Mitten Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mitten Lake Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	13% 11% 4 10% 5% 5% 5% 4 5% 7% 32%	131818
	Night	12% 11% 4 11% 6% 5% 4 4 5% 6% 31%	130646
Transparency	Day	50% 5% 11% 21% 11% 1	135267
	Night	53% 5% 11% 20% 10% 1	135270
Seeing	Day	19% 13% 32% 22% 10% 3	17982
	Night	19% 12% 33% 27% 9% 1	28014

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	8% 15% 6% 20% 11% 7% 5% 4 4 5% 16%	4691
	11th to 20th	Day	6% 17% 6% 15% 9% 9% 10% 6% 5% 5% 11%	4710
	21st to end	Day	6% 12% 8% 14% 6% 6% 9% 4 6% 8% 21%	5193
February	1st to 10th	Day	9% 13% 5% 11% 7% 6% 6% 6% 6% 6% 25%	1890
	11th to 20th	Day	9% 11% 5% 13% 7% 5% 7% 5% 6% 5% 27%	1881
	21st to end	Day	11% 12% 6% 13% 7% 7% 5% 3 5% 6% 23%	1730
March	1st to 10th	Day	13% 9% 3 9% 5% 4 4 3 4 7% 38%	4580
	11th to 20th	Day	10% 11% 4 12% 7% 5% 5% 4 5% 7% 29%	4967
	21st to end	Day	21% 13% 5% 11% 5% 4 4 3 3 4 27%	5991
April	1st to 10th	Day	14% 15% 4 10% 6% 5% 4 4 5% 5% 27%	2520
	11th to 20th	Day	18% 14% 6% 9% 5% 3 3 3 5% 5% 29%	2446
	21st to end	Day	19% 11% 4 7% 6% 5% 4 4 4 7% 30%	2545
May	1st to 10th	Day	15% 8% 2 5% 3 3 4 3 4 5% 47%	6319
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	6319
	21st to end	Day	14% 10% 3 8% 5% 4 4 4 5% 7% 35%	6935
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 4 4 7% 9% 38%	2800
	11th to 20th	Day	15% 8% 3 6% 5% 5% 5% 5% 6% 6% 36%	2800
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 5% 7% 9% 36%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 42%	6360
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 9% 47%	6360
	21st to end	Day	9% 6% 3 6% 5% 4 5% 4 6% 8% 44%	6924
August	1st to 10th	Day	13% 7% 2 7% 4 6% 5% 4 6% 8% 38%	4563
	11th to 20th	Day	13% 6% 2 5% 3 4 4 3 5% 8% 45%	4156
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 5% 8% 37%	4312
September	1st to 10th	Day	13% 8% 3 6% 4 4 5% 3 5% 8% 41%	2400
	11th to 20th	Day	12% 8% 3 6% 5% 4 4 4 6% 8% 39%	2367
	21st to end	Day	16% 9% 3 8% 4 4 4 3 5% 7% 36%	2182
October	1st to 10th	Day	17% 10% 3 6% 4 4 4 4 5% 7% 36%	2000
	11th to 20th	Day	20% 15% 5% 8% 5% 4 5% 3 6% 5% 25%	2000
	21st to end	Day	21% 15% 5% 8% 4 3 4 3 6% 6% 26%	1986
November	1st to 10th	Day	15% 15% 5% 8% 5% 4 4 3 5% 6% 29%	1852
	11th to 20th	Day	14% 18% 6% 13% 6% 5% 4 3 4 5% 22%	1711
	21st to end	Day	14% 15% 6% 16% 9% 4 4 3 5% 5% 17%	1680
December	1st to 10th	Day	8% 17% 8% 28% 10% 5% 4 3 4 3 10%	2690
	11th to 20th	Day	9% 20% 7% 18% 10% 5% 5% 3 3 4 17%	3088
	21st to end	Day	18% 24% 7% 13% 6% 5% 4 2 4 4 14%	4070
January	1st to 10th	Night	9% 14% 6% 20% 11% 8% 5% 3 4 5% 16%	8232
	11th to 20th	Night	6% 17% 6% 17% 10% 8% 7% 7% 6% 5% 12%	8280
	21st to end	Night	6% 13% 7% 16% 7% 6% 6% 4 5% 6% 24%	9018
February	1st to 10th	Night	8% 13% 6% 12% 7% 6% 5% 5% 5% 7% 26%	2730
	11th to 20th	Night	8% 11% 4 11% 7% 7% 6% 4 6% 8% 28%	2626
