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 2026-05-17.

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	12% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	132114
	Night	13% 10% 4 8% 5% 4 4 4 5% 7% 37%	130709
Transparency	Day	29% 39% 8% 13% 9% 2	135825
	Night	29% 38% 8% 14% 9% 2	135068
Seeing	Day	20% 14% 23% 23% 15% 6%	17019
	Night	18% 11% 22% 30% 17% 2	27125

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	12% 17% 7% 10% 5% 3 3 3 4 5% 31%	5258
	11th to 20th	Day	8% 14% 6% 13% 6% 5% 3 2 3 4 36%	5280
	21st to end	Day	12% 14% 6% 10% 6% 4 4 3 4 5% 34%	5780
February	1st to 10th	Day	14% 14% 5% 10% 4 4 4 3 5% 5% 31%	1965
	11th to 20th	Day	18% 14% 5% 9% 4 4 4 3 4 5% 29%	2048
	21st to end	Day	18% 13% 4 8% 3 4 3 3 4 6% 34%	1730
March	1st to 10th	Day	15% 13% 5% 8% 5% 4 4 3 4 5% 34%	4909
	11th to 20th	Day	12% 9% 4 6% 3 3 4 3 5% 6% 45%	5090
	21st to end	Day	15% 9% 3 7% 4 4 4 4 5% 7% 40%	5586
April	1st to 10th	Day	13% 9% 2 6% 4 4 6% 4 6% 7% 40%	2310
	11th to 20th	Day	14% 7% 2 6% 5% 4 5% 3 5% 7% 41%	2542
	21st to end	Day	16% 7% 2 7% 5% 4 5% 4 5% 6% 38%	2730
May	1st to 10th	Day	14% 9% 4 10% 6% 6% 6% 5% 7% 8% 25%	5992
	11th to 20th	Day	16% 8% 3 9% 6% 6% 6% 5% 7% 7% 25%	5939
	21st to end	Day	15% 7% 3 8% 5% 5% 6% 5% 7% 9% 30%	6433
June	1st to 10th	Day	12% 6% 2 6% 6% 6% 6% 6% 7% 9% 34%	2600
	11th to 20th	Day	7% 6% 2 7% 6% 6% 7% 6% 9% 10% 31%	2600
	21st to end	Day	11% 6% 2 6% 6% 6% 7% 5% 8% 10% 34%	2600
July	1st to 10th	Day	7% 5% 2 6% 6% 5% 6% 6% 9% 12% 36%	5900
	11th to 20th	Day	11% 7% 3 8% 6% 6% 7% 6% 8% 10% 29%	5900
	21st to end	Day	14% 9% 3 10% 8% 7% 7% 6% 8% 9% 19%	6424
August	1st to 10th	Day	12% 7% 3 8% 6% 6% 6% 5% 9% 10% 28%	4234
	11th to 20th	Day	11% 8% 3 7% 5% 5% 6% 5% 8% 10% 31%	4174
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 8% 50%	4363
September	1st to 10th	Day	11% 5% 2 7% 5% 6% 5% 5% 6% 8% 40%	2200
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 3 6% 8% 38%	2200
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 5% 5% 7% 39%	2200
October	1st to 10th	Day	13% 5% 2 5% 4 4 3 4 5% 7% 48%	2163
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 5% 5% 51%	2000
	21st to end	Day	15% 9% 2 4 4 3 4 3 3 6% 46%	2200
November	1st to 10th	Day	11% 6% 2 6% 4 3 4 3 5% 5% 51%	1936
	11th to 20th	Day	10% 10% 2 5% 4 4 5% 4 5% 7% 44%	1890
	21st to end	Day	12% 11% 4 7% 4 3 3 3 5% 6% 42%	1890
December	1st to 10th	Day	11% 10% 4 9% 5% 5% 5% 3 5% 6% 37%	3020
	11th to 20th	Day	15% 14% 4 8% 4 5% 5% 4 6% 6% 29%	3463
	21st to end	Day	9% 12% 6% 9% 5% 5% 6% 4 6% 7% 31%	4565
January	1st to 10th	Night	13% 16% 6% 11% 6% 4 3 2 4 5% 30%	7635
	11th to 20th	Night	11% 12% 5% 10% 6% 4 4 3 4 4 38%	7680
	21st to end	Night	14% 12% 4 10% 5% 4 4 3 4 5% 34%	8401
February	1st to 10th	Night	14% 13% 4 7% 4 4 4 4 4 6% 35%	2651
	11th to 20th	Night	18% 12% 5% 10% 4 5% 4 4 5% 4 30%	2458
