Cedar Drive Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cedar Drive Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	12% 8% 3 7% 5% 4 5% 4 6% 7% 39%	126968
	Night	12% 9% 3 8% 5% 4 4 3 5% 7% 39%	123327
Transparency	Day	43% 4 13% 26% 13% 1	130430
	Night	44% 4 12% 24% 14% 2	127926
Seeing	Day	16% 9% 29% 27% 14% 4	17373
	Night	17% 8% 31% 32% 12% 1	27198

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	8% 8% 3 10% 6% 6% 6% 4 6% 6% 37%	3617
	11th to 20th	Day	9% 12% 4 8% 5% 6% 6% 4 5% 7% 34%	3620
	21st to end	Day	11% 12% 4 12% 6% 5% 5% 3 4 5% 32%	3982
February	1st to 10th	Day	9% 11% 4 10% 6% 4 4 4 6% 7% 34%	1767
	11th to 20th	Day	10% 10% 3 8% 4 5% 5% 4 6% 6% 39%	1897
	21st to end	Day	11% 12% 3 7% 6% 4 4 5% 6% 8% 35%	1650
March	1st to 10th	Day	12% 8% 2 5% 3 3 3 4 6% 7% 48%	4480
	11th to 20th	Day	10% 6% 3 7% 4 5% 5% 5% 6% 9% 40%	4688
	21st to end	Day	23% 11% 3 8% 4 4 4 3 4 5% 30%	5967
April	1st to 10th	Day	15% 12% 4 7% 5% 4 5% 4 5% 6% 32%	2400
	11th to 20th	Day	19% 9% 3 6% 5% 4 4 4 5% 5% 36%	2374
	21st to end	Day	17% 10% 4 7% 5% 5% 5% 4 5% 6% 33%	2560
May	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 4 7% 46%	6045
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	6319
	21st to end	Day	15% 7% 3 6% 5% 4 5% 4 6% 7% 39%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 3 6% 7% 44%	2800
	11th to 20th	Day	14% 7% 2 6% 5% 4 5% 4 6% 8% 39%	2800
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	2800
July	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 7% 10% 41%	6360
	11th to 20th	Day	6% 3 2 5% 4 4 5% 4 7% 11% 51%	6360
	21st to end	Day	7% 6% 2 7% 5% 5% 4 4 6% 8% 45%	6924
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 42%	4563
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 9% 47%	4063
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 4 6% 8% 41%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 4 4 4 6% 8% 43%	2400
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 3 6% 8% 47%	2295
	21st to end	Day	12% 7% 3 8% 4 5% 5% 4 5% 8% 41%	2200
October	1st to 10th	Day	13% 8% 3 6% 3 4 4 4 5% 7% 45%	2096
	11th to 20th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 6% 31%	1952
	21st to end	Day	17% 8% 3 9% 4 4 4 3 4 6% 37%	1980
November	1st to 10th	Day	13% 8% 3 7% 4 4 4 3 5% 8% 41%	1780
	11th to 20th	Day	12% 10% 4 9% 7% 5% 5% 3 7% 7% 32%	1782
	21st to end	Day	15% 10% 3 6% 5% 5% 4 5% 5% 7% 35%	1600
December	1st to 10th	Day	9% 12% 4 9% 6% 5% 6% 5% 7% 6% 32%	2610
	11th to 20th	Day	12% 17% 4 9% 5% 5% 4 4 5% 5% 30%	3008
	21st to end	Day	19% 19% 6% 12% 6% 5% 4 3 4 4 19%	3982
January	1st to 10th	Night	9% 12% 5% 8% 4 4 4 3 4 6% 40%	6348
	11th to 20th	Night	10% 12% 5% 12% 6% 5% 5% 3 5% 6% 32%	6360
	21st to end	Night	9% 14% 6% 10% 6% 5% 5% 4 5% 5% 32%	6996
February	1st to 10th	Night	10% 12% 5% 9% 7% 4 4 3 5% 5% 35%	2630
