HUMIDITY _ by pengxiuhui

VIEWS: 112 PAGES: 38

									 HUMIDITY &

• Humidity is the amount of moisture or
    water vapor present in the air.
•   "Relative Humidity" is the ratio of water
    vapor present in a given volume of air at a
    given temperature to the most water
    vapor the air can hold, expressed as a
•   Example: 40% relative humidity means
    40% saturation of the air.
Humidity & Weather
• Humidity depends mainly on where the air has
  come from.
  – Air coming from the ocean = likely to contain a lot of
    water vapour = humid.
  – Air coming from a continent = likely to be dry = low
• The amount of water that air can hold is
  dependent on its temperature.
  – The hotter the air, the more moisture it can hold.
  – This makes water vapour an interesting greenhouse gas.
  – When the humidity is 80% or higher, there is enough
    water in the air for rain or snow.
• When no more water can evaporate
  into the atmosphere, the air is
  saturated (full of water molecules)
  and is said to be at 100% relative
• The rate of evaporation decreases as
  the moisture content of the air
  increases and approaches saturation.
  Humidity decreases with altitude.
• Proximity to large bodies of water
  and the prevalence of moisture-
  bearing winds favor high humidity.
Measuring humidity

• Actual humidity is the actual amount of
 moisture present per volume of air.
• Relative humidity is the ratio of the
 amount of moisture that that particular
 temperature of air can hold before
 condensation begins.
Problems with Humidity:

Too much humidity       Too little humidity
• Mold                  • Static & sparks
• Musty smells          • Breathing problems
• Allergy, asthma       • Damage to furniture
• Damage to furniture

                       Will there be rain?

Time: 3 pm
Air temperature: 25° C
Saturation quantity: 24 grams/cubic
Actual quantity: 6 grams/cubic meter
Relative humidity = 25%

                       Will there be rain?

Time: 12am
Air temperature: 4° C
Saturation quantity: 6 grams/cubic meter
Actual quantity: 6 grams/cubic meter
Relative humidity = 100%


Humidity and Rain
• 100% humidity = the air is completely
 saturated and is holding its maximum
 amount of water vapor.
 Weather: foggy with some drizzle or mist.

• The air DOES NOT have to be fully
 saturated to have rainfall. When it's
 raining, it just means the raindrops have
 become heavy enough to where they're
 heavier than the surrounding air
                          Convectional Rain

 -High temperature
 -High humidity
 -Does not last long

Watch this video: Costa Rica


                         Relief /Orographic Rain

Watch this video: New Zealand
                                                                                                          Andes Mtns


                                                                                          Pacific Ocean

                                Atacama Desert, Chile
                         Frontal Rain

  • 2 air masses of different temperature,
    humidity and density meet
  • Air masses can spread over hundreds of
    square km.

  Watch this video: Europe
                           Warm Front

When warm air approaches
a) warm air advances and rises over cold air, which is denser
and heavier.
b) as warm air rises it is cooled and condensation takes place.
c) continued condensation leads to the growth of water droplets
d) when heavy enough these fall as rain.
e) Light rain falls steadily over a wide area for several hrs.
        Warm front brings fog to Galveston,
                heat to Houston

A few days ago we were measuring snow in southeast Texas. Today,
you're probably tempted to turn the air conditioner back on.
A warm front is blowing in a rush of warm, muggy air. Temperatures are
soaring into the mid 20s, even with a cloudy sky.
The water temperature in the Gulf of Mexico is only 17°C now. As the
warm, moist air blows over the cooler water, the moisture near the
ground is condensing and forming thick sea fog along the coast. A Dense
Fog Advisory is effect for the coastal counties until midnight. The
visibility on the Galveston webcams has been less than a mile all day.
Meantime in the northern part of the state, temperatures are about 40°
colder. A cold front should move through southeast Texas while you're
sleeping tonight. You'll need the jackets again on Wednesday...and maybe
your heater too.
                                Dec 8, 2009
Cold front : when cold air approaches
   and replacing warm air

a) heavier cold air advances and
   pushes beneath a body of lighter
   warm air.
b) Forcing the warm air to rise quickly
c) Water vapour condenses
d) Cumulonimbus clouds are formed
e) Heavy rain falls over a small area
   for a short duration.
        Cold front may bring tornadoes and
                 flooding (Florida)

