Palomar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Palomar 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% 5% 65%	8666
	Night	10% 5% 59%	8502
Transparency	Day	8% 17% 32% 36%	7077
	Night	10% 5% 16% 29% 37%	6670
Seeing	Day	13% 40% 42%	608
	Night	5% 15% 32% 39% 7%	2905

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% 47%	579
	Night	20% 5% 46%	773
February	Day	34% 5% 5% 6% 29%	561
	Night	33% 5% 5% 5% 31%	684
March	Day	10% 6% 59%	664
	Night	11% 5% 56%	698
April	Day	6% 5% 65%	720
	Night	8% 5% 5% 5% 58%	600
May	Day	5% 7% 64%	801
	Night	5% 5% 5% 8% 54%	562
June	Day	85%	780
	Night	5% 9% 73%	537
July	Day	76%	786
	Night	5% 74%	566
August	Day	7% 69%	1114
	Night	5% 6% 70%	929
September	Day	79%	995
	Night	5% 77%	951
October	Day	13% 5% 69%	649
	Night	9% 62%	714
November	Day	14% 55%	468
	Night	12% 5% 56%	698
December	Day	25% 54%	549
	Night	20% 5% 50%	790

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	10% 7% 29% 32% 20%	337
	Night	11% 5% 22% 36% 24%	454
February	Day	20% 26% 29% 20%	233
	Night	15% 9% 28% 27% 18%	290
March	Day	10% 7% 22% 28% 29%	464
	Night	12% 5% 19% 28% 33%	458
April	Day	10% 5% 27% 37% 19%	558
	Night	12% 23% 30% 28%	438
May	Day	10% 6% 21% 29% 31%	675
	Night	14% 6% 12% 26% 39%	421
June	Day	5% 16% 44% 30%	746
	Night	19% 15% 34% 26%	506
July	Day	5% 17% 31% 41%	699
	Night	9% 10% 34% 42%	499
August	Day	10% 5% 16% 28% 38%	957
	Night	11% 7% 15% 28% 38%	791
September	Day	5% 27% 61%	930
	Night	5% 7% 25% 57%	877
October	Day	15% 35% 39%	794
	Night	7% 7% 14% 28% 42%	847
November	Day	15% 6% 22% 33% 22%	343
	Night	11% 7% 31% 22% 27%	574
December	Day	9% 34% 50%	341
	Night	8% 8% 35% 45%	515

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% 34% 38%	26
	Night	7% 19% 40% 27%	207
February	Day	36% 53% 9%	41
	Night	15% 36% 33% 14%	167
March	Day	20% 34% 37% 6%	58
	Night	8% 21% 31% 32% 6%	233
April	Day	28% 50% 18%	53
	Night	10% 18% 40% 27%	224
May	Day	18% 45% 33%	59
	Night	23% 36% 34%	187
June	Day	6% 40% 50%	60
	Night	7% 38% 47%	180
July	Day	30% 67%	62
	Night	21% 61% 13%	194
August	Day	25% 68%	89
	Night	19% 57% 19%	364
September	Day	7% 44% 40% 6%	88
	Night	7% 27% 51% 12%	360
October	Day	9% 47% 34% 8%	72
	Night	10% 31% 46% 7%	351
November	Night	9% 26% 40% 19%	231
December	Night	8% 25% 40% 19% 5%	207

Page generated at 2008-05-11 01:45:52. Version ab11