The Irish Meteorological Service The Sea Breeze An onshore breeze which develops in coastal areas on a warm day. Differential heating between.

The Irish Meteorological Service www.met.ie

The Sea Breeze

• An onshore breeze which develops in coastal areas on a warm day.

• Differential heating between the land and sea.

Forecasting Local Weather

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

Sea breeze formation

Two columns of airAt dawn:

Sea breeze formation

As land heats up a circulation develops

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.

It’s not just a coastal thing

• Sea breezes can occasionally penetrate over 50km inland

• Sea breezes can enhance convection due to convergence, particularly on peninsulas

Wind Flow over Mountains

Mountain Waves from Above

Lenticular Altocumulus

Temperatures: Radiation BalanceTemperatures: Radiation Balance

Typical Diurnal Variation of Temperature

• Min soon after dawn Temp falls until incoming

shorwave >outgoing longvave

• Max after local noon Temp rises Until incoming

shorwave <outgoing longvave

Temperature ForecastingTemperature Forecasting Techniques Techniques

Maximum 850 w Empirical relationship between 850 w 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

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

NOT ALLOWED IN

THE FREE ATMOSPHERE

Saturation of Air

• For a parcel of air to become SATURATED, either1. It must acquire some more moisture, or

2. It must cool down

• The first case can occur if air passes over a body of water

• The second case can have many causes...

Dew – A Brief Diversion

• DEW forms when water vapour condenses out onto the earth’s 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.

Diurnal Variation of Temperature

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

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

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 dawnFreezing 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 < 0c

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.

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.

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.The Irish Meteorological Service www.met.ie

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

Radiation Fog

Sea Fog

Post Cold-Frontal Fog

Sea Fog on the Norwegian Coast

Valley Fog in Norway

Continental Anticyclone

Orientation of Warm Sector

Fog in Switzerland

North Italy / Northern Balkans

Frost Occurrence

Occurs on radiation nights• Clear skies+Slack winds

•Anticyclone •Ridge or •Slack airflow •Long night

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

Frost Definitions

• Air frost -- Air temperatures below zero.• Ground frost – Ground temperatures below zero

• Slight -2o < Td < 0o

• Sharp -5o < Td < -2o

• Severe -10o< Td < -5o

• Very Severe Td < -10o

• Hoar Frost -- deposits ice (through sublimation) onto surfaces.

Formation of Hoar/Rime Frost

Conditions Favouring ‘Wet’ Frost

• Tsurface < 0 °C, Tsurface < Tdewpoint

• Sufficient humidity Tdewpoint -Tair<1.5°C

• Tair ,Tsurface and Tdewpoint are converging

Frost Criteria

Slight 0º to -2º CSharp -2º to -5º CSevere -5 to -10º CVery Severe Below -10 C

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

“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

On long frosty nights a build up of hoar or rime frost can lead to a layer of Ice

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

ground

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

Snow Forecasting Percentage Probability of Snow

90% 70% 50% 30% 10%

T Air -0.3 1.2 1.6 2.3 3.9

1000-850hPa 1281 1290 1293 1298 1303

0° Isotherm 12 25 35 45 61

1000-500hPa 5180 5238 5258 5292 5334

Warm and Cold Clouds

IdealizedThunderstorm

+++ ++

++++ ++ +

++ +++ ++ +

------- --- -- -- --- --- --- --- - - --- -

++++ +

++ ++ ++ ++

- -- -++

-- - ---

- --- ---

- --20° C

-10° C

Non-inductive Charging (NIC) Theory

• 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.

Forecasting Thunderstorms

Stability Indices• Boyden Index I=(800-700hPa)-T700hPa

• Thunder Probable if I > 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 K20

Lightning!

Tropical Cyclones

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.

Hurricane Isabel (2003)

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).

The Irish Meteorological Service The Sea Breeze An onshore breeze which develops in coastal areas on a warm day. Differential heating between.

irish meteorological

radiation balance slide

t d850 t

sea temperatures

tropical cyclones cont

old tropical cyclones

sea breeze formation

w900 t

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