The Sea Breeze

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The Sea Breeze

• An onshore breeze which develops in coastal areas
  on a warm day.
• Differential heating between the land and sea.

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Forecasting Local Weather

• Sea Breeze (Again!)
• Temperature
• Dew
• Fog
• Frost
• Snow
• Thunderstorms
• Tropical Cyclones

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Sea breeze formation
Two columns of air At dawn
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Sea breeze formation
As land heats up a circulation develops
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How and When?

• Land temperatures need to be at least 3.5 oC
  warmer than sea temperatures
• They are very common and strong in tropical
  regions
• In Ireland generally from March to late September.

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Its not just a coastal thing

• Sea breezes can occasionally penetrate over 50km
  inland
• Sea breezes can enhance convection due to
  convergence, particularly on peninsulas

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Wind Flow over Mountains
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Mountain Waves from Above
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Lenticular Altocumulus
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Temperatures Radiation Balance
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Typical Diurnal Variation of Temperature

• Min soon after dawn
• Temp falls until incoming shorwave gtoutgoing
  longvave
• Max after local noon
• Temp rises Until incoming shorwave ltoutgoing
  longvave

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Temperature Forecasting Techniques

• Maximum
• 850 qw Empirical relationship between 850 qw and
  maximum temperature
• 1000-850hPa thickness, using standard tables,
  correction for cloud
• Minimum
• McKenzie Uses Maximum Temperature, Td at time of
  Tmax, and correction for wind/cloud
• Model Output statistics MOS
• Uses model output of temperatures, combined with
  regression techniques containing local
  information

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Moisture in the Atmosphere

• Water can exist in any one of three phases
• Solid (Ice, Hoar Frost)
• Liquid (Raindrops, Cloud drops, Drizzle, Dew)
• Gas (water vapour)
• The amount of WATER VAPOUR that the air can hold
  is heavily dependent upon temperature.
• Measure Water Vapour content in different ways
• Relative Humidity
• Dew Point Temperature
• Wet Bulb Temperature
• Mixing Ratio

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NOT ALLOWED IN THE FREE ATMOSPHERE
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Td
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Saturation of Air

• For a parcel of air to become SATURATED, either
• It must acquire some more moisture, or
• It must cool down
• The first case can occur if air passes over a
  body of water
• The second case can have many causes...

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Dew A Brief Diversion

• DEW forms when water vapour condenses out onto
  the earths surface.
• Night time bring radiation cooling to the ground
• Grass, exposed metal (cars!) etc cool more
  rapidly than roads, footpaths
• On MOST nights the temperature of the ground
  falls below the Dew Point
• Exceptions windy, cloudy weather.

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Diurnal Variation of Temperature
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What is Fog?
Visibility less than 1Km Visibility 1Km to 18Km
Relative Humidity greater than 75 Fog Mist
Relative Humidity less than 75 Haze Haze
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Formation of Fog

• Ground will cool at night, through loss of heat
  through long-wave radiation (clear, calm nights)
• Air at the surface will cool through contact with
  the colder ground.
• Fog forms at the surface initially in a shallow
  layer, then it grows upwards as the top of the
  fog layer loses heat in turn.
• Known as RADIATION FOG

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Fog

• Radiation Fog
• On clear nights air near the surface of the earth
  is cooled due to outgoing radiation

Conditions Favouring Fog Formation Clear skies,
long night ?Tair and Tdewpoint converging Little
or no wind Timing often just after dawn
Freezing Fog is when the temperature is less than
zero and the water droplets in the fog are
supercooled. This is very uncommon in Ireland.
NOT just fog with T lt 0c
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Formation of Radiation Fog

• Why just after dawn?
• Rising sun heats surface of the ground
• ? Evaporation of night-time dew
• ? Injection of moisture into the (cold) lowest
  layers
• ? Condensation into fog droplets
• Usually clears again after a couple of hours.

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Fog

• Sea Fog
• Forms when moist air is cooled to saturation by
  contact with a cool sea surface
• Most common in spring and early summer when the
  sea is at its coldest
• Temperature of sea relative to Dewpoint of the
  Airmass?
• Look for Td greater than 13C or 14C
• Can get Td up to 16C or 17C in Tm air during the
  summer.

