Hidden Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hidden Creek Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	127071
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	124635
Transparency	Day	38% 6% 12% 22% 17% 4	130673
	Night	38% 5% 11% 22% 19% 5%	129102
Seeing	Day	19% 11% 28% 24% 13% 5%	16567
	Night	17% 10% 29% 30% 13% 1	26330

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	16% 5% 2 4 3 3 3 3 4 6% 51%	4097
	11th to 20th	Day	14% 8% 2 5% 3 3 3 3 4 4 51%	4114
	21st to end	Day	15% 7% 2 6% 4 4 4 4 4 5% 44%	4466
February	1st to 10th	Day	14% 7% 2 6% 6% 4 5% 4 6% 6% 40%	1800
	11th to 20th	Day	21% 7% 2 6% 3 3 3 3 4 6% 41%	1783
	21st to end	Day	17% 8% 3 6% 5% 3 5% 3 5% 7% 39%	1683
March	1st to 10th	Day	17% 6% 2 6% 5% 4 5% 4 5% 6% 40%	4935
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 5% 7% 52%	4980
	21st to end	Day	16% 6% 2 6% 5% 5% 4 3 5% 6% 41%	5465
April	1st to 10th	Day	12% 5% 3 5% 4 4 4 4 5% 6% 48%	2392
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 5% 7% 49%	2374
	21st to end	Day	16% 6% 2 5% 4 5% 4 4 6% 6% 43%	2400
May	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 7% 10% 33%	5812
	11th to 20th	Day	20% 8% 2 6% 4 4 5% 4 6% 9% 31%	5861
	21st to end	Day	15% 5% 2 5% 4 4 4 4 6% 10% 41%	6923
June	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 7% 10% 43%	2800
	11th to 20th	Day	12% 4 1 5% 4 5% 5% 4 7% 10% 41%	2800
	21st to end	Day	9% 3 1 5% 6% 4 5% 5% 7% 10% 44%	2800
July	1st to 10th	Day	8% 4 2 5% 4 5% 5% 5% 8% 11% 43%	6227
	11th to 20th	Day	9% 6% 2 7% 6% 6% 6% 6% 10% 13% 29%	5900
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 6% 7% 10% 40%	6424
August	1st to 10th	Day	16% 6% 3 6% 5% 5% 5% 5% 8% 10% 32%	4234
	11th to 20th	Day	9% 5% 2 7% 6% 5% 6% 5% 7% 9% 39%	4024
	21st to end	Day	9% 4 2 5% 3 4 5% 3 6% 10% 49%	4312
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 5% 4 6% 47%	2400
	11th to 20th	Day	13% 6% 2 5% 4 4 4 4 6% 7% 45%	2370
	21st to end	Day	22% 7% 2 5% 5% 3 4 4 6% 7% 36%	2100
October	1st to 10th	Day	14% 6% 2 5% 4 4 3 3 4 6% 50%	2000
	11th to 20th	Day	16% 4 1 5% 4 4 4 3 4 4 52%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 3 3 5% 53%	2200
November	1st to 10th	Day	12% 4 1 4 4 4 4 2 4 5% 55%	2057
	11th to 20th	Day	14% 6% 2 3 3 3 3 3 5% 6% 51%	1860
	21st to end	Day	16% 5% 1 4 4 3 3 4 5% 5% 50%	1680
December	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 5% 5% 50%	2680
	11th to 20th	Day	18% 7% 2 6% 2 2 3 3 4 6% 46%	3070
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 6% 40%	4048
January	1st to 10th	Night	13% 7% 2 4 3 3 3 3 4 5% 53%	6051
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 4 55%	5979
	21st to end	Night	15% 7% 3 5% 3 3 3 3 4 4 49%	6490
February	1st to 10th	Night	14% 7% 2 6% 4 4 5% 4 6% 6% 43%	2600
	11th to 20th	Night	20% 8% 2 5% 3 4 3 3 3 5% 43%	2502
	21st to end	Night	15% 8% 2 6% 4 4 3 2 5% 5% 46%	1945