	21st to end	Night	17% 12% 4 9% 5% 3 4 3 5% 5% 32%	2076
March	1st to 10th	Night	17% 13% 4 7% 4 3 3 3 4 6% 36%	5164
	11th to 20th	Night	12% 7% 3 5% 3 3 4 3 5% 7% 48%	5090
	21st to end	Night	11% 8% 3 6% 5% 5% 5% 4 5% 7% 41%	5588
April	1st to 10th	Night	14% 9% 2 5% 5% 3 5% 4 6% 7% 41%	2310
	11th to 20th	Night	13% 9% 2 6% 4 3 4 3 6% 7% 43%	2043
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 5% 7% 41%	1890
May	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 6% 8% 9% 29%	4123
	11th to 20th	Night	16% 7% 3 9% 6% 6% 6% 5% 7% 8% 27%	4086
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 6% 8% 33%	4421
June	1st to 10th	Night	10% 6% 2 8% 6% 5% 5% 5% 7% 10% 37%	1800
	11th to 20th	Night	7% 6% 2 6% 5% 5% 6% 4 8% 12% 39%	1800
	21st to end	Night	9% 7% 2 7% 5% 4 6% 4 7% 11% 39%	1800
July	1st to 10th	Night	12% 6% 2 6% 4 5% 6% 5% 9% 11% 33%	4058
	11th to 20th	Night	10% 7% 2 7% 5% 6% 6% 6% 8% 11% 32%	4057
	21st to end	Night	11% 9% 5% 11% 7% 6% 7% 5% 8% 9% 22%	4427
August	1st to 10th	Night	11% 6% 2 7% 5% 5% 6% 4 8% 10% 36%	2915
	11th to 20th	Night	13% 7% 3 7% 5% 4 5% 4 5% 9% 38%	2875
	21st to end	Night	6% 4 1 5% 4 4 5% 4 7% 9% 51%	3482
September	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 6% 9% 43%	2200
	11th to 20th	Night	15% 7% 2 6% 3 4 4 4 5% 8% 42%	2200
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 7% 46%	2200
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 6% 50%	2235
	11th to 20th	Night	15% 7% 2 4 3 3 4 2 4 5% 51%	2400
	21st to end	Night	14% 8% 2 4 3 3 3 3 4 7% 49%	2640
November	1st to 10th	Night	13% 8% 2 5% 3 3 4 3 5% 5% 49%	2579
	11th to 20th	Night	12% 10% 2 6% 4 3 3 3 6% 6% 44%	2730
	21st to end	Night	13% 10% 2 6% 3 4 3 3 5% 6% 44%	2730
December	1st to 10th	Night	12% 12% 6% 9% 4 3 3 3 5% 5% 37%	4380
	11th to 20th	Night	16% 12% 6% 12% 4 4 4 3 5% 5% 30%	4952
	21st to end	Night	10% 13% 7% 9% 5% 4 4 4 5% 8% 30%	6633

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	42% 24% 7% 17% 8% 1	5259
	11th to 20th	Day	39% 24% 3 11% 20% 3	5280
	21st to end	Day	42% 24% 5% 17% 11% 2	5780
February	1st to 10th	Day	23% 59% 3 7% 7% 1	1965
	11th to 20th	Day	26% 58% 3 6% 6% 1	2058
	21st to end	Day	21% 59% 3 7% 7% 2	1730
March	1st to 10th	Day	38% 31% 6% 8% 11% 7%	4909
	11th to 20th	Day	24% 30% 4 15% 20% 7%	5090
	21st to end	Day	28% 30% 6% 22% 11% 2	5586
April	1st to 10th	Day	19% 61% 5% 9% 5% 1	2310
	11th to 20th	Day	18% 61% 4 10% 6% 1	2542
	21st to end	Day	19% 60% 4 11% 6% 1	2730
May	1st to 10th	Day	38% 34% 11% 13% 5%	6027
	11th to 20th	Day	38% 34% 12% 13% 2	5943
	21st to end	Day	33% 33% 10% 12% 9% 1	6447
June	1st to 10th	Day	17% 65% 6% 7% 5%	2600
	11th to 20th	Day	16% 66% 7% 6% 5%	2600
	21st to end	Day	17% 64% 7% 8% 4	2600
July	1st to 10th	Day	25% 34% 16% 16% 10%	5835
	11th to 20th	Day	33% 33% 17% 15% 2	5831
	21st to end	Day	43% 33% 9% 9% 5%	6240
August	1st to 10th	Day	30% 41% 12% 14% 3	4221