	21st to end	Night	11% 15% 5% 10% 7% 5% 5% 4 6% 5% 27%	2076
March	1st to 10th	Night	11% 7% 3 8% 4 4 4 4 6% 8% 41%	5493
	11th to 20th	Night	10% 12% 5% 13% 6% 5% 5% 4 6% 7% 27%	5243
	21st to end	Night	20% 13% 4 10% 6% 4 4 3 5% 5% 27%	5179
April	1st to 10th	Night	19% 12% 3 9% 5% 4 4 3 4 7% 29%	2100
	11th to 20th	Night	22% 11% 3 7% 4 3 3 3 5% 6% 34%	2035
	21st to end	Night	19% 10% 3 6% 4 4 4 3 5% 5% 36%	1852
May	1st to 10th	Night	13% 8% 3 6% 3 4 3 3 4 7% 46%	3596
	11th to 20th	Night	11% 7% 3 8% 5% 4 4 3 4 6% 45%	3596
	21st to end	Night	13% 9% 3 8% 4 4 4 4 5% 6% 41%	3942
June	1st to 10th	Night	12% 8% 2 6% 4 4 4 5% 5% 7% 44%	1600
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 5% 5% 7% 39%	1600
	21st to end	Night	12% 5% 2 7% 5% 4 5% 4 6% 9% 41%	1600
July	1st to 10th	Night	11% 6% 2 5% 3 4 4 4 6% 9% 47%	3618
	11th to 20th	Night	11% 5% 2 5% 3 2 4 4 5% 8% 52%	3617
	21st to end	Night	10% 5% 2 6% 3 4 4 4 5% 8% 49%	3947
August	1st to 10th	Night	10% 7% 2 6% 4 5% 5% 4 6% 7% 43%	2597
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 52%	2885
	21st to end	Night	16% 9% 2 6% 4 3 3 2 5% 6% 43%	3586
September	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 5% 7% 49%	2000
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 5% 8% 46%	2012
	21st to end	Night	14% 8% 2 6% 5% 4 4 4 5% 7% 40%	2215
October	1st to 10th	Night	16% 7% 2 6% 4 5% 4 3 4 8% 41%	2400
	11th to 20th	Night	16% 13% 4 7% 4 4 5% 4 6% 6% 31%	2400
	21st to end	Night	19% 14% 3 8% 5% 4 5% 4 5% 7% 26%	2853
November	1st to 10th	Night	15% 13% 4 7% 5% 4 4 3 5% 7% 32%	2666
	11th to 20th	Night	14% 17% 6% 12% 5% 5% 3 3 5% 5% 25%	2903
	21st to end	Night	14% 15% 6% 14% 7% 5% 4 3 5% 5% 21%	2940
December	1st to 10th	Night	10% 13% 7% 21% 10% 6% 4 3 5% 5% 14%	4720
	11th to 20th	Night	10% 17% 10% 16% 7% 5% 5% 3 5% 4 18%	5339
	21st to end	Night	21% 19% 5% 12% 7% 5% 4 3 4 4 16%	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	71% 4 8% 14% 4	4692
	11th to 20th	Day	73% 5% 7% 10% 5%	4710
	21st to end	Day	61% 4 8% 18% 9%	5193
February	1st to 10th	Day	56% 6% 9% 18% 11%	1890
	11th to 20th	Day	56% 6% 8% 22% 7%	1890
	21st to end	Day	62% 5% 8% 15% 9% 1	1730
March	1st to 10th	Day	46% 4 15% 19% 14% 2	4580
	11th to 20th	Day	54% 5% 13% 17% 10% 1	4967
	21st to end	Day	62% 4 8% 14% 11%	5993
April	1st to 10th	Day	58% 4 9% 17% 10% 1	2520
	11th to 20th	Day	57% 4 10% 19% 9% 1	2446
	21st to end	Day	55% 5% 9% 18% 13% 1	2545
May	1st to 10th	Day	40% 5% 9% 27% 18% 2	6357
	11th to 20th	Day	43% 7% 11% 24% 11% 3	6323
	21st to end	Day	47% 6% 9% 22% 17% 1	6949
June	1st to 10th	Day	40% 6% 13% 27% 12% 1	2800
	11th to 20th	Day	46% 7% 12% 21% 13% 1	2800
	21st to end	Day	43% 8% 16% 24% 9% 1	2800
July	1st to 10th	Day	36% 7% 15% 34% 8% 1	6291
	11th to 20th	Day	32% 6% 17% 32% 11% 1	6285
	21st to end	Day	36% 7% 13% 29% 15%	6728
