High Rock Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the High Rock Lake Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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% 6% 2 6% 4 4 4 4 6% 8% 43%	127582
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	124449
Transparency	Day	38% 6% 12% 21% 18% 5%	131201
	Night	38% 6% 10% 19% 21% 6%	128899
Seeing	Day	16% 10% 24% 24% 18% 8%	16995
	Night	16% 8% 25% 31% 18% 2	27127

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	14% 7% 2 5% 3 3 2 2 3 4 55%	4146
	11th to 20th	Day	11% 6% 2 4 3 3 3 2 3 4 59%	4150
	21st to end	Day	16% 8% 3 6% 4 4 3 3 4 4 46%	4565
February	1st to 10th	Day	13% 7% 3 5% 4 4 6% 3 5% 8% 42%	1800
	11th to 20th	Day	21% 6% 2 5% 3 3 3 3 5% 5% 43%	1852
	21st to end	Day	18% 8% 2 7% 4 4 4 3 4 5% 42%	1740
March	1st to 10th	Day	20% 7% 2 6% 4 3 4 3 5% 7% 40%	4935
	11th to 20th	Day	9% 5% 2 6% 3 4 3 3 4 6% 54%	4980
	21st to end	Day	16% 7% 2 6% 4 3 3 3 4 5% 47%	5465
April	1st to 10th	Day	11% 4 1 4 3 3 4 4 6% 8% 52%	2244
	11th to 20th	Day	12% 4 2 6% 3 4 4 4 5% 7% 48%	2374
	21st to end	Day	12% 5% 2 5% 5% 5% 4 4 6% 7% 45%	2498
May	1st to 10th	Day	10% 7% 3 8% 5% 6% 6% 5% 8% 11% 33%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 8% 29%	5861
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 5% 8% 41%	6433
June	1st to 10th	Day	8% 5% 2 7% 6% 5% 5% 5% 8% 10% 39%	2600
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 5% 7% 12% 41%	2600
	21st to end	Day	5% 4 2 6% 6% 6% 6% 6% 8% 11% 40%	2600
July	1st to 10th	Day	9% 6% 2 7% 5% 6% 6% 5% 7% 10% 37%	5900
	11th to 20th	Day	7% 6% 2 7% 6% 6% 7% 6% 10% 14% 29%	5900
	21st to end	Day	9% 7% 3 8% 6% 6% 6% 5% 8% 11% 31%	6424
August	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 5% 7% 11% 25%	4234
	11th to 20th	Day	8% 6% 3 7% 5% 7% 7% 5% 8% 10% 34%	4174
	21st to end	Day	6% 4 2 5% 3 4 5% 4 7% 10% 50%	4319
September	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 4 5% 10% 46%	2315
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 3 5% 8% 43%	2200
	21st to end	Day	19% 7% 2 6% 5% 3 4 4 4 7% 39%	2200
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 6% 53%	2032
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 3 6% 57%	2000
	21st to end	Day	13% 4 2 5% 3 3 3 2 4 5% 56%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 2 3 3 4 5% 55%	2057
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 7% 54%	1984
	21st to end	Day	13% 6% 2 5% 4 3 4 3 4 6% 49%	1890
December	1st to 10th	Day	14% 7% 2 5% 3 3 3 2 4 6% 52%	3020
	11th to 20th	Day	17% 7% 2 7% 4 4 3 3 5% 5% 43%	3463
	21st to end	Day	19% 6% 2 5% 3 3 4 3 5% 7% 44%	4565
January	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 5% 56%	6019
	11th to 20th	Night	12% 7% 2 4 3 3 3 2 3 4 56%	6030
	21st to end	Night	16% 8% 2 5% 4 3 3 3 4 6% 46%	6633
February	1st to 10th	Night	15% 6% 2 6% 4 4 5% 3 4 6% 46%	2601
