Hidden Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hidden Lake Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	17% 9% 3 7% 5% 4 5% 4 5% 7% 35%	131308
	Night	17% 9% 3 7% 5% 4 4 3 5% 6% 36%	130841
Transparency	Day	45% 6% 12% 21% 13% 2	134811
	Night	44% 6% 13% 21% 14% 2	135412
Seeing	Day	26% 19% 29% 17% 7% 2	16712
	Night	24% 18% 29% 21% 7% 1	25871

By Month

Percentage Cloud Cover NE (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	10% 11% 4 12% 7% 7% 5% 4 5% 5% 30%	4661
	11th to 20th	Day	15% 11% 3 9% 5% 3 4 3 4 5% 36%	4650
	21st to end	Day	15% 10% 5% 7% 4 4 4 4 5% 6% 38%	5278
February	1st to 10th	Day	15% 9% 3 9% 6% 5% 5% 4 5% 6% 31%	1920
	11th to 20th	Day	13% 8% 3 8% 7% 6% 5% 4 5% 6% 34%	2048
	21st to end	Day	17% 13% 3 8% 6% 4 4 3 5% 6% 33%	1730
March	1st to 10th	Day	20% 11% 4 8% 5% 4 4 3 4 5% 33%	4576
	11th to 20th	Day	14% 9% 4 7% 5% 5% 5% 4 6% 6% 37%	4761
	21st to end	Day	20% 9% 3 7% 5% 5% 4 3 5% 6% 33%	6102
April	1st to 10th	Day	18% 11% 4 6% 4 5% 4 4 4 6% 34%	2520
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 4 4 6% 38%	2446
	21st to end	Day	19% 8% 3 7% 5% 5% 4 4 6% 6% 34%	2508
May	1st to 10th	Day	31% 10% 3 6% 4 4 4 3 5% 7% 23%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 5% 5% 3 5% 7% 31%	6262
	21st to end	Day	21% 8% 3 6% 5% 4 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	17% 8% 3 5% 5% 6% 6% 5% 6% 9% 32%	2800
	11th to 20th	Day	18% 7% 2 7% 5% 4 4 4 6% 9% 35%	2800
	21st to end	Day	16% 8% 2 7% 5% 5% 5% 4 6% 9% 34%	2800
July	1st to 10th	Day	12% 8% 3 7% 5% 6% 5% 5% 7% 9% 33%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 6% 6% 6% 8% 11% 34%	6360
	21st to end	Day	15% 7% 2 6% 5% 4 5% 4 6% 7% 39%	6926
August	1st to 10th	Day	19% 7% 3 8% 5% 5% 5% 5% 6% 8% 30%	4466
	11th to 20th	Day	14% 6% 3 7% 4 5% 5% 5% 7% 9% 35%	3864
	21st to end	Day	12% 5% 2 7% 5% 4 5% 4 7% 8% 40%	4312
September	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 7% 41%	2400
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 6% 49%	2278
	21st to end	Day	17% 7% 3 5% 3 4 5% 3 6% 7% 39%	2200
October	1st to 10th	Day	21% 7% 3 6% 3 4 4 3 5% 4 39%	2139
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 6% 7% 41%	1990
	21st to end	Day	21% 9% 2 6% 5% 4 4 3 5% 6% 34%	1980
November	1st to 10th	Day	12% 9% 3 6% 5% 3 4 3 4 6% 45%	1851
	11th to 20th	Day	17% 8% 3 5% 4 4 5% 4 5% 7% 39%	1890
	21st to end	Day	17% 10% 3 8% 4 3 4 4 5% 5% 35%	1801
December	1st to 10th	Day	15% 10% 3 7% 5% 4 4 4 4 7% 37%	2680
	11th to 20th	Day	19% 11% 3 6% 4 3 4 4 5% 5% 36%	3066
	21st to end	Day	19% 11% 5% 8% 4 4 3 3 4 6% 31%	4048
January	1st to 10th	Night	14% 9% 3 13% 8% 6% 4 3 6% 6% 29%	8234
	11th to 20th	Night	14% 12% 4 11% 5% 5% 4 4 6% 6% 30%	8235
	21st to end	Night	13% 9% 3 7% 5% 6% 6% 4 5% 6% 36%	8900
February	1st to 10th	Night	15% 9% 3 6% 4 5% 5% 5% 6% 7% 34%	2698
	11th to 20th	Night	13% 10% 3 7% 5% 5% 5% 4 7% 7% 33%	2458
