Download ES.13 - Smyth County Schools

ES.13
The student will investigate and understand that energy transfer between the sun and the earth
and its atmosphere drives weather and climate on Earth.
Electromagnetic radiation from the sun is primarily short wave (due to the 5000°C temperature of
the photosphere) – visible and ultraviolet wavelengths. The atmosphere is largely transparent to
these wavelengths, except for ozone which absorbs ultraviolet radiation. Thus incoming solar
radiation does not heat the atmosphere. It can be reflected or scattered by clouds, dust, and
aerosols in the troposphere, but on a clear day, most of the visible light reaches the surface of the
earth, where it is absorbed, thus raising the temperature of the earth’s surface. As the earth
warms, it radiates infrared (longer waves due to Earth’s lower temperature) which is absorbed by
carbon dioxide and water vapor in the troposphere (lowest layer of the atmosphere), thus raising
the temperature of the air. This is known as the greenhouse effect which drives the weather and
climate on Earth.
Key concepts include
a) observation and collection of weather data
If instrumentation is lacking for the actual collection of data by the students, the Internet is a most
valuable resource for weather data. Please explore the links listed at the end of this section.
Air temperature – varies from day to night with conditions. It is instructive to compare temperature
variation on a clear day and night with that of a cloudy day and night. The clear day will show
dramatic differences in temperature over a 24 hour period, whereas the cloudy day and night will
have even temperatures due to reflection of incoming radiation by clouds and the trapping of
outgoing radiation by clouds. Changes in air temperatures can also mark the passage of a cold or
warm front.
Dew Point Temperature – the temperature at which air is saturated with water vapor; the
temperature at which condensation can occur. The dew point temperature will always be equal to
or less than air temperature. When the two are equal, there is usually some form of condensation
or precipitation occurring. The higher the dew point temperature, the greater the amount of water
vapor in the air. Generally speaking, dew point temperatures greater than 59°F will result in a
prediction of thunderstorms.
Air Pressure – low pressures usually indicate rising air and the passage of a front or a storm
system. It is measured with a barometer or a barograph. High pressures are associated with clear
skies and sinking air. The official units are millibars (950 – 1030 mb), however, local weather often
uses inches of mercury. It can be interesting to record air pressure each day at the same time and
note the kind of weather that occurs. On maps, air pressure is used to draw isobars (lines
connecting equal air pressure readings) and show high and low pressure systems.
Please see the power points on convection and global wind belts for the relationship between air
pressure systems and wind.
Relative Humidity – percent of saturation. This is measured with a psychrometer or a hygrometer.
This is how close the air is to being full of water. As temperature increases, the air can hold more
water vapor. The closer the air temperature and the dew point temperature, the higher the relative
humidity. Relative humidity can be increased as water evaporates or as the temperature drops.
Early during a storm, falling precipitation evaporates. Since evaporation cools the air, as a storm
begins, relative humidity increases. During a snowstorm, snow will not begin to accumulate until
the air near the ground is saturated. It is instructive to plot air temperature, dew point temperature
and relative humidity through a storm. This data is readily available on the Internet for any storm,
but you must plan ahead to collect it. There is an example in the power point on condensation and
precipitation.
Wind speed – measured with an anemometer – The greater the wind speed, the greater the
pressure gradient (how much the air pressure changes over a distance). Thus hurricanes that are
intense low pressure systems have very high wind speeds. On weather maps, this can be
correlated with closely spaced isobars (lines connecting points of equal air pressure).
Wind direction – In the Northern Hemisphere, if you stand with the wind at your back, the high
pressure will be to your right and the low pressure will be to your left. This is the Law of Buys
Ballot and is due to the fact that wind blows from a high to a low and turns to the right in the
Northern Hemisphere. It is fun to go outside and try this on a really windy day. It is good to check
ground observations against the flag in front of your school for wind directions. The flag is usually
more accurate. Use a compass to orient yourself, then come in and get the most recent weather
map on the computer to see if your predictions are correct. Generally speaking, the wind will be
from the SW ahead of a cold front and from the NW behind a cold front. In the Northern
Hemisphere, winds blow clockwise around a high pressure cell and counterclockwise around a low
pressure cell.
Please see the power points on local winds, single convection cell, and on global wind belts.
