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 2024-01-14.

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	12% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74952
	Night	13% 6% 2 5% 5% 3 4 5% 5% 7% 44%	74818
Transparency	Day	38% 7% 12% 21% 18% 4	74674
	Night	38% 7% 11% 20% 19% 5%	74615
Seeing	Day	15% 9% 24% 24% 20% 8%	14220
	Night	16% 8% 24% 31% 19% 2	24369

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% 6% 3 5% 4 3 3 3 4 5% 46%	1706
	11th to 20th	Day	16% 7% 3 5% 4 3 4 4 5% 6% 45%	1647
	21st to end	Day	16% 6% 2 4 4 2 4 4 5% 6% 48%	1782
February	1st to 10th	Day	14% 6% 3 4 5% 3 5% 4 6% 7% 41%	1620
	11th to 20th	Day	20% 6% 3 5% 4 2 3 3 5% 5% 45%	1662
	21st to end	Day	19% 8% 2 6% 4 3 4 4 4 6% 41%	1560
March	1st to 10th	Day	15% 4 2 4 4 2 4 3 5% 6% 51%	1980
	11th to 20th	Day	16% 5% 2 6% 4 3 4 5% 6% 8% 42%	1980
	21st to end	Day	16% 7% 2 6% 5% 3 4 5% 5% 7% 41%	2178
April	1st to 10th	Day	11% 3 1 3 3 3 4 5% 6% 8% 53%	2019
	11th to 20th	Day	13% 4 2 5% 4 3 4 5% 5% 8% 47%	2134
	21st to end	Day	13% 5% 3 4 5% 3 5% 5% 5% 7% 47%	2248
May	1st to 10th	Day	10% 5% 2 5% 6% 3 5% 5% 6% 10% 43%	2340
	11th to 20th	Day	12% 7% 3 5% 6% 3 5% 5% 6% 8% 40%	2301
	21st to end	Day	10% 7% 2 6% 6% 4 5% 6% 6% 9% 39%	2531
June	1st to 10th	Day	8% 4 2 6% 6% 4 6% 6% 7% 11% 39%	2340
	11th to 20th	Day	9% 4 2 5% 5% 3 6% 6% 7% 12% 41%	2340
	21st to end	Day	6% 4 2 6% 7% 5% 6% 7% 8% 11% 38%	2340
July	1st to 10th	Day	7% 5% 3 6% 6% 5% 6% 7% 7% 12% 35%	2340
	11th to 20th	Day	7% 5% 3 5% 7% 4 7% 7% 9% 13% 32%	2340
	21st to end	Day	6% 4 3 7% 7% 4 7% 7% 8% 11% 35%	2547
August	1st to 10th	Day	10% 6% 2 6% 6% 4 7% 7% 8% 12% 34%	2327
	11th to 20th	Day	10% 6% 3 7% 7% 5% 8% 7% 8% 9% 29%	2303
	21st to end	Day	10% 7% 2 7% 5% 3 6% 6% 7% 10% 38%	2383
September	1st to 10th	Day	8% 4 3 5% 6% 3 6% 5% 5% 10% 44%	2084
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 5% 5% 8% 44%	1980
	21st to end	Day	19% 7% 2 5% 5% 2 4 5% 4 6% 40%	1980
October	1st to 10th	Day	14% 6% 2 4 4 2 3 3 3 7% 52%	1830
	11th to 20th	Day	12% 5% 2 4 4 3 3 3 4 6% 55%	1800
	21st to end	Day	13% 4 2 5% 4 2 3 2 4 5% 55%	1980
November	1st to 10th	Day	14% 5% 2 4 3 2 2 3 4 5% 56%	1857
	11th to 20th	Day	12% 5% 2 4 3 2 3 4 4 6% 53%	1796
	21st to end	Day	13% 5% 2 5% 5% 3 4 4 5% 6% 49%	1710
December	1st to 10th	Day	18% 8% 2 5% 4 2 4 4 4 7% 41%	1710
	11th to 20th	Day	16% 5% 2 3 4 2 2 2 5% 6% 54%	2153
	21st to end	Day	17% 7% 2 4 4 3 5% 5% 5% 6% 42%	3124
January	1st to 10th	Night	17% 5% 2 4 4 3 4 4 4 6% 47%	2459
	11th to 20th	Night	16% 7% 3 6% 4 3 4 4 4 5% 44%	2391
	21st to end	Night	16% 6% 2 4 4 2 4 3 5% 5% 49%	2574
February	1st to 10th	Night	16% 6% 2 5% 4 3 5% 4 4 6% 45%	2340
