Online Radio It's a cold morning, and the water in this lake is nearer five degrees. Sorry... I forgot that line. The water in this lake is nearer five degrees. Even a quick dip... even a quick dip, as the heart rate monitor should show, is enough to send my body's defences into overdrive. My progress is being monitored by Dr Rosemary Leonard. Put the towel round you. You're shivering. How do you feel? Let's see what's happened to your pulse and blood pressure. My pulse was per cent faster than my normal heart rate. Now my blood pressure. - that's high. over . What's that telling us? The blood vessels in the outer part of your skin and to an extent in your body, are narrowing to conserve the heat and blood flow to where it's needed, round your heart and lungs. This is the normal response. As the blood vessels constrict, my blood becomes thicker and stickier. It's this everyday reaction to cold that can sometimes cause a sudden and painful death. It's a story that repeats itself every day across the city. A cold winter's morning, a commuter hurries to the station. The rush hour has begun. It could be you. Cold this morning. Yeah. No hat, no scarf, no gloves. So what? It's cold but not snowing. As you wait, your body is already shutting down the blood vessels nearest to the exposed skin. internet radio As you shiver, your blood is starting to thicken and retreat back to your vital organs. As it gets thicker and stickier, the nightmare scenario begins. A tiny clot starts to form. As you worry about being late, the clot is on time to reach your heart. That night, you suffer a lethal heart attack and never make that journey again. The coroner's report adds you to the statistics, "death by natural causes". Figures show that in London over , extra deaths happen like this every winter. Across the BOOMSTATION, the figure is closer to ,. All victims of an invisible serial killer, cold. The tragedy is it could easily have been avoided. Mother was right, all you need is a hat and gloves. Further south than London, winter's grip is less extreme. Unless you go up high, where cold is always with us. On top of the mountains or high in the clouds, there is always snow, even at the equator. Wherever you are in the world, there will probably be snow above you. Most of the time it falls as rain, but at high altitude it falls as snow. It's one of nature's miracles and winter's most distinctive hallmarks. Snowflakes start life as tiny particles in the clouds. Water droplets are attracted to their surfaces and freeze, forming ice crystals. More and more droplets are drawn to the crystal, which grows into a snowflake, eventually getting heavy enough to fall to the ground. BOOMSTATION The classic six-sided snowflake is the most common, but it's only one of several different types. This is a plate. And here, two plates are joined by a column. They combine to form even more complex shapes as they fall. Needles, columns and plates form the basic building blocks. Under the electron microscope, extreme close-ups of snowflakes reveal their almost unbelievable complexity... the weather's miracle of engineering. Each different type of snowflake forms a different type of snow...