Download WEATHER FORECAST Clear skies ahead!

WEATHER FORECAST
Clear skies ahead!
With its 72-meter wingspan and 2,400 kg weight, Solar Impulse
(HB-SIB) is a light stick that has to navigate the skies while
steering clear of strong atmospheric turbulence that could
damage it or make it lose control. This explains why the
weather and accurate forecasts are key for the success of this
unprecedented challenge!
This worksheet will give you the necessary information to
understand major weather events, to decode the principles of
forecasts, and to build the components of your weather station!
Project: EPFL | dgeo | Solar Impulse
Writing: Marie-Noëlle Kaempf
Graphic design: Anne-Sylvie Borter, Repro – EPFL Print Center
Project follow-up: Yolande Berga
1/14
A PRECEDENT...
In March 1999, the team aboard the Breitling
Orbiter 3 completed the first non-stop flight around
the world in a balloon. This performance was not
only the result of the efforts of Bertrand Piccard
and Brian Jones’s team, but also of the work of
meteorologists Pierre Eckert and Luc Trullemans!
Indeed, a balloon without an engine has to be at
the right altitude, sometimes down to the last
meter, to be carried by the winds that will take it in
the right direction. Now, Bertrand Piccard and his
team are building on this experience to tackle Solar
Impulse’s new challenge.
WHAT EXACTLY IS METEOROLOGY?
Meteorology is the science that studies the various parameters that influence the weather (wind, precipitation, temperature, etc.) and develops models to predict them.
Since the beginning of human history, weather forecasts have had to meet specific needs. Farmers,
for example, have to plant and harvest at the right time, and sailors have to avoid open ocean storms.
The earliest meteorological records go back more than three millennia and were found in China. A long
time elapsed before mankind was able to go beyond forecasts that were only based on the observation of local winds and clouds. Indeed, without a way to observe the weather at the scale of an entire
continent, it is impossible to conclude that cloud formations move across large spatial expanses and
Edmund Halley (1656 - 1742) is remembered for the astronomical calculations that allowed him to predict the return of the comet named
after him. Fascinated by astronomy, he started to travel to discover
the southern sky. As a result, he took a liking to the observation of the
ocean and its currents. He invented the diving bell, which allows two
persons to submerge over 10 meters under water and to stay in it for
several hours.
The diving bell’s air was renewed by lowering empty barrels that were
opened under the bell. It was used, for example, in the context of
uncovering works and for repairing dikes. Since he was also a captain, he led numerous scientific expeditions from which he gathered
a great deal of observations. That is how he mapped ocean currents
and winds.
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WEATHER FORECAST
to predict the weather a few days in advance. Only with the appearance of high-speed communication networks (telegraph networks during the mid-19th century) did it become possible to start making
serious forecasts. The quality of the predictions improved considerably when satellites were sent to
space to collect data over the entire surface of the earth and powerful computers were developed to
run complex weather models.
Quiz
The complexity of atmospheric phenomena and
the difficulty of modeling them, as well as the
impossibility of knowing the many minor factors
that can influence the weather, make it impossible to establish long-term forecasts. Projects like
the Breitling 3 Orbiter offered an opportunity to
test and improve innovative software, whose use
was subsequently generalized and applied in other domains. The software that was developed to
guide the balloon now helps track the movement
of pollutants that escape into the atmosphere.
The latest technologies are being put to use for
the Solar Impulse project. It is likely that many
unexpected applications will come out of this experience!
What does each instrument measure?
Pluviometer
•
• Wind speed
Heliometer
•
• Wind direction
Barometer
•
• Atmospheric pressure
Thermometer •
Anemometer
•
Weather vane •
Hygrometer
•
• Precipitation
• Humidity
• Solar radiation
• Temperature
THE WATER CYCLE
Our planet is surrounded by a protective layer of gas that is essential to life. The temperature and
density within the layer vary with the altitude. The outer limit of the atmosphere (the exosphere), which
extends to an altitude of at least 10,000 km, is not clearly defined. At different altitudes, the atmosphere
is referred to using different names.
