Hidden Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hidden Mountain Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	105338
	Night	14% 9% 4 7% 4 4 4 4 6% 7% 37%	103970
Transparency	Day	25% 47% 7% 12% 7% 2	109093
	Night	25% 46% 7% 12% 8% 2	108354
Seeing	Day	19% 13% 23% 23% 15% 6%	15259
	Night	18% 11% 21% 30% 18% 2	25375

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	13% 13% 10% 12% 5% 3 3 3 5% 6% 27%	2926
	11th to 20th	Day	12% 16% 6% 13% 5% 7% 4 3 3 5% 26%	2930
	21st to end	Day	15% 18% 6% 8% 5% 4 4 3 4 5% 30%	3133
February	1st to 10th	Day	15% 15% 4 10% 5% 4 4 3 5% 5% 31%	1777
	11th to 20th	Day	18% 14% 6% 9% 4 4 4 3 4 5% 29%	1848
	21st to end	Day	19% 14% 3 7% 3 4 3 3 4 6% 32%	1570
March	1st to 10th	Day	18% 13% 4 7% 5% 3 4 3 5% 5% 33%	3344
	11th to 20th	Day	14% 8% 3 6% 4 3 4 3 4 5% 45%	3480
	21st to end	Day	17% 10% 3 6% 4 4 4 4 5% 7% 36%	3815
April	1st to 10th	Day	13% 10% 2 6% 4 4 6% 4 6% 7% 40%	2090
	11th to 20th	Day	15% 8% 2 6% 5% 4 5% 3 5% 7% 40%	2297
	21st to end	Day	16% 7% 2 7% 5% 4 5% 4 5% 7% 38%	2470
May	1st to 10th	Day	17% 9% 3 9% 6% 5% 5% 4 6% 7% 28%	4082
	11th to 20th	Day	18% 8% 4 9% 6% 6% 5% 4 6% 6% 29%	4081
	21st to end	Day	11% 5% 3 7% 5% 5% 6% 5% 8% 10% 36%	4476
June	1st to 10th	Day	12% 6% 2 6% 6% 6% 6% 6% 7% 9% 34%	2470
	11th to 20th	Day	7% 6% 2 7% 7% 6% 7% 6% 9% 10% 32%	2470
	21st to end	Day	11% 7% 2 6% 6% 6% 7% 5% 7% 9% 33%	2470
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 6% 9% 12% 33%	4120
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 9% 35%	4120
	21st to end	Day	17% 11% 4 11% 8% 7% 6% 6% 7% 7% 17%	4466
August	1st to 10th	Day	12% 7% 3 8% 6% 6% 6% 5% 8% 11% 28%	4104
	11th to 20th	Day	11% 8% 3 7% 5% 5% 6% 5% 8% 10% 31%	4044
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 6% 7% 50%	4229
September	1st to 10th	Day	12% 5% 2 7% 5% 6% 5% 4 6% 7% 40%	2090
	11th to 20th	Day	15% 7% 2 7% 5% 6% 5% 3 6% 7% 38%	2090
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 5% 7% 39%	2090
October	1st to 10th	Day	14% 5% 2 5% 4 4 3 4 5% 7% 47%	2055
	11th to 20th	Day	15% 6% 2 5% 3 3 3 3 5% 5% 49%	1900
	21st to end	Day	15% 9% 1 4 4 3 4 3 3 6% 47%	2090
November	1st to 10th	Day	11% 6% 2 5% 3 3 4 3 5% 5% 52%	1842
	11th to 20th	Day	11% 10% 2 6% 5% 4 5% 4 5% 6% 43%	1800
	21st to end	Day	12% 11% 4 7% 4 3 3 4 5% 5% 43%	1800
December	1st to 10th	Day	10% 10% 4 9% 5% 5% 5% 3 5% 6% 38%	2930
	11th to 20th	Day	15% 15% 4 8% 4 5% 5% 4 6% 6% 28%	3373
	21st to end	Day	9% 12% 6% 9% 5% 5% 6% 4 6% 7% 32%	4466
January	1st to 10th	Night	16% 17% 7% 11% 5% 3 3 3 5% 4 26%	4239
	11th to 20th	Night	15% 13% 4 9% 6% 5% 4 3 4 5% 33%	4250
	21st to end	Night	18% 13% 4 8% 5% 3 4 3 4 4 32%	4546
February	1st to 10th	Night	14% 13% 4 7% 5% 4 4 4 4 6% 35%	2399
	11th to 20th	Night	19% 12% 5% 9% 5% 5% 3 4 5% 4 30%	2218