	21st to end	Night	17% 9% 3 8% 5% 5% 4 3 5% 6% 34%	2076
March	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 35%	5484
	11th to 20th	Night	16% 9% 2 7% 5% 4 4 3 5% 7% 37%	5386
	21st to end	Night	20% 11% 3 8% 5% 3 4 3 5% 5% 33%	5072
April	1st to 10th	Night	18% 10% 3 8% 5% 4 4 4 6% 7% 31%	2100
	11th to 20th	Night	19% 10% 2 7% 5% 5% 4 3 4 6% 36%	2036
	21st to end	Night	19% 9% 3 7% 4 3 3 2 5% 6% 38%	1890
May	1st to 10th	Night	31% 11% 3 7% 4 3 3 3 4 6% 24%	4054
	11th to 20th	Night	20% 12% 3 8% 5% 4 4 3 5% 6% 30%	3672
	21st to end	Night	22% 9% 3 8% 4 5% 4 4 5% 6% 31%	3926
June	1st to 10th	Night	18% 8% 2 5% 4 3 4 3 6% 7% 40%	1600
	11th to 20th	Night	17% 8% 3 6% 4 5% 4 4 7% 8% 35%	1600
	21st to end	Night	15% 7% 2 6% 4 3 5% 4 6% 8% 42%	1600
July	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 6% 9% 40%	3600
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 6% 8% 35%	3600
	21st to end	Night	12% 7% 2 6% 4 3 4 3 5% 6% 47%	3926
August	1st to 10th	Night	22% 7% 3 7% 4 5% 4 4 5% 6% 34%	2652
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 5% 7% 41%	3179
	21st to end	Night	10% 4 2 6% 4 4 3 3 5% 8% 50%	3586
September	1st to 10th	Night	14% 6% 2 6% 5% 3 4 3 6% 7% 45%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 52%	2120
	21st to end	Night	17% 7% 3 5% 3 3 4 3 5% 6% 44%	2200
October	1st to 10th	Night	19% 6% 2 6% 5% 3 4 3 4 6% 41%	2258
	11th to 20th	Night	14% 8% 2 7% 3 4 4 3 5% 6% 43%	2410
	21st to end	Night	21% 8% 2 6% 4 4 3 4 5% 6% 36%	2860
November	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 5% 46%	2667
	11th to 20th	Night	17% 7% 3 6% 5% 4 4 4 5% 5% 39%	2730
	21st to end	Night	14% 9% 3 6% 5% 5% 4 4 6% 7% 38%	2814
December	1st to 10th	Night	17% 10% 3 8% 4 4 4 4 6% 7% 34%	4720
	11th to 20th	Night	23% 10% 3 5% 4 4 4 3 4 4 35%	5348
	21st to end	Night	18% 11% 5% 8% 5% 4 4 3 4 6% 32%	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	46% 5% 13% 25% 9% 1	4662
	11th to 20th	Day	47% 4 11% 20% 14% 5%	4680
	21st to end	Day	45% 4 11% 22% 17% 1	5278
February	1st to 10th	Day	51% 5% 13% 18% 11% 2	1920
	11th to 20th	Day	47% 6% 10% 22% 13% 2	2057
	21st to end	Day	50% 4 12% 19% 13% 1	1730
March	1st to 10th	Day	50% 4 12% 16% 12% 6%	4579
	11th to 20th	Day	42% 8% 15% 24% 8% 3	4761
	21st to end	Day	49% 5% 7% 16% 19% 5%	6102
April	1st to 10th	Day	50% 5% 10% 20% 13% 3	2520
	11th to 20th	Day	45% 5% 11% 21% 15% 2	2446
	21st to end	Day	47% 6% 9% 20% 15% 4	2508
May	1st to 10th	Day	60% 6% 12% 13% 8% 1	5897
	11th to 20th	Day	51% 6% 13% 19% 10% 1	6266
	21st to end	Day	48% 8% 12% 21% 11% 1	6949
June	1st to 10th	Day	45% 9% 12% 23% 10% 1	2800
	11th to 20th	Day	45% 7% 14% 22% 11% 1	2800
	21st to end	Day	46% 7% 14% 20% 12% 1	2800
July	1st to 10th	Day	46% 8% 12% 21% 13% 1	6290
	11th to 20th	Day	39% 9% 17% 23% 11% 1	6285
	21st to end	Day	42% 6% 13% 23% 16% 1	6728
