Weather and Waves
John Huth
Harvard University
Weather Basics
• Hot air rises (less dense), cold air sinks (more
dense)
• Atmosphere becomes colder the higher up you
go (called adiabatic cooling)
• It gets colder as you go away from the equator
• The Coriolis effect causes air moving away from
the equator to the pole to deflect to the east
• The Coriolis effect causes air moving from the
pole toward the equator to deflect to the west
Driving Forces Behind Wind
•Pressure Gradient
Air flows from high to low pressure (“downhill”)
•Coriolis
Caused by the rotation of the earth, wind deflects to the right in the
northern hemisphere
•Centripital
Present when winds are in rotation
•Friction
Air moving along the Earth’s surface is slowed by friction
Pressure Gradient and Winds
Coriolis “Force” causes path of a moving object to be
deflected to the right in the NH and to the left in SH
relative to the surface of the earth
Weather Basics II
• Emergence of three convection cells in northern and
southern hemisphere dominate wind patterns
–
–
–
–
Doldrums – equator
Trades blow from east to west
Horse latitudes – 30 degrees
Westerlies – 30 to 60 degrees north
• As moist air rises, it condenses and gives off heat
• The planet is approximately in an isobaric equilibrium –
pressure remains roughly constant – regardless of
temperature (density of air changes)
• Prevailing winds tend to drive surface ocean currents
Smaller scale version: land and sea breezes
Temperature contrasts (the result of the differential heating
properties of land and water) are responsible for the formation
of land and sea breezes.
Same effect,
but on a much smaller
scale
Land
Breeze-Sea
Breeze
Wind as direction indicator
• Good over short periods of time – persistent
• Prevailing winds generally useful, but seasonally
dependent
• Weather systems and fronts can affect these
• Surface winds versus winds aloft
• Understand how weather
systems/seasons/diurnal variations affect wind
patterns
Wind roses for Boston Logan International Airport
July
January
Local knowledge: summer wind patterns on Cape Cod
During the months of
June/July/August,
in the absence of fronts,
wind patterns on the Cape
are reasonably stable.
Little wind in the
morning,
picking up around 2 PM
from SW, reaching peak
around 3:30, then
subsiding.
Mainly a sea breeze effect,
coupled with prevailing
SW winds
Wind compass – Taumako, Polynesia
Pukapukan wind compass
Fijian wind compass
Beaufort Scale – land indicators
Force
Strength
km/h
Effect
0
Calm
0-1
Smoke rises vertically
1
Light air
1-5
Smoke drifts slowly
2
Light breeze
6-11
Wind felt on face; leaves rustle
3
Gentle breeze
12-19
Twigs move; light flag unfurls
4
Moderate breeze
20-29
Dust and paper blown about; small branches move
5
Fresh breeze
30-39
Wavelets on inland water; small trees move
6
Strong breeze
40-50
Large branches sway; umbrellas turn inside out
7
Near gale
51-61
Whole trees sway; difficult to walk against wind
8
Gale
62-74
Twigs break off trees; walking very hard
9
Strong gale
75-87
Chimney pots, roof tiles and branches blown down
10
Storm
88-101
Widespread damage to buildings
11
Violent Storm
102-117
Widespread damage to buildings
12
Hurricane
Over 119
Devastation
Beaufort scale – at sea
Using wind
• Winds can be deceiving
– Surface winds can blow in different directions
from winds aloft – you must follow the motion
of high clouds to get prevailing winds
• Winds will shift as fronts pass through –
knowledge of this is important (for many
reasons).
• Safety – high winds from thunderstorms
can be dangerous when at sea.
Wind shifts
• Veering shifts – clockwise shift – typical for
N. hemisphere
• Backing shifts – counterclockwise – typical
for S. Hemisphere
• For approaching cold front – SW wind
steady, veers to N to NW (typical)
• For approaching warm front – NE to SE
winds, veers to SW (typical)
Warm and Cold Air masses
• Warm air masses
– Humid, low pressure, warm - move up from
equatorial regions
• Cold air masses
– Dry, high pressure, cold – move down from
polar regions
• Transitions between air masses are called
“fronts”
Weather signs
• Cloud formations and wind directions are
the most reliable and predictive (often
better than NOAA radio).
