Star Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star Lake Observatory clear sky chart from 2005-11-06 to 2024-11-25.

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% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	100345
	Night	13% 7% 2 6% 4 4 4 4 6% 7% 44%	99065
Transparency	Day	40% 7% 11% 17% 18% 7%	104037
	Night	37% 7% 11% 18% 19% 9%	103500
Seeing	Day	15% 9% 20% 24% 22% 10%	14940
	Night	15% 8% 22% 29% 21% 5%	24644

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	17% 9% 2 5% 4 3 4 3 5% 7% 40%	2847
	11th to 20th	Day	15% 7% 2 4 3 3 3 2 3 5% 53%	2850
	21st to end	Day	23% 10% 3 5% 2 3 4 3 4 5% 39%	3299
February	1st to 10th	Day	19% 8% 3 6% 4 4 4 3 5% 5% 38%	1900
	11th to 20th	Day	19% 8% 2 6% 4 4 3 3 5% 6% 40%	1848
	21st to end	Day	15% 10% 4 8% 5% 4 3 3 5% 6% 36%	1570
March	1st to 10th	Day	22% 11% 3 9% 4 4 4 4 5% 6% 28%	3298
	11th to 20th	Day	17% 8% 3 8% 5% 5% 4 4 6% 7% 34%	3480
	21st to end	Day	13% 8% 3 8% 5% 5% 4 3 5% 5% 39%	3991
April	1st to 10th	Day	11% 7% 2 6% 5% 5% 6% 4 6% 6% 41%	2280
	11th to 20th	Day	16% 7% 2 6% 5% 5% 4 4 6% 6% 39%	2254
	21st to end	Day	9% 6% 2 5% 5% 4 5% 4 6% 8% 46%	2280
May	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 9% 33%	3765
	11th to 20th	Day	14% 6% 2 7% 5% 4 4 4 6% 7% 41%	3764
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	4129
June	1st to 10th	Day	9% 6% 2 8% 6% 7% 7% 5% 9% 10% 32%	2411
	11th to 20th	Day	6% 5% 2 7% 7% 6% 7% 6% 8% 10% 36%	2543
	21st to end	Day	7% 5% 3 8% 8% 7% 8% 6% 8% 11% 30%	2470
July	1st to 10th	Day	6% 6% 3 9% 8% 8% 8% 7% 9% 11% 25%	4120
	11th to 20th	Day	8% 6% 2 8% 7% 7% 8% 7% 9% 12% 25%	4120
	21st to end	Day	10% 8% 4 11% 8% 7% 6% 5% 7% 9% 25%	4216
August	1st to 10th	Day	3 3 1 5% 4 5% 4 3 7% 8% 57%	3785
	11th to 20th	Day	6% 4 2 7% 5% 6% 6% 5% 7% 10% 44%	3729
	21st to end	Day	11% 5% 2 8% 6% 6% 6% 5% 7% 8% 37%	4180
September	1st to 10th	Day	11% 7% 2 8% 6% 7% 6% 5% 6% 7% 34%	2280
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 7% 41%	2237
	21st to end	Day	10% 5% 2 6% 4 3 4 3 6% 8% 51%	2090
October	1st to 10th	Day	6% 5% 2 6% 4 4 4 4 5% 6% 56%	1971
	11th to 20th	Day	9% 4 2 6% 3 4 3 2 5% 7% 55%	1900
	21st to end	Day	11% 5% 2 4 3 3 3 3 4 6% 56%	2090
November	1st to 10th	Day	11% 6% 2 4 4 4 4 3 5% 5% 51%	1958
	11th to 20th	Day	11% 6% 2 6% 4 3 4 2 5% 6% 52%	1890
	21st to end	Day	15% 6% 2 5% 4 3 4 3 5% 6% 47%	1746
December	1st to 10th	Day	16% 7% 2 6% 3 4 4 3 5% 6% 43%	1710
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 3 4 4 41%	2153
	21st to end	Day	16% 8% 2 6% 4 4 5% 4 5% 7% 40%	3191
January	1st to 10th	Night	19% 8% 2 4 3 3 3 2 4 6% 45%	4108
	11th to 20th	Night	17% 7% 2 4 3 3 3 2 4 4 51%	4120
	21st to end	Night	27% 9% 2 4 2 2 2 2 3 5% 42%	4344
February	1st to 10th	Night	17% 8% 3 5% 4 4 4 4 4 6% 41%	2280
	11th to 20th	Night	20% 9% 3 6% 3 3 3 2 5% 5% 42%	2218