	11th to 20th	Night	21% 7% 2 5% 3 3 3 3 4 5% 43%	2429
	21st to end	Night	15% 7% 2 5% 3 3 4 3 5% 6% 47%	1889
March	1st to 10th	Night	18% 7% 3 6% 4 3 3 3 5% 5% 44%	4918
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 5% 6% 55%	4980
	21st to end	Night	14% 5% 2 5% 4 3 4 3 5% 7% 47%	5467
April	1st to 10th	Night	13% 5% 2 5% 4 3 4 4 5% 7% 48%	2155
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 5% 8% 50%	1975
	21st to end	Night	12% 5% 2 8% 4 3 5% 4 4 7% 46%	1917
May	1st to 10th	Night	13% 7% 3 7% 4 5% 5% 4 5% 9% 39%	4033
	11th to 20th	Night	17% 8% 3 8% 4 4 5% 3 6% 9% 34%	4032
	21st to end	Night	14% 5% 2 5% 3 4 4 4 6% 9% 43%	4421
June	1st to 10th	Night	9% 6% 3 7% 5% 5% 6% 5% 7% 11% 37%	1800
	11th to 20th	Night	9% 7% 2 6% 5% 4 4 5% 8% 11% 39%	1800
	21st to end	Night	7% 5% 2 7% 6% 6% 5% 5% 7% 13% 37%	1800
July	1st to 10th	Night	10% 7% 3 9% 6% 6% 7% 5% 7% 10% 30%	4058
	11th to 20th	Night	10% 7% 3 8% 7% 7% 7% 7% 9% 10% 26%	4057
	21st to end	Night	12% 8% 3 8% 6% 6% 6% 5% 9% 11% 27%	4427
August	1st to 10th	Night	20% 9% 3 9% 6% 5% 6% 4 7% 9% 22%	2915
	11th to 20th	Night	9% 7% 3 7% 6% 5% 6% 5% 7% 11% 34%	2875
	21st to end	Night	6% 4 2 6% 4 4 4 4 6% 9% 51%	3544
September	1st to 10th	Night	8% 6% 2 5% 4 4 5% 4 7% 8% 46%	2080
	11th to 20th	Night	14% 6% 3 7% 4 4 4 4 6% 6% 43%	2200
	21st to end	Night	16% 8% 2 7% 4 4 5% 4 6% 6% 38%	2200
October	1st to 10th	Night	13% 6% 2 6% 3 4 3 3 4 7% 49%	2366
	11th to 20th	Night	13% 6% 2 5% 3 3 2 3 4 5% 55%	2400
	21st to end	Night	13% 6% 2 4 3 3 3 2 3 4 56%	2640
November	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 4 5% 53%	2461
	11th to 20th	Night	12% 5% 2 4 3 4 4 3 5% 5% 52%	2632
	21st to end	Night	12% 5% 2 4 3 3 3 4 5% 7% 51%	2730
December	1st to 10th	Night	15% 7% 2 5% 3 3 3 2 3 6% 51%	4380
	11th to 20th	Night	18% 8% 3 7% 4 3 3 3 4 5% 42%	4952
	21st to end	Night	19% 5% 2 4 3 3 3 3 4 6% 47%	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	35% 2 4 22% 30% 7%	4147
	11th to 20th	Day	31% 3 3 17% 37% 9%	4150
	21st to end	Day	42% 2 5% 21% 22% 7%	4565
February	1st to 10th	Day	40% 5% 10% 21% 19% 5%	1800
	11th to 20th	Day	42% 5% 8% 19% 20% 6%	1861
	21st to end	Day	45% 5% 7% 17% 20% 6%	1740
March	1st to 10th	Day	44% 4 6% 15% 19% 12%	4935
	11th to 20th	Day	30% 5% 6% 17% 23% 18%	4980
	21st to end	Day	41% 4 7% 23% 20% 6%	5465
April	1st to 10th	Day	30% 4 12% 23% 24% 6%	2244
	11th to 20th	Day	34% 6% 9% 21% 22% 8%	2374
	21st to end	Day	37% 7% 11% 25% 17% 3	2498
May	1st to 10th	Day	41% 8% 18% 20% 11% 2	5897
	11th to 20th	Day	51% 7% 13% 19% 9% 1	5865
	21st to end	Day	38% 8% 13% 20% 18% 3	6447
June	1st to 10th	Day	36% 9% 16% 24% 14% 1	2600
	11th to 20th	Day	34% 9% 19% 21% 15% 2	2600
	21st to end	Day	33% 11% 18% 22% 15% 2	2600
July	1st to 10th	Day	40% 8% 17% 22% 11% 2	5835
	11th to 20th	Day	42% 9% 30% 16% 4	5831
	21st to end	Day	45% 9% 17% 21% 9%	6240
August	1st to 10th	Day	51% 7% 19% 18% 5%	4221