	11th to 20th	Night	9% 11% 4 9% 6% 5% 4 3 5% 6% 38%	2388
	21st to end	Night	14% 12% 3 8% 5% 5% 4 3 5% 5% 35%	1980
March	1st to 10th	Night	14% 7% 2 5% 4 3 4 3 5% 6% 48%	5373
	11th to 20th	Night	10% 9% 3 6% 5% 4 5% 4 6% 9% 39%	5314
	21st to end	Night	21% 10% 3 9% 5% 4 4 3 4 6% 31%	4962
April	1st to 10th	Night	18% 11% 2 7% 4 4 4 3 5% 6% 35%	2000
	11th to 20th	Night	21% 7% 3 6% 4 4 4 3 5% 6% 38%	1975
	21st to end	Night	16% 9% 3 6% 5% 4 5% 3 5% 7% 37%	1840
May	1st to 10th	Night	14% 8% 3 7% 3 4 4 3 5% 7% 42%	3843
	11th to 20th	Night	9% 6% 2 6% 4 4 4 5% 6% 9% 44%	3596
	21st to end	Night	18% 8% 2 7% 4 4 5% 3 5% 6% 39%	3942
June	1st to 10th	Night	10% 6% 2 5% 5% 4 5% 4 6% 7% 46%	1600
	11th to 20th	Night	13% 7% 2 7% 4 5% 4 4 7% 8% 40%	1600
	21st to end	Night	9% 6% 2 6% 4 4 4 5% 7% 8% 46%	1600
July	1st to 10th	Night	11% 6% 2 6% 4 5% 5% 3 5% 8% 44%	3618
	11th to 20th	Night	7% 4 2 5% 3 3 3 3 6% 9% 55%	3617
	21st to end	Night	5% 4 2 7% 5% 4 4 4 5% 9% 51%	3947
August	1st to 10th	Night	10% 6% 3 7% 4 4 4 3 6% 8% 45%	2597
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 6% 47%	2971
	21st to end	Night	8% 6% 2 6% 5% 4 4 3 6% 6% 48%	3586
September	1st to 10th	Night	8% 5% 2 6% 4 4 3 3 5% 7% 53%	2000
	11th to 20th	Night	12% 4 2 4 3 3 3 3 5% 7% 54%	2085
	21st to end	Night	11% 6% 2 6% 4 4 5% 3 5% 7% 48%	2200
October	1st to 10th	Night	12% 6% 3 5% 3 4 4 3 5% 7% 48%	2300
	11th to 20th	Night	16% 7% 3 6% 4 5% 5% 4 6% 6% 38%	2446
	21st to end	Night	17% 10% 3 7% 4 4 4 4 5% 6% 35%	2860
November	1st to 10th	Night	13% 9% 3 5% 4 4 3 3 6% 8% 43%	2563
	11th to 20th	Night	13% 11% 4 9% 5% 5% 4 3 5% 6% 35%	2615
	21st to end	Night	14% 11% 3 7% 5% 4 4 4 5% 7% 37%	2800
December	1st to 10th	Night	11% 10% 3 8% 4 4 4 4 6% 7% 40%	4580
	11th to 20th	Night	12% 17% 6% 11% 6% 5% 4 2 4 4 29%	5199
	21st to end	Night	21% 16% 5% 10% 5% 4 4 3 4 5% 23%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	46% 3 17% 22% 11% 1	3618
	11th to 20th	Day	51% 3 11% 22% 12% 2	3620
	21st to end	Day	56% 3 10% 19% 12%	3982
February	1st to 10th	Day	49% 4 12% 22% 12% 1	1767
	11th to 20th	Day	46% 4 12% 24% 13% 2	1907
	21st to end	Day	47% 5% 14% 23% 9% 2	1650
March	1st to 10th	Day	37% 2 15% 22% 17% 6%	4480
	11th to 20th	Day	40% 4 15% 24% 16% 2	4688
	21st to end	Day	58% 3 10% 18% 11%	5969
April	1st to 10th	Day	52% 5% 11% 21% 11% 1	2400
	11th to 20th	Day	50% 3 9% 23% 14% 2	2374
	21st to end	Day	52% 4 10% 20% 12% 2	2560
May	1st to 10th	Day	39% 3 11% 28% 17% 2	6080
	11th to 20th	Day	38% 6% 11% 29% 16% 1	6323
	21st to end	Day	43% 5% 10% 26% 16% 1	6949
June	1st to 10th	Day	38% 4 12% 29% 15% 2	2800
	11th to 20th	Day	41% 5% 14% 24% 14% 1	2800
	21st to end	Day	38% 6% 15% 27% 13% 2	2800
July	1st to 10th	Day	37% 5% 14% 33% 10% 1	6291
	11th to 20th	Day	26% 6% 16% 36% 14% 1	6285
	21st to end	Day	34% 6% 14% 26% 19% 1	6728
August	1st to 10th	Day	36% 7% 16% 26% 14% 1	4548