	21st to end	Night	18% 13% 4 8% 5% 3 4 3 6% 6% 31%	1884
March	1st to 10th	Night	22% 13% 4 6% 4 3 3 3 4 6% 31%	3509
	11th to 20th	Night	15% 8% 3 5% 4 3 3 3 5% 7% 44%	3480
	21st to end	Night	14% 7% 2 5% 5% 5% 5% 4 5% 6% 41%	3817
April	1st to 10th	Night	14% 9% 2 6% 5% 3 5% 4 5% 6% 41%	2090
	11th to 20th	Night	13% 9% 3 6% 4 4 5% 3 6% 7% 42%	1846
	21st to end	Night	15% 7% 2 7% 5% 4 4 3 5% 7% 41%	1710
May	1st to 10th	Night	16% 7% 3 7% 5% 5% 5% 5% 6% 8% 32%	2813
	11th to 20th	Night	18% 7% 3 7% 5% 4 4 4 7% 8% 32%	2812
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 7% 9% 38%	3079
June	1st to 10th	Night	10% 6% 2 8% 6% 5% 5% 5% 8% 10% 37%	1710
	11th to 20th	Night	7% 5% 2 5% 5% 5% 6% 4 8% 12% 40%	1710
	21st to end	Night	9% 7% 2 6% 5% 4 6% 5% 6% 11% 39%	1710
July	1st to 10th	Night	11% 6% 2 6% 4 6% 6% 6% 9% 10% 33%	2840
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 11% 39%	2840
	21st to end	Night	12% 10% 5% 12% 7% 6% 7% 5% 7% 9% 20%	3087
August	1st to 10th	Night	10% 6% 2 7% 5% 5% 6% 4 8% 10% 36%	2825
	11th to 20th	Night	13% 7% 3 7% 5% 4 5% 4 5% 9% 38%	2785
	21st to end	Night	6% 4 1 4 4 4 5% 4 7% 10% 51%	3375
September	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 6% 9% 43%	2090
	11th to 20th	Night	16% 7% 2 6% 3 4 4 4 5% 8% 41%	2090
	21st to end	Night	14% 6% 2 5% 4 4 4 2 5% 7% 47%	2090
October	1st to 10th	Night	14% 6% 2 6% 4 3 3 2 4 6% 50%	2123
	11th to 20th	Night	15% 7% 2 4 3 3 4 2 4 5% 50%	2280
	21st to end	Night	14% 7% 2 4 3 3 3 3 5% 7% 49%	2508
November	1st to 10th	Night	13% 8% 2 5% 3 3 4 3 5% 5% 49%	2453
	11th to 20th	Night	12% 10% 2 6% 4 3 4 3 6% 6% 44%	2600
	21st to end	Night	12% 10% 2 6% 3 4 3 3 5% 6% 45%	2600
December	1st to 10th	Night	12% 12% 6% 9% 5% 3 4 3 5% 5% 37%	4250
	11th to 20th	Night	16% 12% 6% 12% 4 4 4 3 4 5% 29%	4822
	21st to end	Night	10% 14% 8% 9% 5% 4 4 4 5% 8% 30%	6490

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	33% 40% 4 10% 10% 2	2927
	11th to 20th	Day	37% 41% 4 7% 11% 1	2930
	21st to end	Day	36% 43% 3 7% 11%	3133
February	1st to 10th	Day	20% 65% 2 5% 6% 1	1777
	11th to 20th	Day	22% 64% 2 5% 6% 1	1858
	21st to end	Day	20% 65% 3 5% 6% 2	1570
March	1st to 10th	Day	33% 44% 6% 8% 6% 3	3344
	11th to 20th	Day	18% 42% 4 18% 15% 3	3480
	21st to end	Day	24% 42% 7% 17% 8% 2	3815
April	1st to 10th	Day	18% 66% 4 6% 5% 1	2090
	11th to 20th	Day	17% 67% 3 7% 5% 1	2297
	21st to end	Day	16% 66% 3 9% 5% 1	2470
May	1st to 10th	Day	33% 45% 8% 11% 3	4120
	11th to 20th	Day	31% 44% 7% 15% 2	4085
	21st to end	Day	22% 45% 11% 12% 8% 2	4489
June	1st to 10th	Day	16% 67% 6% 6% 4	2470
	11th to 20th	Day	15% 69% 6% 6% 5%	2470
	21st to end	Day	18% 66% 6% 7% 4	2470
July	1st to 10th	Day	23% 44% 16% 12% 6%	4055
	11th to 20th	Day	26% 39% 12% 20% 2	4051
	21st to end	Day	44% 41% 5% 8% 3	4324
August	1st to 10th	Day	29% 42% 12% 14% 4	4091
