Rock Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rock Lake clear sky chart from 2005-11-06 to 2026-01-17.

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	16% 12% 4 9% 5% 5% 4 3 5% 6% 30%	128413
	Night	14% 14% 5% 11% 6% 5% 4 3 5% 6% 28%	123261
Transparency	Day	55% 5% 10% 20% 10% 1	131965
	Night	57% 5% 10% 17% 9% 1	127797
Seeing	Day	24% 18% 31% 18% 7% 2	16190
	Night	23% 16% 31% 21% 8% 1	25098

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	11% 16% 7% 17% 8% 8% 7% 4 4 5% 14%	3677
	11th to 20th	Day	9% 19% 8% 19% 9% 7% 5% 3 5% 4 13%	3618
	21st to end	Day	12% 19% 11% 15% 6% 6% 4 3 4 4 18%	4462
February	1st to 10th	Day	9% 18% 8% 15% 10% 6% 4 3 4 4 18%	1800
	11th to 20th	Day	11% 16% 7% 15% 7% 5% 5% 3 4 6% 21%	1753
	21st to end	Day	15% 17% 6% 11% 8% 5% 5% 5% 4 5% 19%	1530
March	1st to 10th	Day	16% 16% 5% 13% 5% 5% 3 3 5% 5% 24%	4833
	11th to 20th	Day	10% 15% 6% 12% 6% 5% 5% 4 5% 7% 26%	4980
	21st to end	Day	14% 16% 6% 10% 4 4 3 2 3 4 33%	5510
April	1st to 10th	Day	20% 16% 4 8% 4 4 4 3 4 6% 27%	2400
	11th to 20th	Day	21% 13% 4 6% 4 4 4 3 4 4 34%	2426
	21st to end	Day	20% 14% 4 8% 4 3 3 3 4 4 33%	2600
May	1st to 10th	Day	20% 10% 4 7% 4 4 4 3 5% 6% 32%	6163
	11th to 20th	Day	15% 11% 3 8% 4 4 4 3 4 6% 37%	6318
	21st to end	Day	21% 10% 3 7% 5% 4 4 3 4 6% 34%	6949
June	1st to 10th	Day	20% 10% 3 7% 5% 5% 4 4 6% 6% 30%	2800
	11th to 20th	Day	15% 8% 2 6% 5% 4 5% 4 5% 8% 37%	2892
	21st to end	Day	19% 8% 3 7% 6% 5% 5% 4 5% 7% 32%	3000
July	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 4 6% 9% 34%	6688
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 7% 44%	6360
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 46%	6889
August	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4234
	11th to 20th	Day	19% 8% 2 7% 4 5% 5% 4 6% 7% 34%	4174
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 10% 40%	4459
September	1st to 10th	Day	17% 7% 3 7% 3 3 5% 3 6% 8% 38%	2400
	11th to 20th	Day	15% 7% 2 7% 4 4 3 3 5% 6% 44%	2365
	21st to end	Day	20% 9% 2 7% 5% 5% 4 4 5% 6% 33%	2110
October	1st to 10th	Day	20% 11% 2 8% 4 3 4 3 5% 6% 33%	2000
	11th to 20th	Day	24% 16% 4 8% 5% 6% 4 3 4 4 23%	2000
	21st to end	Day	26% 17% 5% 9% 5% 4 3 3 3 5% 19%	2168
November	1st to 10th	Day	16% 17% 5% 8% 5% 4 4 3 5% 6% 26%	1701
	11th to 20th	Day	16% 18% 8% 13% 6% 5% 3 3 4 6% 20%	1680
	21st to end	Day	19% 22% 8% 13% 6% 4 4 3 4 3 14%	1680
December	1st to 10th	Day	15% 24% 10% 16% 7% 4 4 3 3 4 11%	2680
	11th to 20th	Day	15% 23% 8% 13% 7% 4 3 2 3 3 18%	3066
	21st to end	Day	19% 21% 9% 13% 8% 3 3 2 3 4 16%	4048
January	1st to 10th	Night	11% 18% 9% 18% 8% 7% 4 3 4 4 13%	6489
	11th to 20th	Night	8% 17% 7% 21% 12% 8% 5% 3 4 4 11%	6400
	21st to end	Night	11% 17% 9% 18% 5% 7% 6% 3 4 4 14%	6490
February	1st to 10th	Night	10% 17% 7% 14% 8% 7% 4 4 5% 6% 18%	2600
