Download meteorology - Science with Ms. Reathaford!

Chapter 12
• When people discuss the weather, they’re describing the current
state of the atmosphere.
• Atmospheric properties are given to describe weather
conditions, such as:
•
•
•
•
Temperature
Air Pressure
Wind speed
Amount of moisture in the air
Temperature
Heat
• A measurement of how
rapidly or slowly molecules
move around
• Faster molecules have a
higher temperature
• Slower molecules are cooler
• Can be measured in degrees
Fahrenheit, degrees Celsius,
or Kelvin.
• The transfer of energy that
occurs because of a
difference in temperature
between substances
• Heat flows from higher
temperatures of an object to
areas of lower temperature
• The dew point is the temperature to which air must be cooled at
constant pressure to reach saturation
• Saturation is when the air holds as much water vapor as
possible.
• Without saturation, condensation cannot occur.
• Condensation occurs when matter changes state from a gas to a
liquid.
• In general, as you go up in the troposphere, the temperature
goes down 10 degrees C for every 1000m increase in altitude.
• Lifted Condensation Level (LCL) is the height at which
condensation occurs.
• The height of LCL often corresponds to the base of clouds.
• Temperature is proportional to pressure
• Higher temperature  higher pressure
• Lower temperature  lower pressure
• Temperature is inversely proportional to density
• Higher temperature  lower density
• Lower temperature  higher density
• There is an exception to this rule, known as temperature
inversion, because the temperature increases with height in an
atmospheric layer
Wind
Relative Humidity
• Cool air is more dense than warm
air so it sinks, forcing warm air
upward. The differences in density
and pressure causes wind
• Ultimately, wind is a rush of air
that makes an attempt to relieve
an imbalance between warm (less
dense) air and cool (more dense)
air
• Wind is faster and stronger at
higher altitudes where there are
fewer structures to provide friction
• The ratio of water vapor in a
volume of air relative to how much
water vapor that volume of air is
capable of holding
• Expressed as a percentage
• 100% is holding all of the water
vapor it can (saturated)
• 50% is holding half of the water
vapor it can
• Varies with temperature
• The warmer the air, the more
moisture it can hold because there
is more space between the air
molecules
• Warm (less dense) air rises and cold (more dense) air sinks.
Clouds will form when warm, moist air rises, expands, and cools
in a convection current.
• As the air reaches its dew point, the water vapor in the air
condenses around condensation nuclei (small particles in the
atmosphere around which cloud droplets can form), they
combine with millions of others to form a cloud.
• Clouds can also form when wind encounters a mountain and has
no choice but to go up. This is referred to as orographic lifting.
• When two different air masses meet, this too can create uplift
and clouds will form (warm air is pushed up).
• How quickly an air mass cools is based on its resistance to rising
(stable air stays put). This is referred to as stability.
• The energy that is transferred to the gas during the change of
state is stored in the water vapor and will not be released until
condensation occurs. This is referred to as latent heat.
• Within a cloud, the process by which cloud droplets collide and
join together to form a larger droplet is called coalescence. As
this process continues, the droplets become too heavy and
gravity eventually takes over… leading to precipitation.
• Clouds are generally
classified according to
a system developed in
1803.
• Low clouds form below
2000m
• Middle clouds form
between 2000m and
6000m
• High clouds form above
6000m
Clouds are named based on
their shape:
1. Cirrus – Wispy, stringy
clouds
2. Cumulus – white, puffy
clouds
3. Stratus – layered sheets of
clouds
4. Nimbus – gray rain clouds,
low in the sky
5. Fog – cloud that forms in
contact with Earth’s surface
When cloud droplets collide and
join together to form a larger
droplet it is referred to as
coalescence
Once droplets become too heavy,
they fall to the Earth as
precipitation.
Precipitation is all forms of water
that fall from the clouds.
Four main types:
1. Rain
2. Snow
3. Sleet
4. Hail
Meteorology is the study of atmospheric phenomena.