August	1st to 10th	Day	43% 7% 15% 22% 12% 1	4548
	11th to 20th	Day	36% 7% 15% 30% 12%	4196
	21st to end	Day	44% 7% 14% 24% 10% 1	4311
September	1st to 10th	Day	41% 5% 14% 28% 11%	2400
	11th to 20th	Day	41% 7% 18% 27% 7%	2367
	21st to end	Day	47% 6% 11% 26% 10%	2182
October	1st to 10th	Day	36% 4 14% 30% 16%	3260
	11th to 20th	Day	54% 4 9% 19% 14%	3240
	21st to end	Day	53% 5% 16% 20% 6%	3186
November	1st to 10th	Day	56% 6% 10% 20% 8% 1	1852
	11th to 20th	Day	66% 4 9% 14% 6%	1711
	21st to end	Day	68% 7% 10% 11% 3 1	1680
December	1st to 10th	Day	79% 4 6% 8% 2	2690
	11th to 20th	Day	72% 4 6% 10% 7%	3085
	21st to end	Day	76% 3 4 11% 6%	4070
January	1st to 10th	Night	72% 4 8% 12% 4	8233
	11th to 20th	Night	71% 5% 8% 10% 5%	8280
	21st to end	Night	60% 5% 7% 18% 10%	9018
February	1st to 10th	Night	57% 5% 7% 19% 10% 1	2730
	11th to 20th	Night	54% 6% 10% 20% 10% 1	2627
	21st to end	Night	58% 5% 10% 17% 8% 1	2076
March	1st to 10th	Night	40% 5% 17% 20% 15% 2	5493
	11th to 20th	Night	55% 5% 12% 19% 8% 1	5243
	21st to end	Night	60% 4 9% 15% 11%	5179
April	1st to 10th	Night	55% 5% 7% 18% 14% 1	2100
	11th to 20th	Night	52% 4 9% 22% 12% 2	2035
	21st to end	Night	49% 5% 10% 20% 15% 1	1852
May	1st to 10th	Night	39% 5% 13% 26% 15% 3	3620
	11th to 20th	Night	39% 8% 11% 22% 17% 3	3597
	21st to end	Night	43% 6% 13% 22% 16% 1	3950
June	1st to 10th	Night	38% 7% 15% 27% 13%	1600
	11th to 20th	Night	44% 7% 11% 23% 15% 1	1600
	21st to end	Night	38% 8% 15% 28% 10% 2	1600
July	1st to 10th	Night	33% 6% 19% 31% 9% 2	3580
	11th to 20th	Night	30% 5% 18% 34% 11% 1	3572
	21st to end	Night	32% 6% 15% 28% 17% 1	3832
August	1st to 10th	Night	38% 8% 13% 24% 15% 2	2588
	11th to 20th	Night	33% 7% 16% 31% 12% 1	2910
	21st to end	Night	42% 5% 14% 25% 13% 2	3586
September	1st to 10th	Night	34% 6% 14% 33% 13% 1	2000
	11th to 20th	Night	36% 6% 16% 34% 8%	2012
	21st to end	Night	43% 6% 13% 27% 11% 1	2215
October	1st to 10th	Night	33% 4 9% 32% 20% 2	3920
	11th to 20th	Night	43% 5% 11% 28% 13%	3899
	21st to end	Night	56% 5% 10% 20% 8%	4603
November	1st to 10th	Night	52% 5% 12% 22% 10%	2666
	11th to 20th	Night	61% 5% 12% 14% 7% 1	2903
	21st to end	Night	65% 5% 11% 14% 4 1	2940
December	1st to 10th	Night	72% 5% 9% 12% 2	4720
	11th to 20th	Night	69% 5% 7% 12% 7%	5341
	21st to end	Night	72% 4 5% 12% 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	17% 15% 34% 26% 6% 2	330
	11th to 20th	Day	20% 17% 35% 21% 7% 1	330
	21st to end	Day	18% 12% 44% 22% 4	363
February	1st to 10th	Day	18% 18% 41% 17% 5% 2	330
	11th to 20th	Day	14% 23% 41% 17% 5% 1	330
	21st to end	Day	19% 19% 50% 11% 1	273
March	1st to 10th	Day	18% 20% 40% 15% 6%	330
	11th to 20th	Day	21% 17% 39% 15% 7% 1	330
	21st to end	Day	26% 13% 36% 17% 6% 2	538
April	1st to 10th	Day	24% 17% 34% 19% 5% 1	540
	11th to 20th	Day	31% 14% 28% 19% 7% 1	516
	21st to end	Day	24% 15% 32% 19% 8% 1	535
