Meteorological Matters...

The first of January saw Mark negotiate LA and head southeast past Long Beach in the direction of San Diego reaching the town of San Clemente by nightfall. It is impossible to do justice to the geography of this sprawling urban area in a blog and so I have decided instead to select a theme suggested by a comment in the web diary yesterday....

"Mark was fighting a big head wind as we have Santa Ana winds blowing today".

Santa Ana winds are often referred to in the context of wildfires in California but until this morning I knew very little else about them. However, a bit of research has produced the following.....

The Santa Ana winds blow seasonally in south California (from autumn through to spring) with a peak in December/January. They originate in the Great Basin (desert) between the Sierra Nevada and the Rockies reaching the Pacific coast in the vicinity of Los Angeles. They are typically very blustery, dry and warm (accounting for the high temperatures Mark noted yesterday ) but may be problematic for the coastal area they cross. As recently as two days ago they were making news in the region. Click here and here to read some newspaper coverage.

Like the Fohn in Europe or the Chinook in the Prairies of Canada, the Santa Ana winds are katabatic. This means that they are descending winds. High pressure forms over the Great Basin at this time of year as cool temperatures cause air to sink. As the air subsides towards the surface of the earth, its humidity drops and then drops again as the air moves outwards from the centre of high pressure down towards the coast. The speed of the wind often increases as it is channelled through gaps in the coastal hills.

The Santa Ana winds are grounded in local legend and literature. In Raymond Chandler's story Red Wind, the title being one of the offshore wind's many nicknames, the Santa Anas were introduced as "those hot dry winds that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch. On nights like that every booze party ends in a fight. Meek little wives feel the edge of the carving knife and study their husbands' necks. Anything can happen." Scary stuff!

In October and November 2007 wildfires fanned by Santa Ana winds caused huge devastation throughout southern California.

The NASA image above shows the plumes of smoke from those fires being blown westwards towards the ocean by the winds. Click the image to read about those fires.

The good news for Mark, according to this report, is that the Santa Ana winds seem set to abate in the near future.

The second 'meteorological matter' of this posting is fog...... not that Mark has encountered any but the Google Earth imagery of the stretch of coastline he has just negotiated clearly shows a bank of fog lying just offshore. Fog is very common on the Californian coast - especially in summer and it is not dissimilar to the haar which frequently spoils nice summer days on the east coast of Scotland! Basically, warm air rises over the hot land and humid air from the sea is drawn in to replace it. However, just off shore, a cold ocean current wells up to the surface of the Pacific. As the warm, moist air crosses it, it condenses giving rise to fog..... like this image of the Californian coast from Flickr...