Altitude, in km
Température, in Celsius degrees
The lowest layer, the troposphere, is the one
that we will study. Most weather events play out 100
Thermosphere
in this layer, and it is also where Solar Impulse,
which cannot climb beyond 10,000 m, will chart
80
its course.
The stratosphere absorbs ultraviolet rays, which
is why its temperature increases with increasing
altitude. The ozone layer is located in the stratosphere. Weather balloons are normally used to
study this environment. Auguste Piccard (1884 1962), Bertrand Piccard’s grandfather, ascended
over 16,000 meters in a balloon, establishing the
manned flight altitude record of the time.
40
Stratosphere
20
0
WEATHER FORECAST
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Mesosphere
60
Troposphere
-80°C
-60°C
-40°C
-20°C
0
20°C
In turn, the mesosphere is characterized by a decrease in temperature with increasing altitude. This
is where meteors burn up. It is beyond the reach of balloons and too low for satellites, making it the
least known layer.
The thermosphere contains dioxygen, a molecule that absorbs ultraviolet rays with very short wavelengths. This again leads to an increase in temperature. Above it lies the exosphere, right on the limit to
outer space. Particles are very rare in this layer and move freely. Some of them escape into space. An
X-15 rocket plane, designed to carry out research on high speed and high altitude air travel, reached a
top speed of over 7,200 km/h at a 95,900 m altitude!
The sun provides energy to our planet. The energy that reaches the earth’s surface warms the oceans, the
vegetation, etc. Water evaporates into the atmosphere. It condenses to form clouds and then returns to
the ground as snow, rain or hail. This is the water cycle powered by the energy from the Sun.
condensation
solar
energy
water vapor
precipitations
evapotranspiration
evaporation
lake
infiltrations
groundwater
flow
surface runoff
ocean
CLOUD FORMATION
But how do clouds form? The air contains water vapor. When the temperature of air that is carried to
higher altitudes by updrafts decreases, water vapor condenses to form droplets or crystals.
When sunlight hits the droplets and ice crystals that make up clouds, it is reflected in all directions.
However, only the blue portion of sun’s spectrum is scattered by the air. This is why clouds are white,
making them stand out nicely against a blue sky. When a cloud is thick enough, light cannot pass
through it, making it look dark and ominous!
4/14
WEATHER FORECAST
Do it yourself
On a warm day
Place a bottle of water in the fridge long enough
for it to cool down.
Take it out and watch it. Explain what happens.
On a cold day
Exhale air inside the building
and then do the same outside.
Explain what happens.
IT’S WINDY!
The atmosphere forms a protective layer of gas that helps regulate the earth’s temperature and protects
the planet from solar radiation. The heat from the sun is not evenly distributed over the surface of the
globe. Indeed, while one half of the earth is plunged into darkness, the other half basks in the sun. Regions close to the equator accumulate more heat than the poles because they are more exposed to the
sun. Also, while the oceans act as a thermal regulator, the temperatures on the continents vary more
significantly. These global temperature differences give rise to large-scale atmospheric circulations.
Where air is heated, it expands and
rises. This creates a depression
called low-pressure zone. In contrast, where air is cooled, it contracts and descends, increasing
the pressure. This is referred to as
a high-pressure area (anticyclone).
To locate these zones that are constantly moving, meteorologists carry
out pressure measurements in multiple locations. Then, they map their
measurements and connect the
places with the same atmospheric
pressure to obtain curves (isobars)
that sometimes close. The average
pressure is 1,015 hPa at sea level.
This value defines the boundary between high and low-pressure areas.
WEATHER FORECAST
20°O
0°
20° E
Quiz
In the figure above, use a L to indicate the center of lowpressure areas and an H for high-pressure areas. Indicate the
direction of the wind using arrows.
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Differences in atmospheric pressure from one place to another set the air masses into motion, giving
rise to high-altitude winds. These winds do not blow in straight lines because of the rotation of the
earth. Instead, they spiral. In the northern hemisphere, the air blows clockwise around an anticyclone
and counter-clockwise around a low-pressure area. The greater the pressure difference between low
and high-pressure areas, the stronger the wind. In the southern hemisphere, the winds blow in the
opposite direction.