Sky cover – percent of the sky that is covered by clouds. This is difficult to accurately measure, but
crudely keeping track over the course of a day, along with air pressure can be instructive. As air
pressure decreases, cloudiness should increase.
Cloud type – There are ten official types of clouds, each type indicates something about the
weather, and many students enjoy learning to recognize the types. Please see the S’COOL
website listed below for a fun project run by NASA that lets students do “ground truthing” for
weather satellites and participate in real research. It also has lots to say about different kinds of
clouds. Clouds are classified on the basis of shape and altitude. There are three shape
categories: cumulus is fluffy and puffy like cauliflower; cirrus is high, thin wispy clouds; stratus is
solid layer clouds. On a more simplistic note, clouds can be divided into two categories: vertical
development clouds (cumulus and cumulonimbus) and horizontal development clouds (layers).
Vertical development clouds form when air rises straight up over a mountain range, ahead of a cold
front in a thunderstorm, or just from ground heating on a clear sunny day. Layer clouds are
associated with large scale storm systems such as winter storms (mid latitude lows). As the storm
approaches, the layers get lower and lower, usually over a 24 hour period ahead of the storm.
Again observation of the type of clouds each day and comparison with air pressure and
precipitation patterns can be instructive.
Please see the power point on cloud classification.
Precipitation type & amount – Precipitation is falling water. Condensation is suspended water
drops or ice crystals. When water drops (or ice crystals) become large enough (by further
condensation or coalescence of colliding droplets) to overcome updrafts within the cloud, they will
fall as precipitation. Initially, since air under clouds is not saturated, precipitation will evaporate &
will not reach the ground until the air is close to saturation. Rain gages measure the amount of
precipitation (official ones melt the snow to get water equivalents). Precipitation types include rain
(warm atmosphere), snow (totally cold atmosphere), sleet (thick layer of freezing air that freezes
raindrops), freezing rain (thin layer of freezing air, water becomes super cooled and freezes on
impact) and hail. Hail is associated with thunderstorms, when raindrops freeze when carried aloft
by turbulence within the cloud. The ice forms in concentric layers, each layer representing a trip up
and down inside the cloud.
Please see the power point on condensation and precipitation.
b) prediction of weather patterns
The suggestions above for weather observations can lead to simplistic prediction models.
Please see the power point on mid-latitude lows.
Here in the mid latitudes, weather comes in the form of mid-latitude lows which track generally from
west to east across the United States. It usually takes about a week for one of these systems to
form and move into our area. It is an intense low which follows the boundary between cold polar
air and warmer subtropical air to the south. With the counterclockwise circulation, a warm front is
ahead of the storm and moves toward the north while a cold front trails to the south, generally
moving from west to east, separating cold air behind the front from warm air ahead of the front.
Layer clouds are associated with the warm front and the storm center while vertical development
clouds are associated with the trailing cold front.
Thus, weather prediction depends to a great extent on the prediction of specific storm tracks. I like
to teach this during the winter, when students are vitally interested in weather patterns. Generally
speaking, storm tracks follow the path of the jet stream. Specifically, a low pressure cell will move
at one half of the speed of the steering winds in the direction of the steering winds. The steering
winds are located at the 500 mb level in the atmosphere directly above the low. There are
numerous web sites which give wind speed and direction at this air pressure level, and students
can check these each day for the location of a low and predict where it will be 24 hours later. Then
come back and check. It really works!
Prediction of winter storms can be very tricky here in Virginia as we all know. In general, if the
storm center passes to the north of us, we will be in the warm sector and will experience rain. If it
passes to the south of us, we may be in the cold sector and can experience snow. If it passes
directly over us, we usually get a wintry mix. There can be great variation even within one county.
The pattern to look for on weather maps is a dip in the jet stream to the south (trough), especially if
it extends down to the Gulf of Mexico. This will place us in the storm track. If the storm turns up
the coast, it becomes a Nor’easter, from the northeast winds preceding the storm center, which
bring moisture in from the ocean.
The importance of storm track can be simplistically illustrated by comparing summer weather and
winter weather in Virginia. In the summer, we actually experience cold front weather because the
storm passes well to the north of us, so we get the trailing cold front and thunderstorms. In the
winter, the storms pass south of us or over us more often and so we experience the warm front
leading the storm, and we get steady precipitation for a longer period of time.
c) severe weather occurrences, such as tornadoes, hurricanes, and major storms
I strongly believe that the most important thing we can do for students with regard to severe
weather is to educate them on the proper safety precautions for each kind of storm.
