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 2026-01-02.

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	10% 9% 3 9% 5% 5% 5% 4 6% 7% 36%	131347
	Night	8% 9% 4 10% 6% 5% 5% 4 6% 7% 35%	128808
Transparency	Day	45% 6% 15% 24% 9%	131060
	Night	47% 6% 14% 23% 10% 1	128677
Seeing	Day	14% 9% 31% 29% 14% 4	17794
	Night	14% 8% 33% 32% 11% 1	27802

By Month

Percentage Cloud Cover NW (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	4 14% 6% 14% 10% 9% 8% 3 7% 8% 17%	3186
	11th to 20th	Day	3 8% 3 12% 10% 9% 12% 12% 10% 7% 13%	3494
	21st to end	Day	5% 11% 5% 15% 8% 6% 6% 4 6% 6% 28%	4004
February	1st to 10th	Day	8% 14% 5% 12% 7% 7% 6% 4 8% 7% 21%	1630
	11th to 20th	Day	5% 9% 4 11% 10% 6% 8% 7% 8% 8% 24%	1781
	21st to end	Day	6% 12% 4 11% 8% 6% 9% 4 6% 7% 26%	1650
March	1st to 10th	Day	9% 11% 5% 10% 7% 5% 5% 4 7% 9% 29%	4500
	11th to 20th	Day	7% 13% 6% 14% 9% 5% 5% 4 6% 8% 23%	4924
	21st to end	Day	9% 12% 6% 15% 7% 6% 6% 4 5% 6% 23%	5547
April	1st to 10th	Day	9% 12% 4 8% 6% 4 4 4 6% 8% 36%	2420
	11th to 20th	Day	13% 12% 4 10% 6% 4 5% 4 6% 7% 31%	2572
	21st to end	Day	15% 10% 4 8% 5% 4 4 4 6% 6% 34%	2600
May	1st to 10th	Day	14% 8% 2 6% 3 3 3 3 5% 7% 45%	5976
	11th to 20th	Day	21% 11% 3 7% 5% 5% 4 4 6% 7% 28%	6617
	21st to end	Day	13% 7% 2 5% 4 3 4 3 5% 6% 48%	7428
June	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 5% 7% 8% 39%	3000
	11th to 20th	Day	14% 6% 2 7% 5% 6% 5% 4 6% 8% 37%	3000
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 6% 7% 9% 38%	3000
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 10% 43%	6800
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 8% 46%	6800
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 7% 10% 44%	6944
August	1st to 10th	Day	10% 5% 2 7% 4 4 4 4 6% 9% 46%	4526
	11th to 20th	Day	11% 5% 2 5% 3 3 4 3 6% 9% 50%	4171
	21st to end	Day	10% 6% 2 5% 3 4 4 3 6% 8% 49%	4675
September	1st to 10th	Day	11% 5% 2 5% 4 4 3 4 5% 7% 51%	2430
	11th to 20th	Day	16% 7% 2 6% 3 4 3 4 5% 6% 43%	2200
	21st to end	Day	13% 6% 2 6% 4 3 4 3 6% 8% 45%	2200
October	1st to 10th	Day	10% 8% 2 5% 4 4 4 3 5% 5% 49%	3459
	11th to 20th	Day	9% 9% 3 7% 4 4 3 3 5% 9% 44%	3025
	21st to end	Day	12% 12% 4 8% 4 4 4 3 6% 7% 35%	3190
November	1st to 10th	Day	15% 15% 5% 10% 4 4 5% 4 4 4 30%	1807
	11th to 20th	Day	8% 15% 5% 15% 6% 5% 4 4 5% 5% 28%	1677
	21st to end	Day	9% 14% 4 13% 7% 6% 5% 4 5% 5% 29%	1470
December	1st to 10th	Day	6% 13% 6% 18% 10% 9% 7% 5% 5% 5% 15%	2360
	11th to 20th	Day	9% 12% 5% 15% 9% 9% 6% 5% 7% 5% 19%	2709
	21st to end	Day	6% 9% 4 15% 7% 6% 6% 5% 6% 6% 31%	3575
January	1st to 10th	Night	4 11% 6% 17% 10% 8% 8% 5% 5% 7% 19%	6803
	11th to 20th	Night	5% 9% 5% 13% 8% 8% 11% 8% 10% 7% 17%	6422
	21st to end	Night	7% 13% 7% 15% 7% 5% 5% 5% 6% 7% 23%	6974
February	1st to 10th	Night	8% 11% 5% 11% 8% 6% 7% 6% 7% 6% 24%	2769
