Heat island

monly used in earthwork as retaining structures or as pilings.

hdr. Abbreviation for header.

head 1. Generally the top, end, or upper portion of a structure or part. 2. A sprinkler head. 3. See static head. 4. See head pressure. 5. The entire leaves and branches above the ground of a woody plant. 6. The upper horizontal piece of a door or window jamb forming the top of the frame. See window head or door head.

header 1. A brick laid crosswise in a wall with its end(s) exposed. 2. A masonry unit placed with its ends exposed and overlapping two or more adjacent withes of masonry in tying them together. 3. A framing member that crosses and supports the ends of joists, rafters, etc., transferring weight to the parallel joists, rafters, etc.

header pool The pool that is highest in elevation in a recirculating water feature.

heading back A method of pruning in which 1-yr old shoots are cut back to buds, or an older branch or a stem is cut back to a stub. Shearing is a form of heading back. This type of pruning ruins the natural shape of plants, especially trees, and promotes weakly attached new growth. Thinning is a better method of pruning trees.

head pressure or head feet pressure An expression of pressure as related to the height of the surface of a column of water (or other liquid) above an elevation, indicated in feet of water. It is equivalent to 0.433 psi per foot of water. The term head is usually understood to mean static head and not dynamic head.

head-to-head coverage In sprinkler irrigation, a layout design of sprinkler heads with the radius or limit of coverage of one head reaching to the next head. As most sprinkler heads do not provide even coverage throughout their radius of reach, this allows a much more even distribu tion of water to the coverage area. Common methods of spacing of sprinkler heads for head-to-head coverage include square spacing and triangular spacing.

headwall A masonry or concrete retaining wall around the outlet or entrance of a drain pipe or culvert. These provide protection against erosion of the surfaces around pipe entrances or exits where water is turbulent.

heart rot A disease usually in old trees caused by fungi that normally rot dead wood. It affects ornamental trees. This disease may be evidenced by yellowish to brown mushroom-like or woody growths on the tree bark exterior. Invisible beneath them is internal decay. Hard growths are called conks.

heartwood or duramen Nonliving wood at the center of a tree that is generally darker, much more durable than sapwood, and more decay resistant than sapwood.

heat exchanger A device that transfers heat from inside to outside air, or from fluid, or from solid to solid, without allowing direct contact between the two.

heat-fusion joint A special pipe and fitting that is heated and placed together to form a bond and a leak-proof joint.

heat gain The net temperature increase in an area or material.

heath Open, level, uncultivated land with poor soils, usually acid, and usually having poor drainage.

heat island The microclimate, area, or patch of warmer air that forms in and over urbanized areas because of paved or impervious surfaces, reflection from upright structures, and buildings gathering heat or generating it and then releasing it.

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