Within a high-pressure area, the
pressure of the descending air increases, ensuring fair, clear and dry
weather. In high-pressure areas, the
winds are light, even at the center.
high pressure (anticyclone)
low pressure
Even though anticyclones move and vary in intensity, there are areas where they are often located. In
Europe, the “Azores high” is an anticyclone that is located near the Portuguese Azores Islands during
the winter and spring. It is called the “Bermuda high” in North America because it usually settles there
during the summer and autumn. This high-pressure system warms Europe, carrying dry air from America and blocking the passage of weather disturbances. However, the Caribbean and Central America
sometimes face rainy weather or hurricanes because they receive humid air from the tropics.
Can the mission begin? The green light depends in part on
weather forecasts. How can weather changes be predicted with
enough precision to ensure the success and safety of a flight?
This is the responsibility of Luke Trullmans and his team.
PORTRAIT
LUC TRULLMANS, METEOROLOGIST
What is Luke passionate about? Certainly the adrenaline rush
during the preparation of a flight, but also the lifelong learning
that comes with his profession, as there is always room to improve weather forecasting tools. He is Solar
Impulse’s meteorologist and is seconded from the Royal Meteorological Institute of Belgium. He earned
a master’s degree in physics at the University of Brussels (ULB), and then carried out an internship in
meteorology. A few years later he participated in the creation of a research group focused on pollution in
the Royal Meteorological Institute. His goal: to study the dispersion of pollutants in order to warn populations about them. After this, Luke and his team created a program to predict the trajectory of pollution
in the troposphere. This same program helped Luc guide Bertrand Piccard and Brian Jones during their
circumnavigation of the world in a balloon in 1999 – and today it is at the service of Solar Impulse.
6/14
WEATHER FORECAST
LOCAL WINDS
So far we have only referred to large-scale air currents. Locally, winds take specific directions following
the features of the landscape. Surface winds develop according to the topography, rock walls, and
bodies of water.
In the mountains, along sunny
slopes, the air close the ground
warms up as the ground is heated
by the sun. This causes it to expand.
Because this air is warmer than the
air higher up in the mountains, it
rises, creating an updraft. This phenomenon is more pronounced when
it happens high in a valley, near cliffs
that are more exposed to the sun.
It then leads to the formation of a
wind that flows up the valley (valley
breeze) that starts at sunrise.
VALLEY BREEZE
upward breeze
longitudinal section
MONTAIN BREEZE
cross section
downward breeze
longitudinal section
cross section
In the evening, the cooling is first felt
at high altitudes. The air contracts
and descends, creating a breeze that flows towards the plain (mountain breeze). In narrow valleys,
these winds can be quite violent (50 km/h). Paragliders and other engineless “sky wanderers” take
advantage of these phenomena to gain altitude.
Breezes do not only blow in the mountains. At the seaside, sea and land breezes shape the days (see
Exercise 4).
We can observe a similar phenomenon in large cities in the form of urban breezes. During the day, cities
accumulate heat. Air that is in contact with the city’s concrete surfaces heats up much more easily than
air above rural areas with more vegetation. Heat absorbed by the stone and the concrete during the day
is released again at night. Also, many buildings are heated during the winter. Together, these effects can
lead to temperature differences of up to 10 °C at night in cities like Paris or London. This can give rise
to a gentle breeze towards the city that rarely exceeds 5 km/h.
WEATHER FORECAST
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IT’S RAINING, IT’S POURING...
The confluence of air masses that differ in temperature and humidity creates weather disturbances.
Because they differ in density, these air masses do not mix. Instead, they push and raise each other.
The boundary between such air masses is called a front. Different configurations can be observed.
When a cold air mass arrives in a region where
the air is milder, it is referred to as a cold front.
Because it is denser, it will push the warmer air
away, quickly forcing it upwards. This rise causes
the temperature of the warm air mass to drop until
it can no longer hold water vapor it contains. The
water vapor condenses, forming nimbostratus or
cumulonimbus clouds, and the weather becomes
rainy or very stormy. Consequently, the arrival of a
cold front marks a quick change of weather. After
its passage, the weather remains unstable.