March	1st to 10th	Night	15% 6% 2 5% 3 3 4 3 5% 6% 47%	4918
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 8% 52%	4980
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 7% 46%	5467
April	1st to 10th	Night	15% 7% 2 6% 4 4 3 3 5% 7% 46%	2007
	11th to 20th	Night	11% 6% 2 4 4 3 4 4 6% 8% 48%	1975
	21st to end	Night	14% 6% 1 5% 4 4 4 4 6% 8% 45%	2000
May	1st to 10th	Night	17% 8% 3 8% 4 4 5% 4 6% 9% 32%	4124
	11th to 20th	Night	22% 8% 3 7% 4 4 4 4 6% 8% 32%	4032
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 9% 37%	3929
June	1st to 10th	Night	10% 6% 2 6% 5% 4 5% 4 7% 10% 43%	1600
	11th to 20th	Night	11% 5% 3 5% 5% 4 5% 4 7% 9% 41%	1600
	21st to end	Night	8% 5% 2 5% 5% 5% 6% 5% 7% 10% 43%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 8% 10% 38%	3726
	11th to 20th	Night	11% 8% 3 9% 6% 6% 6% 5% 7% 11% 27%	4057
	21st to end	Night	7% 7% 3 7% 6% 7% 7% 5% 8% 8% 36%	4427
August	1st to 10th	Night	16% 5% 3 8% 5% 5% 6% 4 6% 9% 33%	2915
	11th to 20th	Night	10% 7% 3 9% 5% 4 5% 5% 6% 9% 37%	2992
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 5% 9% 43%	3586
September	1st to 10th	Night	14% 5% 2 6% 3 3 5% 4 6% 7% 46%	2000
	11th to 20th	Night	12% 6% 2 6% 3 3 4 4 4 7% 47%	2025
	21st to end	Night	22% 7% 3 7% 4 3 3 3 5% 5% 38%	2298
October	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 6% 48%	2400
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 49%	2400
	21st to end	Night	14% 4 2 5% 3 3 3 3 5% 5% 52%	2640
November	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 5% 5% 52%	2461
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 6% 49%	2754
	21st to end	Night	12% 4 1 5% 3 4 4 4 6% 5% 51%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 53%	4720
	11th to 20th	Night	19% 7% 3 5% 4 4 4 3 4 6% 41%	5345
	21st to end	Night	16% 7% 2 5% 3 4 4 3 6% 7% 43%	7150

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	36% 3 5% 26% 21% 10%	4098
	11th to 20th	Day	36% 4 6% 19% 24% 11%	4114
	21st to end	Day	41% 4 7% 25% 19% 4	4466
February	1st to 10th	Day	43% 5% 10% 27% 13% 3	1800
	11th to 20th	Day	44% 4 6% 21% 19% 4	1792
	21st to end	Day	46% 4 10% 17% 18% 5%	1683
March	1st to 10th	Day	43% 5% 8% 19% 16% 9%	4935
	11th to 20th	Day	32% 5% 8% 24% 23% 9%	4980
	21st to end	Day	43% 5% 11% 17% 17% 6%	5465
April	1st to 10th	Day	36% 5% 11% 23% 18% 8%	2392
	11th to 20th	Day	36% 4 11% 20% 22% 6%	2374
	21st to end	Day	41% 6% 11% 24% 14% 4	2400
May	1st to 10th	Day	45% 8% 15% 18% 12% 1	5846
	11th to 20th	Day	48% 7% 12% 21% 11% 1	5865
	21st to end	Day	38% 6% 15% 21% 18% 2	6938
June	1st to 10th	Day	34% 9% 16% 26% 14% 2	2800
	11th to 20th	Day	36% 9% 15% 22% 16% 2	2800
	21st to end	Day	32% 8% 16% 26% 16% 2	2800
July	1st to 10th	Day	34% 8% 21% 20% 13% 4	6159
	11th to 20th	Day	42% 10% 21% 21% 6% 1	5831
	21st to end	Day	37% 7% 14% 22% 19%	6240
August	1st to 10th	Day	45% 7% 17% 24% 7%	4221
