Winds Flashcards
How does the varying heights of buildings have an effect on wind speed, direction and frequency
Buildings exert a powerful frictional drag on moving air over and around them.
= turbulence
Where are average wind speeds lower
In cities, they are also lower in city centres than in suburbs
How do high rise buildings have N effect on wind speed, direction and frequency
They may slow air movement but they also channel air into the ‘canyons’ between them.
What happens on calm and clear nights
Urban heat island effect is at its greatest, there is a surface inflow from the cooler areas outside the city to the warmer areas in the city centre
How can a single building modify an airflow passing over it
Air is displaced upwards and around the sides of the building and is also pushed downwards in the lee of the structure
What happens on the windward side of a building
The air pushes against the wall with relatively high pressures. As the air flows around the sides it becomes separates and sets up suction in these areas. In the windward side of the overpressure, which increases with height, causes a descending flow. = a vortex
What is there in the lee of the building
A zone of lower pressure, causing vortices behind it.
What happens if two separate buildings allow airflow between them
The movement may be subject to the Venturi effect in which the pressure within the gap = wind to pick up speed and reach high velocities
How do some buildings avoid the Venturi effect
They are built with gaps in them, or on stilts, but a reasonable flow of air at street level is essential to remove pollution
What does the disturbance of airflow depend on
As buildings are usually part of a group it depends on the height of the buildings and the spacing between them
What happens if buildings are widely spread
Each building will act as an isolated block
What happens if buildings are closely spread
The wake of each building interfere with the airflow around the next structure and this produces a complex pattern of airflow
What is important when buildings are designed
That pollution emitters (chimneys) are high enough to ensure that pollutants are released into the undisturbed flow above the building and not into the lee eddy or the downward following air near the walls
