Weather Part 4: Moisture in the Air 1

Humidity Evaporation: the process by which water molecules escape into the air at temperatures below boiling Worldwide, the atmosphere holds an average of about 14 million tons of moisture. Humidity is water vapor (moisture) in the air. Relative humidity: the percentage of moisture the air holds relative to the amount it could hold at a particular temperature Saturated: 100% relative humidity 2

Measuring Relative Humidity Psychrometer – a device used to measure relative humidity Consists of two thermometers, the bulb of one covered with a moist cloth (the wet bulb) and other not covered (the dry bulb) Air passing over the wet bulb evaporates the water and cools the bulb In low humidity the water evaporates rapidly and the temperature of the wet bulb thermometer drops In high humidity little water evaporates and the temperature of the wet bulb thermometer stays close to that of the dry bulb thermometer 3

Measuring Relative Humidity The difference between the wet bulb and dry bulb readings is calculated, and a chart used to determine the relative humidity Video: om/watch?v=QbcaCxu A1LI 4

Clouds Warm air can hold more moisture than cold air As warm, moist air rises it cools When it becomes saturated the water vapor will begin to condense and form clouds. Cumulus Clouds 5

Clouds Dew point – the temperature at which water vapor condenses Why do we see dew on the grass some mornings but not others? (2 factors) 6

Clouds Clouds form when moisture in the air condenses on small particles of dust or other solids in the air. A cloud is a mixture of liquid water suspended in air. Classified according to their shape and altitude Cirrus Clouds © Copyright Colin Smith and licensed for reuse under this Creative Commons Licence.Colin SmithreuseCreative Commons Licence 7

Three Main Types of Clouds 1. Cumulus: Fluffy and white with flat bottoms Usually indicate fair weather Grow by free upward convection Cumulonimbus – larger, darker on the bottom; thunderclouds 8

Three Main Types of Clouds 2. Stratus: Smooth, gray clouds that block out the sun Non-convective layered sheets Usually with light rain and drizzle Fog – stratus clouds near the ground Nimbostratus – brings rain and snow 9

Three Main Types of Clouds 3. Cirrus: Feathery or fibrous clouds Sometimes called mare’s tails Form at high altitudes Made of ice crystals Often indicate rain or snow on the way 10

Cloud Names Prefixes Strato- Low clouds (example - stratocumulus) Alto- Midlevel clouds (2 to 7 km) (example – altostratus) Cirro-high level clouds (example – cirrocumulus) Nimbo- Thick, layered clouds that produce moderate to heavy precipitation 11 photo by Bill Hails on FlickrBill HailsFlickr le:Altostratus_undulatus.jpg

Clouds 12

Precipitation Water vapor that condenses and falls from the sky as snow, rain, sleet, or hail Cloud droplets collide and combine to increase in size. When they become too large to remain suspended gravity pulls them to Earth. FYI: the average water droplet has one million times as much water as an average cloud droplet 13

Precipitation Snow: forms when water vapor changes directly into a solid Sleet: forms when falling raindrops pass through an extremely cold layer and freeze into small ice pellets; only reaches Earth in winter 14

Hail Can be very damaging Forms in cumulonimbus clouds (thunderclouds) Formation: Water droplets hit ice pellets in the cloud and freeze. A strong updraft keeps the hailstones in the cloud where they accumulate more and more layers of ice, until they become heavy enough to fall to Earth 15 A large hailstone, approximately 133 mm (5 1/4 inches) in diameter, that fell in Harper, Kansas on May 14, 2004.

Measuring Rainfall Precipitation in the form of rain is measured with a rain gauge. A straight-sided container with a flat bottom Measured in millimeters or centimeters, or inches in the USA 16 File:Regenmesser.jpg