	21st to end	Night	20% 8% 3 7% 5% 4 4 3 5% 5% 37%	1884
March	1st to 10th	Night	22% 13% 3 8% 4 4 4 3 4 6% 30%	3583
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 3 5% 8% 37%	3480
	21st to end	Night	16% 8% 2 5% 4 4 4 3 6% 7% 41%	3640
April	1st to 10th	Night	14% 6% 2 5% 5% 4 3 3 5% 6% 45%	1900
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 5% 6% 46%	1875
	21st to end	Night	9% 5% 2 4 4 2 3 3 6% 7% 55%	1900
May	1st to 10th	Night	14% 8% 2 6% 4 4 4 3 5% 7% 44%	3130
	11th to 20th	Night	15% 6% 2 6% 4 3 4 3 5% 8% 43%	3129
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 4 7% 11% 41%	3426
June	1st to 10th	Night	8% 5% 2 7% 5% 6% 7% 5% 7% 10% 40%	1735
	11th to 20th	Night	4 5% 2 5% 4 5% 5% 6% 8% 9% 49%	1607
	21st to end	Night	5% 5% 2 7% 5% 4 6% 6% 8% 12% 39%	1710
July	1st to 10th	Night	5% 6% 3 7% 7% 6% 6% 6% 10% 12% 31%	2850
	11th to 20th	Night	5% 6% 2 7% 6% 6% 7% 7% 10% 12% 32%	2850
	21st to end	Night	7% 8% 4 9% 7% 6% 7% 5% 9% 11% 27%	3337
August	1st to 10th	Night	2 2 1 4 3 3 4 4 6% 11% 59%	3144
	11th to 20th	Night	4 3 2 5% 4 6% 6% 6% 9% 12% 44%	3100
	21st to end	Night	7% 6% 3 8% 5% 5% 6% 5% 7% 8% 41%	3476
September	1st to 10th	Night	9% 5% 2 7% 5% 5% 5% 5% 8% 10% 41%	1900
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 44%	1924
	21st to end	Night	7% 5% 2 6% 4 4 4 3 5% 7% 53%	2090
October	1st to 10th	Night	7% 4 2 5% 4 3 4 3 4 6% 59%	2204
	11th to 20th	Night	8% 4 2 4 3 3 4 4 4 6% 59%	2280
	21st to end	Night	11% 6% 2 5% 3 3 3 2 4 5% 58%	2508
November	1st to 10th	Night	12% 6% 2 4 3 3 4 2 5% 5% 55%	2340
	11th to 20th	Night	12% 6% 1 4 3 3 3 3 5% 6% 55%	2505
	21st to end	Night	14% 7% 2 5% 4 4 4 2 5% 5% 48%	2529
December	1st to 10th	Night	17% 7% 2 5% 4 4 3 2 5% 5% 46%	2470
	11th to 20th	Night	15% 9% 3 6% 4 4 4 2 5% 5% 42%	3042
	21st to end	Night	19% 10% 3 7% 4 3 3 2 4 4 42%	4447

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	43% 4 5% 17% 25% 7%	2848
	11th to 20th	Day	35% 4 4 18% 30% 10%	2850
	21st to end	Day	49% 3 5% 13% 21% 9%	3299
February	1st to 10th	Day	48% 4 7% 14% 18% 10%	1900
	11th to 20th	Day	44% 4 6% 14% 22% 9%	1858
	21st to end	Day	48% 5% 6% 15% 16% 10%	1570
March	1st to 10th	Day	55% 6% 7% 15% 12% 5%	3298
	11th to 20th	Day	48% 6% 7% 17% 17% 5%	3480
	21st to end	Day	46% 6% 9% 12% 21% 7%	3991
April	1st to 10th	Day	40% 8% 8% 17% 18% 9%	2280
	11th to 20th	Day	44% 5% 7% 17% 19% 7%	2254
	21st to end	Day	34% 8% 10% 21% 23% 4	2280
May	1st to 10th	Day	43% 10% 12% 19% 14% 3	3800
	11th to 20th	Day	41% 6% 11% 18% 21% 3	3768
	21st to end	Day	33% 9% 15% 30% 11% 1	4141
June	1st to 10th	Day	43% 10% 14% 21% 10% 1	2411
	11th to 20th	Day	38% 11% 15% 20% 13% 2	2543
	21st to end	Day	43% 12% 15% 21% 8% 1	2470
July	1st to 10th	Day	47% 13% 20% 15% 5%	4056
	11th to 20th	Day	45% 14% 26% 12% 2 1	4051
	21st to end	Day	53% 11% 14% 16% 6% 1	4082
August	1st to 10th	Day	24% 8% 12% 29% 26% 1	3773
