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OCA Anza Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the OCA Anza Observatory clear sky chart from 2003-08-01 to 2005-11-07.

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	9% 66%	8665
	Night	10% 5% 60%	8505
Transparency	Day	6% 16% 32% 39%	6963
	Night	6% 5% 16% 32% 39%	6543
Seeing	Day	14% 43% 36% 5%	581
	Night	5% 18% 31% 36% 8%	2819

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	19% 5% 46%	577
	Night	20% 5% 5% 43%	775
February	Day	32% 6% 6% 6% 29%	561
	Night	31% 6% 5% 5% 30%	684
March	Day	11% 6% 60%	663
	Night	11% 5% 5% 56%	699
April	Day	5% 7% 66%	720
	Night	8% 5% 7% 56%	600
May	Day	5% 69%	802
	Night	6% 9% 57%	561
June	Day	91%	780
	Night	7% 81%	537
July	Day	76%	789
	Night	5% 73%	564
August	Day	6% 68%	1114
	Night	5% 70%	929
September	Day	81%	994
	Night	80%	952
October	Day	13% 71%	648
	Night	8% 5% 65%	716
November	Day	14% 5% 57%	468
	Night	10% 6% 5% 57%	698
December	Day	24% 54%	549
	Night	21% 51%	790

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	12% 6% 29% 38% 12%	339
	Night	11% 6% 21% 40% 19%	441
February	Day	13% 5% 31% 31% 17%	219
	Night	15% 10% 30% 29% 14%	280
March	Day	7% 9% 21% 32% 28%	464
	Night	8% 6% 19% 32% 32%	452
April	Day	10% 5% 23% 36% 23%	582
	Night	7% 20% 35% 32%	434
May	Day	5% 6% 19% 29% 37%	661
	Night	8% 8% 12% 27% 43%	437
June	Day	17% 36% 40%	748
	Night	17% 37% 38%	504
July	Day	5% 13% 32% 46%	666
	Night	7% 14% 30% 45%	481
August	Day	7% 5% 15% 29% 41%	883
	Night	5% 17% 34% 39%	729
September	Day	6% 21% 66%	933
	Night	7% 25% 60%	877
October	Day	12% 38% 41%	789
	Night	5% 14% 31% 43%	847
November	Day	12% 23% 35% 23%	338
	Night	7% 7% 30% 26% 27%	551
December	Day	9% 36% 49%	341
	Night	9% 41% 41%	510

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	23% 52% 23%	21
	Night	8% 23% 36% 27%	202
February	Day	44% 47% 7%	38
	Night	11% 40% 30% 16%	154
March	Day	23% 34% 25% 16%	55
	Night	9% 23% 28% 32% 5%	221
April	Day	27% 49% 21%	51
	Night	9% 33% 29% 25%	218
May	Day	19% 51% 28%	56
	Night	5% 30% 38% 21%	181
June	Day	8% 45% 44%	59
	Night	15% 36% 43%	180
July	Day	40% 52%	61
	Night	25% 57% 12%	190
August	Day	35% 58%	87
	Night	21% 55% 18%	352
September	Day	10% 45% 36% 6%	87
	Night	8% 34% 44% 11%	358
October	Day	12% 43% 31% 10%	66
	Night	10% 32% 42% 9%	349
November	Night	10% 29% 35% 21%	217
December	Night	11% 22% 39% 19% 6%	197

Page generated at 2008-05-14 10:55:49. Version ab11