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 2024-11-28.

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% 42%	101408
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	98082
Transparency	Day	38% 7% 12% 21% 18% 5%	105071
	Night	38% 6% 10% 20% 20% 6%	102558
Seeing	Day	15% 9% 24% 25% 19% 8%	15441
	Night	16% 8% 24% 31% 18% 2	25524

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	18% 5% 2 4 3 3 3 2 4 5% 51%	2836
	11th to 20th	Day	13% 7% 2 5% 3 3 3 2 4 4 53%	2840
	21st to end	Day	22% 8% 2 5% 3 3 3 3 4 4 41%	3124
February	1st to 10th	Day	13% 7% 3 5% 4 4 5% 3 5% 7% 42%	1710
	11th to 20th	Day	19% 6% 2 5% 3 3 3 3 5% 5% 44%	1757
	21st to end	Day	18% 9% 2 7% 4 4 4 3 4 5% 40%	1655
March	1st to 10th	Day	22% 7% 2 6% 4 3 4 3 5% 7% 37%	3435
	11th to 20th	Day	10% 4 2 5% 3 3 4 3 5% 6% 54%	3480
	21st to end	Day	20% 8% 2 5% 4 3 3 3 4 5% 43%	3815
April	1st to 10th	Day	10% 4 1 4 3 3 4 4 6% 8% 53%	2132
	11th to 20th	Day	12% 4 2 6% 3 4 4 4 6% 7% 48%	2254
	21st to end	Day	13% 5% 2 4 5% 4 4 4 6% 7% 46%	2373
May	1st to 10th	Day	11% 7% 3 8% 5% 6% 6% 4 7% 10% 34%	4082
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 5% 8% 35%	4081
	21st to end	Day	7% 5% 1 6% 4 5% 5% 4 6% 9% 48%	4476
June	1st to 10th	Day	8% 5% 2 7% 6% 6% 5% 5% 8% 10% 39%	2470
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 5% 7% 12% 41%	2470
	21st to end	Day	6% 5% 2 7% 6% 6% 6% 6% 9% 11% 38%	2470
July	1st to 10th	Day	7% 6% 2 7% 5% 6% 6% 5% 8% 11% 36%	4120
	11th to 20th	Day	8% 7% 2 7% 6% 6% 6% 6% 9% 12% 31%	4120
	21st to end	Day	11% 9% 3 9% 6% 6% 6% 5% 8% 10% 26%	4466
August	1st to 10th	Day	18% 8% 2 7% 5% 5% 5% 5% 7% 11% 25%	4104
	11th to 20th	Day	8% 6% 2 7% 6% 7% 7% 6% 8% 10% 34%	4044
	21st to end	Day	5% 4 2 5% 3 4 5% 4 7% 10% 51%	4187
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 5% 10% 45%	2199
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 5% 8% 42%	2090
	21st to end	Day	19% 7% 2 6% 4 3 4 4 4 6% 39%	2090
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 3 7% 53%	1931
	11th to 20th	Day	12% 5% 1 5% 3 3 3 3 3 6% 57%	1900
	21st to end	Day	13% 4 2 5% 4 3 2 2 4 5% 56%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 2 3 3 4 5% 54%	1957
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 6% 53%	1890
	21st to end	Day	13% 5% 2 5% 4 3 4 3 5% 6% 49%	1773
December	1st to 10th	Day	18% 8% 2 6% 3 3 4 3 5% 7% 41%	1710
	11th to 20th	Day	16% 5% 1 4 3 2 2 2 5% 6% 54%	2153
	21st to end	Day	17% 7% 2 5% 3 4 5% 4 5% 6% 41%	3124
January	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 4 5% 47%	4109
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 4 5% 49%	4120
	21st to end	Night	22% 8% 2 5% 3 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	15% 6% 2 6% 4 4 5% 3 4 6% 46%	2470
	11th to 20th	Night	21% 7% 2 5% 4 3 3 3 4 5% 44%	2305
	21st to end	Night	16% 7% 2 5% 3 3 5% 3 5% 6% 45%	1798
