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

The Irish Meteorological Service Forecasting Local Weather Sea Breeze (Again!) Temperature Dew Fog Frost Snow Thunderstorms Tropical Cyclones

The Irish Meteorological Service Sea breeze formation Two columns of air At dawn:

The Irish Meteorological Service Sea breeze formation As land heats up a circulation develops

The Irish Meteorological Service How… and When? Land temperatures need to be at least 3.5 o C warmer than sea temperatures … They are very common and strong in tropical regions In Ireland generally from March to late September.

The Irish Meteorological Service 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 The Irish Meteorological Service

Mountain Waves from Above The Irish Meteorological Service

Lenticular Altocumulus The Irish Meteorological Service

Temperatures: Radiation Balance

The Irish Meteorological Service 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

The Irish Meteorological Service Temperature Forecasting Techniques  Maximum  850  w Empirical relationship between 850  w and maximum temperature  hPa thickness, using standard tables, correction for cloud  Minimum  McKenzie: Uses Maximum Temperature, T d at time of T max, 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 The Irish Meteorological Service

NOT ALLOWED IN THE FREE ATMOSPHERE

The Irish Meteorological Service

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Saturation of Air For a parcel of air to become SATURATED, either 1.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... The Irish Meteorological Service

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. The Irish Meteorological Service

Diurnal Variation of Temperature The Irish Meteorological Service

What is Fog? Visibility less than 1Km Visibility 1Km to 18Km Relative Humidity greater than 75% FogMist Relative Humidity less than 75% Haze The Irish Meteorological Service

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 The Irish Meteorological Service

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  T air and T dewpoint 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 < 0 c

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. The Irish Meteorological Service

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 T d greater than 13C or 14C Can get T d up to 16C or 17C in T m 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

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 The Irish Meteorological Service

Radiation Fog The Irish Meteorological Service

Sea Fog The Irish Meteorological Service

Post Cold-Frontal Fog The Irish Meteorological Service

Sea Fog on the Norwegian Coast The Irish Meteorological Service

Valley Fog in Norway The Irish Meteorological Service

Continental Anticyclone The Irish Meteorological Service

Orientation of Warm Sector The Irish Meteorological Service

Fog in Switzerland The Irish Meteorological Service

North Italy / Northern Balkans The Irish Meteorological Service

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 The Irish Meteorological Service

Frost Definitions Air frost -- Air temperatures below zero. Ground frost – Ground temperatures below zero Slight -2 o < T d < 0 o Sharp -5 o < T d < -2 o Severe -10 o < T d < -5 o Very Severe T d < -10 o Hoar Frost -- deposits ice (through sublimation) onto surfaces. The Irish Meteorological Service

Formation of Hoar/Rime Frost Conditions Favouring ‘Wet’ Frost T surface < 0 °C, T surface < T dewpoint Sufficient humidity T dewpoint -T air <1.5°C T air,T surface and T dewpoint are converging Frost Criteria Slight 0º to -2º C Sharp -2º to -5º C Severe -5 to -10º C Very 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

The Irish Meteorological Service Widespread Snow Rare Most likely in showers More frequent on higher ground 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

The Irish Meteorological Service Snow Forecasting

Warm and Cold Clouds The Irish Meteorological Service

Idealized Thunderstorm ° 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.

The Irish Meteorological Service Forecasting Thunderstorms Stability Indices Boyden Index I=( hPa)-T 700hPa Thunder Probable if I > 4/95 Radcliff Index T=  w900 – T 500 Thunder Probable if T  29/30 Potential Instability P=  w500 -  w850 Thunder possible if P  -2 (summer) K Index K = (T 850 – T 500 ) + T d850 – (T 700 – T d700 ) Thunder possible for K  20

Lightning! The Irish Meteorological Service

Tropical Cyclones The Irish Meteorological Service

Tropical Cyclones The Irish Meteorological Service

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. The Irish Meteorological Service

Hurricane Isabel (2003) The Irish Meteorological Service

Hurricane Isabel (2003) The Irish Meteorological Service

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