City Weather (From Worcester News)

City Weather

11:57am Monday 18th May 2009

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The weather within our cities can be very different to the climate of our countryside. Large cities like London are often referred to as heat islands. The heat rises vertically creating a huge bubble of high temperature, especially through the summer months. These heat islands often trigger thunderstorms when the atmosphere is unstable with flash flooding as the drainage system struggles to cope with the huge volume of water which descends upon the city.

These built up spots are warmer than the surrounding rural locations, with night values often much higher. At night, the relative humidity in the city can be as much as 35% lower than the nearby countryside. The dryness of the air affects the temperature. Much of the Sun's energy we receive is used to evaporate water. Much of the air warming is due not to direct solar heat, but to latent heat of condensation. Due to the fact that the air is drier, more of the Sun's heat is available to warm the ground, which in turn warms the air in contact with it. You can compare how quickly a hard surface, such as a pavement, dries in comparison to a porous one, such as soil.

The heat from our cities rise, as it ascends it cools and descends into the countryside where at night it chills, when it reaches dew point (saturation level) it condenses into dew, mist and fog if the weather conditions are stable. Mist, fog and dew formation is therefore likely to occur within the rural setting.

Smog can form within are large cities during hot summer heatwaves, affecting those individuals suffering from breathing conditions. Gusty winds can also whistle around tall office blocks and flats, creating a vortex of swirling debris.

It is very rare to experience deep snow within our cities due to higher temperatures. Deep snow cover is often reserved for surrounding rural locations.

Next Week: Weather Fronts.

Glossary: Heat Capacity: The ratio of the amount of heat supplied to a body and the change in the temperature of the body. The higher the heat capacity, the more heat must be applied to raise the temperature.

Weather for coming week Monday 18th May-Friday 22nd May

Low pressure to the west/north-west influencing our weather through this coming week.

A mix of sunshine and showers, those showers on Monday and Tuesday heavy and squally at times with the risk of local hail and thunder in places. Very windy at first with winds from the south-west. Fewer showers, longer drier and sunnier spells Wednesday onwards with lighter winds and warming temperatures.

Maximum temperature 15-16c 59-61f but 17-18c 63-64f later.

Minimum temperature 8-11c 46-52f.

Last Week's Observations (Monday-Sunday)

Highest day temperature......17.8c Tuesday 12th May

Lowest night temperature.......5.8c Monday 11th May

Wettest day............................11.2 mm Thursday 14th May

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