• Best predictor: tomorrow will be like today
(true 80% of the time). You can improve on
this by being observant.
• Some signs: “red sky at night” are next to
useless – unless you know the cloud
formations causing them.
Important North American Air Masses
In mid-latitudes, fronts develop as Rossby waves,
Typically seen as undulations in the jet-stream.
Isolated pockets can develop as low and high
pressure cells
Warm fronts
• Slow in coming
• Sequence of clouds – build up of moisture in
upper atmosphere, slowly coming down in
height
–
–
–
–
–
–
–
Jet contrails at 40,000 ft tend to stick around
Moon or sun dogs (rings) – from ice crystals
Cirrus clouds (mares’ tails)
Cirro-stratus (mackerel scales) – 20,000
Alto-cumulus (rollers) 15,000-20,000
Stratus (sheet-like) 5000-10,000
Nimbo-stratus (rain clouds) 5000 or lower
• Rain usually lasts for a longer time
Profile of a Warm Front
Lingering jet contrail against a backdrop of
cirrus clouds
If contrail breaks up -> low humidity
If contrail remains -> high humidity (approaching
Warm front)
Sundogs – rings around the sun (or moon)
Caused by ice crystals in the upper atmosphere
Cirro-stratus (high, layered clouds)
22 degree halo around sun/moon
Mares tails – cirrus clouds
(reading wind: watch cloud
motion relative to foreground
object)
Higher wind speed
Lower wind speed
Mackerel scales – cirrocumulus clouds
Old saying: “mackerel scales and mares tails make lofty ships carry low sails”.
-> Approaching warm front
Altocumulus clouds – “rollers”
Faster moving air
Eddies
Slower moving air
Clouds inside
eddies
Stratus clouds – means “layered” in latin
Flat, grey, clouds, covering large areas of the sky
Nimbostratus – rain clouds associated with a
warm front
Cold Fronts
•
•
•
•
•
•
Abrupt transitions
Veering winds (moving clockwise at front)
Strong downdrafts
Squall-lines
Lightning
Development of storms more rapid,
unpredictable, violent, and local than in
warm fronts
Profile of a Cold Front
Wind shifts
• Veering shifts – clockwise shift – typical for
N. hemisphere
• Backing shifts – counterclockwise – typical
for S. Hemisphere
• For approaching cold front – SW wind
steady, veers to N to NW (typical)
• For approaching warm front – NE to SE
winds, veers to SW (typical)
Veering winds as front approaches
(typical for NE)
Fig. 11.7
THUNDERSTORM
CUMULUS STAGE
• CUMULUS STAGE
• REQUIRES CONTINUOUS SOURCE OF
WARM MOIST AIR
• EACH NEW SURGE OF WARM AIR RISES
HIGHER THAN THE LAST
• STRONG UPDRAFTS
• FALLING PRECIPITATION DRAGS AIR DOWN
- DOWNDRAFT
• ENTRAINMENT
Fair weather cumulus clouds
(flat, little vertical structure)
General character of convection
Rising column of hot air (fluid)
Surrounding air is cooler and cooler
At higher altitudes
Hot air rises, at cold enough
Temperatures, it begins to mix
Development of vertical structure
Rising air column
Incoming humid air
Building cumulus clouds can be a sign of
land – high up, seen from further away
Building thunderheads
Start of anvil-head formation
Air column reaches tropopause and spreads
Mature anvil-head
Air column frequently overshoots tropopause,
“bubbles out” high cirrostratus
Fig. 11.2a
THUNDERSTORM
MATURE STAGE
• SHARP COOL GUSTS AT SURFACE SIGNAL
DOWNDRAFTS
• UPDRAFTS EXIST SIDE BY SIDE WITH
DOWNDRAFTS
• IF CLOUD TOP REACHES TROPOPAUSE
UPDRAFTS SPREAD LATERALLY - ANVIL
SHAPE
• TOP OF ICE LADEN CIRRUS CLOUDS
• GUSTY WINDS, LIGHTNING, HEAVY
PRECIPITATION, HAIL
Multicell line storms consist of
a line of storms with a continuous,
well developed gust front at the
leading edge of the line. An
approaching multicell line often
appears as a dark bank of clouds
covering the western horizon. The
great number of closely-spaced
updraft/downdraft couplets qualifies
this complex as multicellular,
although storm structure is quite
different from that of the multicell
cluster storm.
