-safe Outdoor Environments in Queensland, pg 92. (Available from Preventive Health Unit, Queensland Health)

Solstices

After March 21, the sun becomes lower in the sky and the length of the day is shorter until the winter solstice occurs on June 21 each year. Shadows are longest at this time. After June 21 the sun’s path gets higher in the sky each day and shadows become shorter. During the cooler half of the year from March 21 until September 23, the sun continues to rise higher in the sky and the hours of sunlight are longer each day until the summer solstice (the longest day of the year) that occurs on December 22. At this time, it can be seen that shade is cast well to the north of the object and that south faces are exposed to direct sunlight throughout the day.

The fact that the sun is lower in the sky after March 21 and higher in the sky after September 22 is something the designer must always take into consideration when building shade systems.