When a mass of warm air arrives to an area where
the air is colder, we speak of a warm front. This
less dense warm air will gradually rise above the
cold air layer, forcing it upwards. This happens
progressively. An observer on the ground will first
see the formation of cirrus clouds, then medium
altitude clouds, before finally nimbostratus clouds
are formed due to the condensation caused by
the cooling of the air. Then it rains. This change of
weather is slow and may take a day.
COLD FRONT
warm air raised and then pushed
by the cold front
Altocumulus
Cumulonimbus
cumulus
stratocumulus
rear sky
cold air
WARM FRONT
Cirrostratus
warm air
Cirrus
Altostratus
Nimbostratus
cold air pushed
by the warm air
rear sky
Sometimes, when fronts are not very pronounced, they are called weak fronts. We might observe a
change in the direction of the wind, but no significant change of the weather. Stationary fronts can
have the characteristics of cold or warm fronts but only move slightly. In Europe, we often experience
stationary fronts with polar air masses. This can cause major regional contrasts with heavy snowfall in
one place and warmer and drier weather somewhere else.
During the passage of a depression, a
warm front precedes a cold front. Because the cold front moves faster than
the warm front, it catches up with it and
forms an occluded front or an occlusion. In this case, the warm air mass is
trapped between two cold air masses.
Cirrus
Cirrostratus
Altocumulus
Cumulus
Cirrus
Cumulonimbus
Stratocumulus
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Cumulonimbus
Altostratus
Nimbostratus
Stratocumulus
WEATHER FORECAST
HOW TO READ A WEATHER MAP
There are several indications on a synoptic weather chart. You can read not only the isobars
representing the pressure measurements, but also the position of the high-pressure centers (H) and
low-pressure centers (L).
There are also lines that represent the various types of fronts.
Cold front
Cold fronts are indicated by a line with triangles and warm
fronts with semicircles. Stationary fronts are represented by
a line with both types of marks distributed on both sides of the
line. Occlusions have alternating semicircles and triangles on
the same side, towards the direction of the front.
Warm front
Occlusion
Stationary front
direction from which
the wind blows
indicator of the
wind speed
The direction and strength of the winds are also represented.
The shaft indicates the direction from which the wind is coming.
West wind
indicator of
cloudiness
The symbol indicates the wind
speed depending on how many
feathers it contains. The wind speed
is given in knots. 1 knot corresponds
to 1.852 km/h.
5 knots
10 knots
15 knots
Nord-East wind
20 knots
50 knots
55 knots
The small circle indicates whether the sky is clear or not. Here are some of the symbols:
Clear sky
Mostly clear
< 10% cloudy
WEATHER FORECAST
Partly cloudy
20 to 30% cloudy
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Partly cloudy
50% cloudy
Very cloudy
60% cloudy
Overcast sky
Quiz
What can you say about the weather in a place
where this symbol is represented?
Tuesday 12.11.2013
Here is a synoptic map
of Europe on which
you can find different
weather symbols.
source : MétéoSuisse
10/14
WEATHER FORECAST
ALL THIS IN NUMBERS…
Exercise 1
Here is a world map with isobars. Indicate the areas with high (H) and low pressure (L). Also draw the
direction of the wind.
Exercise 2
Precipitation is usually measured in mm. This measurement represents the thickness of the water layer
that would have accumulated if it had not infiltrated into the ground. The record for the most abundant
precipitation in 10 minutes in Switzerland was in Locarno, where 33.6 mm fell on 29 August 2003*.
a) How much rainwater would we drain if the gutter of a building measuring 25 m by 8 m and with a
height of 10 m were blocked during these 10 minutes?
b) The most abundant rain in one day was recorded in Camedo in 1983: 414 l/m2. By how much would
Camedo’s record have been broken if Locarno’s 2003 storm had continued with the same intensity
all day long?
Exercise 3
Students built a rain gauge using a graduated cylinder with a capacity of 500 ml fitted with a funnel with
a diameter of 18 centimeters. They want to measure daily precipitation and chart it in mm and l/m2 to
compare their measurements to the statistics provided by the weather agency. Determine the calculations needed. Is the device adapted to record rainfall records?