	11th to 20th	Day	33% 5% 14% 35% 12%	4103
	21st to end	Day	39% 7% 14% 24% 15% 1	4311
September	1st to 10th	Day	38% 5% 14% 27% 14% 1	2400
	11th to 20th	Day	35% 4 18% 32% 11% 1	2295
	21st to end	Day	42% 4 13% 30% 11%	2200
October	1st to 10th	Day	26% 2 11% 35% 25% 1	3408
	11th to 20th	Day	45% 4 9% 28% 14% 1	3155
	21st to end	Day	43% 4 14% 29% 11%	3180
November	1st to 10th	Day	41% 4 13% 27% 14% 1	1780
	11th to 20th	Day	51% 4 14% 20% 10% 1	1782
	21st to end	Day	49% 5% 17% 23% 6% 1	1600
December	1st to 10th	Day	50% 6% 16% 23% 5% 1	2610
	11th to 20th	Day	56% 4 10% 15% 13% 1	3005
	21st to end	Day	71% 3 5% 15% 6%	3982
January	1st to 10th	Night	46% 3 10% 27% 13% 1	6348
	11th to 20th	Night	54% 3 9% 18% 15% 1	6360
	21st to end	Night	54% 4 12% 18% 12%	6996
February	1st to 10th	Night	51% 3 9% 23% 12% 2	2630
	11th to 20th	Night	47% 4 10% 24% 13% 2	2389
	21st to end	Night	51% 3 12% 21% 11% 2	1980
March	1st to 10th	Night	38% 3 15% 26% 14% 3	5373
	11th to 20th	Night	42% 3 16% 23% 14% 2	5314
	21st to end	Night	55% 3 10% 18% 12% 1	4962
April	1st to 10th	Night	50% 4 7% 19% 19% 1	2000
	11th to 20th	Night	47% 3 8% 23% 15% 3	1975
	21st to end	Night	46% 3 11% 24% 14% 1	1840
May	1st to 10th	Night	42% 3 12% 21% 18% 3	3870
	11th to 20th	Night	36% 6% 14% 26% 17% 2	3597
	21st to end	Night	47% 3 10% 24% 13% 3	3950
June	1st to 10th	Night	36% 5% 12% 31% 15% 1	1600
	11th to 20th	Night	42% 4 11% 25% 17% 2	1600
	21st to end	Night	34% 6% 16% 25% 15% 3	1600
July	1st to 10th	Night	36% 6% 15% 30% 9% 4	3580
	11th to 20th	Night	26% 5% 16% 35% 16% 2	3572
	21st to end	Night	30% 5% 16% 25% 22% 2	3832
August	1st to 10th	Night	35% 5% 13% 27% 18% 1	2588
	11th to 20th	Night	36% 6% 13% 29% 15% 1	2997
	21st to end	Night	34% 6% 14% 24% 20% 3	3586
September	1st to 10th	Night	31% 4 12% 32% 20% 1	2000
	11th to 20th	Night	31% 3 14% 36% 15% 1	2085
	21st to end	Night	36% 3 13% 31% 15% 1	2200
October	1st to 10th	Night	28% 3 8% 39% 19% 2	3755
	11th to 20th	Night	36% 5% 12% 29% 16% 2	3982
	21st to end	Night	47% 4 10% 24% 16%	4610
November	1st to 10th	Night	40% 4 12% 26% 15% 2	2563
	11th to 20th	Night	50% 4 12% 20% 13% 1	2615
	21st to end	Night	48% 4 14% 26% 8% 1	2800
December	1st to 10th	Night	43% 5% 22% 23% 6% 1	4580
	11th to 20th	Night	60% 3 10% 14% 12% 1	5201
	21st to end	Night	64% 4 8% 15% 8%	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 10% 33% 31% 11% 2	300
	11th to 20th	Day	19% 10% 34% 29% 7% 1	300
	21st to end	Day	19% 10% 29% 32% 8% 1	330
February	1st to 10th	Day	15% 9% 39% 27% 8% 2	300
	11th to 20th	Day	11% 14% 42% 22% 10% 1	300
	21st to end	Day	18% 14% 40% 25% 3	249
March	1st to 10th	Day	16% 15% 36% 23% 9%	300
	11th to 20th	Day	19% 11% 32% 28% 8% 2	314
	21st to end	Day	27% 8% 27% 27% 7% 3	550
April	1st to 10th	Day	23% 14% 31% 23% 8% 1	500
	11th to 20th	Day	27% 15% 27% 20% 10% 2	492