March	1st to 10th	Night	22% 9% 3 7% 4 3 4 3 5% 5% 35%	3418
	11th to 20th	Night	11% 6% 2 5% 4 3 3 2 5% 6% 53%	3480
	21st to end	Night	18% 5% 2 5% 3 3 4 3 4 6% 46%	3817
April	1st to 10th	Night	13% 5% 2 5% 4 3 4 4 5% 7% 48%	2047
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 5% 8% 49%	1875
	21st to end	Night	12% 5% 2 7% 4 3 5% 4 4 7% 47%	1821
May	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 8% 42%	2813
	11th to 20th	Night	15% 8% 3 8% 5% 4 5% 3 5% 7% 38%	2812
	21st to end	Night	7% 5% 2 5% 4 5% 5% 5% 7% 11% 45%	3079
June	1st to 10th	Night	9% 6% 3 6% 5% 5% 6% 4 7% 12% 37%	1710
	11th to 20th	Night	9% 7% 2 6% 5% 4 4 5% 8% 11% 39%	1710
	21st to end	Night	7% 5% 2 7% 6% 6% 5% 5% 7% 13% 36%	1710
July	1st to 10th	Night	7% 7% 3 9% 6% 6% 7% 5% 7% 10% 32%	2840
	11th to 20th	Night	10% 7% 3 7% 6% 6% 6% 6% 8% 10% 32%	2840
	21st to end	Night	15% 9% 3 8% 6% 6% 5% 5% 8% 9% 25%	3087
August	1st to 10th	Night	20% 9% 3 9% 6% 5% 6% 4 7% 9% 22%	2825
	11th to 20th	Night	9% 7% 3 7% 5% 5% 5% 5% 7% 11% 34%	2785
	21st to end	Night	6% 4 2 6% 4 4 4 4 6% 9% 51%	3435
September	1st to 10th	Night	9% 6% 2 5% 5% 5% 5% 4 7% 8% 45%	1976
	11th to 20th	Night	15% 6% 3 7% 4 5% 4 4 6% 6% 42%	2090
	21st to end	Night	16% 8% 2 7% 4 4 5% 4 6% 6% 39%	2090
October	1st to 10th	Night	13% 7% 2 6% 3 4 3 3 4 6% 49%	2247
	11th to 20th	Night	13% 6% 2 4 2 3 3 3 4 5% 55%	2280
	21st to end	Night	12% 6% 2 5% 3 3 3 2 3 4 56%	2508
November	1st to 10th	Night	14% 7% 2 4 3 3 3 3 4 5% 53%	2341
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 5% 5% 51%	2506
	21st to end	Night	12% 5% 2 4 3 3 3 4 5% 7% 51%	2562
December	1st to 10th	Night	18% 8% 3 6% 4 4 3 2 4 6% 42%	2470
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	3042
	21st to end	Night	20% 6% 2 4 3 3 4 4 5% 7% 43%	4532

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	37% 3 5% 28% 20% 6%	2837
	11th to 20th	Day	35% 4 5% 19% 31% 6%	2840
	21st to end	Day	45% 3 6% 17% 23% 5%	3124
February	1st to 10th	Day	40% 5% 9% 21% 19% 5%	1710
	11th to 20th	Day	41% 5% 8% 19% 21% 6%	1766
	21st to end	Day	46% 5% 7% 17% 18% 6%	1655
March	1st to 10th	Day	46% 5% 7% 17% 16% 8%	3435
	11th to 20th	Day	29% 6% 8% 20% 24% 13%	3480
	21st to end	Day	44% 4 7% 21% 21% 3	3815
April	1st to 10th	Day	29% 4 12% 23% 25% 7%	2132
	11th to 20th	Day	34% 6% 9% 20% 22% 8%	2254
	21st to end	Day	36% 7% 11% 25% 18% 3	2373
May	1st to 10th	Day	42% 10% 16% 18% 12% 2	4120
	11th to 20th	Day	45% 8% 12% 20% 13% 1	4085
	21st to end	Day	30% 9% 15% 22% 20% 4	4489
June	1st to 10th	Day	36% 9% 16% 24% 14% 1	2470
	11th to 20th	Day	34% 9% 19% 22% 15% 2	2470
	21st to end	Day	34% 11% 17% 22% 14% 2	2470
July	1st to 10th	Day	39% 10% 19% 18% 12% 2	4055
	11th to 20th	Day	41% 10% 24% 20% 5% 1	4051
	21st to end	Day	50% 10% 15% 17% 8%	4324
August	1st to 10th	Day	51% 7% 19% 18% 5%	4091
