Hidden Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hidden Hill Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	4 3 1 4 3 3 3 2 4 5% 68%	135052
	Night	4 4 1 4 3 3 3 3 4 6% 64%	132104
Transparency	Day	19% 2 6% 17% 27% 28%	130004
	Night	21% 3 9% 17% 24% 27%	128135
Seeing	Day	8% 11% 22% 32% 21% 6%	16639
	Night	8% 6% 16% 32% 30% 8%	25992

By Month

Percentage Cloud Cover SW (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	7% 7% 3 6% 4 4 4 3 6% 7% 51%	3793
	11th to 20th	Day	7% 4 2 6% 5% 5% 5% 3 3 4 56%	4100
	21st to end	Day	10% 7% 3 8% 5% 5% 4 3 4 7% 45%	4510
February	1st to 10th	Day	13% 8% 3 7% 4 4 3 3 4 5% 45%	1800
	11th to 20th	Day	8% 7% 3 7% 5% 5% 5% 3 5% 7% 45%	1894
	21st to end	Day	11% 6% 2 6% 6% 4 4 4 5% 6% 46%	1650
March	1st to 10th	Day	8% 9% 3 10% 7% 7% 7% 5% 5% 6% 34%	4770
	11th to 20th	Day	6% 6% 2 7% 5% 4 4 4 5% 8% 48%	5000
	21st to end	Day	7% 8% 3 7% 6% 6% 6% 5% 7% 8% 38%	5651
April	1st to 10th	Day	9% 8% 2 7% 4 4 4 3 5% 6% 47%	2400
	11th to 20th	Day	8% 4 1 4 4 4 4 3 5% 7% 56%	2369
	21st to end	Day	4 4 2 3 3 3 3 3 4 8% 63%	2400
May	1st to 10th	Day	3 2 1 2 1 2 2 2 4 5% 76%	5455
	11th to 20th	Day	1 1 2 2 2 2 2 3 5% 80%	6091
	21st to end	Day	1 1 1 1 1 1 2 3 5% 82%	6902
June	1st to 10th	Day	3 3 1 2 2 3 3 3 4 7% 69%	2800
	11th to 20th	Day	1 1 1 1 1 2 1 4 6% 82%	2800
	21st to end	Day	1 1 1 1 1 1 1 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 2 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 90%	6026
	21st to end	Day	1 1 1 1 1 2 3 91%	6402
August	1st to 10th	Day	1 1 1 1 1 1 1 3 4 86%	5777
	11th to 20th	Day	1 1 1 2 94%	5407
	21st to end	Day	1 1 1 1 2 1 2 3 86%	5984
September	1st to 10th	Day	1 1 1 1 1 2 1 1 1 3 88%	2400
	11th to 20th	Day	1 1 2 1 2 2 2 4 7% 77%	2400
	21st to end	Day	3 1 1 1 1 2 2 2 3 4 80%	2150
October	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 76%	3280
	11th to 20th	Day	2 1 1 3 2 2 3 2 3 5% 77%	3242
	21st to end	Day	2 2 1 3 3 3 4 3 5% 7% 67%	3448
November	1st to 10th	Day	5% 4 2 5% 4 4 4 4 6% 7% 54%	1875
	11th to 20th	Day	5% 5% 2 6% 5% 4 5% 3 6% 8% 53%	1800
	21st to end	Day	9% 6% 1 5% 4 3 3 3 5% 8% 54%	1705
December	1st to 10th	Day	4 5% 2 6% 4 5% 3 3 5% 9% 54%	2620
	11th to 20th	Day	13% 9% 3 7% 6% 5% 5% 4 5% 7% 36%	3028
	21st to end	Day	7% 9% 3 9% 7% 7% 7% 5% 7% 7% 32%	4004
January	1st to 10th	Night	6% 5% 2 6% 4 3 4 4 6% 9% 51%	6157
	11th to 20th	Night	7% 6% 2 7% 5% 5% 4 3 5% 5% 52%	5880
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 5% 6% 47%	6468
February	1st to 10th	Night	12% 7% 2 5% 4 3 4 4 6% 6% 47%	2600
	11th to 20th	Night	8% 5% 2 5% 5% 5% 4 4 7% 9% 47%	2392
	21st to end	Night	7% 6% 2 4 4 4 4 4 5% 7% 52%	1980
March	1st to 10th	Night	7% 9% 2 8% 6% 6% 6% 5% 6% 8% 37%	5122