	11th to 20th	Night	21% 6% 2 4 4 2 4 4 4 6% 44%	2180
	21st to end	Night	16% 6% 3 5% 3 2 5% 3 5% 6% 46%	1695
March	1st to 10th	Night	13% 5% 2 4 4 2 3 4 5% 6% 51%	1980
	11th to 20th	Night	15% 6% 3 6% 6% 3 4 4 5% 6% 42%	1980
	21st to end	Night	16% 6% 3 6% 5% 3 5% 4 5% 7% 41%	2178
April	1st to 10th	Night	13% 5% 3 4 5% 3 4 4 5% 7% 49%	1940
	11th to 20th	Night	13% 4 2 4 4 3 3 4 5% 8% 50%	1775
	21st to end	Night	13% 4 2 6% 5% 2 5% 4 4 7% 47%	1726
May	1st to 10th	Night	10% 5% 2 5% 5% 3 5% 6% 5% 8% 46%	1620
	11th to 20th	Night	12% 6% 3 7% 6% 3 6% 4 7% 8% 39%	1592
	21st to end	Night	10% 7% 3 5% 4 4 6% 6% 7% 9% 40%	1744
June	1st to 10th	Night	9% 5% 3 5% 6% 3 6% 5% 7% 12% 37%	1620
	11th to 20th	Night	10% 6% 2 5% 6% 3 4 6% 8% 11% 39%	1620
	21st to end	Night	8% 4 3 6% 7% 4 6% 6% 7% 12% 36%	1620
July	1st to 10th	Night	6% 6% 3 7% 7% 4 6% 7% 8% 12% 34%	1620
	11th to 20th	Night	6% 5% 3 5% 6% 4 7% 8% 9% 10% 35%	1620
	21st to end	Night	6% 5% 3 5% 7% 5% 7% 7% 9% 12% 35%	1771
August	1st to 10th	Night	9% 7% 3 7% 8% 4 7% 7% 8% 10% 31%	1611
	11th to 20th	Night	10% 7% 3 7% 7% 4 6% 6% 7% 9% 34%	1598
	21st to end	Night	9% 6% 2 6% 6% 3 6% 7% 8% 11% 37%	1953
September	1st to 10th	Night	9% 6% 2 4 5% 3 5% 5% 7% 8% 45%	1872
	11th to 20th	Night	13% 5% 2 6% 5% 3 5% 4 5% 6% 43%	1980
	21st to end	Night	15% 8% 2 6% 5% 3 5% 5% 6% 7% 39%	1980
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 4 4 6% 49%	2128
	11th to 20th	Night	13% 6% 3 4 3 2 3 4 4 5% 54%	2160
	21st to end	Night	12% 6% 2 4 3 2 3 3 3 5% 56%	2376
November	1st to 10th	Night	14% 7% 2 4 3 2 3 3 4 5% 54%	2221
	11th to 20th	Night	13% 5% 2 4 4 3 4 4 5% 5% 52%	2380
	21st to end	Night	13% 5% 2 3 3 2 3 5% 5% 7% 51%	2470
December	1st to 10th	Night	18% 8% 3 5% 4 3 3 3 4 6% 42%	2470
	11th to 20th	Night	17% 6% 2 4 4 2 3 4 3 5% 49%	3042
	21st to end	Night	20% 6% 2 3 4 2 4 5% 5% 7% 44%	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	42% 4 8% 22% 19% 5%	1707
	11th to 20th	Day	41% 5% 7% 21% 22% 5%	1647
	21st to end	Day	37% 4 8% 23% 22% 7%	1782
February	1st to 10th	Day	41% 6% 10% 21% 17% 5%	1620
	11th to 20th	Day	41% 5% 8% 20% 20% 6%	1671
	21st to end	Day	46% 6% 7% 17% 18% 6%	1560
March	1st to 10th	Day	35% 5% 8% 21% 25% 7%	1980
	11th to 20th	Day	40% 7% 10% 20% 17% 6%	1980
	21st to end	Day	43% 6% 10% 19% 18% 3	2178
April	1st to 10th	Day	29% 4 12% 24% 25% 7%	2019
	11th to 20th	Day	34% 6% 9% 21% 21% 8%	2134
	21st to end	Day	36% 6% 11% 25% 19% 4	2248
May	1st to 10th	Day	36% 7% 17% 20% 16% 4	2340
	11th to 20th	Day	41% 9% 17% 19% 12% 2	2305
	21st to end	Day	39% 9% 14% 24% 13% 1	2531
June	1st to 10th	Day	36% 9% 16% 25% 14% 1	2340
	11th to 20th	Day	35% 9% 19% 21% 15% 2	2340
	21st to end	Day	35% 12% 17% 21% 14% 2	2340