Thunderstorms- These storms form when warm humid air rises straight up. As the air rises, it
cools and water vapor condenses, releasing heat into the developing cloud, causing it to further
rise. The most severe thunderstorms can overshoot up into the stratosphere, where there is
normally no weather. As precipitation falls, it cools the air which sinks and flows out ahead of the
thunderstorm in a gust front. This has the effect of cooling the ground ahead of the storm, so each
thunderstorm contains the seeds of its own destruction. By giving lift to the atmosphere, the gust
front also sews the seeds for the next storm in the squall line. Thunderstorms usually cover ten
miles or so and are in existence for maybe an hour. Hazards from these storms include heavy
rain, flash flooding, hail, high winds, and most importantly, lightning. Most student have very little
idea of how to be safe from lightning, which is one of the major hazards in this area (Perrowville
Road in Forest is a lightning hot spot – rivaling Florida – for cloud to ground lightning). Students
should stay off the phone and the computer, sit well away from the TV and windows, and stay out
of the bathroom. Wireless devices are OK. Outside, they should seek shelter, not under a large
tree or overhanging cliff. Automobiles are good shelter. If you are about to be struck by lightning,
your hair will stand on end (even very long hair), and you should crouch immediately and minimize
contact with the ground. Severe burns occur where lightning exits your body to the ground.
Tornadoes – These storms form in thunderstorms, in particular, supercell thunderstorms.
Supercell thunderstorms are very large and long lasting (day or more) and contain a rotating
column of clouds called a mesocyclone in the core of the cumulonimbus cloud. Tornadoes
generally move from the SW to the NE and are usually on the ground for less than a half hour. In
some instances, however, very large ones can be on the ground for hours. Besides the usual
thunderstorm hazards, the greatest hazard to life from a tornado is flying debris. I have personally
seen a soda straw driven into a telephone pole in a relatively small tornado. Buildings are
destroyed by high winds (up to 300 mph) in the funnel, not by the low pressure as previously
thought. A tornado watch means that conditions are right for a tornado to occur; a tornado warning
means that one is on the ground and you should take cover immediately. In contrast to a lightning
storm your car or a trailer (even a double wide on a foundation) is a very bad place to be in a
tornado. The SW corner of a basement is best, with a blanket pulled over you for protection from
debris. If you do not have a basement, an interior closet or hallway, or even laying flat in a bathtub
can provide protection – again with a blanket pulled over you. If you are outside, in a trailer or a
car, get out, lay flat in a ditch on your stomach and protect your head. An underpass is a very bad
place to be; winds become funneled at an underpass and increase in wind speed. Do not try to
outrace a tornado or drive away from one; it is virtually impossible to really tell where they are
going and you can drive directly into its path. We occasionally get tornadoes in this area;
encourage students to make a plan with their families before an emergency is on them.
Hurricanes - These large tropical storms consist of concentric rain bands of thunderstorms
rotating around a calm eye. The fastest wind speed and the most rain occur in the NE quadrant of
the storm and the eye wall. The central low pressures in hurricanes are the lowest recorded on
Earth (850’s in some large category 5’s). These form over tropical oceans and obtain their energy
from a cycle of evaporation and condensation. For full development, they require water
temperatures above 80°F down to depths of 200 m. and little or no wind shear in the upper
troposphere. Their rotation is counterclockwise in the lower half of the storm and switch to
clockwise around an upper level high at the top of the troposphere. They travel from the SE toward
the NW, hooking around to the N or NE as they encounter the Prevailing Westerlies. There is no
part of the eastern seaboard or the Gulf of Mexico coastline that has not experienced one of these
storms. Most damage and loss of life in hurricanes comes as a result of storm surge (not wind),
the rising level of water pushed onshore by the winds, aided by tides and the low pressure in the
eye itself. Most insurance policies only cover falling water and/or wind, so flood insurance for rising
water is essential if you live in a hurricane prone region. Inland, hazards are those associated with
thunderstorms with the added problem of extensive flooding (Hurricane Camille, 1969) associated
with large amounts of rain as the storm dissipates over land. With today’s computer technology
and software and better observational tools, the landfall point of hurricanes can now be predicted
accurately several days in advance. There should be zero loss of life because the only reasonable
precaution for those living or vacationing in coastal areas is evacuation.
d) weather phenomena and the factors that affect climate, including radiation and convection
Weather is the condition of the atmosphere at a single time and place. Climate is the long term
average weather at a location. Thus measures of climate include some measure of temperature
and some measure of moisture.