	11th to 20th	Night	7% 6% 2 7% 4 4 4 3 5% 8% 50%	5000
	21st to end	Night	6% 8% 2 6% 5% 5% 5% 5% 7% 8% 42%	5324
April	1st to 10th	Night	6% 6% 2 5% 5% 4 4 4 6% 7% 50%	2000
	11th to 20th	Night	5% 3 2 5% 4 3 4 3 6% 8% 57%	1979
	21st to end	Night	3 3 1 3 2 3 4 4 5% 8% 63%	2000
May	1st to 10th	Night	1 2 2 1 2 2 2 3 6% 78%	4527
	11th to 20th	Night	1 1 1 2 1 1 2 4 6% 81%	3810
	21st to end	Night	1 1 1 1 1 2 3 5% 84%	3972
June	1st to 10th	Night	2 3 1 3 2 2 2 3 4 6% 73%	1600
	11th to 20th	Night	1 1 1 1 1 2 2 4 4 83%	1600
	21st to end	Night	1 1 1 2 1 1 2 3 88%	1600
July	1st to 10th	Night	1 1 1 1 96%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 91%	3916
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	4472
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	4035
	11th to 20th	Night	1 1 1 2 94%	4458
	21st to end	Night	1 1 2 1 1 2 3 87%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 4 88%	2000
	11th to 20th	Night	2 1 1 2 2 1 2 2 2 5% 81%	2000
	21st to end	Night	2 2 1 2 1 1 1 1 3 5% 82%	2245
October	1st to 10th	Night	2 1 2 2 2 2 2 3 6% 79%	3920
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 6% 76%	3873
	21st to end	Night	3 2 1 3 2 2 3 3 5% 7% 70%	4121
November	1st to 10th	Night	4 3 1 3 3 3 4 4 8% 9% 59%	2420
	11th to 20th	Night	5% 4 1 5% 4 3 4 4 6% 8% 55%	2600
	21st to end	Night	6% 5% 2 5% 4 4 4 3 5% 7% 54%	2690
December	1st to 10th	Night	5% 4 2 4 3 4 4 3 6% 6% 59%	4560
	11th to 20th	Night	12% 9% 3 7% 5% 4 5% 4 7% 10% 34%	5176
	21st to end	Night	10% 8% 4 10% 7% 6% 6% 5% 6% 7% 31%	6952

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	31% 3 17% 26% 15% 8%	3793
	11th to 20th	Day	31% 4 9% 12% 24% 20%	4100
	21st to end	Day	39% 3 9% 12% 20% 17%	4510
February	1st to 10th	Day	39% 4 7% 19% 20% 11%	1800
	11th to 20th	Day	35% 4 11% 21% 21% 8%	1904
	21st to end	Day	37% 4 9% 22% 20% 8%	1650
March	1st to 10th	Day	44% 4 11% 14% 22% 4	4770
	11th to 20th	Day	31% 4 10% 32% 15% 9%	5000
	21st to end	Day	37% 6% 15% 26% 14% 2	5651
April	1st to 10th	Day	34% 5% 11% 23% 20% 7%	2400
	11th to 20th	Day	26% 5% 11% 26% 24% 8%	2372
	21st to end	Day	20% 3 11% 31% 28% 6%	2400
May	1st to 10th	Day	11% 2 5% 26% 35% 20%	5455
	11th to 20th	Day	9% 1 3 24% 45% 16%	6094
	21st to end	Day	7% 2 7% 20% 48% 16%	6904
June	1st to 10th	Day	15% 4 9% 22% 35% 15%	2800
	11th to 20th	Day	5% 2 6% 18% 38% 31%	2800
	21st to end	Day	7% 2 2 10% 28% 51%	2800
July	1st to 10th	Day	2 1 3 17% 77%	6251
	11th to 20th	Day	6% 1 1 3 13% 75%	5953
	21st to end	Day	3 1 1 11% 32% 52%	6217
August	1st to 10th	Day	8% 1 1 7% 25% 58%	4206
	11th to 20th	Day	2 1 1 7% 29% 60%	3911
	21st to end	Day	7% 1 8% 33% 51%	4311
September	1st to 10th	Day	6% 2 2 9% 30% 52%	2400
	11th to 20th	Day	9% 2 4 20% 39% 26%	2400
	21st to end	Day	10% 2 3 18% 38% 29%	2150