July	1st to 10th	Day	38% 12% 17% 18% 11% 3	2275
	11th to 20th	Day	37% 13% 22% 18% 8% 1	2271
	21st to end	Day	37% 12% 19% 21% 10% 1	2405
August	1st to 10th	Day	40% 11% 19% 22% 8%	2314
	11th to 20th	Day	45% 11% 17% 17% 9% 1	2303
	21st to end	Day	39% 10% 18% 21% 10% 3	2383
September	1st to 10th	Day	34% 9% 15% 20% 18% 4	2084
	11th to 20th	Day	38% 7% 13% 18% 21% 3	1980
	21st to end	Day	44% 6% 9% 20% 17% 4	1980
October	1st to 10th	Day	34% 5% 9% 22% 25% 4	1830
	11th to 20th	Day	32% 5% 8% 27% 21% 8%	1800
	21st to end	Day	33% 3 7% 22% 26% 9%	1980
November	1st to 10th	Day	31% 4 7% 21% 29% 8%	1857
	11th to 20th	Day	31% 4 8% 22% 27% 7%	1796
	21st to end	Day	36% 4 8% 22% 24% 6%	1710
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	39% 4 9% 18% 23% 7%	2459
	11th to 20th	Night	43% 5% 7% 17% 23% 5%	2391
	21st to end	Night	38% 3 6% 21% 26% 6%	2574
February	1st to 10th	Night	41% 4 10% 20% 20% 6%	2340
	11th to 20th	Night	42% 4 8% 18% 21% 7%	2182
	21st to end	Night	40% 5% 10% 19% 19% 7%	1695
March	1st to 10th	Night	34% 5% 8% 21% 23% 9%	1980
	11th to 20th	Night	43% 5% 9% 18% 19% 6%	1980
	21st to end	Night	43% 7% 9% 20% 17% 4	2178
April	1st to 10th	Night	35% 6% 9% 19% 25% 6%	1940
	11th to 20th	Night	33% 5% 10% 22% 22% 8%	1775
	21st to end	Night	37% 6% 11% 21% 20% 5%	1726
May	1st to 10th	Night	33% 8% 15% 23% 17% 3	1620
	11th to 20th	Night	42% 9% 13% 19% 15% 2	1593
	21st to end	Night	38% 10% 14% 25% 13% 1	1745
June	1st to 10th	Night	38% 11% 15% 22% 12% 2	1620
	11th to 20th	Night	36% 10% 17% 22% 14% 1	1620
	21st to end	Night	38% 12% 16% 18% 12% 4	1620
July	1st to 10th	Night	38% 14% 17% 17% 11% 3	1575
	11th to 20th	Night	36% 14% 21% 19% 8% 2	1566
	21st to end	Night	37% 12% 21% 21% 9% 1	1672
August	1st to 10th	Night	43% 12% 18% 18% 9% 1	1600
	11th to 20th	Night	44% 11% 18% 17% 10% 1	1598
	21st to end	Night	37% 11% 19% 22% 8% 2	1953
September	1st to 10th	Night	33% 9% 15% 20% 16% 6%	1872
	11th to 20th	Night	39% 9% 11% 18% 21% 2	1980
	21st to end	Night	42% 10% 11% 16% 17% 4	1980
October	1st to 10th	Night	35% 5% 9% 21% 24% 5%	2128
	11th to 20th	Night	32% 5% 8% 23% 23% 8%	2160
	21st to end	Night	33% 4 6% 21% 26% 11%	2376
November	1st to 10th	Night	34% 5% 6% 19% 24% 12%	2221
	11th to 20th	Night	34% 5% 7% 17% 26% 11%	2380
	21st to end	Night	33% 5% 6% 24% 26% 6%	2470
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	23% 20% 33% 17% 6%	249
	21st to end	Day	19% 22% 39% 15% 5%	264
February	1st to 10th	Day	19% 21% 38% 15% 8%	240
	11th to 20th	Day	31% 19% 37% 11% 3	240
	21st to end	Day	24% 22% 31% 19% 3	278
March	1st to 10th	Day	19% 15% 35% 23% 7%	420
	11th to 20th	Day	19% 16% 41% 18% 6%	420
	21st to end	Day	22% 15% 34% 22% 7% 1	462