• Weather – the current state of the atmosphere
• Climate – long-term variations in weather for a particular area;
average weather over a long period of time
A Question of Balance
• The surface of the Earth heats up unevenly by the sun since it is
curved and sits on its axis
• Those areas that receive the majority of the sun’s radiation over the
year are warmer
An air mass is a large body of air
that takes on the characteristics of
the area over which it forms.
• The area from which air
masses form comes from is
called a source region
• Air masses are either
Maritime (form over water) or
Continental (form over land)
• Air masses are also either
tropical (warm) or polar (cold)
• So when they are classified
they are labeled as “maritime
tropical,” which tells us where
they form and their
temperature
Air Mass Modification – when air
mass moves, it starts to acquire
some of the characteristics of the
new surface beneath it. Essentially,
they transfer heat from one location
to another
Coriolis Effect – the rotation
of the Earth causes moving
particles such as air to be
deflected to the right in
the northern hemisphere
and to the left in the
southern hemisphere
Doldrums
• primarily windless zones at the
equator (they are calm and
sometimes light)
Trade Winds
• occurs between the equator
and 30oN and S latitude
Prevailing Westerlies
• flows between 30o and 60o N
and S of the Equator
Polar Easterlies
• between 60o and the N and S
poles
Jet Stream
• narrow bands of fast, highaltitude, westerly winds
• Narrow region separating two air masses of different densities.
• Typically, air masses have different temperature and moisture.
Types of Fronts:
1.
Cold Front – Where cool dense air meets warm air, moving it steeply
upward
•
•
•
2.
Fastest of all front systems
Move furthest while maintaining
identity
Can result in clouds, showers and storms
Warm Front – Where warm air replaces cooler air and gradually rises
•
•
•
Tend to move slowly
Less violent than cold front
Can result in extensive
cloudiness and precipitation
3. Stationary Front – When a front does not move, or barely
moves as the differences between the air masses are small. Air
masses “stall”
•
Patterns similar to a warm front
4. Occluded Front – When a rapidly moving cold air mass
overtakes a warm front wedging the warm air upward
•
Can cause precipitation
Rising air is associated with low pressure
Sinking air is associated with high pressure
This movement combined with the Coriolis effect results in rotating pressure systems.
High-Pressure Systems
Sinking air reaches the surface and spreads away from the center
The Coriolis effect causes it to spin in a clockwise direction in the northern
hemisphere
Associated with fair weather
Low-Pressure Systems
As air rises, there is a flow of air into the center to replace it.
The Coriolis effect causes it to rotate counter-clockwise in the northern hemisphere
Associated with clouds and precipitation
Thermometer – device used to measure
temperature
Barometer – device used to measure air
pressure
Anemometer – device used to measure
wind speed
Hygrometer - device used to measure
relative humidity
Ceilometer – device used to measure the
height of cloud layers and estimates the
amount of sky covered by clouds
To make accurate forecasts,
meteorologists must gather
atmospheric data at heights up
to 30,000m
At present, the instrument of
choice is a balloon-borne
package of sensors called a
Radiosonde
• Sensors measure temperature, air
pressure, and humidity.
• Readings are constantly sent back
by radio signal, for tracking
• Provide accurate snapshots of
atmospheric conditions
• Very expensive
The term radar stands for “radio detecting and ranging”
Weather Radar – radio waves detect where rain is
falling at any given point.
• range of 400km in diameter
Doppler Radar
• Doppler Effect – the change in wave frequency that
occurs in energy as the energy moves toward or
away from an observer.
• Used to plot the speed at which raindrops move
toward or away from a radar station
• Provides good estimation of wind speeds
associated with precipitation areas
• Provides a distinct advantage over conventional
radar systems
One of the main uses of
satellites in orbit around Earth
is to observe weather.
Mounted cameras take photos of
Earth at regular intervals.