	11th to 20th	Day	38% 8% 16% 24% 14% 1	4063
	21st to end	Day	30% 6% 17% 28% 17% 2	4312
September	1st to 10th	Day	37% 6% 12% 18% 21% 4	2400
	11th to 20th	Day	37% 6% 8% 25% 19% 4	2370
	21st to end	Day	46% 6% 11% 18% 16% 4	2100
October	1st to 10th	Day	35% 4 7% 19% 31% 5%	3260
	11th to 20th	Day	26% 3 6% 22% 31% 12%	3240
	21st to end	Day	22% 3 9% 29% 24% 13%	3538
November	1st to 10th	Day	33% 4 8% 24% 26% 4	2057
	11th to 20th	Day	34% 3 11% 22% 22% 7%	1860
	21st to end	Day	35% 4 9% 25% 22% 4	1680
December	1st to 10th	Day	35% 4 9% 26% 18% 8%	2680
	11th to 20th	Day	40% 3 13% 24% 15% 4	3066
	21st to end	Day	42% 5% 12% 24% 15% 1	4048
January	1st to 10th	Night	33% 4 6% 22% 26% 9%	6051
	11th to 20th	Night	35% 3 5% 22% 26% 10%	5979
	21st to end	Night	39% 3 6% 26% 21% 5%	6490
February	1st to 10th	Night	41% 5% 10% 24% 16% 3	2600
	11th to 20th	Night	44% 3 7% 20% 19% 6%	2504
	21st to end	Night	41% 3 9% 21% 17% 8%	1945
March	1st to 10th	Night	37% 4 8% 23% 21% 8%	4918
	11th to 20th	Night	30% 5% 8% 21% 27% 9%	4980
	21st to end	Night	38% 4 9% 21% 22% 6%	5467
April	1st to 10th	Night	39% 5% 9% 18% 22% 7%	2007
	11th to 20th	Night	33% 6% 13% 21% 21% 6%	1975
	21st to end	Night	37% 6% 12% 23% 17% 5%	2000
May	1st to 10th	Night	48% 7% 16% 17% 10% 2	4152
	11th to 20th	Night	48% 8% 12% 20% 10% 1	4033
	21st to end	Night	43% 7% 14% 24% 12% 1	3936
June	1st to 10th	Night	36% 9% 13% 25% 17% 1	1600
	11th to 20th	Night	37% 9% 18% 22% 14% 1	1600
	21st to end	Night	33% 8% 15% 23% 18% 3	1600
July	1st to 10th	Night	39% 10% 19% 16% 14% 2	3686
	11th to 20th	Night	49% 10% 20% 16% 5% 1	4006
	21st to end	Night	42% 9% 14% 20% 15%	4297
August	1st to 10th	Night	48% 7% 16% 22% 6% 1	2904
	11th to 20th	Night	42% 7% 15% 24% 12%	3019
	21st to end	Night	37% 6% 12% 28% 15% 1	3586
September	1st to 10th	Night	37% 8% 13% 21% 15% 7%	2000
	11th to 20th	Night	35% 7% 10% 24% 21% 3	2025
	21st to end	Night	48% 5% 10% 18% 16% 3	2298
October	1st to 10th	Night	40% 4 9% 18% 22% 6%	3920
	11th to 20th	Night	25% 3 8% 22% 30% 11%	3899
	21st to end	Night	22% 3 6% 31% 26% 12%	4252
November	1st to 10th	Night	34% 5% 8% 22% 25% 6%	2461
	11th to 20th	Night	36% 5% 7% 20% 24% 7%	2754
	21st to end	Night	34% 4 10% 24% 23% 5%	2940
December	1st to 10th	Night	31% 4 9% 27% 24% 4	4720
	11th to 20th	Night	45% 4 10% 19% 16% 6%	5348
	21st to end	Night	39% 5% 12% 26% 16% 1	7150

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	26% 25% 29% 18% 3	319
	11th to 20th	Day	26% 28% 32% 12% 3	318
	21st to end	Day	24% 22% 40% 11% 3	330
February	1st to 10th	Day	23% 24% 40% 9% 3 1	300
	11th to 20th	Day	40% 22% 29% 9% 1	330
	21st to end	Day	29% 18% 31% 18% 3	414
March	1st to 10th	Day	23% 18% 37% 18% 4	500
	11th to 20th	Day	23% 21% 35% 16% 5% 1	500
	21st to end	Day	26% 17% 36% 17% 4 1	550
April	1st to 10th	Day	24% 19% 32% 18% 5% 1	500
	11th to 20th	Day	20% 15% 30% 22% 9% 3	492
