Climate in cities

Basics

2. Urban climate
Wind in the city

Worksheet 3:
Experiments about wind in the city

1. Wind in the city

Air warms up faster in cities compared to rural areas, particularly on summer days.  This leads to low pressure areas in the cities and higher air pressures in the surrounding countryside.    As a result of this pressure gradient, air moves into the cities from the rural areas.  The air follows the corridors made by the main roads and its speed decreases by up to 50%.  Once in the centre of the city, the wind speed rises by up to 20% as it moves around street corners and eddies may be formed.

In the centre of the city the temperature of the air increases and the warmer air rises along the walls of the houses higher into the atmosphere.  It then moves back towards the suburbs as a so-called anti-breeze. It then cools down, gets heavier and sinks and comes back to the city as an urban breeze.

Compare the rural-urban wind patterns with the circulation of air in tropical regions or the circulation of air over the land and the sea. Ask your geography teacher for details.

2. Experiment

You can find out how the wind generates eddies with this experiment

2.1 Safety Information

- Only conduct these experiments in a physics or chemistry laboratory or outside in a sand box.
- Make sure that an adult is present at all times.
- Follow the instructions very carefully.  Fires and electrical currents are dangerous.
- Have sand, a fire blanket, and a fire extinguisher near by.
- Never extinguish a fire with water.  Use sand, a fire blanket or the fire extinguisher.
- If you can't extinguish the fire, sound the fire alarm, leave the building and call the fire brigade.

 You will need:

1. Equipment needed for the experiment.  Photograph: Gert Bauer.

Material:

1. concrete or asbestos tile to use as a base
2. wool, pair of scissors and glue to fix the bricks into place
3. bricks (or old juice boxes filled with water) to act as model houses
4. brick covered with aluminium foil to put the fuel on
5. lighter and fir cones to use as fuel and generate smoke
6. hair drier with cardboard tube attached to it with adhesive tape to use as the wind
7. spray bottle to moisten the fir cones and generate smoke
8. a blackboard for the background.

 2.2 Setting up the experiment

2. Experimental set-up.  Photograph: Gert Bauer.

Attach small pieces of string to the sides of the bricks so that you can see how the air travels past them.  Set up the bricks on the concrete or asbestos base to represent the buildings in a city.  Put a blackboard behind them to act as a background. 

2.3 Carrying out the experiments:

1. Put the fir cones onto the brick covered with aluminium foil and set light to them. 
2. Once they are alight, moisten them with a small amount of water to generate smoke.
3. Blow the smoke towards the bricks using the hair dryer with the cardboard tube attached to it.  Try the different strength settings on the hair dryer.  
4. Watch how the smoke moves around the bricks.  Use these observations to help you work out how air moves around cities.

Experiment 1

First we look at how the wind moves around a multi-storey building.

The wind can either rise up the building or circle around the building and this movement causes the wind speed to increase by up to 20%.

First blow the smoke towards the long side of the first high brick in the row and watch how the smoke moves around it. Vary the distance and angle between the hair dryer and the brick.  Watch how the smoke then moves around the next brick.

Click on the icon to see a short movie showing how air flows around a building. Movie: G. Bauer and B. Wohlhöfer.

Attention:
If you cannot see anything, please install Microsoft Media Player (TM) or RealPlayer (TM) first.
If you have a slow modem it is better to save the file onto your hard disk and run it later. If you have a fast connection, then just run the movie.

5. Air movement around buildings.  Photograph: Gert Bauer.

Click on the icon to see a movie about the experiment! Movie: G. Bauer and B. Wohlhöfer.

 Experiment 2

In this experiment we will look at how air moves around a row of houses and how it whirls on the leeward (sheltered) side of each building. 

Use the hairdryer to blow the smoke above the top of the bricks.  Watch how the smoke blows over and around the buildings and whirls around in the sheltered areas between the bricks.

8. Simplified stream of air along a row of houses.  Photograph: Gert Bauer.

Experiment 3

Set out the bricks so that the bricks at the back are approximately three times higher than the the bricks at the front and so there is some kind of square in the centre of the experimental setup.  The experiment aims to show how the wind moves around street corners and in squares and how its speed increases by up to 20%. 

Blow the smoke from the left into the model of the city.  In our experiments we decided that the whirls  where strongest when the smoke was blown into the street at an angle of 45°.

9. Idealised airstream in a city.  Photograph: Gert Bauer.

Click on the icon to see a movie of the experiment.  Movie: G. Bauer and B. Wohlhöfer.

3. Importance of wind in cities

3.1 Wind tunnel tests

Before new parts of a city are built, tests are made using models to look at how the air flow will change in the city and if strong winds are generated at particular corners.  The image below shows a wind tunnel  model of the newly built Potsdamer Platz in Berlin.

11. Photo taken by Elmar Uherek at the climate exhibiton at the German Museum, Munich.

The most important parameters which influence the wind in a city are the wind profile and the roughness of the ground.  These parameters are simulated in the wind tunnel tests.  With the help of this simulation, whole areas of a city or single buildings can be tested in detail.

The results of a wind tunnel test look like this:

12. Construction plan of a new area within a city.  The plan shows the areas (in red, yellow and purple) where the windspeed is likely to increase.  Author: Mr. Müller, Stuttgart.  Source: www.stadtklima.de/stuttgart/websk21/Heft13/kap3.HTM

Further links:

www.ruscheweyh.de/bauwesen.php
www.gfa.de/de/Windlasten/windlast.html

3.2. Windload on a building

The windload on a building changes with time and space. It works directly on the outside surface of closed building and indirectly via openings on inside walls.  It depends on the dominant wind direction and wind speed. The following animation from an aerophysics company shows the how the windload on a building changes with time:

13. Windload on a building.  Author: Gesellschaft für Aerophysik mbH, Munich. 
Animation source: www.gfa.de/de/Windlasten/windlast.html

About this page:

authors: G. Bauer and B. Wohlhöfer- University of Nürnberg, Germany
educational reviewing: Prof. Dr. Helmut Schrettenbrunner and Julia Heres - University of Nürnberg, Germany
last update: 2003-09-09