	11th to 20th	Day	41% 7% 21% 20% 9% 2	4083
	21st to end	Day	28% 6% 15% 34% 14% 2	4187
September	1st to 10th	Day	34% 9% 14% 20% 19% 4	2199
	11th to 20th	Day	40% 6% 12% 18% 20% 3	2090
	21st to end	Day	45% 6% 9% 19% 17% 4	2090
October	1st to 10th	Day	30% 4 7% 20% 28% 11%	3204
	11th to 20th	Day	20% 3 5% 21% 25% 26%	3140
	21st to end	Day	26% 3 7% 25% 23% 17%	3428
November	1st to 10th	Day	33% 4 8% 20% 27% 8%	1957
	11th to 20th	Day	32% 4 8% 22% 27% 7%	1890
	21st to end	Day	36% 4 8% 22% 24% 6%	1773
December	1st to 10th	Day	44% 4 10% 21% 17% 4	1710
	11th to 20th	Day	33% 3 8% 21% 27% 9%	2150
	21st to end	Day	40% 6% 8% 24% 19% 2	3124
January	1st to 10th	Night	38% 5% 7% 20% 25% 6%	4109
	11th to 20th	Night	38% 5% 7% 16% 29% 6%	4120
	21st to end	Night	44% 3 5% 17% 24% 7%	4532
February	1st to 10th	Night	40% 4 10% 20% 21% 5%	2470
	11th to 20th	Night	43% 4 7% 17% 21% 7%	2307
	21st to end	Night	39% 5% 10% 19% 20% 7%	1798
March	1st to 10th	Night	48% 6% 9% 15% 16% 6%	3418
	11th to 20th	Night	33% 5% 7% 17% 24% 15%	3480
	21st to end	Night	40% 5% 7% 22% 20% 7%	3817
April	1st to 10th	Night	35% 6% 9% 19% 25% 7%	2047
	11th to 20th	Night	32% 6% 10% 21% 22% 8%	1875
	21st to end	Night	36% 6% 12% 22% 20% 4	1821
May	1st to 10th	Night	39% 8% 15% 22% 14% 2	2840
	11th to 20th	Night	45% 7% 11% 20% 15% 1	2813
	21st to end	Night	29% 10% 14% 30% 16% 1	3087
June	1st to 10th	Night	38% 11% 15% 22% 13% 2	1710
	11th to 20th	Night	35% 10% 16% 23% 14% 1	1710
	21st to end	Night	38% 11% 16% 19% 13% 4	1710
July	1st to 10th	Night	45% 10% 15% 15% 13% 2	2795
	11th to 20th	Night	43% 11% 25% 15% 5% 1	2786
	21st to end	Night	53% 10% 15% 17% 5%	2988
August	1st to 10th	Night	56% 7% 18% 14% 5%	2814
	11th to 20th	Night	41% 7% 24% 18% 8% 1	2812
	21st to end	Night	28% 7% 13% 28% 20% 3	3435
September	1st to 10th	Night	34% 9% 15% 20% 17% 6%	1976
	11th to 20th	Night	41% 8% 11% 17% 21% 2	2090
	21st to end	Night	43% 9% 11% 16% 17% 4	2090
October	1st to 10th	Night	36% 5% 8% 19% 26% 7%	3754
	11th to 20th	Night	22% 3 6% 21% 28% 20%	3779
	21st to end	Night	28% 3 5% 24% 24% 17%	4120
November	1st to 10th	Night	34% 5% 6% 20% 24% 11%	2341
	11th to 20th	Night	34% 5% 7% 17% 26% 10%	2506
	21st to end	Night	32% 5% 7% 24% 26% 7%	2562
December	1st to 10th	Night	44% 4 8% 20% 18% 5%	2470
	11th to 20th	Night	38% 4 7% 18% 24% 9%	3044
	21st to end	Night	39% 6% 9% 22% 23% 3	4532

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% 18% 34% 20% 4	270
	11th to 20th	Day	25% 20% 33% 16% 6%	270
	21st to end	Day	20% 21% 39% 15% 5%	297
February	1st to 10th	Day	17% 20% 38% 16% 8%	270
	11th to 20th	Day	31% 19% 37% 10% 3	270
	21st to end	Day	25% 22% 31% 19% 3	310
March	1st to 10th	Day	20% 14% 34% 24% 7% 1	460
	11th to 20th	Day	18% 16% 42% 18% 6%	460
	21st to end	Day	22% 16% 34% 21% 6% 1	506
April	1st to 10th	Day	20% 14% 33% 19% 12% 1	460
	11th to 20th	Day	15% 17% 26% 24% 15% 3	452