* www.meteosuisse.admin.ch/web/fr/climat/climat_en_suisse/en_suisse.html
WEATHER FORECAST
11/14
Exercise 4
People who live on coastlines are subject to the tides but also to the breeze. When in the course of a
day does the breeze blow towards the sea? Complement your response with a diagram to explain this
phenomenon.
Thursday 21.06.2012
source : MétéoSuisse
Exercise 5
In Switzerland and in neighboring France, the
“bise” is a wind that blows over the plateau from
the north to the northeast (that is: it blows from
that direction). It is a cold, dry wind that usually
occurs following weather disturbances and announces the return of good weather.
Which of the three synoptic maps represents the
aforementioned bise?
A
Friday 08.12.2006
Monday 25.11.2013
source : MétéoSuisse
B
source : MétéoSuisse
C
Exercice 6
The following table contains monthly rainfall records in a number of cities in different continents. The
measurements are provided in mm. These are average values collected over a period of thirty years.
Create a graph containing all these records.
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
TOTAL
Geneva
82
76
67
65
71
81
73
79
80
81
86
81
922
Quebec
95
81
85
79
98
110
118
121
117
95
112
104
1,215
Cambera
51
55
53
49
42
37
41
58
61
71
65
52
635
Tombouctou
0
0
0
0
2
45
51
35
6
0
0
0
139
Calcutta
12
34
41
68
139
277
389
343
280
144
41
17
1,785
Sao Paolo
237
219
168
109
76
62
40
51
78
122
143
187
1,492
12/14
WEATHER FORECAST
TECHNOLOGY: BUILD YOUR OWN WEATHER STATION!
Meteorologists conduct regular surveys of the atmospheric conditions not only to look at the weather
objectively and to avoid resting on misguiding memories, but also to predict the weather. Here are three
basic types of measurement devices that you can build with basic material.
THE WEATHER VANE
Materials required
• A pole or tool handle
• Washers (or beads)
• A stick
• A compass
• Nails
• A blade (knife or cutter)
• A plastic sheet
Procedure
Cut a slit in each end of the stick.
Cut the point and tail of the arrow out of the plastic sheet and insert them
into the slits. You can use a nail to fix them.
Determine the center of gravity of the arrow and make a hole in it.
Attach the arrow to the top of the pole using a long nail, after sliding washers between the stick and the
pole to reduce friction (you can also use beads, if you find beads with large enough holes).
Place the weather vane outside. Use the compass to mark the cardinal points on the base or on the
floor.
THE RAIN GAUGE
Materials required
• A funnel
• A measuring cylinder
• Cellar tape
Procedure
Attach the funnel to the graduated cylinder with the tape.
You only need to calculate the surface on which the water
collected by the funnel falls and work out a correspondence
between the cylinder’s graduation and the number of mm of
rainfall. Take a look at exercise 3 if you need help.
Make sure that the mouth of the funnel mouth is horizontal.
You can make many other devices. You can find many online.
Thermometers or barometers are often in science classes.
WEATHER FORECAST
13/14
THE ANEMOMETER
Materials required
• A bamboo pole
• Two light sticks
• Staples or nails
• A long wood screw (considerably
longer than two times the thickness of the stick)
• Two hollow spheres (e.g. tennis
balls)
• A small metal tube whose inner
diameter is slightly larger than
the screw’s diameter (a ballpoint
pen body will do if you do not
find anything better)
Procedure
Use the screw to attach the two sticks to each other perpendicularly. They
must cross at the center.
Cut each ball into two hemispheres. Mark one of them to make it easily distinguishable from the others.
Stable a hemisphere to each end of the sticks. They should be positioned so that the fixing screw is
exactly at the center of gravity of the cross. They should be on the same side of the sticks as the tip
of the screw.
Pierce the bamboo pole to just the right depth to support the screw at the bottom of the hole. If done
well, the anemometer will be able to rotate freely in the wind. You might have to enlarge the hole in the
bamboo to slide in the tube. File the screw so that its tip does not crack the wood while moving and to
reduce friction to a minimum.
To calibrate the device, you will either need a vehicle with a speedometer or a second calibrated anemometer. Make a table representing the number of spins (over 20 seconds for example) depending on
the wind speed.
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WEATHER FORECAST