Able to track clouds, but not
necessarily precipitation
Infrared Imagery
• Detects differences in thermal
energy, which are used to map
either cloud cover or surface
temperatures
• Objects that radiate warmth at
slightly different frequencies show
up as different colors
• Allows meteorologists to determine
the temperature of a cloud, which is
useful in detecting strong
thunderstorms and their potential to
produce severe weather
Station Model – a record of weather data for a particular site at
a particular time
• Allows meteorologists to fit a large amount of data into a small space
• Gives meteorologists a uniform way of communicating weather data
(Ex)
Isobars – pressure
Isotherms - temperature
• Meteorologists use isobars, isotherms, and station
model data to analyze current weather conditions
before they can move on to forecasting.
• Isotherms help identify frontal systems
• Isobars help identify pressure systems
Short-Term Forecast:
• Digital Forecast – relies on
numerical data from
computers, satellites, and
models
• Analog Forecast –involves
comparing current weather
patterns to patterns that
took place in the past
(assuming that the weather
will behave in a similar
fashion)
• The more data collected,
the more accurate the shortterm forecast will be
Long-Term Forecast:
• Usually can’t be trusted if
they are more than three
days out
SEVERE WEATHER
THUNDERSTORMS
At any given moment, nearly 2000 thunderstorms are occurring around the
world.
1. Abundant source of moisture in the lower levels of the atmosphere
2. A mechanism to lift the air to allow moisture to condense and release
latent heat
3. The portion of the atmosphere where clouds grow must be unstable,
to allow rising and cooling of air necessary to stop the cloud growth.
Conditions of Formation of
Thunderstorms
Lightning is electricity caused by the rapid
rush of air in a cumulonimbus cloud.
A lightning bolt forms when friction
between updrafts and downdrafts
separates electrons from some of their
atoms creating positive ions and negative
ions.
This creates regions of air with opposite
charges.
A channel of negatively charged air moves
toward the ground and a channel of
positively charged ions rushes upward from
the ground to meet it, creating an
illumination called lightning.
Lightning
■ Lightning heats the air to around 30000°C, which is five times
HOTTER than the surface of the Sun!
■ As the super-heated air rapidly expands and contracts, it creates
a sound called thunder.
■ Because sound travels more slowly than light, you typically see
lightning before you hear thunder even though they are actually
generated at the same time!
Thunder
■ Hail: precipitation in the
form of balls or lumps of
ice, occurring most
frequently in the central
US. Most common during
Spring
■ Flood: occur when a
thunderstorm moves
slowly over one location,
dumping all its rain in one
place
Other Dangers of
Thunderstorms
The rising, moist updrafts and the
falling, cool downdrafts form a
convection cell that produces the
wind gusts that are associated
with thunderstorms.
Sometimes, these convection
cells become more drastic in size
and form what is referred to as a
supercell.
Supercells are deep-rotating
updrafts with potential to
generate severe / extreme
weather.
Supercells
Tornadoes
A tornado is a violent,
whirling column of air
in contact with the
ground
■ Before reaching the ground,
it’s called a funnel cloud
■ Often associated with a super
cell, which is the most severe
thunderstorm
Characteristics
■ Air is made visible by dust and
debris drawn into the swirling
column or by the condensation
of water vapor into a visible
cloud
■ Over the area it covers, few
storms on Earth can match a
tornado’s violence
■ A tornado is caused by wind
shear, which is when wind
speed and direction change
suddenly with height – causing
a horizontal rotation near the
Earth’s surface
■ An updraft can shift this
rotating air into the vertical
Formation
position
■ As updrafts speed up the
rotation, air pressure in the
center decreases, creating a
pressure gradient between the
inside and outside of the
tornado – creating violent winds
■ Although devastating, tornadoes
typically last only a few minutes.
