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Chapter 18 – Water in the atmosphere

Chapter 18 – Water in the atmosphere. Page 388 How would you describe the cloud in this photo? If you saw this cloud in the sky, what type of weather would you expect? How do clouds form? Why do clouds sometimes produce rain?. Chapter 18.1. Humidity and Condensation.

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Chapter 18 – Water in the atmosphere

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  1. Chapter 18 – Water in the atmosphere • Page 388 • How would you describe the cloud in this photo? • If you saw this cloud in the sky, what type of weather would you expect? • How do clouds form? • Why do clouds sometimes produce rain?

  2. Chapter 18.1

  3. Humidity and Condensation • Water vapor strongly affects the weather

  4. Characteristics of water • water is the only substance that commonly exists in all 3 states • Solid 0 Celsius or lower • Liquid between 0 Celsius and 100 Celsius • Above 100 Celsius water vapor • condensation gas  Liquid - Release heat • evaporation liquid gas - absorbs heat

  5. Humidity • Specific humidity – the actual amount of water vapor in the air. 20 grams/kilograms of air is very humid • Saturated air – rare of evaporation = the rate of Condensation • Warmer air can hold more water vapor • Relative humidity – How near the air is to maximum capacity. 0 – 100% • Measuring Humidity – Psychrometer – wet and dry bulb tem. If they both read the same – no water is evaporating 100%

  6. Condensation • At Night as air cools it ability to hold water decreases. If it cools past the point of saturation condensation occurs, forming clouds, dew, or fog • Dew Point - the temperature at which saturation occurs and condensation begins • Cooling and condensation – water vapor must have something to condense onto once it goes below dew point

  7. Condensation nuclei, such as salt, sulfate particles, or nitrate particles are in the air. • Dew vs. Frost – above freezing surface dew will form – Below freezing frost • Formation of Fog – Occurs when cold surfaces cool warm moist air • Slight air movements keep the droplets suspended • Advection Fog – warm moist air blows over cool surface. Northern U.S. Southerly winds blow over snow

  8. Discussion – Page 394 • Why would the melting of ice on the fruit in the morning be a cause for concern?

  9. Chapter 18.2

  10. Clouds P 396 • Clouds form when air cools to its dew point. Can form at any altitude in troposphere

  11. Types of Clouds – Cloud names are formed from one or more of the same 5 words • Stratus and strato – clouds that form in layers • Cumulus and cumulo – describe clouds that grow upward (heap) Fluffy clouds with flat bases • Cirrus and Cirro - Describe feathery clouds (curl of hair) high feathery ice clouds • Alto – between 2000-7000 meters • Nimbus and Nimbo – Dark rain clouds

  12. Cloud Formation • If cloud temp is below -20C cloud is made mostly of ice and snow • As warm air rises it cools. When it reaches a height where it hits the dew point, clouds form – Condensation Level • Unsaturated air cools at a rate of 10C for every kilometer it rises – dry–adiabatic lapse rate • Saturated air cools 5-9C/Km - moist-adiabatic lapse rate • The cooling as air rises is due to the expansion of the air

  13. Cumulonimbus Clouds - Heat from condensation keeps the air inside a cumulus cloud less dense that the surrounding air which makes it grow vertically • Meteorologist can predict cloud height if they know: Temperature of the clouds, dew point and the rate at which the rising air cools. • Layer clouds or stratiform form in stable hair • Stable air: If the temp of rising air inside the cloud decrease more quickly than the temp outside the cloud

  14. Chapter 18.3

  15. Precipitation • Any form of water that falls form a cloud to the earth’s surface

  16. How precipitation forms • Growth of water droplets in a cloud occurs by bumping • Growth of Ice Crystals – temp in the upper clouds are cold enough to freeze the droplets, as the ice falls it collects more droplets • Kinds of Precipitation • Sleet – frozen rain droplets • Freezing rain – occurs when rain hits a surface and freezes – ice sheet or glaze will form • Hail – Ball shaped ice from thunderstorms

  17. Measuring Precipitation • Measured in a rain gauge by 1\100th of an inch • Snow is measured by using a ruler or snow stick. 1 inch of rain is about 10 inches of snow

  18. Weather Modification • Dry Ice (frozen CO2) into clouds which cools the cloud and form ice crystals • Artificial nuclei such as a silver iodide is dropped into the cloud for more condensation to occur

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