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Other types of fog

• Advection Fog
• Warm air passing over cold ground (e.g. Warm
  sector reaching snow-covered ground).
• Frontal Fog
• Frontal precipitation falls through a dry layer
  of air, where it evaporates. The consequent
  increase in the water vapour can trigger fog.
  Typical of weak, slow-moving fronts in the summer
  months.
• Advected sea or radiation fog
• Fog which has formed in one place but been
  transported to another by a (usually gentle)
  breeze.

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Fog formation

• Very heavily influenced by the topography
• Exposed upslopes (south and southwest of Ireland)
• River valleys (e.g. Po valley in Italy)
• Flat bogland
• Steep valleys which lead to cold-air pooling and
  consequent inversions

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Radiation Fog
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Sea Fog
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Post Cold-Frontal Fog
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Sea Fog on the Norwegian Coast
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Valley Fog in Norway
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Continental Anticyclone
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Orientation of Warm Sector
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Fog in Switzerland
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North Italy / Northern Balkans
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Frost Occurrence

• Occurs on radiation nights
• Clear skiesSlack winds
• Anticyclone
• Ridge or
• Slack airflow
• Long night

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Frost Conditions

• Often a choice between frost and fog
• Cold, dry air ? Favours frost
• More moist air ? Fog more likely
• Unusual to have both together

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Frost Definitions

• Air frost -- Air temperatures below zero.
• Ground frost Ground temperatures below zero
• Slight -2o lt Td lt 0o
• Sharp -5o lt Td lt -2o
• Severe -10olt Td lt -5o
• Very Severe Td lt -10o
• Hoar Frost -- deposits ice (through sublimation)
  onto surfaces.

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Formation of Hoar/Rime Frost
Radiation night with clear skies and slack
winds

• We also require a source of moisture
• This is present in the air as a gas Water
  Vapour
• Cooler air holds less moisture

Dewpoint Temperature to which air must be
cooled to become saturated with
respect to water

• Conditions Favouring Wet Frost
• Tsurface lt 0 C, Tsurface lt Tdewpoint
• Sufficient humidity Tdewpoint -Tairlt1.5C
• Tair ,Tsurface and Tdewpoint are converging

Frost Criteria Slight 0º to -2º
C Sharp -2º to -5º C Severe
-5 to -10º C Very Severe Below -10 C
On long frosty nights a build up of hoar or rime
frost can lead to a layer of Ice
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Snow

• Snow Scenario
• Warm front approaching from South,
  
• cold surface (Easterly) airflow
• Cold front turns to snow before clearing
• Showers in cold west to north airflow,
• or Easterly airflow

• Widespread Snow Rare
• Most likely in showers
• More frequent on higher ground

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Snow Forecasting
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Warm and Cold Clouds
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Idealized Thunderstorm

• Charge separation most likely occurs during
  rebounding collisions between ice crystals and
  large ice hydrometeors such as graupel and hail
  that remain suspended in the mixed phase zone by
  the updraft of a growing thunderstorm.

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Non-inductive Charging (NIC) Theory
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Forecasting Thunderstorms

• Stability Indices
• Boyden Index I(800-700hPa)-T700hPa
• Thunder Probable if I gt 4/95
• Radcliff Index T?w900 T500
• Thunder Probable if T ? 29/30
• Potential Instability P?w500 - ?w850
• Thunder possible if P ? -2 (summer)
• K Index K (T850 T500) Td850 (T700
  Td700)
• Thunder possible for K?20

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Lightning!
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Tropical Cyclones
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Tropical Cyclones
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Tropical Cyclones

• Need Sea Temperature above 26.5 C
• Low levels of vertical wind shear
• Easterly Wave in the trade wind flow (Atlantic)
• Differences of degree...
• Tropical Depression
• Tropical Storm
• Hurricane
• Bring vast amounts of moisture into the upper
  atmosphere.

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Hurricane Isabel (2003)
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Hurricane Isabel (2003)
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Tropical Cyclones / cont

• Most damage / deaths caused by coastal flooding
• Weaken quickly over land
• but.... can bring very heavy rain inland leading
  to flash floods
• We watch out for old tropical cyclones that get
  caught up in mid-latitude weather systems
• Tend to bring very heavy rain (rather than strong
  winds).