	11th to 20th	Day	25% 39% 14% 16% 5%	4213
	21st to end	Day	15% 38% 10% 29% 8%	4363
September	1st to 10th	Day	16% 60% 6% 9% 8% 2	2200
	11th to 20th	Day	18% 59% 6% 10% 6% 1	2200
	21st to end	Day	19% 60% 9% 8% 3	2200
October	1st to 10th	Day	17% 38% 5% 12% 20% 8%	3527
	11th to 20th	Day	15% 36% 3 17% 13% 16%	3240
	21st to end	Day	15% 36% 4 22% 14% 10%	3538
November	1st to 10th	Day	19% 55% 5% 11% 10% 1	1936
	11th to 20th	Day	18% 55% 3 9% 12% 3	1890
	21st to end	Day	22% 55% 5% 10% 8% 1	1890
December	1st to 10th	Day	30% 37% 5% 10% 17% 2	3020
	11th to 20th	Day	38% 32% 7% 12% 8% 2	3460
	21st to end	Day	35% 30% 12% 16% 7% 1	4565
January	1st to 10th	Night	45% 23% 8% 17% 6% 1	7636
	11th to 20th	Night	38% 22% 5% 14% 19% 3	7680
	21st to end	Night	41% 24% 5% 14% 14% 2	8401
February	1st to 10th	Night	21% 59% 4 8% 6% 1	2651
	11th to 20th	Night	25% 58% 4 7% 5% 1	2459
	21st to end	Night	21% 59% 4 8% 5% 2	2076
March	1st to 10th	Night	37% 30% 6% 14% 8% 3	5164
	11th to 20th	Night	19% 29% 7% 15% 21% 9%	5090
	21st to end	Night	27% 30% 10% 21% 10% 2	5588
April	1st to 10th	Night	17% 61% 6% 9% 5% 1	2310
	11th to 20th	Night	18% 61% 4 9% 6% 1	2043
	21st to end	Night	17% 61% 5% 11% 7%	1890
May	1st to 10th	Night	33% 35% 13% 14% 5%	4150
	11th to 20th	Night	39% 33% 13% 13% 2	4087
	21st to end	Night	35% 34% 11% 14% 7%	4429
June	1st to 10th	Night	15% 65% 9% 6% 4 1	1800
	11th to 20th	Night	14% 66% 8% 8% 3	1800
	21st to end	Night	16% 64% 7% 9% 5%	1800
July	1st to 10th	Night	31% 32% 15% 16% 5%	4015
	11th to 20th	Night	31% 32% 20% 14% 2	4006
	21st to end	Night	43% 32% 10% 9% 6%	4297
August	1st to 10th	Night	27% 41% 15% 15% 2	2904
	11th to 20th	Night	26% 39% 14% 17% 4	2902
	21st to end	Night	12% 38% 11% 30% 8% 1	3482
September	1st to 10th	Night	15% 59% 7% 9% 9% 1	2200
	11th to 20th	Night	15% 60% 7% 10% 7% 1	2200
	21st to end	Night	17% 59% 8% 11% 5%	2200
October	1st to 10th	Night	21% 37% 5% 14% 19% 5%	3651
	11th to 20th	Night	17% 36% 5% 12% 16% 14%	3899
	21st to end	Night	13% 35% 3 23% 15% 11%	4252
November	1st to 10th	Night	21% 54% 5% 12% 7% 1	2579
	11th to 20th	Night	19% 54% 3 9% 11% 3	2730
	21st to end	Night	21% 55% 4 12% 8% 1	2730
December	1st to 10th	Night	33% 36% 4 11% 14% 1	4380
	11th to 20th	Night	40% 32% 8% 11% 8% 1	4954
	21st to end	Night	36% 31% 10% 13% 9% 1	6633

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	28% 28% 31% 12% 1	330
	11th to 20th	Day	36% 26% 23% 12% 2	330
	21st to end	Day	30% 28% 32% 8% 2	363
February	1st to 10th	Day	25% 27% 33% 12% 2	330
	11th to 20th	Day	38% 32% 23% 7% 1	330
	21st to end	Day	38% 20% 32% 8% 1	273
March	1st to 10th	Day	28% 22% 27% 18% 5% 1	477
	11th to 20th	Day	28% 23% 29% 16% 3 1	540
	21st to end	Day	26% 23% 27% 18% 6%	594
April	1st to 10th	Day	24% 17% 27% 21% 9% 1	540
	11th to 20th	Day	21% 19% 28% 20% 10% 2	532
	21st to end	Day	21% 16% 28% 21% 11% 3	540
