Hidden Waters Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hidden Waters Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	137765
	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 44%	134903
Transparency	Day	40% 5% 11% 23% 17% 4	141371
	Night	37% 5% 12% 24% 18% 4	139367
Seeing	Day	21% 14% 26% 22% 12% 5%	17124
	Night	17% 12% 26% 31% 13% 1	26860

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	11% 6% 3 6% 5% 5% 4 4 5% 6% 45%	5258
	11th to 20th	Day	8% 9% 3 7% 5% 4 4 3 4 5% 48%	5280
	21st to end	Day	10% 7% 2 6% 5% 6% 5% 4 6% 6% 43%	5780
February	1st to 10th	Day	10% 9% 3 8% 6% 4 4 4 6% 7% 40%	1890
	11th to 20th	Day	14% 9% 3 6% 5% 4 5% 3 4 7% 39%	1843
	21st to end	Day	13% 9% 2 7% 4 4 5% 4 5% 6% 40%	1784
March	1st to 10th	Day	14% 8% 2 7% 4 4 4 3 5% 7% 41%	5045
	11th to 20th	Day	10% 6% 2 5% 4 4 3 3 5% 7% 51%	5090
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 4 5% 7% 39%	5586
April	1st to 10th	Day	10% 7% 3 6% 6% 4 5% 4 6% 7% 42%	2463
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 5% 7% 46%	2494
	21st to end	Day	17% 7% 2 6% 4 4 5% 4 5% 7% 40%	2612
May	1st to 10th	Day	14% 6% 3 7% 6% 6% 6% 5% 7% 8% 32%	7616
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 4 7% 9% 35%	7626
	21st to end	Day	19% 5% 2 6% 5% 5% 6% 4 6% 7% 35%	8826
June	1st to 10th	Day	11% 5% 2 5% 4 5% 6% 5% 7% 9% 41%	2940
	11th to 20th	Day	10% 6% 3 6% 5% 5% 7% 6% 8% 10% 36%	2940
	21st to end	Day	9% 5% 2 6% 5% 4 6% 6% 7% 11% 40%	2940
July	1st to 10th	Day	6% 4 2 7% 6% 5% 6% 5% 7% 10% 42%	6220
	11th to 20th	Day	8% 5% 2 7% 6% 7% 7% 6% 9% 12% 29%	5965
	21st to end	Day	9% 6% 3 8% 5% 6% 7% 5% 7% 9% 36%	6424
August	1st to 10th	Day	14% 5% 2 7% 6% 6% 7% 5% 8% 9% 31%	4234
	11th to 20th	Day	8% 5% 2 7% 5% 7% 6% 6% 8% 9% 37%	4151
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	4312
September	1st to 10th	Day	13% 5% 2 6% 5% 4 5% 3 5% 7% 45%	2400
	11th to 20th	Day	13% 5% 2 6% 5% 4 4 4 4 7% 46%	2285
	21st to end	Day	22% 6% 2 7% 5% 4 4 4 5% 6% 34%	2150
October	1st to 10th	Day	15% 6% 2 6% 4 3 4 4 5% 7% 46%	2000
	11th to 20th	Day	15% 6% 2 6% 4 4 3 4 3 5% 49%	2000
	21st to end	Day	13% 6% 2 5% 3 4 3 4 5% 6% 49%	2200
November	1st to 10th	Day	8% 6% 2 5% 4 4 4 3 5% 6% 53%	2057
	11th to 20th	Day	12% 6% 2 4 4 3 4 4 7% 6% 47%	1811
	21st to end	Day	12% 6% 2 5% 5% 4 4 4 5% 6% 46%	1680
December	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 5% 6% 49%	2680
	11th to 20th	Day	16% 5% 2 6% 5% 5% 5% 3 4 6% 42%	3070
	21st to end	Day	16% 7% 2 7% 4 4 5% 5% 7% 8% 36%	4113
January	1st to 10th	Night	8% 7% 2 6% 4 5% 5% 4 6% 7% 45%	7635
	11th to 20th	Night	10% 7% 3 5% 3 4 3 3 5% 6% 51%	7680
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 6% 50%	8401
February	1st to 10th	Night	11% 7% 3 6% 5% 5% 5% 5% 7% 7% 42%	2730
	11th to 20th	Night	15% 9% 2 6% 5% 4 5% 5% 6% 6% 39%	2663
	21st to end	Night	12% 8% 3 7% 4 4 5% 4 5% 8% 40%	2017
