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d ~ 154'368'000 km: day 60

Today is St David's day. A day for celebrating all things connected to Welsh life and culture - eisteddfod, rugby and mountains to name the most obvious. But there is another aspect to Wales that we could well be celebrating.

Not many countries could be said to have a national weather-type but certainly for people journeying from the south of England the weather type culturally associated with Wales is rain. Rainwater seeps down the mountainsides and gushes along rocky stream beds watering lush grasses on steep slopes and down into fertile mountain valleys.

But why so much rain? Well it's caused by the mountains themselves. They act as a physical barrier to the prevailing westerly wind and air is forced upwards over the craggy peaks. High up there temperatures are cooler and so water vapour in the air condenses turning from an invisible gas into millions of droplets of liquid water or clouds. As more and more water condenses the droplets become bigger and heavier and often will eventually fall out of the cloud as rain.

But why do many holiday makers from the east find this Welsh rainfall so particularly unpleasant? That's all to do with typical weather in the east. After this air has passed over the mountains it descends again and moves towards the east of the country. But even though clouds may form it's much less likely that rain will fall now - like wringing out a sponge the journey up over the mountains has dried out the air and there simply isn't enough water left in it to give the same amounts of rain. Land which lies downwind of mountains like these is said to be in their "rain shadow". London lies in the rain shadow of the Welsh mountains so the wet welsh weather actually keeps our capital dry - drier than Rome in fact.

Every day is different of course but this is the typical setup of rain over the southern half of UK brought about because of our landscape. Even smaller-scale variations in geography across the country can lead to localised tendencies in the weather called "microclimates". The particular location of a hill may mean that one town is often flooded and rainy while another is usually dry. These microclimates exist all over the UK.

So if you're considering enjoying some lavabread or barabrith tonight then take a few moments to think about whether you yourself live in a microclimate and what the cause of your local climate may be. If there was ever a day for celebrating our glorious tapestry of regional weather variations (and that life-giving rain in particular) then St David's day could well be it.

Dydd Gwyl Dewi hapus

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