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Notes for the week of 10/03/11
Textbook Chapter 9-1
Using a psychrometer to find Relative Humidity
 Remember that Relative Humidity is the comparison of how much water
vapor the air is actually holding compared to how much it could hold at a
given temperature
 The psychrometer has two thermometers. One has a piece of cloth wrapped
around the bulb (wet bulb) and the other has no cloth (dry bulb).
 First, wet the cloth. Then swing the psychrometer for a few minutes (until the
wet bulb temperature stops changing). Read and record the wet bulb and dry
bulb temperatures.
 Subtract the wet bulb temperature from the dry bulb temperature. Write this
number down as wet bulb-dry bulb temperature.
 Look at the Relative Humidity chart on page 256 of the textbook.
 Example Problem
Dry Bulb = 20C Wet Bulb = 17C
Procedure: Subtract wet bulb temperature from dry bulb temperature.
20C - 17C = 3
Use the chart on page 256 of the text,
First look for the number three (3) across the top of the chart. Place your
right finger on it. Then along the left column, find the dry bulb temp of
20C. Place your left finger on it. Move your left finger horizontally (left
to right) and your right finger down. Where the two fingers meet will be
the relative humidity. You should have gotten 74%.
Try it. Dry Bulb = 23C
Wet Bulb = 18C
Show your work here.
Relative humidity = ___________
How do clouds form?
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Key terms: EVAPORATION, CONDENSATION, DEW POINT,
CLOUD DROPLETS
First, EVAPORATION takes place as the Suns energy energizes water
molecules from plants, puddles, streams, rivers, lakes, oceans and soil.
The air (warm, moist air) is less dense than cooler, dryer air, so it rises up into
the atmosphere.
(Cloud formation continued)
 As air rises, it cools and expands (due to less pressure). This is known as
adiabatic cooling.
 At some elevation, the warm, moist air reaches it’s DEW POINT ( the
temperature at which gas turns back to liquid) and CONDENSATION occurs
around tiny particles of dust.
 These tiny droplets of water (known as CLOUD DROPLETS) form clouds
when millions of these droplets cluster together.
 As these droplets bump into one another, the droplets stick together and get
larger. When the get too big and heavy to remain suspended in the
atmosphere, they fall as precipitation.
 Precipitation is either rain, snow sleet or hail.
What determines the type of precipitation that falls?
 Precipitation type, for the most part, depends on the temperature of the
atmosphere the droplets fall through.
 Droplets that fall through temperatures above freezing fall as rain.
 Snow forms when air temperature is below freezing and water vapor turns
directly into snow.
 If snow flakes pass through a layer of warm air and melt, then pass through a
layer of below freezing air, they re-freeze turning into sleet.
 Hail forms in cumulonimbus clouds where air currents are very active. The
water droplets are blown up into the cloud where they freeze, then they fall
down into warmer air nearer the bottom of the cloud where a new layer of
water sticks to the small ice pellet. This mixture of ice and water then blows
back up into the cloud where it freezes again. This process is repeated several
times and the ice pellet grows until it is to large to be suspended by the strong
winds that are present inside of the cumulonimbus cloud. At this point, the ice
(known as a hailstone) falls to earth.
What are the three main types of clouds?
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CUMULUS clouds are the puffy white clouds that often accompany fair
weather. They can grow into much taller and more troublesome
cumulonimbus clouds that bring heavy rain, strong winds, thunder and
lightning, even hail and tornadoes.
STRATUS clouds are like blankets that cover the sky. These clouds
accompany dreary weather and can bring rain showers.
CIRRUS clouds are found at really high altitudes. They are the thin, wispy
ice clouds that usually occur on nice days. Watch for these clouds to bunch
up and increase in number, this indicates coming storms.