	11th to 20th	Day	25% 40% 14% 16% 5%	4083
	21st to end	Day	14% 39% 10% 28% 8%	4229
September	1st to 10th	Day	15% 63% 5% 8% 7% 2	2090
	11th to 20th	Day	18% 62% 5% 9% 6% 1	2090
	21st to end	Day	17% 62% 9% 8% 4	2090
October	1st to 10th	Day	17% 38% 5% 11% 20% 8%	3419
	11th to 20th	Day	15% 37% 3 17% 12% 16%	3140
	21st to end	Day	14% 37% 3 22% 14% 10%	3428
November	1st to 10th	Day	18% 57% 4 10% 9% 2	1842
	11th to 20th	Day	18% 58% 2 8% 12% 3	1800
	21st to end	Day	20% 57% 4 9% 8% 2	1800
December	1st to 10th	Day	29% 38% 5% 9% 17% 2	2930
	11th to 20th	Day	38% 33% 7% 12% 8% 2	3370
	21st to end	Day	35% 31% 12% 15% 7% 1	4466
January	1st to 10th	Night	39% 39% 4 8% 8% 2	4239
	11th to 20th	Night	31% 38% 6% 10% 14% 2	4250
	21st to end	Night	34% 42% 5% 8% 11% 1	4546
February	1st to 10th	Night	18% 65% 3 7% 6% 1	2399
	11th to 20th	Night	22% 64% 3 6% 5% 1	2219
	21st to end	Night	19% 65% 3 7% 4 2	1884
March	1st to 10th	Night	36% 44% 8% 8% 4 1	3509
	11th to 20th	Night	18% 41% 8% 15% 11% 7%	3480
	21st to end	Night	22% 43% 7% 16% 10% 2	3817
April	1st to 10th	Night	16% 67% 5% 7% 4 1	2090
	11th to 20th	Night	17% 67% 4 6% 5% 1	1846
	21st to end	Night	13% 66% 4 10% 6%	1710
May	1st to 10th	Night	29% 47% 8% 12% 3	2840
	11th to 20th	Night	29% 44% 11% 14% 2	2813
	21st to end	Night	22% 47% 11% 14% 6%	3087
June	1st to 10th	Night	13% 68% 8% 6% 4 1	1710
	11th to 20th	Night	12% 69% 8% 8% 3	1710
	21st to end	Night	15% 67% 6% 7% 4	1710
July	1st to 10th	Night	26% 42% 16% 13% 3	2795
	11th to 20th	Night	21% 40% 17% 19% 3	2786
	21st to end	Night	41% 40% 8% 9% 2	2988
August	1st to 10th	Night	26% 42% 15% 15% 2	2814
	11th to 20th	Night	25% 40% 14% 17% 4	2812
	21st to end	Night	11% 39% 11% 30% 7% 1	3375
September	1st to 10th	Night	14% 62% 7% 9% 8% 1	2090
	11th to 20th	Night	15% 62% 6% 9% 7% 1	2090
	21st to end	Night	15% 62% 8% 10% 5%	2090
October	1st to 10th	Night	21% 38% 5% 13% 19% 5%	3539
	11th to 20th	Night	17% 37% 5% 12% 15% 15%	3779
	21st to end	Night	12% 36% 3 23% 15% 11%	4120
November	1st to 10th	Night	20% 57% 5% 11% 6% 1	2453
	11th to 20th	Night	19% 57% 3 8% 10% 3	2600
	21st to end	Night	19% 58% 4 11% 8% 1	2600
December	1st to 10th	Night	32% 37% 4 11% 15% 1	4250
	11th to 20th	Night	40% 33% 8% 11% 8% 1	4824
	21st to end	Night	36% 32% 10% 13% 9%	6490

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	29% 28% 30% 12% 1	300
	11th to 20th	Day	35% 27% 23% 13% 2	300
	21st to end	Day	29% 27% 32% 9% 3	300
February	1st to 10th	Day	26% 26% 33% 13% 3	270
	11th to 20th	Day	38% 33% 21% 7% 1	270
	21st to end	Day	41% 20% 32% 5% 1	225
March	1st to 10th	Day	26% 22% 27% 18% 6% 1	403
	11th to 20th	Day	27% 23% 29% 15% 4 1	460
	21st to end	Day	26% 23% 26% 18% 7%	506
April	1st to 10th	Day	26% 17% 26% 20% 9% 1	460
	11th to 20th	Day	21% 20% 25% 21% 11% 2	452
	21st to end	Day	21% 16% 28% 20% 11% 3	460