Estimating distances to storms
• Base of clouds in thunderstorm is typically
5000 ft.
– Use range techniques to find distance
• Difference between lightning and thunder
arrival times (light is faster than sound)
– 5 seconds per mile of distance
• Prevailing winds –
– Is the storm track moving toward you, or will it
pass by?
Thunderstorm/squall issues
• General direction is indicated by high cirrus
clouds at top of anvil head
– NOT surface winds (often blow toward the storm)
• If a storm misses you (passes to the side), be
alert for more storms moving in the same
direction.
• Wind is biggest issue
– Lightning is less of a hazard, but shouldn’t be ignored.
Basic Pressure Systems: 1.Low
L
Basic Pressure Systems: 2.High
H
Cyclones
High pressure
systems
and
“shed” air
Anticyclones
Low pressure systems
Cyclones and anti-cyclones
“suck” air
Coriolis force generates
circulation
Structure of a Hurricane
Low pressure system over NE – March 20th 08
Low pressure systems in the N. Pacific
High and low pressure systems in N. Atlantic
(www.oceanweather.com)
Advection Fog: formed by movement
of warm air over cooler surface
Radiation Fog: forms when land
surface cools as a result of outgoing
radiation and in turn, cools overlying air
Wave Parameters
(Figure 7-1a)
What Causes Waves?
• Wind
• Submarine disturbance
• Gravitational attraction of sun and moon
(tides – very long wavelength waves)
Motion of Water Particles Beneath
Waves
(Figure 7-3b)
Deep Water Waves
(Figure 7-4a)
Waves do not interact with the seafloor
Orbits of the water molecules are circular.
Shallow Water Waves
(Figure 7-4b)
Waves interact with the seafloor are known as Orbits
of the water molecules become elliptical.
Characteristics of water waves
• Velocity depends on wavelength *or* water
depth
– Unlike sound or light – velocity is independent of
wavelength for these
• Waves become unstable when height is 1/7th of
wavelength – whitecaps (120 degree interior
angle)
• Longer wavelength waves hold more energy
• Depth for “shallow” versus “deep” is about 2
times wavelength
gL
V
2
Deep
V  gd
Shallow
g
Gravitation 32 ft/sec/sec
d
Water depth (ft)
L
Wave length (ft)
Instability – when h > 1/7 L
OR – when interior angle is less 120 degrees
120o
h
L
Wind Generation of Waves
• The type of wave generated by wind is
determined by:
– Wind velocity
– Wind duration
– Fetch (distance over which wind blows)
• Simply put, wave size increases as the strength
and duration of the wind, and distance over
which it blows increases.
Cat’s paw
Fetch Conditions
• Time and distance
• Small waves buildup, break
• Larger waves begin – hold more energy before
breaking
• Generally a range of wavelengths
– High wind velocity produces more uniform and longer
wavelength waves
• Typically for NE waters – fully developed seas
only for 10 knot winds
– Larger seas in open ocean
• Swells travel huge distances unaffected
Comments on Swells
• Product of distant storms
– Can travel thousands of miles without losing energy
– Period of swell indicates severity of storm –
• Longer period – more severe storm
– 4 seconds – small
– 8-10 seconds – hurricane
• Mid ocean – can have multiple swells crossing
• In New England, sheltering of coast line limits
significant swell direction
– E.g. Gulf of Maine typically will only see SE swells
– Rhode Island catches a lot of Atlantic storms
– Newport beaches/surfing
Transformation of Shallow-water
Waves
(Figure 7-7b)
Reflecting Swells at Great Wass Island
(Jonesport)
Angle of incidence equals angle of reflection
Wave Refraction
• Bending of the
wave crest as
waves enter
shallow water. It
is due to
– Drag along the
bottom.
– Differential
speed along the
crest.
(Figure 7-8a)
Wave Refraction at Chatham Inlet
Gradual transition between deep and shallow water
Shallow water
Deep Water
Extreme refraction at Baker Island
(Mt. Desert)
Swell patterns around an atoll
reflections
Main
swell
Refractions
Crossing swell patterns between
islands
Multi-swell patterns around island
Polynesian stick chart – illustrating
swell patterns from two islands