October	1st to 10th	Day	12% 2 4 11% 24% 47%	3280
	11th to 20th	Day	12% 2 4 21% 37% 25%	3242
	21st to end	Day	18% 1 4 18% 37% 21%	3448
November	1st to 10th	Day	25% 4 9% 22% 26% 14%	1875
	11th to 20th	Day	26% 4 11% 23% 26% 9%	1800
	21st to end	Day	28% 3 8% 20% 30% 11%	1705
December	1st to 10th	Day	27% 2 7% 18% 35% 11%	2620
	11th to 20th	Day	44% 3 8% 18% 20% 8%	3028
	21st to end	Day	45% 4 15% 20% 12% 4	4004
January	1st to 10th	Night	26% 3 20% 25% 20% 5%	6157
	11th to 20th	Night	31% 4 11% 12% 25% 18%	5880
	21st to end	Night	34% 4 10% 14% 17% 22%	6468
February	1st to 10th	Night	33% 4 9% 20% 21% 14%	2600
	11th to 20th	Night	30% 3 12% 21% 23% 10%	2393
	21st to end	Night	29% 3 11% 28% 21% 9%	1980
March	1st to 10th	Night	37% 5% 14% 15% 23% 6%	5122
	11th to 20th	Night	29% 3 14% 25% 17% 11%	5000
	21st to end	Night	34% 5% 18% 22% 17% 5%	5324
April	1st to 10th	Night	29% 5% 13% 23% 21% 9%	2000
	11th to 20th	Night	23% 3 13% 26% 22% 12%	1979
	21st to end	Night	17% 3 12% 30% 29% 10%	2000
May	1st to 10th	Night	9% 2 9% 23% 36% 22%	4524
	11th to 20th	Night	6% 1 5% 20% 39% 29%	3810
	21st to end	Night	5% 1 7% 18% 41% 29%	3972
June	1st to 10th	Night	13% 3 9% 18% 33% 24%	1600
	11th to 20th	Night	6% 1 7% 14% 28% 44%	1600
	21st to end	Night	5% 1 2 9% 28% 54%	1600
July	1st to 10th	Night	2 1 2 12% 84%	3600
	11th to 20th	Night	5% 1 1 3 10% 81%	3866
	21st to end	Night	6% 1 1 7% 27% 58%	4345
August	1st to 10th	Night	7% 1 1 7% 21% 62%	2929
	11th to 20th	Night	3 1 1 5% 23% 67%	3221
	21st to end	Night	5% 1 2 8% 28% 57%	3608
September	1st to 10th	Night	5% 1 2 7% 22% 63%	2000
	11th to 20th	Night	8% 2 5% 17% 34% 34%	2000
	21st to end	Night	9% 1 3 16% 36% 35%	2245
October	1st to 10th	Night	10% 2 4 11% 19% 55%	3920
	11th to 20th	Night	11% 1 5% 21% 36% 26%	3873
	21st to end	Night	14% 1 5% 17% 37% 26%	4121
November	1st to 10th	Night	18% 4 11% 23% 26% 19%	2420
	11th to 20th	Night	23% 4 11% 22% 29% 11%	2600
	21st to end	Night	26% 3 10% 20% 25% 16%	2690
December	1st to 10th	Night	24% 3 6% 19% 31% 17%	4560
	11th to 20th	Night	42% 3 12% 14% 23% 7%	5176
	21st to end	Night	47% 4 14% 20% 10% 5%	6952

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	27% 17% 23% 22% 10% 1	263
	11th to 20th	Day	13% 17% 25% 30% 13% 4	400
	21st to end	Day	16% 10% 28% 25% 19% 3	440
February	1st to 10th	Day	20% 20% 23% 20% 14% 4	400
	11th to 20th	Day	17% 20% 26% 27% 10%	468
	21st to end	Day	17% 16% 28% 28% 11% 1	414
March	1st to 10th	Day	19% 18% 31% 24% 7% 1	500
	11th to 20th	Day	19% 20% 22% 23% 13% 3	500
	21st to end	Day	17% 19% 30% 22% 10% 1	550
April	1st to 10th	Day	16% 18% 28% 23% 13% 2	500
	11th to 20th	Day	11% 21% 28% 26% 12% 2	492
	21st to end	Day	6% 17% 28% 30% 13% 6%	500
May	1st to 10th	Day	9% 16% 29% 29% 13% 4	503
