The Sea Breeze

1. 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 www.met.ie

2. Forecasting Local Weather • Sea Breeze (Again!) • Temperature • Dew • Fog • Frost • Snow • Thunderstorms • Tropical Cyclones The Irish Meteorological Service www.met.ie

3. Sea breeze formation Two columns of air At dawn: The Irish Meteorological Service www.met.ie

4. Sea breeze formation As land heats up a circulation develops The Irish Meteorological Service www.met.ie

5. 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. The Irish Meteorological Service www.met.ie

6. 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 The Irish Meteorological Service www.met.ie

7. Wind Flow over Mountains The Irish Meteorological Service www.met.ie

8. Mountain Waves from Above The Irish Meteorological Service www.met.ie

9. Lenticular Altocumulus The Irish Meteorological Service www.met.ie

10. Temperatures: Radiation Balance The Irish Meteorological Service www.met.ie

11. 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 www.met.ie

12. Temperature Forecasting Techniques • Maximum • 850 qwEmpirical 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 The Irish Meteorological Service www.met.ie

13. 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 www.met.ie

14. The Irish Meteorological Service www.met.ie

15. NOTALLOWED IN THE FREE ATMOSPHERE The Irish Meteorological Service www.met.ie

16. The Irish Meteorological Service www.met.ie

17. Td The Irish Meteorological Service www.met.ie

18. 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... The Irish Meteorological Service www.met.ie

19. 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 www.met.ie

20. Diurnal Variation of Temperature The Irish Meteorological Service www.met.ie

21. What is Fog? The Irish Meteorological Service www.met.ie

22. 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 www.met.ie

23. 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 < 0c The Irish Meteorological Service www.met.ie

24. 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 www.met.ie

25. 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. The Irish Meteorological Service www.met.ie

26. 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

27. 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 www.met.ie

28. Radiation Fog The Irish Meteorological Service www.met.ie

29. Sea Fog The Irish Meteorological Service www.met.ie

30. Post Cold-Frontal Fog The Irish Meteorological Service www.met.ie

31. Sea Fog on the Norwegian Coast The Irish Meteorological Service www.met.ie

32. Valley Fog in Norway The Irish Meteorological Service www.met.ie

33. Continental Anticyclone The Irish Meteorological Service www.met.ie

34. Orientation of Warm Sector The Irish Meteorological Service www.met.ie

35. Fog in Switzerland The Irish Meteorological Service www.met.ie

36. North Italy / Northern Balkans The Irish Meteorological Service www.met.ie

37. Frost Occurrence • Occurs on radiation nights • Clear skies+Slack winds • Anticyclone • Ridge or • Slack airflow • Long night The Irish Meteorological Service www.met.ie

38. 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 www.met.ie

39. 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. The Irish Meteorological Service www.met.ie

40. 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< 0 °C, Tsurface < Tdewpoint • Sufficient humidity Tdewpoint -Tair<1.5°C • Tair ,TsurfaceandTdewpoint 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 The Irish Meteorological Service www.met.ie

41. 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 www.met.ie

42. Snow Forecasting The Irish Meteorological Service www.met.ie

43. Warm and Cold Clouds The Irish Meteorological Service www.met.ie

44. 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. + + + + + - - - - - - - -20° C - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -10° C + + + + + + + + + + + + + The Irish Meteorological Service www.met.ie Non-inductive Charging (NIC) Theory

45. Forecasting Thunderstorms Stability Indices • Boyden Index I=(800-700hPa)-T700hPa • Thunder Probable if I > 4/95 • Radcliff IndexT=w900 – T500 • Thunder Probable if T 29/30 • Potential InstabilityP=w500 - w850 • Thunder possible if P  -2 (summer) • K Index K = (T850 – T500) + Td850 – (T700 – Td700) • Thunder possible for K20 The Irish Meteorological Service www.met.ie

46. Lightning! The Irish Meteorological Service www.met.ie

47. Tropical Cyclones The Irish Meteorological Service www.met.ie

48. Tropical Cyclones The Irish Meteorological Service www.met.ie

49. 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 www.met.ie

50. Hurricane Isabel (2003) The Irish Meteorological Service www.met.ie