	11th to 20th	Night	10% 16% 7% 14% 7% 5% 5% 3 5% 5% 23%	2533
	21st to end	Night	10% 18% 5% 13% 7% 6% 5% 3 5% 6% 21%	2099
March	1st to 10th	Night	14% 15% 5% 12% 5% 4 4 4 4 5% 26%	5005
	11th to 20th	Night	8% 14% 6% 10% 6% 5% 5% 5% 6% 8% 26%	4979
	21st to end	Night	15% 13% 5% 11% 6% 4 4 3 4 5% 29%	5408
April	1st to 10th	Night	19% 17% 5% 7% 4 3 3 3 5% 5% 30%	2000
	11th to 20th	Night	18% 12% 4 7% 4 3 3 3 4 5% 37%	1941
	21st to end	Night	20% 11% 4 8% 5% 4 4 4 4 6% 30%	1800
May	1st to 10th	Night	20% 10% 2 6% 4 4 4 4 6% 7% 32%	3777
	11th to 20th	Night	16% 7% 2 5% 4 4 3 3 5% 8% 43%	3575
	21st to end	Night	19% 10% 3 6% 5% 4 4 4 4 6% 36%	3926
June	1st to 10th	Night	17% 9% 4 6% 4 5% 4 4 6% 6% 35%	1600
	11th to 20th	Night	14% 6% 2 6% 4 3 4 4 5% 7% 45%	1500
	21st to end	Night	17% 8% 3 7% 5% 4 5% 3 5% 7% 37%	1400
July	1st to 10th	Night	15% 8% 3 6% 5% 4 5% 5% 6% 7% 38%	3278
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 5% 6% 46%	3600
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 7% 50%	3961
August	1st to 10th	Night	12% 4 2 6% 4 4 4 5% 6% 7% 47%	2915
	11th to 20th	Night	17% 8% 2 5% 4 4 4 3 5% 6% 42%	2875
	21st to end	Night	14% 5% 2 4 3 4 4 3 6% 9% 46%	3406
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 7% 46%	2000
	11th to 20th	Night	14% 8% 2 6% 3 3 3 3 4 7% 48%	2032
	21st to end	Night	18% 9% 3 6% 3 3 4 4 4 6% 39%	2290
October	1st to 10th	Night	18% 11% 2 6% 3 4 4 3 4 6% 38%	2400
	11th to 20th	Night	21% 15% 4 7% 5% 4 4 3 4 6% 27%	2400
	21st to end	Night	23% 17% 5% 8% 5% 4 3 3 5% 4 23%	2670
November	1st to 10th	Night	15% 18% 5% 8% 5% 4 4 3 4 5% 28%	2814
	11th to 20th	Night	16% 16% 6% 10% 6% 5% 4 3 5% 5% 24%	2940
	21st to end	Night	17% 22% 7% 12% 6% 5% 5% 3 4 4 15%	2940
December	1st to 10th	Night	15% 23% 8% 16% 8% 4 4 3 4 4 10%	4720
	11th to 20th	Night	14% 21% 8% 15% 6% 5% 4 3 3 4 17%	5348
	21st to end	Night	19% 21% 8% 14% 8% 4 4 3 3 4 13%	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	73% 5% 8% 9% 6%	3678
	11th to 20th	Day	74% 5% 5% 8% 7% 1	3648
	21st to end	Day	72% 3 5% 10% 8% 2	4462
February	1st to 10th	Day	69% 5% 5% 13% 7% 1	1800
	11th to 20th	Day	64% 5% 9% 12% 9% 1	1762
	21st to end	Day	66% 5% 7% 13% 9% 1	1530
March	1st to 10th	Day	62% 4 10% 8% 14% 2	4836
	11th to 20th	Day	58% 5% 13% 15% 7% 2	4980
	21st to end	Day	55% 4 6% 19% 14% 3	5510
April	1st to 10th	Day	59% 4 8% 17% 10% 2	2400
	11th to 20th	Day	54% 4 7% 21% 12% 1	2426
	21st to end	Day	56% 4 7% 17% 13% 3	2600
May	1st to 10th	Day	52% 5% 9% 20% 9% 5%	6172
	11th to 20th	Day	48% 6% 8% 23% 15% 1	6322
	21st to end	Day	52% 5% 10% 18% 12% 3	6949
June	1st to 10th	Day	52% 7% 10% 20% 11% 1	2800
	11th to 20th	Day	45% 6% 11% 24% 13% 2	2892
	21st to end	Day	49% 7% 12% 20% 10% 1	3000
July	1st to 10th	Day	44% 7% 14% 25% 9% 1	6617
	11th to 20th	Day	38% 7% 12% 33% 10%	6285
	21st to end	Day	34% 6% 15% 33% 12%	6691
