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4. Discussion

The Effect of Daylight on Air Temperature

The lowest temperatures in the experiment occurred at both sites in the early mornings. During the night, whilst it was dark, there was no direct solar heating, however, the ground continued to radiate the heat that it had absorbed during the day together with its own inherent warmth. By the end of the night (ie predawn), the residual heat was at its lowest, hence air temperatures were at their lowest value. On Tuesday 15^th, the temperature appeared to be at its lowest at 09:00hr, however this may have been an experimental error at the 07:00hr reading; this reading took place in the dark and in appalling weather conditions and was probably affected by our own body warmth, which made the temperature appear slightly higher than it actually was.

At both locations the temperature was at its highest at either the 13:00hr or 15:00hr reading, the only exception occurred at Cardiff on Tuesday 15^th, when the peak temperature was reached at 19:00hr. This was probably as a result of thick cloud, wet and windy weather much of the day, but from mid-afternoon onwards the cloud thinned until by 19:00 hr the rain and wind had eased off, and hence the rise in temperature. Usually the highest air temperature would be expected to occur in the mid-afternoon, as the sun is high in the sky and the ground is reflecting back the absorbed heat of the day. However, this is not always the case, as with Tuesday, and on a cloudy day the highest temperature may actually occur when the cloud thins sufficiently to permit the greatest solar penetration.

The Effect of Cloud Cover on Air Temperature

As already discussed earlier, varying the thickness and extent of cloud cover has a marked effect on the temperature. On Tuesday 15^th the temperature showed a slight rise as the cloud cover thinned in the late afternoon at Cardiff; whereas at Offwell the cloud cover thinned from late morning onwards and the temperature rose in the early afternoon (reaching its maximum of 15°C at 13:00hr). When, however, the cloud thickened again later in the afternoon, the temperature subsequently dropped. On Wednesday 16^th, the persistent cloud cover all day at Offwell meant there was little variation in air temperature; although a similar pattern was seen in Cardiff, there was some sun mid-afternoon and the air temperature rose. By contrast, on Thursday 17^th, the lowest temperature of the entire experimental period, at both sites, was recorded at 07:00hr, when it was dark and the skies were clear/only partially cloudy. Indicating a greater heat loss had occurred overnight when there was little or no cloud present. This suggests that at night cloud cover has the opposite effect of during the day. So, in conclusion, we observed that in the day cloud cover depressed air temperature by obscuring solar penetration, but in darkness, cloud cover prevented further heat loss by having a blanketing effect helping to retain radiated warmth.

The Effect of Sunshine on Air Temperature

The sun is always above the Earth; it is only obscured by clouds and the Earth’s rotation. Sunshine is the main factor that decides the variations in air temperature. Direct sunshine produces a higher air temperature than when it is obscured by cloud or shaded. Throughout the experiment it has been noticeable that when the sun shone, the temperature rose. For example, on Tuesday 15^th, at Offwell, the sun came out in the early afternoon, and at the 13:00hr reading there was a noticeable difference in temperature, as the cloud had thinned and the sun’s heat was able to penetrate more; a similar situation was observed on Wednesday 16^th afternoon at Cardiff where weak sunshine at 15:00hr accompanied a rise in air temperature to 10°C. Thursday was the sunniest day in the experiment and the highest temperatures were recorded. Every time the sun came out, we saw a rise in temperature providing the thermometer was not in shade. To measure air temperature accurately, the thermometer needed to be shielded from direct sunlight. If the thermometer had not been housed, but left in direct sun, it would have measured the effect of the sun on the thermometer, not the air temperature itself. However, the position of the thermometer with respect to shade from trees or buildings is very important. In this experiment, sunshine was not something we saw too much of!

The Effect of Local Topography

Generally, the temperature profiles of both sites for each day were similar, though the temperature at Offwell was always higher than at Cardiff despite the two sites being only 50 miles apart and sharing a similar longitude. Both sites have a unique microclimate due to their particular topography. The topography of a site is the shape of the landscape; it can have a pronounced affect on the regional climate (the microclimate). The Cardiff site is approximately 3.5 miles north of the city centre and at 50m above sea level, it is slightly higher than the city itself. To the NNW of the site there is a ridge of wooded hills (rising to 250m above sea level), whilst to the SSE is Cardiff bay and beyond that the sea. This means that in one direction the site is sheltered and in the other direction more exposed. As a generalisation, the rain and wind usually come from a westerly direction. The ridge of hills may have a sheltering influence, though often low cloud and mist tend to cling to the hillside. The Offwell site is in a wooded valley in the middle of countryside. Being in a wooded valley means that it is sheltered by the sides of the valley and by the trees, which may act as a windbreak and also affect solar warming by acting as radiators of solar energy. This helps to account for the higher temperature at Offwell even under similar weather conditions.

Seasonal Effects

The temperature range for each day gives an indication of the variation in air temperature (see Table 4).

Table 4 Comparisons of the Daily Minimum and Maximum Temperatures at Cardiff and Offwell and Temperature Range.

 In the duration of the experiment, the minimum temperature at Cardiff was 4°C and the maximum temperature was 10.25°C, a difference of 6.25°C. The minimum temperature at Offwell was 7.2°C and the maximum temperature was 15°C, a difference of 7.8°C. Although the weather during the experimental period was often cloudy, occasional periods of light cloud or clear sky were also present. Although under these conditions there was an increase in air temperature, the maximum temperature observed was not elevated by a large amount. This was probably a seasonal effect, as the autumn sun is lower in the sky and less powerful than during the summer months. However, clear skies had a more marked effect on minimum temperature. The temperature at Offwell was always a few degrees higher than at Cardiff, reaching a 6°C difference at times. Offwell site is near a small village, whereas Cardiff is a busy city; one might have expected the warmth from the city to produce higher air temperature values than at Offwell, but the sheltering influence of the valley was more significant than the warming effect of the city. It would be interesting to repeat this experiment under different seasonal conditions, so for example, in the winter when the trees are bare, does the valley at Offwell still have such a marked sheltering influence; or in the summer, what effect would the higher sun have on the temperature range comparing cloudy and clear conditions.

Conclusion

In conclusion, the air temperature profiles for Cardiff and Offwell were broadly similar to each other, though Offwell had higher temperatures. Temperature range was not particularly great but fairly similar between sites.

There are many factors, which influence the general air temperature. The two main contributing factors are: firstly, the Earth’s inherent heat; secondly, direct solar heating, and indirectly radiated heat, heat which the ground has absorbed from the sun during the day is then radiated back at night.  The weather, especially cloud cover, also has an important effect. At night cloud cover acts as a blanket trapping heat in, whilst in the day, it has a screening effect and inhibits solar warming. Another factor is the season. This experiment took place in the autumn, which meant that the mornings and evenings were colder and darker, and the weather was naturally windier and wetter than if it had taken place in spring or summer. Also, the sun was lower in the sky and therefore, weaker. The topography of a site can also make a marked contribution. Both sites in this experiment have a particular microclimate.

5. Comments

            I thought that, in general, the experiment worked very well and could not be improved upon. The weakest point was probably the satellite pictures. I found the whole experiment very enjoyable with the hardest part being the write-up! The fact that is has been the pilot project is very exciting.

                                                                                                                      5^th November, 2002

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