	11th to 20th	Night	5% 9% 3 10% 7% 6% 7% 6% 7% 7% 33%	2523
	21st to end	Night	6% 10% 4 10% 7% 6% 6% 4 7% 7% 33%	1980
March	1st to 10th	Night	8% 10% 3 9% 6% 7% 6% 5% 7% 8% 31%	5374
	11th to 20th	Night	8% 11% 4 12% 7% 5% 5% 4 6% 7% 32%	5036
	21st to end	Night	9% 13% 6% 14% 6% 5% 4 3 5% 6% 28%	5385
April	1st to 10th	Night	7% 7% 4 8% 5% 5% 6% 4 6% 8% 40%	1980
	11th to 20th	Night	12% 12% 4 7% 5% 4 5% 4 5% 7% 34%	1779
	21st to end	Night	14% 8% 3 8% 5% 4 6% 5% 6% 6% 37%	1800
May	1st to 10th	Night	14% 8% 2 6% 5% 4 5% 4 5% 6% 41%	4002
	11th to 20th	Night	18% 10% 2 6% 4 4 5% 4 7% 8% 33%	3283
	21st to end	Night	13% 6% 2 6% 3 2 3 3 5% 6% 50%	3470
June	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 5% 7% 9% 45%	1400
	11th to 20th	Night	11% 6% 2 6% 5% 4 6% 5% 6% 7% 41%	1400
	21st to end	Night	9% 5% 2 5% 3 3 4 6% 7% 10% 46%	1400
July	1st to 10th	Night	6% 3 2 5% 4 3 5% 4 6% 10% 52%	3180
	11th to 20th	Night	7% 4 1 5% 3 4 4 4 6% 8% 53%	3180
	21st to end	Night	4 5% 2 6% 4 4 5% 4 7% 10% 48%	3918
August	1st to 10th	Night	6% 5% 2 5% 4 4 5% 3 5% 7% 54%	2634
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 7% 10% 51%	2888
	21st to end	Night	9% 5% 2 5% 3 4 3 3 5% 7% 54%	3234
September	1st to 10th	Night	8% 5% 1 4 4 4 5% 3 4 7% 54%	1967
	11th to 20th	Night	12% 5% 2 6% 4 3 4 3 6% 7% 48%	2200
	21st to end	Night	13% 8% 2 5% 4 4 4 3 5% 7% 47%	2200
October	1st to 10th	Night	11% 7% 2 6% 4 3 5% 3 5% 7% 48%	3687
	11th to 20th	Night	10% 8% 3 6% 4 4 4 4 6% 8% 42%	4118
	21st to end	Night	7% 10% 3 8% 5% 5% 5% 4 6% 6% 43%	4610
November	1st to 10th	Night	10% 12% 5% 8% 5% 5% 5% 5% 6% 7% 33%	2709
	11th to 20th	Night	8% 13% 6% 14% 7% 5% 4 3 5% 6% 29%	2940
	21st to end	Night	9% 14% 4 12% 8% 5% 5% 4 5% 6% 29%	3150
December	1st to 10th	Night	6% 11% 5% 15% 10% 10% 7% 5% 5% 6% 19%	5050
	11th to 20th	Night	8% 14% 5% 13% 9% 7% 6% 4 4 7% 22%	5718
	21st to end	Night	6% 10% 4 14% 8% 8% 7% 5% 6% 5% 28%	7645

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	63% 5% 11% 16% 4	3186
	11th to 20th	Day	61% 6% 10% 15% 7%	3518
	21st to end	Day	55% 6% 16% 19% 4	4004
February	1st to 10th	Day	60% 4 10% 17% 7% 1	1630
	11th to 20th	Day	53% 6% 11% 20% 10% 1	1790
	21st to end	Day	54% 8% 12% 18% 8%	1650
March	1st to 10th	Day	50% 5% 18% 18% 9%	4500
	11th to 20th	Day	58% 6% 14% 17% 5%	4926
	21st to end	Day	60% 6% 11% 18% 5%	5547
April	1st to 10th	Day	45% 5% 16% 25% 9% 1	2420
	11th to 20th	Day	52% 5% 13% 19% 10% 1	2572
	21st to end	Day	51% 5% 15% 20% 8% 1	2600
May	1st to 10th	Day	38% 5% 14% 26% 17% 1	5976
	11th to 20th	Day	53% 6% 10% 23% 8% 1	6621
	21st to end	Day	37% 6% 9% 25% 22% 1	7428
June	1st to 10th	Day	41% 8% 17% 23% 11% 1	3000
	11th to 20th	Day	44% 7% 16% 23% 10% 1	3000
	21st to end	Day	40% 9% 20% 25% 6% 1	3000
July	1st to 10th	Day	34% 7% 27% 27% 4	6726
	11th to 20th	Day	35% 6% 21% 33% 5%	6719
	21st to end	Day	34% 6% 23% 28% 9%	6750