	11th to 20th	Day	33% 10% 18% 27% 12% 1	3765
	21st to end	Day	43% 10% 14% 16% 15% 2	4179
September	1st to 10th	Day	45% 10% 12% 15% 13% 5%	2280
	11th to 20th	Day	41% 8% 10% 18% 16% 7%	2237
	21st to end	Day	32% 6% 10% 23% 20% 9%	2090
October	1st to 10th	Day	25% 5% 6% 11% 22% 31%	3270
	11th to 20th	Day	18% 2 6% 15% 28% 31%	3140
	21st to end	Day	22% 3 5% 13% 38% 20%	3428
November	1st to 10th	Day	34% 5% 7% 16% 26% 12%	1958
	11th to 20th	Day	34% 4 6% 14% 29% 13%	1890
	21st to end	Day	38% 4 7% 17% 22% 12%	1746
December	1st to 10th	Day	42% 5% 8% 18% 18% 8%	1710
	11th to 20th	Day	47% 4 6% 11% 24% 8%	2150
	21st to end	Day	42% 6% 7% 19% 22% 3	3191
January	1st to 10th	Night	41% 3 6% 14% 27% 9%	4108
	11th to 20th	Night	38% 3 5% 18% 23% 12%	4120
	21st to end	Night	48% 2 5% 15% 22% 7%	4344
February	1st to 10th	Night	45% 4 7% 15% 19% 9%	2280
	11th to 20th	Night	45% 4 5% 14% 21% 11%	2219
	21st to end	Night	49% 7% 6% 14% 13% 12%	1884
March	1st to 10th	Night	54% 5% 8% 13% 13% 6%	3583
	11th to 20th	Night	46% 6% 8% 16% 18% 6%	3480
	21st to end	Night	41% 5% 10% 19% 20% 5%	3640
April	1st to 10th	Night	40% 6% 9% 17% 18% 11%	1900
	11th to 20th	Night	38% 5% 10% 19% 18% 10%	1875
	21st to end	Night	28% 6% 10% 22% 27% 7%	1900
May	1st to 10th	Night	38% 8% 13% 21% 16% 4	3160
	11th to 20th	Night	39% 6% 12% 23% 18% 2	3130
	21st to end	Night	33% 9% 17% 27% 13% 2	3435
June	1st to 10th	Night	37% 10% 17% 23% 13% 1	1735
	11th to 20th	Night	28% 9% 19% 27% 14% 3	1607
	21st to end	Night	33% 13% 20% 21% 12% 1	1710
July	1st to 10th	Night	40% 16% 25% 15% 4 1	2805
	11th to 20th	Night	37% 16% 32% 12% 2 1	2796
	21st to end	Night	47% 15% 18% 15% 4	3231
August	1st to 10th	Night	17% 9% 20% 28% 25% 1	3132
	11th to 20th	Night	27% 13% 21% 29% 9% 1	3130
	21st to end	Night	38% 11% 14% 18% 15% 4	3476
September	1st to 10th	Night	36% 10% 18% 17% 13% 6%	1900
	11th to 20th	Night	34% 10% 15% 17% 16% 8%	1924
	21st to end	Night	30% 6% 12% 24% 18% 10%	2090
October	1st to 10th	Night	23% 5% 7% 12% 23% 31%	3672
	11th to 20th	Night	17% 3 5% 19% 24% 32%	3779
	21st to end	Night	21% 3 5% 12% 37% 23%	4120
November	1st to 10th	Night	31% 4 8% 18% 23% 16%	2340
	11th to 20th	Night	30% 4 7% 14% 29% 16%	2505
	21st to end	Night	38% 3 6% 19% 21% 13%	2529
December	1st to 10th	Night	40% 5% 8% 21% 20% 7%	2470
	11th to 20th	Night	45% 4 6% 11% 22% 12%	3044
	21st to end	Night	46% 4 6% 17% 20% 7%	4447

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% 20% 34% 17% 3 1	270
	11th to 20th	Day	25% 20% 30% 19% 6% 1	270
	21st to end	Day	26% 25% 37% 9% 3	297
February	1st to 10th	Day	30% 23% 29% 14% 4	270
	11th to 20th	Day	33% 19% 29% 16% 3	270
	21st to end	Day	23% 32% 33% 11% 1	225
March	1st to 10th	Day	27% 20% 31% 20% 3	270
	11th to 20th	Day	24% 22% 35% 16% 3	270
	21st to end	Day	19% 19% 32% 22% 7% 1	397
April	1st to 10th	Day	19% 16% 32% 23% 10%	460
