City of Irvine
2020 Local Hazard Mitigation Plan
Table 3-16: Measuring Heavy Rain Events
Rain Type
Description
Heavy rain
More than 4 mm per hour but less than 8 mm per hour
Very heavy rain
Greater than 8 mm per hour
Moderate shower
Greater than 2 mm, but less than 10 mm per hour
Heavy shower
Greater than 10 mm per hour, but less than 50 mm per hour
Violent shower
Greater than 50 mm per hour
Source: https://water.usgs.gov/edu/activity-howmuchrain-metric.html mm = millimeter
Severe Wind In Southern California, the most common type of severe wind event is called the Santa Ana winds. High pressure over Nevada and Utah, often during the fall and winter months, forces air down from the high desert toward the ocean. As the winds descend, they heat up and increase in speed, sometimes carrying particulate matter and aggravating the respiratory health of those who have allergies. 101,102 Irvine is often affected by Santa Ana winds blowing through the Santa Ana Mountain range. Santa Ana winds are a leading cause of wildfires in California. More information on this is available in the “Wildfire” section. Generally, winds are measured using the Beaufort scale, developed in 1805, which categorizes wind events on a force scale from 0 to 12 using their speed and impacts. Any wind that is classified as force nine or above is generally considered to be a severe wind event. Table 3-17 shows how the Beaufort scale classifies wind events in detail.
Table 3-17: Beaufort Scale
Speed (mph)
Force
Description
0
0 to 1
Calm: Smoke rises vertically, and the sea is flat
1
1 to 3
Light air: The direction of wind is shown by smoke drift, but not wind vanes
Light breeze: Wind is felt on the face, leaves rustle, and wind vanes are moved. Small wavelets appear on the ocean, but do not break Gentle breeze: Leaves and small twigs are in motion, and light flags are extended. Large wavelets appear on the ocean, and crests begin to break Moderate breeze: Dust and loose paper become airborne, and small branches are moved. Small waves appear on the ocean
2
4 to 7
3
8 to 12
4
13 to 18
5
19 to 24
Fresh breeze: Small trees begin to sway and moderate waves form
Strong breeze: Large branches are in motion, and using an umbrella becomes difficult. Large waves begin to form Near gale: Whole trees are in motion, and walking against the wind can be hard. Foam from breaking waves is blown in streaks
6
25 to 31
7
32 to 38
8
39 to 46
Gale: Walking is difficult, and twigs break off trees
9
47 to 54
Severe gale: Slight structural damage. Crests of waves begin to topple
Storm: Trees are uprooted and considerable damage to structures. Very high waves form in long, overhanging crests Violent storm: Widespread damage. Exceptionally high waves form, and the ocean is completely covered in foam
10
55 to 63
11
63 to 72
12
73 and above
Hurricane: Devastating damage. On the ocean, the air is filled with foam and spray
Source: https://www.weather.gov/mfl/beaufort.
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