	21st to end	Day	17% 12% 28% 25% 15% 3	460
May	1st to 10th	Day	13% 13% 30% 24% 16% 5%	463
	11th to 20th	Day	15% 8% 21% 26% 22% 9%	487
	21st to end	Day	13% 5% 17% 26% 28% 10%	529
June	1st to 10th	Day	9% 5% 17% 34% 26% 10%	490
	11th to 20th	Day	10% 3 19% 26% 27% 16%	490
	21st to end	Day	8% 2 14% 34% 28% 14%	490
July	1st to 10th	Day	10% 1 11% 25% 34% 19%	485
	11th to 20th	Day	8% 1 10% 30% 30% 21%	484
	21st to end	Day	7% 2 11% 33% 31% 16%	532
August	1st to 10th	Day	13% 4 16% 23% 29% 15%	486
	11th to 20th	Day	12% 2 9% 31% 27% 18%	479
	21st to end	Day	9% 2 12% 26% 31% 20%	539
September	1st to 10th	Day	10% 2 14% 23% 34% 17%	490
	11th to 20th	Day	15% 4 13% 24% 28% 15%	490
	21st to end	Day	20% 6% 21% 27% 20% 7%	490
October	1st to 10th	Day	18% 6% 17% 26% 24% 9%	490
	11th to 20th	Day	11% 7% 24% 29% 22% 7%	490
	21st to end	Day	14% 9% 23% 29% 17% 7%	539
November	1st to 10th	Day	19% 8% 28% 26% 16% 4	495
	11th to 20th	Day	16% 12% 34% 25% 9% 3	390
	21st to end	Day	12% 12% 34% 25% 14% 3	291
December	1st to 10th	Day	25% 11% 37% 20% 7%	270
	11th to 20th	Day	19% 11% 34% 23% 12% 1	270
	21st to end	Day	22% 15% 31% 22% 9% 2	297
January	1st to 10th	Night	21% 15% 32% 26% 6%	760
	11th to 20th	Night	21% 12% 36% 26% 5%	760
	21st to end	Night	18% 12% 39% 27% 3	836
February	1st to 10th	Night	18% 13% 35% 31% 3	760
	11th to 20th	Night	26% 15% 36% 21% 1	743
	21st to end	Night	19% 17% 36% 25% 3	628
March	1st to 10th	Night	18% 13% 33% 31% 4	760
	11th to 20th	Night	18% 13% 35% 25% 8%	760
	21st to end	Night	19% 11% 36% 28% 6%	836
April	1st to 10th	Night	16% 15% 34% 28% 8%	760
	11th to 20th	Night	15% 13% 30% 28% 14%	751
	21st to end	Night	15% 6% 32% 36% 11%	681
May	1st to 10th	Night	12% 8% 31% 35% 13%	570
	11th to 20th	Night	14% 6% 21% 38% 20% 2	561
	21st to end	Night	12% 3 12% 38% 31% 2	617
June	1st to 10th	Night	11% 2 17% 39% 29% 3	570
	11th to 20th	Night	10% 5% 15% 31% 34% 6%	570
	21st to end	Night	8% 1 13% 38% 36% 3	570
July	1st to 10th	Night	8% 3 10% 32% 40% 7%	555
	11th to 20th	Night	9% 1 7% 33% 43% 7%	552
	21st to end	Night	7% 2 10% 36% 38% 7%	624
August	1st to 10th	Night	13% 1 11% 31% 36% 8%	567
	11th to 20th	Night	12% 3 8% 33% 39% 6%	564
	21st to end	Night	10% 2 8% 31% 42% 7%	808
September	1st to 10th	Night	11% 3 9% 30% 41% 7%	760
	11th to 20th	Night	17% 3 12% 26% 37% 6%	760
	21st to end	Night	20% 3 17% 33% 25% 2	760
October	1st to 10th	Night	15% 6% 17% 30% 26% 6%	760
	11th to 20th	Night	16% 7% 23% 33% 18% 2	760
	21st to end	Night	14% 9% 23% 33% 19% 2	836
November	1st to 10th	Night	16% 7% 25% 34% 16% 2	780
	11th to 20th	Night	15% 11% 29% 33% 12%	800
	21st to end	Night	15% 9% 30% 33% 13% 1	789
December	1st to 10th	Night	21% 12% 29% 32% 6%	760
	11th to 20th	Night	19% 9% 29% 33% 10%	760
	21st to end	Night	25% 11% 25% 31% 7%	836

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