C L O U D S

1. C L O U D S

2. look how impressive and fascinating these pictures are ! from the silky filaments of high altitude cirrus to the towering, threatening mass of storm – bearing cumulonimbus; clouds are as varied as weather itself ;

3. apart from their beauty and interest, clouds provide useful indications on weather conditions and forecast; moreover they are a good opportunity to revise some school topics; if temperature drops below a critical value [dew point ] moisture condenses into droplets that form on microscopic dust particles [condensation nuclei]; about one million droplets are contained in only one rain‑drop.

4. clouds origin from water, that covers 70 per cent of the earth's surface; millions of tons of water vapour rise daily from oceans, lakes, rivers, and planttranspiration; this moistly air moves aloft encountering lower pressures, so that it expands becoming cooler ;

5. mind map

6. anyway, in clouds we find together water in the form of gas, liquid and solid : -- in ice water molecules are locked into hexagonal crystals and do not move freely; -- in liquid they are moving fast enough to break free from the crystal structure, but are still attached to each other; -- in the state of gas molecules are free to move about.

7. the changes water can undergo are: • - evaporation: from liquid to gas (water vapour); • - condensation: from gas (water vapour) to liquid; • - freezing: from liquid to solid (ice); • - melting: from solid to liquid; • - sublimation: from gas directly to solid; • deposition: from vapour directly to solid; • what makes water change its state?

8. the main difference in the three states is the average speed of the molecules that are : -- moving faster in a gas such as water vapour; -- slower in the liquid phase ; -- much slower in the solid phase (ice);

9. mind map :

10. let's revise some basics : • all substances can exist as solid, liquid or gas, according to the speed of molecular motion; • but different substances change from one phase to another at vastly different temperatures: • nitrogen ( gas ) freezes at (−) 200 °C , so that it • never happens naturally on earth; • metals, on the contrary, remain solid above (+) • 200°C, that is much hotter than any survival limit;

11. among these, water is special, because it's the only substance that can exist in three phases at earth's ordinary temperatures, as in clouds ; at the end , the only thing that makes millions of water molecules act differently when they get together to shape the three different phases, is energy;

12. this leads us to another point : water's changes of state always add or take up energy in the form of heath; this energy [ latent heat ] is a basic element to drive weather conditions, because energy cannot be created or destroyed, but only change form ; therefore, when water molecules slow down enough to change from vapour to liquid or ice, their motion energy [kinetic energy ] changes into heat;

13. mind map :

14. the latent heat released by billions of vapour molecules as they condense into water warms the air enough to make it rise faster ; this way more and more air flows in, creating wind; it is the release of latent heat that supplies most of the energy needed for rain showers, thunderstorms, and even hurricanes; for those who are concerned, something more about clouds :

15. cirrostratus: above 6000 m; often signal rain or snow within 24 hours; altostratus: grey or bluish above 2000 m; produce continuous precipitations; Cumulonimbus: generally produce showers of rain or snow.

16. Nimbostratus: Dark grey layer producing continuous rain or snow stratocumulus;: Low, lumpy layers ; below 2000 m ; rarely give precipitations

17. questions : -- running in a hot day makes you sweat and if you stop you quickly dry up and feel fresh ; explain why ; -- why does condensation occur on the outside of a can of Coke soon after it has been taken out of a fridge and placed in a warm room?

18. -- why is there often condensation on the windows of cars that have been left standing out of doors overnight? -- why does condensation occur in front of your face when you breathe out on a cold day?