May	1st to 10th	Day	22% 17% 27% 19% 10% 4	463
	11th to 20th	Day	19% 11% 22% 27% 15% 5%	487
	21st to end	Day	15% 9% 20% 24% 21% 11%	529
June	1st to 10th	Day	12% 7% 23% 28% 22% 8%	490
	11th to 20th	Day	9% 6% 20% 28% 26% 11%	490
	21st to end	Day	11% 8% 26% 25% 19% 11%	490
July	1st to 10th	Day	12% 5% 16% 26% 27% 15%	485
	11th to 20th	Day	11% 3 17% 29% 27% 13%	484
	21st to end	Day	14% 4 14% 29% 25% 13%	532
August	1st to 10th	Day	14% 6% 19% 26% 23% 13%	486
	11th to 20th	Day	14% 4 18% 26% 25% 13%	479
	21st to end	Day	12% 4 12% 26% 28% 19%	539
September	1st to 10th	Day	10% 4 14% 30% 28% 14%	490
	11th to 20th	Day	15% 6% 13% 25% 29% 12%	490
	21st to end	Day	18% 8% 16% 32% 22% 5%	490
October	1st to 10th	Day	17% 7% 23% 27% 20% 6%	490
	11th to 20th	Day	17% 14% 23% 26% 18% 3	490
	21st to end	Day	21% 13% 22% 28% 14% 2	539
November	1st to 10th	Day	16% 15% 26% 28% 12% 2	380
	11th to 20th	Day	22% 18% 30% 22% 7% 1	300
	21st to end	Day	20% 21% 32% 19% 7% 1	300
December	1st to 10th	Day	29% 22% 34% 12% 3	300
	11th to 20th	Day	25% 20% 30% 20% 5%	300
	21st to end	Day	21% 24% 34% 16% 6%	330
January	1st to 10th	Night	24% 21% 27% 24% 4	800
	11th to 20th	Night	26% 18% 28% 22% 5%	800
	21st to end	Night	22% 18% 34% 23% 3	841
February	1st to 10th	Night	20% 17% 29% 30% 4	760
	11th to 20th	Night	27% 20% 34% 18% 1	743
	21st to end	Night	29% 22% 28% 20% 1	628
March	1st to 10th	Night	21% 18% 30% 27% 4	760
	11th to 20th	Night	23% 18% 29% 23% 7% 1	760
	21st to end	Night	21% 15% 31% 26% 8%	836
April	1st to 10th	Night	20% 17% 27% 27% 8%	760
	11th to 20th	Night	17% 14% 29% 27% 13%	636
	21st to end	Night	18% 9% 27% 37% 9% 1	570
May	1st to 10th	Night	16% 12% 23% 32% 15% 1	570
	11th to 20th	Night	16% 5% 21% 36% 20% 2	561
	21st to end	Night	11% 4 16% 34% 32% 5%	617
June	1st to 10th	Night	9% 3 13% 42% 29% 4	570
	11th to 20th	Night	6% 4 14% 39% 31% 6%	570
	21st to end	Night	12% 2 12% 37% 33% 4	570
July	1st to 10th	Night	12% 3 9% 32% 40% 5%	555
	11th to 20th	Night	10% 1 7% 31% 44% 7%	552
	21st to end	Night	9% 3 9% 32% 38% 8%	624
August	1st to 10th	Night	11% 3 8% 31% 41% 6%	567
	11th to 20th	Night	12% 3 8% 35% 38% 5%	564
	21st to end	Night	10% 3 7% 31% 41% 8%	665
September	1st to 10th	Night	12% 3 11% 31% 36% 6%	760
	11th to 20th	Night	17% 4 13% 28% 33% 5%	760
	21st to end	Night	16% 5% 15% 33% 29% 3	760
October	1st to 10th	Night	17% 6% 16% 31% 26% 6%	760
	11th to 20th	Night	18% 8% 21% 33% 18% 2	760
	21st to end	Night	18% 10% 19% 37% 14% 2	836
November	1st to 10th	Night	16% 7% 21% 37% 15% 3	780
	11th to 20th	Night	18% 14% 23% 32% 11% 2	800
	21st to end	Night	18% 15% 24% 32% 10% 1	800
December	1st to 10th	Night	23% 16% 29% 27% 6%	800
	11th to 20th	Night	23% 13% 26% 31% 6% 1	800
	21st to end	Night	24% 16% 27% 28% 4	880

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