May	1st to 10th	Day	21% 16% 31% 22% 7% 2	703
	11th to 20th	Day	18% 13% 33% 23% 11% 3	725
	21st to end	Day	20% 14% 30% 22% 11% 2	789
June	1st to 10th	Day	15% 13% 32% 25% 12% 3	730
	11th to 20th	Day	22% 10% 34% 21% 9% 3	730
	21st to end	Day	17% 12% 32% 22% 13% 4	730
July	1st to 10th	Day	10% 6% 30% 31% 17% 8%	721
	11th to 20th	Day	12% 9% 31% 25% 16% 6%	718
	21st to end	Day	9% 7% 33% 28% 15% 9%	794
August	1st to 10th	Day	14% 8% 26% 29% 18% 5%	726
	11th to 20th	Day	17% 7% 21% 29% 21% 5%	680
	21st to end	Day	17% 10% 24% 26% 18% 5%	583
September	1st to 10th	Day	18% 13% 23% 25% 15% 6%	530
	11th to 20th	Day	15% 10% 26% 31% 15% 3	530
	21st to end	Day	22% 12% 24% 26% 12% 4	512
October	1st to 10th	Day	20% 17% 29% 21% 9% 3	330
	11th to 20th	Day	25% 22% 34% 12% 6%	330
	21st to end	Day	29% 17% 35% 13% 4 1	363
November	1st to 10th	Day	26% 16% 35% 17% 5% 1	330
	11th to 20th	Day	26% 20% 38% 13% 3 1	330
	21st to end	Day	23% 18% 42% 13% 3	330
December	1st to 10th	Day	17% 15% 45% 19% 4	330
	11th to 20th	Day	20% 19% 41% 17% 3	330
	21st to end	Day	27% 20% 33% 16% 3 1	363
January	1st to 10th	Night	15% 14% 43% 22% 6%	1050
	11th to 20th	Night	17% 14% 41% 21% 6% 1	1050
	21st to end	Night	19% 11% 38% 25% 6%	1155
February	1st to 10th	Night	16% 15% 40% 23% 7% 1	1050
	11th to 20th	Night	15% 16% 39% 24% 6% 1	992
	21st to end	Night	18% 14% 39% 24% 4	692
March	1st to 10th	Night	17% 13% 35% 28% 6% 1	840
	11th to 20th	Night	18% 14% 40% 21% 7%	705
	21st to end	Night	20% 13% 30% 27% 9% 2	693
April	1st to 10th	Night	25% 11% 35% 23% 6%	630
	11th to 20th	Night	28% 10% 26% 29% 6%	611
	21st to end	Night	24% 8% 32% 27% 8% 1	600
May	1st to 10th	Night	21% 12% 29% 31% 7% 1	600
	11th to 20th	Night	19% 9% 29% 32% 9% 2	591
	21st to end	Night	16% 9% 28% 34% 12%	650
June	1st to 10th	Night	16% 7% 31% 35% 12% 1	600
	11th to 20th	Night	17% 10% 28% 35% 10%	600
	21st to end	Night	14% 7% 26% 37% 14% 2	600
July	1st to 10th	Night	10% 3 24% 38% 23% 2	585
	11th to 20th	Night	13% 6% 29% 31% 19% 1	582
	21st to end	Night	10% 2 24% 43% 18% 3	657
August	1st to 10th	Night	10% 4 26% 36% 23% 2	597
	11th to 20th	Night	15% 5% 20% 33% 25% 3	594
	21st to end	Night	17% 4 25% 30% 22% 2	660
September	1st to 10th	Night	17% 7% 24% 32% 18% 3	600
	11th to 20th	Night	15% 7% 26% 35% 16% 2	612
	21st to end	Night	19% 10% 28% 30% 13% 1	800
October	1st to 10th	Night	20% 11% 30% 26% 13% 1	800
	11th to 20th	Night	24% 14% 32% 23% 6%	800
	21st to end	Night	26% 11% 29% 26% 8%	880
November	1st to 10th	Night	23% 13% 36% 23% 5% 1	820
	11th to 20th	Night	23% 19% 33% 19% 6%	1013
	21st to end	Night	23% 19% 33% 20% 4	1050
December	1st to 10th	Night	19% 18% 37% 21% 5% 1	1050
	11th to 20th	Night	20% 19% 36% 19% 6% 1	1050
	21st to end	Night	27% 14% 35% 19% 4	1155

Page generated at 2026-04-14 01:44:29. Version ac07

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