August	1st to 10th	Day	49% 7% 14% 20% 9% 1	4452
	11th to 20th	Day	42% 7% 16% 26% 8%	3900
	21st to end	Day	36% 7% 13% 29% 13% 1	4312
September	1st to 10th	Day	40% 7% 11% 25% 16% 1	2400
	11th to 20th	Day	35% 5% 10% 26% 22% 2	2278
	21st to end	Day	43% 5% 11% 22% 16% 3	2200
October	1st to 10th	Day	41% 4 9% 19% 23% 3	3477
	11th to 20th	Day	34% 5% 10% 23% 22% 5%	3218
	21st to end	Day	36% 4 14% 26% 16% 4	3180
November	1st to 10th	Day	40% 5% 14% 24% 15% 2	1851
	11th to 20th	Day	41% 5% 11% 23% 17% 2	1890
	21st to end	Day	47% 5% 14% 23% 9% 2	1801
December	1st to 10th	Day	45% 5% 15% 26% 9% 1	2680
	11th to 20th	Day	46% 5% 14% 19% 15% 1	3066
	21st to end	Day	51% 5% 9% 16% 14% 5%	4048
January	1st to 10th	Night	48% 5% 14% 21% 10% 2	8234
	11th to 20th	Night	48% 4 14% 17% 12% 4	8280
	21st to end	Night	44% 5% 13% 17% 18% 2	8900
February	1st to 10th	Night	45% 6% 14% 22% 11% 2	2698
	11th to 20th	Night	45% 7% 12% 20% 14% 2	2460
	21st to end	Night	47% 5% 12% 23% 11% 3	2076
March	1st to 10th	Night	47% 4 13% 18% 15% 3	5494
	11th to 20th	Night	43% 6% 18% 22% 8% 3	5387
	21st to end	Night	48% 6% 7% 15% 21% 3	5072
April	1st to 10th	Night	50% 6% 10% 20% 12% 2	2100
	11th to 20th	Night	48% 6% 9% 19% 17% 1	2036
	21st to end	Night	46% 5% 10% 20% 15% 4	1890
May	1st to 10th	Night	59% 6% 13% 12% 8% 1	4060
	11th to 20th	Night	53% 7% 13% 16% 9% 2	3673
	21st to end	Night	50% 8% 14% 18% 10% 1	3927
June	1st to 10th	Night	42% 7% 13% 25% 12% 1	1600
	11th to 20th	Night	43% 7% 16% 22% 11%	1600
	21st to end	Night	38% 8% 16% 24% 14% 1	1600
July	1st to 10th	Night	41% 6% 16% 21% 15% 1	3560
	11th to 20th	Night	43% 9% 16% 21% 11% 1	3552
	21st to end	Night	36% 7% 13% 26% 17% 1	3809
August	1st to 10th	Night	49% 8% 14% 21% 8% 1	2642
	11th to 20th	Night	41% 7% 15% 29% 8%	3209
	21st to end	Night	29% 6% 15% 35% 14% 1	3586
September	1st to 10th	Night	38% 6% 12% 26% 17% 1	2000
	11th to 20th	Night	32% 6% 12% 26% 23% 1	2120
	21st to end	Night	40% 5% 11% 25% 17% 2	2200
October	1st to 10th	Night	41% 5% 13% 23% 17% 2	3687
	11th to 20th	Night	31% 4 12% 25% 22% 5%	3921
	21st to end	Night	35% 4 16% 29% 14% 2	4610
November	1st to 10th	Night	38% 5% 13% 22% 19% 3	2667
	11th to 20th	Night	42% 5% 12% 21% 17% 3	2730
	21st to end	Night	42% 6% 16% 26% 9% 1	2814
December	1st to 10th	Night	44% 5% 14% 29% 7%	4720
	11th to 20th	Night	48% 5% 15% 15% 16% 2	5348
	21st to end	Night	50% 5% 9% 17% 14% 5%	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	33% 29% 29% 7% 2	220
	11th to 20th	Day	37% 27% 27% 7% 1	220
	21st to end	Day	33% 29% 29% 6% 2 1	270
February	1st to 10th	Day	36% 31% 28% 5% 1	360
	11th to 20th	Day	37% 27% 28% 7% 1	536
	21st to end	Day	33% 29% 26% 11% 2	446
March	1st to 10th	Day	35% 24% 29% 10% 2	540
	11th to 20th	Day	35% 25% 29% 10% 1 1	540
	21st to end	Day	39% 25% 24% 8% 3 2	594
April	1st to 10th	Day	36% 22% 26% 9% 5% 1	540
	11th to 20th	Day	33% 25% 26% 11% 4 1	516