Weather phenomena I interpret as including warm and cold fronts, cyclones and anticyclones (lows
and highs), and air masses. These can be examined in detail in any decent Earth Science
textbook. Generally speaking, for 3 days out of 4, a place experiences air mass weather, so air
masses are the primary control of climate.
In general, climate is correlated with latitude. There is a temperature gradient from the equator to
the poles, so climate becomes more seasonal and cooler the higher the latitude. Rising air
produces clouds and precipitation. Sinking air produces clear sky and evaporation. Around the
equator there is a belt of rising air, so this is a very wet climate. Around 30° N and S are the
subtropical high pressures, where air sinks. Most major deserts are located in this dry belt
centered on 20° N and S (Sahara, Namibian, Australian, etc.). The mid latitudes from 30 to 60° N
and S are the belts most affected by mid-latitude lows. Precipitation comes with the passage of
these storms; since their tracks change seasonally, these latitudes can have a strong seasonal
component to their precipitation. Polar Regions are largely affected by the sinking air of the polar
high pressure cell. Consequently, Polar Regions can be considered to be desert-like in their
precipitation patterns.
This general pattern is modified locally as a result of wind direction, the presence of mountain
ranges and large bodies of water. When air has to rise over a mountain range, the windward side
(toward the wind) will receive precipitation from the rising air and the leeward side where the air
sinks will experience drying conditions leading to the formation of a rain shadow desert. The coast
ranges of the western U.S. and the Sierra Nevada Mountains are prime examples. When the wind
blows off of the ocean as in the western U.S., moist air tends to moderate the climate leading to
more even temperatures, both seasonally and daily. This is a maritime climate. Further inland, the
moisture has been precipitated from the air. Dry air leads to a large temperature range, both daily
and seasonally, and creates a continental climate, as in the central and eastern portions of the
United States. The effect of large bodies of water can be seen on the eastern shores of the Great
Lakes which experience milder conditions than the western shores. Going to higher altitudes in
mountain ranges is the equivalent of going to higher latitudes. Above the tree line on most
mountain ranges a tundra type ecosystem exists.
Impacts of radiation have already been discussed in many sections. However, convection has only
been alluded to in other places. Convection is one of the most important physical concepts in
Earth Sciences. Convection is the vertical and horizontal motion of a substance in response to
unequal heating and the resultant density differences. Convection in Earth’s outer core is
responsible for Earth’s magnetic field. Convection in the mantle drives plate tectonics and is
responsible for volcanic activity and mountain building. Convection in Earth’s atmosphere is
responsible for wind and weather. Convection in the sun transfers energy from the core to the
photosphere. The SOL’s refer to convection in the ocean, however, this is misleading. For
convection to occur a substance must be heated from the bottom and the oceans are largely
heated from the top by solar radiation. The oceans, like everything, are organized into layers
based on density, and vertical motions do occur as a result of density differences, but technically
this is not convection (see ES.11). For a full discussion of convection and its relationship to wind
and weather, please see the attached three power points: Single Convection Cell; Local Winds,
Cyclones, and Anticyclones; and Global Wind Belts.
Relevant Internet Resources for ES.12 & ES.13
http://www.erh.noaa.gov/rnk/
Can get local weather forecasts, data from last 2 days in tabular or graphic form
http://www.ametsoc.org/amsedu/dstreme/
Web page that supports an AMS teacher education program (a free grad course), also has
many resources that can be used in the classroom
http://www.nhc.noaa.gov/
Want to follow a hurricane? – This is the place
http://asd-www.larc.nasa.gov/SCOOL/
An opportunity for your students to participate in real atmospheric research
http://ww2010.atmos.uiuc.edu/(Gh)/home.rxml
Lots of maps and background, up to date & hourly archives for weather maps
http://rst.gsfc.nasa.gov/Sect14/Sect14_1c.html
Good overview of atmospheric circulation