May	1st to 10th	Day	22% 18% 28% 20% 9% 4	543
	11th to 20th	Day	20% 11% 23% 27% 14% 5%	551
	21st to end	Day	17% 11% 20% 23% 19% 10%	573
June	1st to 10th	Day	12% 7% 25% 28% 21% 8%	530
	11th to 20th	Day	9% 6% 21% 28% 26% 10%	530
	21st to end	Day	10% 8% 25% 26% 20% 11%	530
July	1st to 10th	Day	11% 5% 15% 28% 26% 15%	525
	11th to 20th	Day	11% 3 16% 30% 26% 14%	524
	21st to end	Day	14% 5% 14% 28% 26% 13%	576
August	1st to 10th	Day	14% 6% 18% 25% 23% 13%	526
	11th to 20th	Day	14% 4 18% 26% 26% 12%	519
	21st to end	Day	11% 4 13% 27% 27% 18%	583
September	1st to 10th	Day	10% 4 15% 31% 26% 13%	530
	11th to 20th	Day	14% 5% 13% 26% 30% 12%	530
	21st to end	Day	18% 9% 16% 32% 21% 5%	530
October	1st to 10th	Day	16% 7% 22% 27% 22% 6%	530
	11th to 20th	Day	16% 13% 23% 28% 17% 2	530
	21st to end	Day	23% 13% 22% 27% 13% 2	583
November	1st to 10th	Day	16% 16% 26% 29% 11% 2	414
	11th to 20th	Day	23% 18% 31% 21% 7% 1	330
	21st to end	Day	22% 22% 31% 18% 7% 1	330
December	1st to 10th	Day	30% 21% 33% 13% 3	330
	11th to 20th	Day	26% 20% 30% 19% 4	330
	21st to end	Day	22% 23% 34% 15% 5%	363
January	1st to 10th	Night	24% 21% 27% 24% 4	840
	11th to 20th	Night	26% 19% 29% 22% 4	840
	21st to end	Night	23% 18% 36% 22% 2	924
February	1st to 10th	Night	21% 17% 29% 29% 4	840
	11th to 20th	Night	27% 20% 35% 17% 1	823
	21st to end	Night	27% 22% 29% 21% 1	692
March	1st to 10th	Night	21% 17% 31% 27% 4	840
	11th to 20th	Night	23% 18% 29% 23% 6% 1	840
	21st to end	Night	20% 16% 32% 25% 7%	924
April	1st to 10th	Night	19% 17% 28% 28% 8%	840
	11th to 20th	Night	17% 14% 28% 29% 12%	704
	21st to end	Night	17% 9% 27% 38% 8% 1	630
May	1st to 10th	Night	16% 11% 25% 32% 14% 1	630
	11th to 20th	Night	15% 6% 22% 37% 19% 2	609
	21st to end	Night	12% 5% 16% 33% 30% 5%	650
June	1st to 10th	Night	10% 4 13% 41% 29% 4	600
	11th to 20th	Night	6% 4 14% 40% 31% 6%	600
	21st to end	Night	11% 2 12% 37% 34% 4	600
July	1st to 10th	Night	12% 3 9% 32% 40% 4	585
	11th to 20th	Night	11% 1 7% 30% 45% 7%	582
	21st to end	Night	9% 3 9% 31% 39% 9%	657
August	1st to 10th	Night	12% 3 9% 31% 40% 6%	597
	11th to 20th	Night	12% 3 8% 35% 38% 5%	594
	21st to end	Night	10% 3 8% 32% 39% 8%	700
September	1st to 10th	Night	13% 3 11% 32% 35% 6%	800
	11th to 20th	Night	16% 4 13% 28% 34% 5%	800
	21st to end	Night	16% 5% 15% 34% 28% 2	800
October	1st to 10th	Night	16% 6% 15% 32% 26% 6%	800
	11th to 20th	Night	17% 8% 21% 34% 17% 2	800
	21st to end	Night	19% 10% 20% 36% 13% 2	880
November	1st to 10th	Night	16% 8% 22% 37% 15% 3	820
	11th to 20th	Night	18% 14% 24% 32% 10% 2	840
	21st to end	Night	19% 15% 24% 31% 10% 1	840
December	1st to 10th	Night	24% 15% 29% 26% 6%	840
	11th to 20th	Night	23% 13% 27% 31% 6% 1	840
	21st to end	Night	25% 16% 28% 27% 4	924

Page generated at 2026-05-14 01:59:00. Version ac07

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