          EMILY FUGGETTA, Posted: Wednesday, December 2, 2009 12:15 am
A Tuesday night cold front may leave locals scrambling for sweaters and
rain boots through Thursday.
Storms developing ahead of the cold front may put a large area of north
and central Florida at risk for severe weather, according to a press
As the cold front moves east, people should be prepared for strong winds,
possible tornadoes and as much as 6 to 8 inches of rain before Thursday
On Tuesday, the National Oceanic and Atmospheric Administration’s
National Weather Service also issued a warning that increased winds will
create large waves and dangerous flooding.
The warning also said that boaters should consider postponing trips into
the Gulf of Mexico until later in the week to avoid dangerous water
conditions. The coastal flood watch will be in effect until Thursday
Monsoon Rain
• The word "monsoon," which comes from
    mausim, the Arabic word for seasons, refers to
    complex seasonal winds that sweep across
    Asia and other areas.
•   In winter Asia's monsoon winds are fairly dry.
•   In summer, however, the winds bring drenching
    rains on which several billion Asians rely.
               January, winter

               Dry season

                Dry winds

    Winds filled w/ moisture

July, summer

 Wet season
Winter MONSOON: (N. Hem.)
Leaves China as NW monsoon
Reaches Spore as NE mons.
Reaches Australia as NW
(N. Hem.)
Leaves Aust as SE mons.
Arrives Spore as SW mons.
Arrives China as SE mons.
From Times Online

                    Changes in the Monsoon Patterns…
June 16, 2008

Monsoon rains arrive early in India
• India’s annual monsoon rains have swept across
    the country about two weeks ahead of schedule,
    their earliest for over a century.
•   The rains are essential for the farms that
    provide a livelihood for 60 per cent of India's 1.1
    billion people.
•   Rainfall over the first two weeks of June was
    more than 40 per cent above average for the
    time of year, good news for consumer hit by
    rising food prices.
• The monsoon usually begins sweeping
 across the subcontinent in early June, but
 rarely reaches New Delhi and the rest of
 northern India before the beginning of
 July. In one district of Assam, at least
 50,000 people had be taken to higher
 ground by rescuers in motorboats and
 rowing boats.
Weakened Monsoon Season Predicted For
South Asia, Due To Rising Temperatures

    ScienceDaily (Mar. 2, 2009) — The South Asian summer monsoon - critical to
agriculture in Bangladesh, India, Nepal and Pakistan - could be weakened and delayed
due to rising temperatures in the future, according to a recent climate modeling study.

                                  Read this article:
        Cloud Seeding
   Even in areas with very low humidity, water is present in the sky and in
   clouds. A rainstorm happens after moisture collects around particles in
   the air, causing it to reach a level of saturation at which point it can no
   longer hold in that moisture. Cloud seeding essentially helps that process
   along, providing "nuclei" around which water condenses. These nuclei can
   be salts, calcium chloride, dry ice or silver iodide, which the Chinese use.
   Silver iodide is used because its form is similar to ice crystals. Calcium
   chloride is often used in warm or tropical areas.
                                                                  The Beijing Weather Modification
                                                                  Office is researching how to prevent
                                                                  rain in the city on Aug. 8, 2008,
                                                                  during the opening ceremonies of
                                                                  the 2008 Summer Olympics. The
                                                                  government has even guaranteed
                                                                  clear skies for the event.
                                                                  They plan to do it by tracking cloud
                      Image courtesy Jurie Maree/
                                                                  formations and causing rain in the
The Chinese government believes launching shells containing       days leading up to the ceremonies.
silver iodide pellets into clouds will curb rainfall and smog,
ensuring clear skies for the 2008 Summer Olympics
                                                                  However, one official admits that
opening ceremonies in Beijing.                                    although cloud seeding could be
                                                                  effective in preventing light rain on
                                                                  Aug. 8, it could not curb the onset
                                                                  of a moderate to heavy storm.
    China's Weather Manipulation Brings
      Crippling Snowstorm to Beijing
By Clay DillowPosted 11.11.2009 at 6:08 pm

 To do so, they've turned to cloud seeding, a controversial
 practice that involves launching (or dropping) chemicals into
 the atmosphere -- silver iodide in China, though dry ice and
 liquid propane also work -- that cause water vapor in the air
 to crystallize at temperatures it otherwise would not. Its
 effectiveness is dubious; while it's generally accepted that it
 works to some degree, it can only increase precipitation by
 20 percent.
 Weather manipulation is actually not as rare as one might
 think. Currently, 24 countries practice some kind of cloud
 seeding. China prefers to deliver its silver iodide to the sky
 via rockets or artillery.
For the residents of Beijing, those armaments deliver all the
headaches that go along with big snowstorms

•   traffic problems,
•   flight delays,
•   cancelled classes
•   highways in and around Beijing were closed
•   many roads were blocked
•   bringing day-to-day commerce to a grinding halt.

• in 2005, a snow melting agent killed 10,000 trees in
  Beijing, and experts worry it could be eroding the city's
  infrastructure. As of last night,


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