	21st to end	Day	22% 10% 31% 21% 14% 2	500
May	1st to 10th	Day	18% 10% 32% 28% 9% 3	582
	11th to 20th	Day	17% 11% 30% 27% 11% 4	725
	21st to end	Day	17% 9% 27% 28% 13% 7%	789
June	1st to 10th	Day	10% 8% 34% 29% 14% 4	730
	11th to 20th	Day	17% 9% 32% 26% 13% 4	730
	21st to end	Day	13% 7% 31% 30% 15% 4	730
July	1st to 10th	Day	9% 4 21% 32% 25% 9%	721
	11th to 20th	Day	8% 5% 26% 33% 20% 8%	718
	21st to end	Day	7% 5% 24% 33% 22% 9%	794
August	1st to 10th	Day	8% 5% 22% 33% 24% 9%	726
	11th to 20th	Day	13% 5% 17% 29% 26% 11%	565
	21st to end	Day	13% 6% 21% 27% 23% 10%	583
September	1st to 10th	Day	16% 7% 22% 28% 20% 8%	530
	11th to 20th	Day	11% 3 22% 30% 25% 8%	530
	21st to end	Day	14% 8% 28% 24% 21% 6%	530
October	1st to 10th	Day	15% 11% 27% 27% 16% 4	426
	11th to 20th	Day	19% 17% 33% 24% 8%	330
	21st to end	Day	21% 14% 32% 25% 7% 1	363
November	1st to 10th	Day	17% 12% 38% 24% 9% 1	306
	11th to 20th	Day	16% 17% 38% 20% 8% 1	300
	21st to end	Day	24% 8% 39% 23% 5%	300
December	1st to 10th	Day	15% 15% 41% 23% 6%	300
	11th to 20th	Day	21% 15% 39% 17% 6% 1	300
	21st to end	Day	28% 13% 32% 18% 7% 1	330
January	1st to 10th	Night	15% 11% 37% 29% 7%	1000
	11th to 20th	Night	19% 12% 37% 25% 7% 1	1000
	21st to end	Night	18% 8% 37% 29% 7% 1	1100
February	1st to 10th	Night	15% 11% 38% 28% 7% 1	1000
	11th to 20th	Night	15% 11% 39% 26% 8%	833
	21st to end	Night	20% 10% 35% 30% 4	660
March	1st to 10th	Night	18% 9% 34% 31% 8% 1	800
	11th to 20th	Night	15% 12% 36% 28% 7% 1	767
	21st to end	Night	22% 7% 32% 30% 8% 1	660
April	1st to 10th	Night	23% 8% 37% 27% 4 1	600
	11th to 20th	Night	26% 7% 31% 29% 7%	593
	21st to end	Night	20% 7% 32% 32% 8% 1	600
May	1st to 10th	Night	22% 6% 33% 31% 8% 1	600
	11th to 20th	Night	17% 7% 33% 33% 10%	591
	21st to end	Night	14% 5% 26% 38% 16% 1	650
June	1st to 10th	Night	11% 7% 27% 41% 14% 1	600
	11th to 20th	Night	16% 7% 26% 39% 12% 1	600
	21st to end	Night	11% 5% 26% 42% 16% 1	600
July	1st to 10th	Night	7% 2 19% 48% 22% 2	585
	11th to 20th	Night	8% 4 22% 41% 24% 1	582
	21st to end	Night	6% 2 21% 48% 23% 1	657
August	1st to 10th	Night	9% 2 16% 47% 25% 1	597
	11th to 20th	Night	10% 3 15% 41% 28% 3	594
	21st to end	Night	9% 3 20% 42% 23% 2	660
September	1st to 10th	Night	11% 4 24% 35% 23% 4	600
	11th to 20th	Night	13% 5% 22% 34% 23% 3	685
	21st to end	Night	14% 6% 23% 37% 20% 2	800
October	1st to 10th	Night	15% 9% 26% 32% 18% 1	800
	11th to 20th	Night	18% 10% 34% 29% 8%	800
	21st to end	Night	22% 10% 31% 27% 10% 1	880
November	1st to 10th	Night	17% 10% 39% 27% 7% 1	789
	11th to 20th	Night	18% 14% 38% 23% 6%	815
	21st to end	Night	19% 12% 36% 26% 7%	1000
December	1st to 10th	Night	18% 11% 39% 25% 7%	1000
	11th to 20th	Night	21% 10% 36% 25% 7% 1	1000
	21st to end	Night	26% 11% 33% 26% 4	1100

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