August	1st to 10th	Day	35% 6% 19% 27% 13% 1	4510
	11th to 20th	Day	32% 5% 20% 35% 8%	4210
	21st to end	Day	33% 5% 19% 34% 9%	4674
September	1st to 10th	Day	32% 6% 17% 32% 12% 1	2430
	11th to 20th	Day	42% 6% 17% 24% 10% 1	2200
	21st to end	Day	36% 6% 16% 31% 11%	2200
October	1st to 10th	Day	37% 4 10% 37% 10% 1	3459
	11th to 20th	Day	39% 4 10% 36% 12%	3025
	21st to end	Day	48% 4 11% 27% 9%	3190
November	1st to 10th	Day	57% 4 11% 19% 9% 1	1807
	11th to 20th	Day	58% 5% 15% 15% 7% 1	1677
	21st to end	Day	58% 6% 11% 18% 7%	1470
December	1st to 10th	Day	69% 6% 10% 11% 4	2360
	11th to 20th	Day	63% 5% 12% 15% 5%	2710
	21st to end	Day	52% 5% 9% 21% 12% 1	3575
January	1st to 10th	Night	63% 6% 9% 17% 4	6803
	11th to 20th	Night	58% 8% 11% 16% 7%	6460
	21st to end	Night	59% 6% 11% 19% 4	6974
February	1st to 10th	Night	56% 6% 12% 16% 10% 1	2769
	11th to 20th	Night	47% 6% 14% 20% 13%	2524
	21st to end	Night	48% 6% 13% 23% 11% 1	1980
March	1st to 10th	Night	48% 5% 17% 20% 9% 1	5374
	11th to 20th	Night	50% 5% 14% 20% 11%	5036
	21st to end	Night	56% 6% 10% 21% 7%	5385
April	1st to 10th	Night	40% 5% 13% 25% 15% 2	1980
	11th to 20th	Night	50% 5% 11% 20% 13% 1	1779
	21st to end	Night	45% 7% 14% 22% 12% 2	1800
May	1st to 10th	Night	42% 5% 10% 26% 17%	4002
	11th to 20th	Night	46% 7% 11% 24% 10% 2	3283
	21st to end	Night	34% 5% 9% 25% 25% 2	3470
June	1st to 10th	Night	35% 8% 18% 28% 10% 2	1400
	11th to 20th	Night	40% 8% 16% 24% 12% 2	1400
	21st to end	Night	32% 8% 21% 33% 6% 1	1400
July	1st to 10th	Night	27% 7% 28% 32% 6% 1	3145
	11th to 20th	Night	28% 6% 20% 38% 8%	3138
	21st to end	Night	30% 5% 24% 30% 9% 1	3808
August	1st to 10th	Night	29% 5% 16% 33% 15% 1	2626
	11th to 20th	Night	28% 5% 21% 34% 11% 1	2915
	21st to end	Night	30% 4 18% 35% 12% 1	3234
September	1st to 10th	Night	30% 5% 14% 35% 15% 2	1967
	11th to 20th	Night	35% 6% 18% 29% 12% 1	2200
	21st to end	Night	38% 5% 12% 32% 13% 1	2200
October	1st to 10th	Night	37% 5% 12% 30% 15% 2	3687
	11th to 20th	Night	40% 4 12% 29% 15%	4118
	21st to end	Night	42% 5% 12% 33% 8%	4610
November	1st to 10th	Night	49% 6% 14% 20% 9% 2	2709
	11th to 20th	Night	56% 6% 13% 16% 7% 1	2940
	21st to end	Night	55% 6% 15% 17% 6%	3150
December	1st to 10th	Night	65% 6% 14% 11% 4	5050
	11th to 20th	Night	62% 4 12% 16% 4 1	5716
	21st to end	Night	55% 5% 10% 19% 10% 1	7645

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	9% 10% 36% 29% 13% 4	303
	11th to 20th	Day	11% 9% 35% 33% 10% 2	300
	21st to end	Day	16% 8% 39% 25% 10% 2	330
February	1st to 10th	Day	14% 11% 40% 27% 5% 3	300
	11th to 20th	Day	9% 9% 35% 34% 11% 1	300
	21st to end	Day	6% 9% 39% 31% 13% 2	249
March	1st to 10th	Day	13% 12% 34% 29% 10% 2	300
	11th to 20th	Day	13% 11% 34% 27% 13% 2	300
	21st to end	Day	15% 8% 33% 26% 14% 4	362
April	1st to 10th	Day	14% 12% 32% 31% 9% 2	520
	11th to 20th	Day	19% 10% 32% 24% 12% 1	690