	11th to 20th	Day	6% 15% 30% 26% 19% 5%	528
	21st to end	Day	6% 9% 25% 32% 22% 5%	573
June	1st to 10th	Day	5% 10% 24% 32% 19% 9%	530
	11th to 20th	Day	2 10% 28% 36% 18% 5%	530
	21st to end	Day	2 6% 21% 37% 26% 9%	530
July	1st to 10th	Day	2 14% 41% 33% 10%	526
	11th to 20th	Day	1 11% 39% 34% 15%	524
	21st to end	Day	1 9% 44% 30% 15%	575
August	1st to 10th	Day	2 3 17% 38% 29% 12%	527
	11th to 20th	Day	1 19% 39% 28% 13%	517
	21st to end	Day	1 1 10% 39% 35% 14%	583
September	1st to 10th	Day	1 4 14% 39% 34% 8%	530
	11th to 20th	Day	2 6% 20% 38% 25% 8%	530
	21st to end	Day	4 6% 18% 40% 26% 6%	530
October	1st to 10th	Day	3 11% 23% 31% 26% 6%	530
	11th to 20th	Day	6% 10% 20% 32% 26% 6%	523
	21st to end	Day	6% 9% 24% 35% 21% 5%	539
November	1st to 10th	Day	7% 13% 20% 31% 25% 4	411
	11th to 20th	Day	7% 12% 21% 31% 21% 8%	300
	21st to end	Day	11% 9% 26% 30% 19% 4	253
December	1st to 10th	Day	12% 15% 19% 29% 15% 11%	200
	11th to 20th	Day	17% 16% 17% 29% 16% 7%	200
	21st to end	Day	16% 21% 25% 21% 12% 5%	220
January	1st to 10th	Night	17% 6% 20% 28% 21% 9%	800
	11th to 20th	Night	13% 7% 16% 34% 22% 9%	800
	21st to end	Night	15% 8% 15% 32% 24% 7%	880
February	1st to 10th	Night	19% 9% 16% 29% 21% 6%	800
	11th to 20th	Night	13% 10% 19% 30% 19% 7%	784
	21st to end	Night	12% 10% 21% 36% 18% 3	660
March	1st to 10th	Night	16% 12% 19% 31% 19% 4	800
	11th to 20th	Night	13% 12% 20% 27% 23% 5%	800
	21st to end	Night	13% 11% 24% 31% 20% 3	880
April	1st to 10th	Night	11% 9% 25% 30% 22% 4	800
	11th to 20th	Night	8% 13% 22% 32% 21% 5%	791
	21st to end	Night	5% 10% 25% 30% 23% 7%	800
May	1st to 10th	Night	4 7% 27% 31% 25% 7%	793
	11th to 20th	Night	3 4 17% 35% 35% 5%	484
	21st to end	Night	2 6% 17% 37% 33% 5%	433
June	1st to 10th	Night	4 3 17% 36% 34% 7%	400
	11th to 20th	Night	1 4 14% 44% 34% 4	400
	21st to end	Night	1 1 8% 38% 45% 8%	400
July	1st to 10th	Night	1 7% 36% 47% 8%	390
	11th to 20th	Night	1 2 28% 56% 13%	506
	21st to end	Night	2 3 28% 54% 13%	657
August	1st to 10th	Night	2 1 6% 30% 49% 13%	597
	11th to 20th	Night	7% 38% 43% 11%	769
	21st to end	Night	1 4 28% 54% 12%	880
September	1st to 10th	Night	1 2 8% 33% 47% 10%	800
	11th to 20th	Night	2 3 10% 36% 40% 10%	800
	21st to end	Night	3 3 11% 32% 39% 13%	800
October	1st to 10th	Night	3 4 12% 36% 35% 11%	800
	11th to 20th	Night	6% 5% 11% 33% 34% 11%	792
	21st to end	Night	6% 4 14% 31% 28% 16%	836
November	1st to 10th	Night	5% 8% 16% 31% 31% 11%	780
	11th to 20th	Night	7% 5% 19% 34% 27% 9%	800
	21st to end	Night	8% 7% 18% 34% 26% 8%	800
December	1st to 10th	Night	14% 8% 17% 31% 24% 7%	800
	11th to 20th	Night	16% 9% 17% 30% 22% 5%	800
	21st to end	Night	14% 13% 23% 28% 18% 4	880

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