Fall 2002 Gary Austin gaustin@uidaho.edu

Click thumbnails for a detailed view
Conditions vary according to latitude due to the tilt of the earth relative to the the sun.  Each position on earth is presented differently each day of the year since the earth is in motion.

### Solar Access

Horizontal Position

This is measured as an azimuth or as a South bearing and defines the horizontal variation in the position of the sun as it travels across the sky during the day.

Altitude

The angle of sun in the sky is defined by the altitude.  When the azimuth and the altitude are both known then the sunlight striking windows and the length of shadows cast can be estimated.  These are, of course, important for energy conservation in buildings and the creation of microclimates outside the building.

 Winter Sun Path Summer Sun Path

### Solar Path Diagrams

Solar Path Diagram 48 Degrees

This diagram would be used for Moscow since we are at about latitude 46.8N at latitude 116.7W.  It is used to plot the position of the sun and provide azimuth (bearing in this case) and altitude.  Note than the makers of various diagrams each create a slightly different graphic system.

### Definition of the Sun's Position (Azimuth and Altitude Angles)

For a certain location, for a certain day and hour, azimuth and altitude angles may be defined by the following procedure. For this purpose the sun path diagram prepared for this location should be used.

Example : Define the position of the sun in a town near latitude 36 at 9:00 am of December 21.

 Step 1: Select the sun path diagram for the site latitude (or nearest latitude). 36˚ North latitude is used in this in this example. Step 2: Find the date curve for December 21. Step 3: Find the hour line for 9:00 am and mark its intersection with the curve of December 21. Step 4: Lay a straight-edge from the center of the chart from the observation point) through the marked hour point to the perimeter circle. Read the Azimuth Angle from the perimeter scale. For this example (α) = 137.5˚. Step 5: On he straight line, measure the distance in millimeters between the perimeter circle and the marked point. Each millimeter represents one degree of altitude angle. This distance will be measured 16.5 mm. This means the altitude of the sun at 9:00 am of December 21 is (θ) = 16.5˚.

### Solar Path Animation

New software tools allow more accurate calculation of sun position and three dimensional representations of the path at various time of day and year.

One such tool is available at the Weather Tool site.
http://www.squ1.com/software/weatool/features.html

Sun angle calculator on the web
http://www.susdesign.com/sunangle/

Building Shape and Orientation for Energy Conservation

Rectangular: 1:1.6 proportion

17 ½ degrees east of south for temperate climates

The building design proposal illustrated in these sections is based on the use of a sun path diagram created by the Libbey Owens Ford company.  It is a more complex tool that provides profile angles, solar incidence and day-lighting information not available on the simpler diagrams that we have studied.

We will spend more time using this system once we have preliminary building footprints on our West Farm Project.  The instructions on the use of the Sun Angle Calculator is provided on line at
http://www.sbse.org/resources/sac
and provides an expanded consideration of the sun's path for site analysis.

## Wind

Weather stations data at the University of Idaho

Search for "Inside Idaho"

PULLMAN-MOSCOW  AVERAGE WIND SPEED - MPH

Jan  Feb Mar Apr May Jun  Jul  Aug  Sep Oct  Nov Dec  Ann
7.3  6.8  6.6  6.6  5.8  5.1  4.1  4.3  4.6  5.7  7.2  7.9   6.0

Weather stations collect wind direction and speed data.  Most airports have on-site weather stations and are a good source of local information.

Data on wind is often presented in tabular form but a "wind rose" diagram is especially useful and readable by visual thinkers

 For each of the sectors the outermost (blue) wedges show the wind frequency distribution. The middle (black) wedges show the distribution of the product of the two columns, i.e. the wind speeds times their frequency. The innermost (red) wedges show the distribution of the wind speeds cubed (i.e. the energies) multiplied by their frequencies.

 The weather on the lee slope of a hill is usually quieter than on the weather slope. However, this may be reversed if the weather slope is steeper than the lee slope. When the boundary layer of air is compressed as it passes over a ridge, wind speed is usually 20 percent greater on the top of the ridge than on the slopes.
 Weather data Average High JAN  FEB  MAR  APR  MAY  JUN  JUL  AUG  SEP  OCT  NOV  DEC YEAR  35.6 41.3 49.0 57.5 65.9 73.1 82.6 84.0 74.4 60.5 43.1 35.5  58.5 Average Low JAN  FEB  MAR  APR  MAY  JUN  JUL  AUG  SEP  OCT  NOV  DEC YEAR  23.2 26.8 31.2 35.4 40.6 45.2 48.4 48.7 42.9 36.0 29.9 23.6  36.0 Monthly Precip JAN  FEB  MAR  APR  MAY  JUN  JUL  AUG  SEP  OCT  NOV  DEC YEAR  2.99 2.52 2.57 2.52 2.62 1.87 1.12 1.19 1.28 2.01 3.54 3.14 27.37