	21st to end	Day	19% 9% 30% 26% 13% 2	700
May	1st to 10th	Day	16% 10% 26% 29% 16% 3	703
	11th to 20th	Day	20% 8% 31% 25% 13% 3	725
	21st to end	Day	19% 9% 21% 30% 18% 4	789
June	1st to 10th	Day	15% 10% 30% 28% 12% 6%	730
	11th to 20th	Day	14% 8% 35% 28% 11% 4	730
	21st to end	Day	13% 9% 30% 29% 15% 4	730
July	1st to 10th	Day	9% 5% 28% 31% 19% 8%	721
	11th to 20th	Day	10% 7% 29% 30% 19% 5%	718
	21st to end	Day	7% 4 26% 37% 18% 8%	794
August	1st to 10th	Day	10% 5% 24% 34% 22% 6%	726
	11th to 20th	Day	13% 6% 24% 29% 20% 7%	715
	21st to end	Day	12% 7% 26% 31% 20% 4	803
September	1st to 10th	Day	13% 8% 34% 28% 13% 4	560
	11th to 20th	Day	19% 9% 26% 34% 11% 2	330
	21st to end	Day	18% 12% 31% 25% 12% 2	330
October	1st to 10th	Day	22% 13% 27% 26% 10% 2	330
	11th to 20th	Day	21% 10% 37% 23% 7% 2	330
	21st to end	Day	24% 8% 37% 23% 6% 2	363
November	1st to 10th	Day	25% 12% 32% 20% 9% 2	330
	11th to 20th	Day	16% 15% 35% 27% 8%	330
	21st to end	Day	15% 15% 40% 23% 6% 2	330
December	1st to 10th	Day	15% 10% 37% 26% 10% 2	330
	11th to 20th	Day	16% 16% 36% 26% 5% 1	330
	21st to end	Day	13% 9% 39% 26% 10% 3	363
January	1st to 10th	Night	11% 10% 37% 29% 11% 1	1006
	11th to 20th	Night	14% 10% 39% 29% 8% 1	1000
	21st to end	Night	13% 12% 38% 30% 7% 1	1100
February	1st to 10th	Night	13% 10% 36% 31% 9% 1	1000
	11th to 20th	Night	10% 9% 36% 32% 11% 2	969
	21st to end	Night	12% 8% 34% 34% 11% 2	660
March	1st to 10th	Night	14% 8% 35% 29% 11% 3	800
	11th to 20th	Night	12% 7% 37% 31% 11% 1	630
	21st to end	Night	14% 6% 32% 33% 13% 2	660
April	1st to 10th	Night	10% 6% 38% 34% 11% 1	600
	11th to 20th	Night	18% 9% 31% 33% 9% 1	593
	21st to end	Night	17% 6% 32% 31% 13% 1	600
May	1st to 10th	Night	17% 7% 31% 32% 13% 2	600
	11th to 20th	Night	19% 8% 27% 33% 13%	591
	21st to end	Night	20% 7% 25% 33% 14%	650
June	1st to 10th	Night	12% 8% 33% 33% 13% 1	600
	11th to 20th	Night	13% 6% 35% 30% 15% 1	600
	21st to end	Night	13% 3 26% 40% 18%	600
July	1st to 10th	Night	6% 3 25% 42% 23% 1	585
	11th to 20th	Night	8% 5% 27% 41% 17% 1	582
	21st to end	Night	6% 2 23% 48% 19% 2	657
August	1st to 10th	Night	8% 3 22% 47% 18% 1	597
	11th to 20th	Night	8% 3 26% 39% 21% 3	594
	21st to end	Night	10% 5% 27% 41% 16% 1	660
September	1st to 10th	Night	10% 5% 31% 34% 17% 3	600
	11th to 20th	Night	16% 6% 31% 36% 10% 1	600
	21st to end	Night	17% 8% 32% 31% 13% 1	600
October	1st to 10th	Night	19% 8% 33% 31% 9%	658
	11th to 20th	Night	21% 10% 30% 30% 8%	930
	21st to end	Night	16% 11% 36% 29% 7% 1	1100
November	1st to 10th	Night	17% 11% 36% 28% 7% 1	1025
	11th to 20th	Night	15% 11% 38% 26% 8% 2	1050
	21st to end	Night	17% 13% 39% 23% 6% 1	1050
December	1st to 10th	Night	11% 10% 38% 30% 10% 2	1050
	11th to 20th	Night	18% 12% 34% 27% 8% 2	1050
	21st to end	Night	17% 6% 35% 31% 9% 2	1155

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