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San Francisco
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Francisco clear sky chart from 2005-11-05 to 2008-11-27.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	8% 5% 9% 51%	12261
	Night	10% 6% 9% 47%	12170
Transparency	Day	13% 22% 28% 16%	9416
	Night	13% 21% 25% 16%	9266
Seeing	Day	14% 32% 39% 8%	850
	Night	8% 16% 30% 29% 9%	3692

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	17% 7% 5% 43%	823
	Night	18% 6% 6% 41%	1214
February	Day	17% 6% 5% 5% 6% 8% 36%	763
	Night	16% 6% 6% 5% 5% 7% 38%	1016
March	Day	9% 5% 5% 6% 7% 10% 38%	1019
	Night	13% 6% 5% 5% 5% 9% 40%	1021
April	Day	12% 5% 8% 46%	1077
	Night	11% 5% 5% 5% 5% 5% 9% 42%	899
May	Day	6% 11% 56%	1233
	Night	6% 9% 56%	812
June	Day	9% 63%	1260
	Night	5% 10% 59%	720
July	Day	6% 5% 11% 57%	1237
	Night	9% 15% 48%	804
August	Day	5% 10% 61%	1152
	Night	7% 5% 5% 14% 46%	885
September	Day	10% 64%	1031
	Night	8% 6% 8% 56%	943
October	Day	8% 6% 58%	920
	Night	6% 5% 5% 60%	1113
November	Day	13% 9% 5% 46%	1002
	Night	10% 7% 9% 47%	1443
December	Day	25% 5% 6% 7% 33%	744
	Night	24% 6% 5% 6% 38%	1300

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	10% 16% 16% 22% 6%	619
	Night	6% 17% 21% 18% 9%	928
February	Day	5% 20% 32% 18% 5%	503
	Night	7% 19% 25% 19% 7%	701
March	Day	12% 23% 29% 16%	714
	Night	7% 25% 31% 18%	672
April	Day	5% 24% 37% 19%	726
	Night	5% 25% 31% 23%	597
May	Day	20% 32% 24% 9%	1017
	Night	18% 27% 33% 10%	632
June	Day	11% 19% 36% 25%	1036
	Night	7% 15% 32% 31%	578
July	Day	11% 36% 34%	778
	Night	12% 32% 36%	547
August	Day	10% 46% 27%	1033
	Night	12% 34% 29%	772
September	Day	18% 36% 26%	946
	Night	13% 32% 30%	846
October	Day	8% 20% 24% 22%	810
	Night	8% 20% 24% 24%	991
November	Day	5% 16% 18% 22% 12%	762
	Night	6% 16% 22% 20% 13%	1166
December	Day	8% 24% 25% 19%	472
	Night	6% 18% 24% 23%	836

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Day	9% 37% 28% 12%	75
	Night	10% 20% 32% 23%	350
February	Day	16% 25% 32% 16%	67
	Night	11% 16% 32% 27% 6%	286
March	Day	26% 41% 25%	84
	Night	16% 25% 33% 19%	336
April	Day	30% 38% 25%	76
	Night	13% 28% 30% 20%	325
May	Day	20% 32% 35% 6%	88
	Night	9% 17% 31% 31% 7%	248
June	Day	15% 43% 32% 6%	88
	Night	6% 23% 34% 30% 5%	169
July	Day	29% 61% 6%	78
	Night	9% 23% 42% 15%	216
August	Day	26% 67%	89
	Night	11% 25% 41% 14%	315
September	Day	5% 25% 54% 13%	87
	Night	9% 24% 40% 18%	350
October	Day	14% 29% 31% 13%	91
	Night	7% 14% 25% 33% 14%	359
November	Day	14% 25% 25% 22%	27
	Night	6% 11% 35% 32% 9%	420
December	Night	12% 17% 34% 21% 6%	318

Page generated at 2008-11-27 10:47:51. Version ab11