	11th to 20th	Day	41% 7% 21% 20% 9% 2	4213
	21st to end	Day	28% 6% 15% 34% 14% 2	4319
September	1st to 10th	Day	33% 8% 15% 21% 19% 4	2315
	11th to 20th	Day	39% 6% 13% 19% 20% 3	2200
	21st to end	Day	45% 6% 10% 19% 16% 4	2200
October	1st to 10th	Day	30% 4 7% 21% 28% 10%	3305
	11th to 20th	Day	20% 3 5% 21% 26% 25%	3240
	21st to end	Day	27% 3 7% 24% 23% 17%	3538
November	1st to 10th	Day	32% 4 8% 21% 28% 8%	2057
	11th to 20th	Day	31% 4 8% 23% 27% 7%	1984
	21st to end	Day	36% 4 8% 22% 24% 5%	1890
December	1st to 10th	Day	36% 3 7% 19% 26% 8%	3020
	11th to 20th	Day	44% 3 9% 16% 23% 6%	3460
	21st to end	Day	40% 4 9% 28% 17% 2	4565
January	1st to 10th	Night	32% 4 5% 18% 32% 9%	6019
	11th to 20th	Night	33% 3 5% 15% 36% 8%	6030
	21st to end	Night	40% 3 6% 18% 27% 7%	6633
February	1st to 10th	Night	40% 4 10% 20% 20% 5%	2601
	11th to 20th	Night	43% 4 8% 17% 21% 7%	2431
	21st to end	Night	38% 5% 9% 19% 22% 8%	1889
March	1st to 10th	Night	41% 5% 6% 14% 25% 9%	4918
	11th to 20th	Night	30% 4 6% 16% 27% 18%	4980
	21st to end	Night	37% 4 7% 25% 20% 7%	5467
April	1st to 10th	Night	35% 5% 9% 19% 24% 7%	2155
	11th to 20th	Night	32% 6% 10% 22% 22% 8%	1975
	21st to end	Night	37% 6% 12% 21% 19% 4	1917
May	1st to 10th	Night	42% 6% 16% 20% 15% 1	4060
	11th to 20th	Night	47% 7% 14% 18% 13% 1	4033
	21st to end	Night	36% 8% 12% 28% 15% 3	4429
June	1st to 10th	Night	38% 11% 15% 21% 13% 2	1800
	11th to 20th	Night	36% 10% 17% 22% 14% 1	1800
	21st to end	Night	37% 11% 17% 19% 13% 3	1800
July	1st to 10th	Night	47% 8% 16% 16% 11% 1	4015
	11th to 20th	Night	49% 13% 23% 11% 3 1	4006
	21st to end	Night	48% 8% 18% 19% 6%	4297
August	1st to 10th	Night	57% 7% 17% 14% 5%	2904
	11th to 20th	Night	41% 7% 24% 18% 8% 1	2902
	21st to end	Night	29% 7% 13% 28% 20% 3	3544
September	1st to 10th	Night	33% 9% 14% 21% 17% 5%	2080
	11th to 20th	Night	40% 8% 11% 18% 21% 2	2200
	21st to end	Night	44% 9% 12% 16% 16% 4	2200
October	1st to 10th	Night	36% 5% 8% 20% 26% 6%	3873
	11th to 20th	Night	23% 3 6% 21% 28% 19%	3899
	21st to end	Night	28% 2 5% 23% 25% 16%	4252
November	1st to 10th	Night	34% 5% 6% 20% 24% 11%	2461
	11th to 20th	Night	34% 4 7% 17% 27% 10%	2632
	21st to end	Night	33% 5% 7% 24% 26% 7%	2730
December	1st to 10th	Night	37% 3 6% 20% 27% 6%	4380
	11th to 20th	Night	46% 4 9% 16% 17% 8%	4954
	21st to end	Night	37% 4 12% 23% 22% 2	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	25% 21% 33% 18% 3	330
	11th to 20th	Day	27% 22% 32% 15% 5%	330
	21st to end	Day	21% 22% 39% 14% 4	363
February	1st to 10th	Day	16% 21% 41% 15% 7%	300
	11th to 20th	Day	34% 18% 35% 10% 3	300
	21st to end	Day	23% 22% 32% 20% 4	339
March	1st to 10th	Day	21% 15% 33% 23% 6% 1	500
	11th to 20th	Day	18% 16% 40% 19% 6%	500
	21st to end	Day	21% 17% 35% 21% 6% 1	550
April	1st to 10th	Day	21% 15% 34% 18% 11% 1	500
	11th to 20th	Day	14% 18% 28% 23% 14% 2	492
