Mapping Shade: Trees, Neighbors, and Winter Shadows

A site that is sunny in June can be in shade all December. The winter sun is lower and it lives closer to the southern horizon, so a tree, a neighbor's roof, or your own eave that the high summer sun clears will intercept the sun you wanted for heat. Draw that shade before you buy glazing.

It continues the site survey and uses the true-south line from the compass guide. Summer shade, which you want, is the subject of the shade-first guide. Do not erase a tree that is doing that job because a winter sketch looks crowded.

Difficulty

Level

Beginner

Time

One clear day, longer if you measure several trees

Skills

Pacing, a phone inclinometer or a protractor, and a sketch

Cost estimate

Free

A tape, a notebook, and an angle measured from the place the collector would sit. No shade tool is required for a first decision.

Materials

Item	Qty	Notes
The true-south line	1	From the stick at solar noon, not from an uncorrected compass.
Tape or a paced distance	1	From the proposed collector to each obstacle.
A way to measure an angle above the horizon	1	A phone inclinometer sighted along a straight edge, or a protractor and a string.

Measuring the obstacle angle

Stand where the collector or the window will be. Sight the top of the obstacle. The angle of that line above the horizontal is the obstacle altitude. If the sun, at the season you care about, is lower than that angle in that direction, the obstacle shades you. If the sun is higher, it clears.

At solar noon near the winter solstice, the sun's altitude is about 90 degrees minus your latitude minus 23.5 degrees. At 40 degrees north that is about 26 degrees. A tree whose top sits 40 degrees above the horizon, due south of the wall, blocks noon sun in late December. The same tree may clear a June noon sun, which at 40 degrees north is up around 73 degrees.

You can also work from height and distance when you cannot sight from the collector. Shadow length at noon is about the obstacle height divided by the tangent of the solar altitude. At 26 degrees the tangent is about one half, so a 20 foot tree throws a noon shadow on the order of 40 feet. If the tree is closer than that, the wall behind it is in noon shade. This is the due-south noon check. Morning and afternoon sun is lower and off to the side, so a tree southeast of the site can matter even when the noon check looks fine.

What to put on the sketch

For each obstacle that stands in the southern half of the sky, write the direction relative to true south, the distance, the height as well as you can get it, and the altitude angle from the collector. Photograph it with a person in the frame for scale. Date the page. A June photograph is not a December map.

Include things that are not trees. A neighbor's ridge, your porch roof, a chimney on the same house, a fence, and a hill. Include things that are not there yet if you can see them coming: a sapling under the south eave, a neighbor's allowed second story, a planned addition. A collector that works until the maple fills in is a collector with a deadline.

Bare branches still block some winter sun. If the branches already cover the sightline, count them. Do not assume leaf drop solves a tree that stands due south of a glazing wall.

What the map is for

If the winter altitude of the obstacles is higher than the winter sun across the middle of the day, that surface is a poor heating site. Pick another wall, or spend the project on conservation and on summer shade. If the obstacles only clip the first hour after sunrise, the site is still usable and you should say so in hours, not as a yes or no.

PV is less forgiving of a hard shadow that crosses a string of modules, and more able to use a roof that is only shaded in the early morning. Note the season either way. The mounting-spot guide is next if the shade map leaves you more than one sunny surface.

1. Stand at the future collector

   Not at the curb. Shade at the sidewalk is a different shade from shade on the wall.

2. Record altitude and direction

   For each obstacle, write the angle above the horizon and how far it is from true south.

3. Compare with the winter sun

   Use the noon altitude for your latitude as the first test, then think about the hours before and after noon, when the sun is lower.

4. Draw the summer case separately

   High sun and west sun. That sketch feeds the shade project, not the collector size.

5. Date it and name what will grow

   A map without a date will be misread in five years.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

• https://www.builditsolar.com/SiteSurvey/site_survey.htm
• https://www.builditsolar.com/SiteSurvey/FindingSouth.htm
• https://www.builditsolar.com/SiteSurvey/MagDecl.htm
• https://www.builditsolar.com/SiteSurvey/StepByStep.htm
• https://www.builditsolar.com/SiteSurvey/challenges.htm
• https://www.builditsolar.com/GettingStarted/GettingStarted.htm
• https://www.builditsolar.com/GettingStarted/ChoosingProjects.htm
• https://www.builditsolar.com/References/SunChartRS.htm