	21st to end	Day	22% 12% 37% 21% 8% 1	500
May	1st to 10th	Day	20% 17% 33% 19% 9% 3	503
	11th to 20th	Day	22% 9% 32% 22% 12% 2	527
	21st to end	Day	19% 6% 28% 26% 18% 3	573
June	1st to 10th	Day	11% 5% 29% 30% 20% 4	530
	11th to 20th	Day	14% 6% 24% 32% 17% 7%	530
	21st to end	Day	10% 5% 26% 32% 22% 5%	530
July	1st to 10th	Day	11% 3 18% 34% 26% 9%	525
	11th to 20th	Day	6% 2 16% 39% 27% 10%	524
	21st to end	Day	10% 4 21% 31% 23% 11%	576
August	1st to 10th	Day	13% 3 21% 29% 24% 9%	526
	11th to 20th	Day	11% 4 19% 31% 26% 9%	519
	21st to end	Day	10% 5% 18% 29% 25% 13%	583
September	1st to 10th	Day	14% 5% 20% 27% 20% 14%	530
	11th to 20th	Day	16% 3 19% 31% 21% 9%	530
	21st to end	Day	28% 9% 21% 28% 10% 5%	530
October	1st to 10th	Day	19% 7% 24% 25% 17% 8%	530
	11th to 20th	Day	17% 13% 23% 27% 17% 2	530
	21st to end	Day	17% 15% 31% 26% 9% 2	583
November	1st to 10th	Day	16% 12% 32% 26% 10% 3	535
	11th to 20th	Day	20% 13% 34% 23% 9% 2	398
	21st to end	Day	22% 17% 32% 20% 8% 1	220
December	1st to 10th	Day	22% 18% 36% 17% 6%	220
	11th to 20th	Day	22% 15% 35% 19% 7% 1	220
	21st to end	Day	25% 19% 31% 17% 7%	242
January	1st to 10th	Night	20% 16% 41% 20% 3	840
	11th to 20th	Night	21% 17% 35% 24% 3	825
	21st to end	Night	20% 15% 42% 21% 2	880
February	1st to 10th	Night	19% 15% 38% 25% 3	800
	11th to 20th	Night	26% 18% 38% 17% 1	783
	21st to end	Night	21% 18% 38% 21% 3	660
March	1st to 10th	Night	18% 18% 37% 24% 3	800
	11th to 20th	Night	22% 17% 35% 21% 6%	800
	21st to end	Night	18% 16% 39% 22% 5%	880
April	1st to 10th	Night	19% 17% 38% 22% 5%	800
	11th to 20th	Night	14% 14% 36% 27% 8%	791
	21st to end	Night	17% 11% 35% 32% 6%	800
May	1st to 10th	Night	19% 9% 33% 27% 12%	640
	11th to 20th	Night	19% 6% 29% 31% 14% 1	591
	21st to end	Night	14% 2 22% 38% 21% 2	443
June	1st to 10th	Night	10% 3 21% 42% 24% 1	400
	11th to 20th	Night	11% 4 19% 45% 21% 1	400
	21st to end	Night	8% 3 23% 44% 21% 1	400
July	1st to 10th	Night	12% 3 16% 38% 27% 3	438
	11th to 20th	Night	7% 3 12% 40% 32% 7%	582
	21st to end	Night	7% 2 18% 41% 28% 4	657
August	1st to 10th	Night	12% 2 12% 40% 30% 3	597
	11th to 20th	Night	14% 3 12% 40% 28% 3	659
	21st to end	Night	11% 2 15% 39% 27% 5%	880
September	1st to 10th	Night	15% 2 14% 36% 28% 4	800
	11th to 20th	Night	14% 3 15% 37% 27% 3	800
	21st to end	Night	25% 5% 20% 31% 18% 2	800
October	1st to 10th	Night	21% 5% 20% 30% 21% 4	800
	11th to 20th	Night	18% 9% 26% 31% 15% 1	800
	21st to end	Night	16% 8% 31% 31% 13% 1	880
November	1st to 10th	Night	13% 9% 33% 31% 13% 2	820
	11th to 20th	Night	18% 12% 34% 28% 8%	840
	21st to end	Night	14% 12% 35% 30% 8% 1	840
December	1st to 10th	Night	21% 12% 36% 24% 7%	840
	11th to 20th	Night	19% 11% 36% 29% 5%	840
	21st to end	Night	21% 12% 32% 33% 3	924

Page generated at 2026-01-14 01:14:26. Version ac07

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