April	1st to 10th	Day	19% 14% 33% 20% 13% 1	420
	11th to 20th	Day	15% 16% 27% 24% 16% 3	412
	21st to end	Day	17% 12% 28% 25% 15% 3	420
May	1st to 10th	Day	13% 13% 31% 22% 16% 4	423
	11th to 20th	Day	15% 7% 21% 26% 22% 9%	447
	21st to end	Day	14% 5% 16% 25% 29% 11%	485
June	1st to 10th	Day	10% 5% 15% 33% 27% 10%	450
	11th to 20th	Day	10% 4 19% 25% 26% 16%	450
	21st to end	Day	8% 3 15% 33% 28% 14%	450
July	1st to 10th	Day	10% 2 11% 24% 34% 19%	445
	11th to 20th	Day	7% 1 11% 30% 31% 21%	444
	21st to end	Day	6% 2 10% 32% 33% 17%	488
August	1st to 10th	Day	11% 3 16% 23% 30% 16%	446
	11th to 20th	Day	12% 3 7% 31% 28% 19%	439
	21st to end	Day	10% 2 12% 25% 31% 20%	495
September	1st to 10th	Day	10% 2 14% 21% 34% 18%	450
	11th to 20th	Day	14% 4 12% 24% 30% 16%	450
	21st to end	Day	20% 5% 20% 27% 21% 7%	450
October	1st to 10th	Day	19% 6% 16% 25% 25% 9%	450
	11th to 20th	Day	12% 6% 24% 28% 22% 8%	450
	21st to end	Day	14% 10% 24% 30% 16% 5%	495
November	1st to 10th	Day	18% 8% 27% 25% 17% 4	455
	11th to 20th	Day	16% 12% 35% 25% 10% 3	356
	21st to end	Day	12% 11% 34% 25% 14% 4	270
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% 27% 5%	736
	21st to end	Night	18% 12% 40% 27% 3	792
February	1st to 10th	Night	19% 13% 35% 30% 3	720
	11th to 20th	Night	26% 15% 36% 22% 1	703
	21st to end	Night	18% 16% 36% 26% 3	592
March	1st to 10th	Night	18% 13% 35% 32% 3	720
	11th to 20th	Night	18% 13% 35% 25% 9%	720
	21st to end	Night	19% 11% 35% 28% 6%	792
April	1st to 10th	Night	16% 14% 33% 28% 8%	720
	11th to 20th	Night	15% 12% 30% 28% 14%	711
	21st to end	Night	15% 6% 32% 35% 12%	646
May	1st to 10th	Night	11% 8% 31% 36% 13%	540
	11th to 20th	Night	14% 5% 21% 37% 21% 2	531
	21st to end	Night	13% 3 12% 38% 32% 2	584
June	1st to 10th	Night	11% 2 16% 39% 29% 3	540
	11th to 20th	Night	10% 4 15% 30% 34% 6%	540
	21st to end	Night	8% 1 13% 38% 36% 3	540
July	1st to 10th	Night	7% 2 10% 33% 40% 8%	525
	11th to 20th	Night	8% 1 7% 33% 44% 8%	522
	21st to end	Night	7% 2 10% 35% 39% 8%	591
August	1st to 10th	Night	12% 1 10% 31% 38% 8%	537
	11th to 20th	Night	12% 3 7% 32% 40% 6%	534
	21st to end	Night	11% 2 8% 31% 41% 7%	765
September	1st to 10th	Night	10% 3 8% 30% 42% 7%	720
	11th to 20th	Night	15% 3 12% 27% 38% 5%	720
	21st to end	Night	19% 3 16% 33% 26% 2	720
October	1st to 10th	Night	15% 6% 18% 29% 26% 6%	720
	11th to 20th	Night	17% 7% 23% 32% 19% 2	720
	21st to end	Night	15% 10% 24% 33% 17% 2	792
November	1st to 10th	Night	16% 7% 25% 32% 17% 3	740
	11th to 20th	Night	15% 11% 29% 33% 12%	760
	21st to end	Night	15% 8% 30% 33% 13% 1	760
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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