	21st to end	Day	15% 11% 29% 27% 16% 3	500
May	1st to 10th	Day	13% 13% 30% 24% 15% 4	503
	11th to 20th	Day	17% 8% 22% 25% 21% 8%	527
	21st to end	Day	15% 7% 18% 25% 26% 10%	573
June	1st to 10th	Day	9% 5% 18% 33% 25% 9%	530
	11th to 20th	Day	9% 3 19% 27% 26% 15%	530
	21st to end	Day	7% 2 13% 33% 29% 15%	530
July	1st to 10th	Day	10% 1 11% 25% 35% 18%	525
	11th to 20th	Day	7% 1 10% 30% 31% 22%	524
	21st to end	Day	6% 2 10% 32% 33% 16%	576
August	1st to 10th	Day	15% 4 16% 23% 28% 14%	526
	11th to 20th	Day	12% 2 9% 31% 28% 18%	519
	21st to end	Day	9% 2 13% 27% 31% 19%	583
September	1st to 10th	Day	10% 2 16% 24% 32% 16%	530
	11th to 20th	Day	14% 4 14% 25% 28% 15%	530
	21st to end	Day	21% 5% 22% 26% 19% 7%	530
October	1st to 10th	Day	17% 6% 17% 26% 25% 9%	530
	11th to 20th	Day	12% 7% 25% 28% 21% 6%	530
	21st to end	Day	16% 10% 24% 27% 16% 7%	583
November	1st to 10th	Day	19% 8% 28% 26% 15% 3	535
	11th to 20th	Day	16% 13% 34% 25% 9% 2	424
	21st to end	Day	13% 15% 32% 24% 12% 3	330
December	1st to 10th	Day	26% 13% 36% 19% 6%	330
	11th to 20th	Day	21% 10% 36% 22% 10% 1	330
	21st to end	Day	23% 15% 30% 23% 8% 1	363
January	1st to 10th	Night	21% 16% 32% 25% 5%	840
	11th to 20th	Night	21% 13% 37% 25% 5%	840
	21st to end	Night	18% 13% 40% 27% 2	924
February	1st to 10th	Night	18% 13% 36% 30% 3	801
	11th to 20th	Night	27% 16% 35% 21% 1	783
	21st to end	Night	18% 17% 36% 26% 3	660
March	1st to 10th	Night	18% 13% 33% 31% 4	800
	11th to 20th	Night	17% 14% 36% 25% 8%	800
	21st to end	Night	18% 12% 37% 27% 6%	880
April	1st to 10th	Night	16% 15% 34% 27% 8%	800
	11th to 20th	Night	15% 13% 31% 28% 13%	791
	21st to end	Night	14% 6% 32% 36% 12%	717
May	1st to 10th	Night	12% 8% 31% 36% 13%	600
	11th to 20th	Night	14% 6% 21% 38% 19% 2	591
	21st to end	Night	13% 4 13% 37% 30% 2	650
June	1st to 10th	Night	10% 3 17% 39% 28% 3	600
	11th to 20th	Night	10% 5% 16% 31% 33% 6%	600
	21st to end	Night	8% 1 13% 38% 37% 4	600
July	1st to 10th	Night	8% 3 10% 33% 39% 7%	585
	11th to 20th	Night	9% 1 7% 33% 42% 8%	582
	21st to end	Night	7% 2 10% 35% 39% 8%	657
August	1st to 10th	Night	15% 1 11% 31% 35% 7%	597
	11th to 20th	Night	12% 3 8% 32% 39% 6%	594
	21st to end	Night	10% 2 9% 32% 41% 6%	851
September	1st to 10th	Night	10% 3 10% 31% 40% 6%	800
	11th to 20th	Night	16% 3 12% 27% 36% 6%	800
	21st to end	Night	20% 3 17% 33% 25% 2	800
October	1st to 10th	Night	14% 6% 18% 31% 26% 6%	800
	11th to 20th	Night	17% 7% 23% 34% 18% 2	800
	21st to end	Night	15% 9% 23% 33% 18% 2	880
November	1st to 10th	Night	16% 7% 25% 34% 16% 2	820
	11th to 20th	Night	15% 11% 29% 33% 11%	840
	21st to end	Night	15% 9% 31% 32% 12%	840
December	1st to 10th	Night	22% 12% 30% 31% 6%	840
	11th to 20th	Night	20% 9% 30% 33% 9%	840
	21st to end	Night	24% 12% 26% 31% 7%	924

Page generated at 2026-01-29 01:21:45. Version ac07

Copyright © 2026 Attilla Danko.