The Fujita Tornado Intensity Scale ranks tornadoes according to the path of
destruction, wind speed, and duration
– Bottom range is F0 - winds up to 118km/hr
– Top range is F5 – winds of more than 500km/hr
– Most tornadoes do not exceed the F1 category
Tornadoes can occur at any time and at any place, although some times and
locations are more conducive to their formation
– Most occur in the spring, during late afternoon and evening, when
temperature differences are greatest
– Most common in the central United States due to colliding cP and mT
air masses
– More than 700 tornadoes touch down each year in the United States
Classification and Distribution
TROPICAL STORMS
The most violent storm on Earth is within the calm, sunny tropics.
Tropical Cyclones
■ Large, rotating, low-pressure storm
■ The strongest tropical cyclones are known in the United States,
Mexico, and the Caribbean as hurricanes
■ In the western Pacific Ocean, these storms are referred to as typhoons
■ People living near the Indian Ocean refer to these storms as cyclones
Characteristics
Tropical Cyclones / Hurricanes
require two basic conditions to form:
1. Abundant supply of warm water
2. Some sort of disturbance to lift
the warm air and keeprising it
These conditions exist in all tropical
oceans except the South Atlantic
Ocean and the Pacific Ocean west of
the South American coast because
waters in these areas are cooler.
Formation
1. As water evaporates from the
ocean, heat is stored in the form
of latent heat
2. The heat is released as the air
rises and water vapor condenses
into clouds and rain
3. The rising air creates a low
pressure system at the ocean
surface and more air moves in to
replace it
4. The Coriolis Effect causes the air
to turn counterclockwise,
resulting in the rotation of the
cyclone
5. Air moving towards center rises
and rotates faster, further
decreasing pressure in the center
Tropical Disturbance - beginning stages, causes the air to rise
Tropical Depression - when a disturbance acquires a cyclonic rotation around a
center of low pressure
Tropical Storm - when winds of the depression reach 65km/hr
Hurricane - when winds of the storm reach 120km/hr
Eye - calm center of the storm
Eyewall - a band immediately surrounding the eye where the winds are the
strongest
Stages of a Hurricane
■ Saffir-Simpson Hurricane
Scale - classifies hurricanes
according to wind speed, air
pressure in center, and
potential for property damage
■ Ranges from Category 1 (74
mph winds) to Category 5
(+155 mph winds)
■ At Category 3 it is classified as
a major hurricane
Classifying Hurricanes
■ Hurricanes cause a lot of
damage, especially along
coastal areas
■ Much of the damage results
from violent winds
■ Strong winds move onshore
and are responsible for
another major threat, storm
surges, which is when winds
move a mound of water over
land
■ Storm surges can be as high
as 6m above normal sea
level
Hurricane Hazards
■ Heat released through
condensation of vast
amounts of water vapor fuels
hurricanes. This
condensation produces great
amounts of rain
RECURRING WEATHER
Persistent or repetitive weather can negatively affect agriculture, transportation,
and recreation.
Floods
■ Natural occurrence in which water spills over the sides of a stream’s
banks onto land
■ Can be caused by long thunderstorms, hurricanes, and mild storms
that stay in the same area for extended times
■ The main cause of thunderstorm-related deaths in the United States
each year
Droughts
■ A drought is an extended period of below average rainfall
■ Caused by large high-pressure systems in an area for an extended
period of time
■ One of the most extreme droughts occurred during the 1930’s in the
central United States (Dust Bowl)
Heat Wave
Cold Wave
■ A Heat Wave is an extended period
of above normal temperatures
■ A Cold Wave is an extended
period of below normal
temperatures, which can be
caused by high pressure systems
of continental polar (cP) or Arctic
air
■ Can result from long term highpressure systems with few clouds
to block the blazing Sun
■ System barely moves because the
air currents guiding the highpressure system are weak
■ Increases rate of humidity, leading
to a higher heat index (how warm it
feels to the human body)
■ Can cause heatstroke, sunstroke,
and/or death
■ Wind-Chill: because wind
transports heat away from the
body, the temperature may feel
cooler than it really is
■ The wind-chill factor tells how
cold it actually feels to the human
body