August	1st to 10th	Day	47% 8% 13% 24% 7% 1	4221
	11th to 20th	Day	47% 7% 13% 27% 6% 1	4213
	21st to end	Day	38% 7% 12% 32% 10%	4459
September	1st to 10th	Day	43% 7% 13% 25% 11% 1	2400
	11th to 20th	Day	40% 6% 12% 29% 13%	2365
	21st to end	Day	52% 7% 9% 23% 9%	2110
October	1st to 10th	Day	50% 6% 11% 24% 8% 1	3260
	11th to 20th	Day	57% 5% 9% 19% 10% 1	3240
	21st to end	Day	66% 5% 11% 14% 4	3482
November	1st to 10th	Day	58% 6% 12% 18% 5% 1	1701
	11th to 20th	Day	67% 5% 9% 11% 7% 1	1680
	21st to end	Day	76% 4 8% 7% 4 1	1680
December	1st to 10th	Day	79% 4 6% 8% 3	2680
	11th to 20th	Day	73% 3 4 11% 8% 1	3066
	21st to end	Day	74% 4 4 11% 6%	4048
January	1st to 10th	Night	75% 5% 8% 8% 5%	6489
	11th to 20th	Night	75% 6% 6% 7% 6% 1	6445
	21st to end	Night	73% 4 5% 8% 9% 1	6490
February	1st to 10th	Night	66% 5% 9% 12% 6% 1	2600
	11th to 20th	Night	64% 4 8% 16% 8% 1	2535
	21st to end	Night	62% 5% 7% 17% 8% 1	2099
March	1st to 10th	Night	60% 4 8% 12% 11% 4	5015
	11th to 20th	Night	55% 7% 15% 15% 7% 1	4980
	21st to end	Night	56% 5% 7% 17% 12% 2	5408
April	1st to 10th	Night	57% 4 8% 18% 12% 1	2000
	11th to 20th	Night	51% 4 6% 24% 14% 1	1941
	21st to end	Night	56% 5% 7% 16% 15% 2	1800
May	1st to 10th	Night	50% 6% 10% 20% 11% 4	3784
	11th to 20th	Night	40% 6% 13% 23% 16% 2	3576
	21st to end	Night	50% 6% 10% 21% 12% 1	3927
June	1st to 10th	Night	48% 6% 12% 20% 13% 1	1600
	11th to 20th	Night	38% 6% 12% 27% 16% 1	1500
	21st to end	Night	46% 7% 15% 21% 12%	1400
July	1st to 10th	Night	43% 7% 16% 23% 11%	3241
	11th to 20th	Night	39% 6% 12% 33% 11%	3552
	21st to end	Night	33% 6% 11% 38% 12% 1	3844
August	1st to 10th	Night	34% 7% 17% 27% 13% 1	2904
	11th to 20th	Night	42% 7% 18% 25% 8% 1	2902
	21st to end	Night	34% 7% 16% 32% 10% 2	3406
September	1st to 10th	Night	34% 9% 16% 28% 12% 1	2000
	11th to 20th	Night	37% 5% 13% 31% 13%	2032
	21st to end	Night	45% 7% 12% 25% 11%	2290
October	1st to 10th	Night	41% 6% 11% 27% 15%	3920
	11th to 20th	Night	52% 5% 12% 18% 12%	3899
	21st to end	Night	58% 5% 17% 14% 6%	4306
November	1st to 10th	Night	58% 6% 12% 17% 7%	2814
	11th to 20th	Night	62% 6% 11% 14% 6% 1	2940
	21st to end	Night	72% 6% 9% 10% 3	2940
December	1st to 10th	Night	77% 6% 8% 6% 2	4720
	11th to 20th	Night	71% 4 5% 10% 9%	5348
	21st to end	Night	76% 4 4 9% 7%	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	24% 25% 35% 11% 3 1	220
	11th to 20th	Day	31% 19% 34% 12% 3 1	212
	21st to end	Day	26% 22% 34% 13% 4	328
February	1st to 10th	Day	30% 30% 31% 6% 3	300
	11th to 20th	Day	27% 34% 34% 6%	300
	21st to end	Day	35% 25% 32% 7% 1	294
March	1st to 10th	Day	28% 22% 36% 13% 1	500
	11th to 20th	Day	28% 21% 34% 12% 4 1	500
	21st to end	Day	31% 23% 28% 14% 3 1	550
April	1st to 10th	Day	29% 23% 32% 12% 4 1	500
	11th to 20th	Day	35% 20% 22% 14% 7% 2	492
	21st to end	Day	31% 18% 32% 12% 5% 2	500