March	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 5% 6% 47%	5028
	11th to 20th	Night	9% 4 2 4 3 3 4 4 6% 8% 53%	5090
	21st to end	Night	10% 6% 2 5% 5% 5% 5% 4 6% 7% 46%	5588
April	1st to 10th	Night	12% 7% 2 6% 4 5% 5% 4 6% 6% 42%	2156
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 3 6% 8% 46%	2075
	21st to end	Night	16% 5% 2 4 4 4 4 4 6% 7% 45%	2025
May	1st to 10th	Night	15% 7% 3 6% 4 5% 5% 4 6% 9% 36%	5237
	11th to 20th	Night	18% 5% 2 6% 4 4 5% 4 6% 8% 37%	5234
	21st to end	Night	20% 6% 3 6% 4 6% 5% 4 6% 8% 34%	5311
June	1st to 10th	Night	9% 5% 2 6% 4 5% 4 4 7% 8% 46%	1680
	11th to 20th	Night	10% 4 2 6% 6% 5% 5% 4 7% 10% 42%	1680
	21st to end	Night	9% 5% 1 6% 4 4 5% 5% 8% 9% 45%	1680
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 8% 11% 42%	3952
	11th to 20th	Night	8% 6% 3 9% 6% 6% 6% 6% 8% 10% 31%	4102
	21st to end	Night	6% 6% 3 8% 5% 5% 6% 5% 8% 10% 37%	4427
August	1st to 10th	Night	15% 5% 2 6% 4 5% 4 5% 7% 10% 36%	2915
	11th to 20th	Night	8% 6% 2 7% 4 6% 6% 5% 7% 8% 41%	2875
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 9% 48%	3575
September	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	12% 5% 2 5% 4 4 3 3 6% 7% 47%	2110
	21st to end	Night	22% 5% 2 6% 4 3 4 3 5% 6% 39%	2250
October	1st to 10th	Night	17% 4 2 6% 4 3 4 3 4 6% 46%	2400
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 51%	2400
	21st to end	Night	13% 5% 2 4 3 4 3 3 5% 6% 50%	2640
November	1st to 10th	Night	9% 7% 2 6% 3 3 3 3 5% 6% 52%	2461
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 6% 6% 48%	2801
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 6% 6% 46%	2940
December	1st to 10th	Night	8% 5% 3 6% 4 4 4 3 5% 8% 51%	4720
	11th to 20th	Night	14% 7% 3 7% 4 5% 4 4 5% 7% 41%	5345
	21st to end	Night	13% 5% 3 7% 5% 6% 5% 4 7% 9% 35%	7080

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	39% 5% 7% 22% 20% 6%	5259
	11th to 20th	Day	38% 3 6% 18% 22% 14%	5280
	21st to end	Day	40% 4 7% 27% 19% 4	5780
February	1st to 10th	Day	42% 3 11% 23% 17% 4	1890
	11th to 20th	Day	46% 3 8% 21% 19% 4	1852
	21st to end	Day	43% 3 11% 21% 17% 5%	1784
March	1st to 10th	Day	42% 4 8% 20% 15% 9%	5045
	11th to 20th	Day	33% 4 8% 26% 22% 7%	5090
	21st to end	Day	44% 4 10% 18% 18% 6%	5586
April	1st to 10th	Day	41% 3 10% 21% 18% 7%	2463
	11th to 20th	Day	38% 3 11% 22% 20% 5%	2494
	21st to end	Day	44% 4 10% 24% 15% 4	2612
May	1st to 10th	Day	45% 7% 12% 21% 13% 2	7651
	11th to 20th	Day	45% 6% 12% 25% 12% 1	7630
	21st to end	Day	47% 5% 12% 19% 15% 3	8841
June	1st to 10th	Day	38% 5% 14% 23% 19% 1	2940
	11th to 20th	Day	40% 6% 17% 22% 14% 2	2940
	21st to end	Day	35% 7% 16% 26% 15% 2	2940
July	1st to 10th	Day	36% 9% 16% 23% 13% 4	6155
	11th to 20th	Day	42% 12% 16% 23% 6% 1	5896
	21st to end	Day	41% 6% 12% 25% 15% 1	6240
August	1st to 10th	Day	45% 7% 16% 24% 8%	4221
