Weather and Waves

John HuthHarvard 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.

Land Breeze-Sea BreezeSame effect, but on a much smaller scale

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 ofJune/July/August, in the absence of fronts,wind patterns on the Capeare reasonably stable.

Little wind in the morning,picking up around 2 PM from SW, reaching peakaround 3:30, then subsiding.

Mainly a sea breeze effect, coupled with prevailing SW winds

Wind compass – Taumako, Polynesia

Pukapukan wind compass

Fijian wind compass

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 – land indicators

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 highpressure 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 ofcirrus clouds

If contrail breaks up -> low humidityIf contrail remains -> high humidity (approachingWarm front)

Sundogs – rings around the sun (or moon)Caused by ice crystals in the upper atmosphereCirro-stratus (high, layered clouds) 22 degree halo around sun/moon

Mares tails – cirrus clouds(reading wind: watch cloudmotion relative to foregroundobject)

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”

Slower moving air

Faster moving air

Eddies

Clouds insideeddies

Stratus clouds – means “layered” in latinFlat, grey, clouds, covering large areas of the sky

Nimbostratus – rain clouds associated with awarm 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 coolerAt higher altitudes

Hot air rises, at cold enoughTemperatures, it begins to mix

Incoming humid air

Rising air column

Development of vertical structure

Building cumulus clouds can be a sign ofland – high up, seen from further away

Building thunderheads

Start of anvil-head formation

Air column reaches tropopause and spreads

Mature anvil-head

Fig. 11.2a

Air column frequently overshoots tropopause,“bubbles out” high cirrostratus

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 and

Anticyclones

Cyclones and anti-cyclones

High pressure systems “shed” air

Low pressure systems“suck” air

Coriolis force generatescirculation

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

2

gLV

gdV

Deep

Shallow

g

d

L

Gravitation 32 ft/sec/sec

Water depth (ft)

Wave length (ft)

h

L

Instability – when h > 1/7 LOR – when interior angle is less 120 degrees

120o

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

• Bending of the wave crest as waves enter shallow water. It is due to– Drag along the

bottom.– Differential

speed along the crest.

Wave Refraction(Figure 7-8a)

Wave Refraction at Chatham InletGradual transition between deep and shallow water

Shallow water

Deep Water

Extreme refraction at Baker Island(Mt. Desert)

Swell patterns around an atoll

reflections

Mainswell

Refractions

Crossing swell patterns between islands

Multi-swell patterns around island

Polynesian stick chart – illustratingswell patterns from two islands