	21st to end	Day	29% 23% 30% 13% 4 1	500
May	1st to 10th	Day	34% 22% 25% 12% 5% 1	503
	11th to 20th	Day	22% 24% 31% 17% 4 1	527
	21st to end	Day	23% 14% 31% 19% 9% 2	573
June	1st to 10th	Day	19% 16% 34% 20% 9% 2	530
	11th to 20th	Day	20% 15% 34% 20% 8% 2	530
	21st to end	Day	18% 19% 32% 22% 8% 2	530
July	1st to 10th	Day	14% 11% 35% 26% 11% 2	525
	11th to 20th	Day	13% 8% 35% 30% 11% 2	524
	21st to end	Day	14% 13% 39% 24% 9% 1	576
August	1st to 10th	Day	16% 10% 28% 31% 11% 5%	526
	11th to 20th	Day	16% 10% 30% 25% 12% 6%	519
	21st to end	Day	13% 13% 31% 26% 13% 4	583
September	1st to 10th	Day	19% 11% 28% 23% 15% 5%	530
	11th to 20th	Day	14% 12% 26% 25% 15% 8%	530
	21st to end	Day	21% 14% 31% 19% 11% 4	530
October	1st to 10th	Day	26% 15% 25% 20% 10% 5%	530
	11th to 20th	Day	23% 22% 27% 22% 5% 2	524
	21st to end	Day	32% 17% 30% 15% 6% 1	462
November	1st to 10th	Day	26% 23% 24% 19% 7% 1	425
	11th to 20th	Day	32% 25% 25% 14% 3 1	430
	21st to end	Day	35% 23% 25% 14% 3 1	341
December	1st to 10th	Day	31% 28% 27% 10% 3	220
	11th to 20th	Day	45% 21% 28% 5% 1	220
	21st to end	Day	31% 27% 29% 11% 1 1	242
January	1st to 10th	Night	26% 29% 29% 14% 2	840
	11th to 20th	Night	32% 22% 30% 14% 2	840
	21st to end	Night	25% 25% 32% 16% 2	924
February	1st to 10th	Night	28% 23% 30% 17% 2	840
	11th to 20th	Night	28% 26% 34% 11% 1	823
	21st to end	Night	31% 25% 28% 14% 2	692
March	1st to 10th	Night	29% 23% 31% 14% 3	840
	11th to 20th	Night	27% 25% 30% 16% 2	840
	21st to end	Night	29% 24% 29% 15% 4 1	924
April	1st to 10th	Night	30% 20% 33% 14% 3	840
	11th to 20th	Night	26% 22% 29% 16% 5%	816
	21st to end	Night	28% 21% 29% 18% 4	690
May	1st to 10th	Night	30% 16% 32% 17% 6%	600
	11th to 20th	Night	22% 16% 34% 23% 4	439
	21st to end	Night	20% 10% 30% 30% 9%	433
June	1st to 10th	Night	21% 11% 33% 27% 9%	400
	11th to 20th	Night	18% 10% 29% 34% 9% 1	400
	21st to end	Night	17% 12% 28% 34% 10% 1	400
July	1st to 10th	Night	15% 6% 28% 35% 16% 1	390
	11th to 20th	Night	16% 5% 25% 38% 15% 1	388
	21st to end	Night	10% 8% 28% 40% 13%	438
August	1st to 10th	Night	14% 5% 24% 44% 13% 1	438
	11th to 20th	Night	15% 6% 26% 33% 18% 2	772
	21st to end	Night	15% 9% 25% 33% 17% 2	880
September	1st to 10th	Night	15% 9% 29% 31% 15% 2	800
	11th to 20th	Night	15% 10% 24% 29% 19% 4	800
	21st to end	Night	22% 9% 26% 29% 12% 3	800
October	1st to 10th	Night	25% 11% 26% 25% 11% 2	800
	11th to 20th	Night	22% 18% 28% 25% 7% 1	800
	21st to end	Night	28% 18% 28% 21% 5%	880
November	1st to 10th	Night	21% 21% 30% 21% 7% 1	820
	11th to 20th	Night	28% 22% 26% 18% 6% 1	840
	21st to end	Night	25% 21% 29% 21% 4 1	840
December	1st to 10th	Night	28% 25% 29% 15% 3	840
	11th to 20th	Night	31% 22% 29% 15% 2	840
	21st to end	Night	31% 25% 29% 12% 3	924

Page generated at 2026-04-14 02:44:24. Version ac07

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