	11th to 20th	Day	38% 7% 13% 28% 13% 1	4190
	21st to end	Day	35% 5% 16% 30% 13% 1	4312
September	1st to 10th	Day	38% 4 14% 18% 21% 4	2400
	11th to 20th	Day	38% 3 7% 27% 20% 4	2285
	21st to end	Day	49% 5% 7% 20% 17% 2	2150
October	1st to 10th	Day	32% 4 8% 22% 28% 6%	3260
	11th to 20th	Day	27% 3 8% 29% 21% 12%	3240
	21st to end	Day	24% 2 9% 30% 25% 10%	3538
November	1st to 10th	Day	32% 3 10% 25% 26% 5%	2057
	11th to 20th	Day	36% 3 9% 22% 23% 7%	1811
	21st to end	Day	37% 4 10% 26% 21% 2	1680
December	1st to 10th	Day	35% 4 11% 25% 20% 5%	2680
	11th to 20th	Day	43% 4 10% 24% 15% 4	3066
	21st to end	Day	44% 5% 15% 19% 15% 1	4113
January	1st to 10th	Night	37% 5% 8% 24% 22% 4	7636
	11th to 20th	Night	33% 3 7% 23% 23% 10%	7680
	21st to end	Night	35% 3 8% 29% 22% 4	8401
February	1st to 10th	Night	40% 3 12% 26% 16% 2	2730
	11th to 20th	Night	45% 3 9% 22% 17% 5%	2665
	21st to end	Night	42% 4 12% 23% 15% 6%	2017
March	1st to 10th	Night	38% 3 9% 24% 17% 9%	5028
	11th to 20th	Night	28% 4 10% 28% 24% 7%	5090
	21st to end	Night	38% 4 13% 22% 19% 5%	5588
April	1st to 10th	Night	40% 5% 10% 21% 19% 4	2156
	11th to 20th	Night	36% 4 13% 24% 20% 4	2075
	21st to end	Night	38% 4 12% 28% 15% 3	2025
May	1st to 10th	Night	44% 7% 14% 21% 11% 3	5264
	11th to 20th	Night	43% 6% 14% 26% 10% 1	5235
	21st to end	Night	48% 6% 14% 19% 9% 3	5318
June	1st to 10th	Night	35% 5% 15% 28% 16% 1	1680
	11th to 20th	Night	38% 6% 17% 25% 14% 1	1680
	21st to end	Night	34% 6% 18% 22% 18% 2	1680
July	1st to 10th	Night	36% 10% 19% 21% 13% 2	3909
	11th to 20th	Night	46% 11% 16% 18% 7% 1	4051
	21st to end	Night	38% 9% 18% 24% 11%	4297
August	1st to 10th	Night	43% 7% 17% 25% 7% 1	2904
	11th to 20th	Night	37% 8% 15% 26% 14%	2902
	21st to end	Night	33% 4 17% 31% 13% 1	3575
September	1st to 10th	Night	35% 5% 15% 21% 19% 5%	2000
	11th to 20th	Night	36% 3 10% 30% 20% 2	2110
	21st to end	Night	46% 4 9% 21% 17% 2	2250
October	1st to 10th	Night	38% 4 9% 22% 24% 3	3920
	11th to 20th	Night	25% 4 11% 22% 30% 8%	3899
	21st to end	Night	22% 3 10% 30% 28% 6%	4252
November	1st to 10th	Night	33% 3 11% 24% 24% 6%	2461
	11th to 20th	Night	35% 4 7% 26% 22% 5%	2801
	21st to end	Night	37% 4 11% 25% 20% 4	2940
December	1st to 10th	Night	33% 4 11% 29% 21% 2	4720
	11th to 20th	Night	43% 4 12% 19% 17% 5%	5348
	21st to end	Night	41% 6% 14% 21% 16% 1	7080

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	35% 26% 25% 11% 3	330
	11th to 20th	Day	35% 24% 26% 13% 2	330
	21st to end	Day	33% 26% 29% 9% 2	363
February	1st to 10th	Day	31% 26% 31% 11% 2	330
	11th to 20th	Day	44% 25% 25% 5% 2	330
	21st to end	Day	33% 22% 29% 15% 2	400
March	1st to 10th	Day	24% 24% 27% 21% 4	540
	11th to 20th	Day	28% 22% 27% 18% 4 1	540
	21st to end	Day	26% 20% 30% 17% 6% 1	594
April	1st to 10th	Day	24% 24% 26% 18% 7% 1	540
	11th to 20th	Day	23% 19% 27% 20% 9% 3	532
	21st to end	Day	27% 16% 29% 17% 10% 1	540
