A cowl is a usually hood-shaped covering used to increase the draft of a chimney

Chimney

A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the...

 and prevent backflow. The cowl, usually made of galvanized iron, is fitted to the chimney pot to prevent wind blowing the smoke

Smoke

Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...

 back down into the room beneath. No doubt named for the resemblance of many designs to the cowl

Cowl

This article is about the garment used by monks and nuns. For other uses, see Cowl or Cowling .The cowl is an item of clothing consisting of a long, hooded garment with wide sleeves. Originally it may have referred simply to the hooded portion of a cloak...

 garment worn by monks, they have been in use for centuries.

When using an open fire to heat a room the smoke rises through a flue

Flue

A flue is a duct, pipe, or chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. In the United States, they are also known as vents and for boilers as breeching for water heaters and modern furnaces...

 to a chimney pot on the roof. Under normal conditions the warm air from the fire will rise up the chimney taking the smoke with it and dispersing it at rooftop level where it is less of a nuisance.

In strong winds the force of the wind may overwhelm the updraft and push the airflow in reverse down the flue. Smoke will then fill the room it is intended to heat posing a health and fire risk, causing discomfort and dirtying furnishings.

When raw coal

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 rather than smokeless fuel is burnt, which was the case for centuries before clean air legislation

Clean Air Act

A Clean Air Act is one of a number of pieces of legislation relating to the reduction of airborne contaminants, smog and air pollution in general. The use by governments to enforce clean air standards has contributed to an improvement in human health and longer life spans...

 took effect, then the amount of smoke is considerable and measures to prevent backflow occurring are a necessity.

Another secondary function is to prevent birds and squirrels from nesting in the chimney. They often also act as a rain guard to keep rain from going down the chimney. A metal wire mesh is sometimes added as a spark arrestor. Wooden cowls

Cowl (oast)

thumb|Cowls on a maltings at [[Ware, Hertfordshire|Ware]], Herts. A Cowl is a device used on a kiln to keep the weather out of, and induce a flow of air through, the kiln...

 were used on oasts to prevent the ingress of rain into kilns, and create a flow of air through the kiln.

A H-style cap (cowl) is a chimney top constructed from chimney pipes shaped like the letter H. It is an age-old method to regulate draft in situations where prevailing winds or turbulence cause downdraft and backpuffing. Although the H-cap has a distinctive advantage over most other downdraft caps, it fell out of favor because of its bulky looks. It is found mainly in marine use but has been gaining popularity again for its energy saving functionality. The H-cap stabilizes the draft rather than increases it. Other downdraft caps are based on the Venturi effect

Venturi effect

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...

, solving downdraft problems by increasing the updraft constantly resulting in much higher fuel consumption.