2.1 Buildings in the UK Climate – Climate and Weather

We will be focusing mainly on climate at both the large and small scales. However, it is important to understand the main distinction between weather and climate – the time over which the conditions are experienced.

The Met Office offers the following description of weather and climate:

“Weather is how the atmosphere is behaving on a day to day basis including temperature, rain and wind. These can change hour by hour, day by day. Climate on the other hand looks at how the weather changes over a long period of time, typically over 30 years but sometimes over hundreds of thousands of years.
Climate is what you expect, weather is what you get”

Essential Reading: MetOffice_Guide_to_Climate_Science1 – This is a very helpful introductory guide, but please note that it is not up to date with events and data since the time of its release around 2006. Therefore, the boxed text on page 7 is now out of date and there have been warmer years on record since. For an update about the warmest years on record, please see this NASA Analysis about the Warmest Year on Record.

Optional Reading: A short video and further information about climate science is available here on the Met Office website: http://www.metoffice.gov.uk/climate-guide  

3. How Our Climate Works

We cannot know for certain the exact nature of our future climate. But we can base our assumptions in a sound understanding of the elemental workings of weather and climate.

We will start with the global scale and gradually move right down to the micro-climate scale within and around buildings, their components and layers.

The global climate is driven primarily by the following factors:

Solar radiation
This is the ultimate energy source that drives global climate through differential heating of land, air and water. The heated water, in turn, creates air currents / wind which drive weather patterns.

Solar radiation also heats at every level right down to an individual building. It is up to designers to control solar radiation – to provide heat in cold weather and to minimise overheating in hot weather.

Angle of Earth’s axis
Earth orbits around the sun in an elliptical orbit and with its axis tilted at 23.5 degrees off vertical – this creates the seasonal temperature differences:

• When the northern hemisphere tilts away from the sun it is winter in northern hemisphere & summer in the southern hemisphere.
• When the northern hemisphere tilts towards the sun it is summer in northern hemisphere & winter in the southern hemisphere.

Rotation of Earth on its axis
Global rotation induces the “Coriolis effect”. This effect is the rotation of Earth’s surface creating air movement that is generally clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere.

Other factors also influence how air and moisture moves across the globe. Some of these operate on a national or regional scale, and some can also be very localised:

Humidity
Moisture is present in the air – from bodies of water and precipitation. The lower the air humidity, the greater potential it has to take up and transport large amounts of moisture across the globe (and deposit that water somewhere else).

The surface over which air is moving will also influence the characteristics of the air flow (i.e. faster laminar or smooth flow typically occurs over plains or oceans vs. turbulent flow through urban, hilly or mountainous terrain).

On and off shore breezes
During the day, the dominant direction of wind will be on shore, and at night the dominant wind direction is off shore (see diagrams below).

Diurnal air flow patterns
These can also be found inland, amongst hills or mountains and valleys, for example. During the day, the wind flows up valleys. While at night the wind flows in reverse.

Changes in elevation
Height above sea-level affects temperature. It will be widely known that in situations similar to those shown above, the temperature will be highest at the lower points in the landscape and cooler at the tops of hills and mountains. Globally, the temperature falls by 6.5ºC for every 1000m increase in altitude.

Rain shadow effect

One further process to be aware of is the “rain shadow” effect. Moist air is cooled as it rises over high ground, leading it to deposit some of its moisture burden as rain or snow. The resulting dry air gives rise to a drier zone in the lee of the mountains – see diagram below.

All of the factors above are some of the fundamental and unchanging processes of climate that will operate, regardless of the severity of any changes in climate.