	11th to 20th	Day	24% 14% 28% 23% 11% 1	452
	21st to end	Day	12% 12% 29% 31% 14% 3	460
May	1st to 10th	Day	13% 11% 29% 27% 16% 4	463
	11th to 20th	Day	15% 5% 22% 31% 18% 9%	487
	21st to end	Day	7% 4 22% 30% 26% 12%	529
June	1st to 10th	Day	13% 3 13% 29% 28% 14%	566
	11th to 20th	Day	8% 3 15% 30% 34% 10%	680
	21st to end	Day	10% 3 12% 27% 31% 18%	680
July	1st to 10th	Day	7% 2 9% 22% 43% 17%	671
	11th to 20th	Day	8% 1 6% 23% 37% 24%	668
	21st to end	Day	10% 2 7% 25% 35% 21%	591
August	1st to 10th	Day	5% 2 6% 23% 36% 29%	486
	11th to 20th	Day	11% 2 5% 17% 39% 26%	479
	21st to end	Day	13% 4 7% 22% 31% 24%	539
September	1st to 10th	Day	11% 2 11% 24% 34% 19%	490
	11th to 20th	Day	12% 4 11% 25% 36% 12%	490
	21st to end	Day	12% 3 13% 30% 33% 10%	490
October	1st to 10th	Day	7% 7% 22% 35% 21% 8%	371
	11th to 20th	Day	10% 6% 26% 31% 21% 6%	300
	21st to end	Day	18% 10% 20% 30% 17% 5%	330
November	1st to 10th	Day	17% 7% 29% 30% 14% 2	300
	11th to 20th	Day	14% 15% 34% 24% 12%	300
	21st to end	Day	17% 18% 33% 20% 10% 3	282
December	1st to 10th	Day	22% 11% 33% 24% 8% 1	270
	11th to 20th	Day	23% 16% 36% 19% 5% 1	270
	21st to end	Day	25% 19% 28% 21% 6% 1	297
January	1st to 10th	Night	24% 15% 31% 25% 6%	950
	11th to 20th	Night	23% 13% 32% 26% 6%	950
	21st to end	Night	26% 13% 32% 25% 4	935
February	1st to 10th	Night	21% 14% 36% 24% 5%	760
	11th to 20th	Night	29% 14% 27% 26% 4	743
	21st to end	Night	25% 18% 36% 19% 2	628
March	1st to 10th	Night	20% 14% 34% 27% 6%	675
	11th to 20th	Night	18% 12% 34% 28% 8% 1	570
	21st to end	Night	16% 11% 36% 28% 9%	627
April	1st to 10th	Night	17% 11% 35% 26% 9% 1	570
	11th to 20th	Night	17% 12% 31% 27% 13% 1	563
	21st to end	Night	11% 9% 29% 36% 15% 1	570
May	1st to 10th	Night	12% 5% 27% 38% 16% 2	570
	11th to 20th	Night	10% 4 21% 39% 22% 3	561
	21st to end	Night	6% 3 20% 35% 29% 6%	617
June	1st to 10th	Night	9% 2 12% 37% 34% 7%	570
	11th to 20th	Night	5% 1 11% 30% 43% 9%	570
	21st to end	Night	7% 2 9% 28% 40% 14%	570
July	1st to 10th	Night	6% 2 4 26% 46% 17%	555
	11th to 20th	Night	6% 2 4 21% 48% 20%	552
	21st to end	Night	6% 1 6% 25% 47% 15%	624
August	1st to 10th	Night	3 1 3 21% 49% 23%	567
	11th to 20th	Night	6% 1 5% 20% 50% 18%	564
	21st to end	Night	11% 3 4 24% 39% 19%	627
September	1st to 10th	Night	9% 2 7% 26% 43% 13%	570
	11th to 20th	Night	11% 4 8% 30% 39% 8%	594
	21st to end	Night	10% 2 12% 32% 38% 5%	760
October	1st to 10th	Night	9% 3 18% 36% 31% 3	760
	11th to 20th	Night	9% 5% 24% 35% 23% 4	760
	21st to end	Night	14% 8% 20% 35% 21% 2	836
November	1st to 10th	Night	13% 7% 22% 34% 20% 3	780
	11th to 20th	Night	15% 8% 28% 33% 15% 1	800
	21st to end	Night	18% 9% 27% 30% 15% 1	779
December	1st to 10th	Night	21% 11% 28% 28% 11%	760
	11th to 20th	Night	23% 8% 32% 29% 9%	760
	21st to end	Night	28% 12% 26% 28% 6%	997

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