May	1st to 10th	Day	25% 17% 28% 17% 9% 4	543
	11th to 20th	Day	22% 14% 29% 20% 10% 4	567
	21st to end	Day	21% 9% 27% 22% 16% 6%	617
June	1st to 10th	Day	14% 10% 25% 30% 14% 6%	570
	11th to 20th	Day	17% 9% 26% 25% 17% 6%	570
	21st to end	Day	11% 9% 28% 32% 15% 5%	570
July	1st to 10th	Day	10% 5% 20% 29% 25% 11%	565
	11th to 20th	Day	6% 4 19% 35% 24% 11%	544
	21st to end	Day	12% 4 19% 30% 24% 10%	576
August	1st to 10th	Day	15% 7% 19% 29% 19% 12%	526
	11th to 20th	Day	13% 5% 20% 28% 23% 10%	519
	21st to end	Day	10% 5% 20% 29% 21% 14%	583
September	1st to 10th	Day	16% 8% 20% 24% 22% 11%	530
	11th to 20th	Day	16% 4 18% 27% 25% 10%	530
	21st to end	Day	29% 9% 21% 23% 14% 3	530
October	1st to 10th	Day	20% 6% 22% 28% 19% 5%	530
	11th to 20th	Day	20% 16% 22% 26% 14% 2	530
	21st to end	Day	20% 16% 32% 23% 8% 2	583
November	1st to 10th	Day	18% 15% 30% 25% 11% 2	535
	11th to 20th	Day	27% 16% 31% 19% 6% 1	321
	21st to end	Day	27% 17% 37% 15% 4	220
December	1st to 10th	Day	26% 22% 35% 13% 3	220
	11th to 20th	Day	27% 20% 33% 15% 5%	220
	21st to end	Day	29% 20% 33% 13% 5%	256
January	1st to 10th	Night	21% 20% 35% 22% 2	840
	11th to 20th	Night	21% 18% 34% 25% 3	840
	21st to end	Night	17% 18% 42% 22% 2	924
February	1st to 10th	Night	18% 18% 36% 25% 3	840
	11th to 20th	Night	27% 20% 34% 18% 1	823
	21st to end	Night	23% 18% 37% 20% 2	692
March	1st to 10th	Night	18% 19% 36% 24% 3	840
	11th to 20th	Night	23% 18% 30% 25% 4 1	840
	21st to end	Night	21% 18% 33% 23% 5% 1	924
April	1st to 10th	Night	18% 20% 31% 25% 6%	840
	11th to 20th	Night	17% 16% 32% 26% 9% 1	831
	21st to end	Night	19% 11% 29% 33% 8%	765
May	1st to 10th	Night	19% 9% 26% 32% 12% 1	630
	11th to 20th	Night	17% 7% 24% 35% 15% 1	621
	21st to end	Night	16% 2 22% 39% 19% 2	529
June	1st to 10th	Night	12% 4 20% 43% 21% 1	420
	11th to 20th	Night	12% 5% 15% 48% 19% 2	420
	21st to end	Night	9% 5% 15% 50% 20% 1	420
July	1st to 10th	Night	11% 3 10% 37% 34% 5%	523
	11th to 20th	Night	8% 3 10% 39% 35% 6%	597
	21st to end	Night	7% 3 11% 43% 32% 4	657
August	1st to 10th	Night	11% 2 10% 42% 31% 3	597
	11th to 20th	Night	12% 4 9% 39% 33% 3	594
	21st to end	Night	12% 4 12% 39% 29% 4	869
September	1st to 10th	Night	16% 3 14% 36% 26% 5%	800
	11th to 20th	Night	14% 4 14% 37% 27% 4	800
	21st to end	Night	24% 5% 20% 30% 20% 2	800
October	1st to 10th	Night	19% 8% 18% 31% 21% 3	800
	11th to 20th	Night	17% 10% 29% 30% 14% 1	800
	21st to end	Night	18% 12% 25% 34% 10% 1	880
November	1st to 10th	Night	16% 10% 28% 35% 10% 1	820
	11th to 20th	Night	19% 14% 28% 30% 9% 1	840
	21st to end	Night	18% 15% 31% 29% 6% 1	840
December	1st to 10th	Night	18% 16% 34% 28% 3	840
	11th to 20th	Night	19% 11% 35% 30% 4	840
	21st to end	Night	18% 14% 33% 32% 3	924

Page generated at 2026-07-13 03:14:51. Version ac07

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