May	1st to 10th	Day	26% 17% 32% 17% 6% 2	503
	11th to 20th	Day	22% 18% 32% 17% 8% 2	527
	21st to end	Day	22% 19% 29% 20% 7% 2	573
June	1st to 10th	Day	25% 15% 35% 20% 5% 1	530
	11th to 20th	Day	18% 15% 37% 21% 8% 2	622
	21st to end	Day	22% 16% 35% 19% 7% 1	730
July	1st to 10th	Day	14% 12% 35% 29% 9% 1	670
	11th to 20th	Day	15% 14% 34% 24% 10% 3	524
	21st to end	Day	12% 14% 36% 27% 9% 3	576
August	1st to 10th	Day	17% 9% 31% 28% 10% 5%	526
	11th to 20th	Day	16% 9% 29% 28% 12% 5%	519
	21st to end	Day	17% 13% 32% 23% 11% 4	583
September	1st to 10th	Day	19% 12% 32% 21% 11% 5%	530
	11th to 20th	Day	17% 10% 23% 23% 16% 10%	530
	21st to end	Day	22% 17% 25% 20% 12% 3	530
October	1st to 10th	Day	24% 19% 25% 17% 11% 3	530
	11th to 20th	Day	29% 24% 31% 11% 4 1	530
	21st to end	Day	37% 18% 28% 14% 3	564
November	1st to 10th	Day	34% 20% 31% 11% 3	275
	11th to 20th	Day	33% 20% 32% 11% 4 1	220
	21st to end	Day	32% 26% 32% 8% 1 1	220
December	1st to 10th	Day	33% 20% 32% 15% 1	220
	11th to 20th	Day	35% 25% 30% 8% 1 1	220
	21st to end	Day	31% 27% 25% 13% 4 1	242
January	1st to 10th	Night	25% 21% 36% 15% 3	840
	11th to 20th	Night	28% 20% 31% 17% 4 1	825
	21st to end	Night	17% 20% 35% 21% 6% 1	880
February	1st to 10th	Night	20% 24% 38% 15% 3	800
	11th to 20th	Night	21% 21% 40% 16% 3	783
	21st to end	Night	22% 23% 34% 18% 3	660
March	1st to 10th	Night	22% 25% 32% 19% 3	800
	11th to 20th	Night	21% 20% 38% 15% 5% 1	800
	21st to end	Night	19% 20% 34% 19% 6% 1	880
April	1st to 10th	Night	28% 17% 33% 18% 4	800
	11th to 20th	Night	25% 18% 29% 18% 9% 1	757
	21st to end	Night	25% 14% 30% 21% 10%	600
May	1st to 10th	Night	25% 10% 27% 29% 9% 1	480
	11th to 20th	Night	21% 10% 35% 27% 7%	394
	21st to end	Night	18% 10% 34% 28% 10%	433
June	1st to 10th	Night	20% 13% 33% 29% 7%	400
	11th to 20th	Night	19% 8% 32% 32% 10%	400
	21st to end	Night	21% 9% 27% 32% 11% 1	400
July	1st to 10th	Night	13% 6% 31% 36% 14% 1	390
	11th to 20th	Night	15% 6% 29% 34% 14% 1	388
	21st to end	Night	11% 6% 35% 36% 12% 1	448
August	1st to 10th	Night	13% 5% 24% 37% 19% 2	597
	11th to 20th	Night	13% 6% 23% 35% 21% 3	594
	21st to end	Night	16% 8% 30% 28% 16% 2	765
September	1st to 10th	Night	13% 10% 31% 30% 16% 2	800
	11th to 20th	Night	19% 8% 24% 26% 19% 5%	800
	21st to end	Night	22% 11% 23% 27% 15% 2	800
October	1st to 10th	Night	24% 15% 26% 22% 11% 2	800
	11th to 20th	Night	28% 17% 30% 17% 8% 1	800
	21st to end	Night	32% 18% 29% 17% 4	880
November	1st to 10th	Night	31% 17% 28% 19% 5%	820
	11th to 20th	Night	26% 23% 28% 16% 7%	840
	21st to end	Night	31% 21% 28% 17% 3	840
December	1st to 10th	Night	29% 21% 33% 12% 4	840
	11th to 20th	Night	25% 23% 34% 15% 2 1	840
	21st to end	Night	34% 19% 29% 15% 3 1	924

Page generated at 2026-01-14 01:14:30. Version ac07

Copyright © 2026 Attilla Danko.