Swan Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Swan Lake Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	131837
	Night	12% 6% 2 6% 4 4 4 4 6% 7% 44%	130654
Transparency	Day	37% 8% 12% 20% 18% 6%	135624
	Night	36% 7% 11% 19% 20% 7%	134937
Seeing	Day	15% 9% 21% 23% 20% 12%	16834
	Night	15% 7% 22% 30% 22% 4	26914

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% 8% 3 6% 4 4 4 3 4 4 43%	5258
	11th to 20th	Day	11% 6% 2 4 2 2 2 2 4 6% 58%	5280
	21st to end	Day	23% 9% 3 7% 4 3 4 2 4 4 37%	6056
February	1st to 10th	Day	14% 7% 3 7% 4 4 4 4 5% 7% 40%	2100
	11th to 20th	Day	21% 7% 2 5% 5% 4 4 3 4 5% 42%	2048
	21st to end	Day	18% 7% 3 6% 4 4 4 4 5% 5% 41%	1730
March	1st to 10th	Day	20% 9% 2 7% 5% 4 4 4 5% 5% 36%	4739
	11th to 20th	Day	9% 5% 2 6% 3 4 4 4 5% 7% 51%	5090
	21st to end	Day	15% 6% 2 7% 4 3 4 3 4 5% 45%	5586
April	1st to 10th	Day	11% 6% 2 5% 5% 4 4 3 6% 7% 46%	2352
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 5% 6% 50%	2494
	21st to end	Day	8% 5% 2 5% 4 5% 4 4 6% 8% 49%	2683
May	1st to 10th	Day	12% 6% 2 7% 5% 4 5% 4 7% 8% 39%	5875
	11th to 20th	Day	17% 7% 3 7% 5% 4 6% 5% 7% 8% 31%	5861
	21st to end	Day	11% 6% 2 7% 4 4 5% 4 6% 9% 42%	6433
June	1st to 10th	Day	8% 6% 2 8% 6% 5% 6% 5% 7% 11% 35%	2600
	11th to 20th	Day	4 4 2 6% 4 5% 5% 5% 8% 13% 44%	2600
	21st to end	Day	5% 5% 2 5% 5% 6% 6% 6% 8% 12% 40%	2600
July	1st to 10th	Day	7% 5% 2 8% 6% 7% 8% 6% 9% 11% 31%	5900
	11th to 20th	Day	5% 4 2 6% 5% 5% 7% 6% 9% 12% 39%	5900
	21st to end	Day	7% 7% 3 8% 7% 7% 7% 6% 9% 11% 27%	6424
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 9% 12% 36%	4234
	11th to 20th	Day	7% 4 2 5% 5% 5% 5% 4 6% 9% 47%	3890
	21st to end	Day	8% 4 2 4 4 4 5% 4 7% 10% 48%	4296
September	1st to 10th	Day	10% 7% 2 7% 4 4 5% 4 7% 8% 41%	2200
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 6% 7% 42%	2200
	21st to end	Day	13% 5% 2 5% 4 4 4 4 5% 6% 48%	2200
October	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 4 6% 54%	2200
	11th to 20th	Day	11% 5% 1 4 3 3 3 3 4 6% 58%	2090
	21st to end	Day	13% 4 2 4 3 4 4 3 4 5% 55%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 4 54%	1890
	11th to 20th	Day	13% 4 1 5% 3 3 3 3 5% 6% 53%	1890
	21st to end	Day	12% 6% 2 5% 3 4 5% 4 5% 7% 47%	1890
December	1st to 10th	Day	17% 7% 2 5% 4 3 4 2 6% 5% 46%	3020
	11th to 20th	Day	15% 9% 3 6% 4 5% 4 3 5% 5% 43%	3463
	21st to end	Day	21% 7% 2 5% 3 3 4 3 5% 6% 41%	4565
January	1st to 10th	Night	14% 5% 3 7% 4 4 4 3 5% 6% 45%	7635
	11th to 20th	Night	12% 7% 3 6% 3 3 4 3 5% 6% 50%	7680
	21st to end	Night	20% 9% 3 6% 4 3 3 3 4 5% 41%	8122
February	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	2520
	11th to 20th	Night	20% 7% 2 6% 4 3 3 3 4 7% 42%	2458
	21st to end	Night	18% 7% 2 5% 4 4 4 3 4 6% 43%	2076
March	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 6% 36%	5320
	11th to 20th	Night	9% 5% 2 5% 3 3 3 4 5% 7% 53%	5090
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 46%	5588
April	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 7% 48%	2284
	11th to 20th	Night	11% 5% 2 5% 3 3 3 4 5% 7% 52%	2075
	21st to end	Night	8% 5% 2 6% 4 4 4 4 5% 7% 52%	1936
May	1st to 10th	Night	8% 6% 3 8% 6% 5% 5% 4 7% 10% 38%	4042
	11th to 20th	Night	17% 6% 2 7% 5% 5% 6% 5% 7% 8% 33%	4032
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 6% 10% 44%	4421
June	1st to 10th	Night	7% 6% 2 6% 6% 5% 6% 5% 9% 11% 38%	1800
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 4 8% 13% 45%	1800
	21st to end	Night	5% 3 2 6% 5% 5% 6% 6% 8% 12% 42%	1800
July	1st to 10th	Night	8% 6% 3 8% 6% 6% 7% 6% 8% 12% 32%	4058
	11th to 20th	Night	4 4 2 6% 5% 5% 7% 6% 10% 15% 35%	4057
	21st to end	Night	8% 7% 3 9% 6% 6% 6% 6% 8% 11% 29%	4427
August	1st to 10th	Night	8% 5% 3 8% 5% 6% 7% 6% 8% 13% 32%	2915
	11th to 20th	Night	7% 5% 2 5% 5% 4 5% 5% 6% 10% 46%	3153
	21st to end	Night	8% 5% 2 5% 3 4 4 4 6% 8% 51%	3586
September	1st to 10th	Night	8% 5% 2 7% 5% 4 5% 5% 7% 10% 41%	2181
	11th to 20th	Night	13% 6% 2 5% 4 4 5% 3 7% 7% 45%	2200
	21st to end	Night	11% 5% 2 6% 4 4 4 4 6% 7% 47%	2200
October	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 5% 5% 53%	2200
	11th to 20th	Night	10% 5% 2 4 3 3 3 2 3 5% 60%	2308
	21st to end	Night	11% 5% 2 4 3 3 3 3 4 5% 57%	2640
November	1st to 10th	Night	14% 5% 2 4 3 4 3 2 4 5% 56%	2625
	11th to 20th	Night	14% 5% 2 4 3 2 4 3 3 6% 55%	2730
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2730
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 52%	4380
	11th to 20th	Night	16% 6% 2 6% 3 4 4 3 4 6% 47%	4952
	21st to end	Night	19% 7% 2 6% 4 3 4 3 5% 5% 42%	6633

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% 5% 7% 21% 19% 5%	5259
	11th to 20th	Day	29% 3 5% 21% 30% 12%	5280
	21st to end	Day	51% 3 5% 14% 19% 8%	6056
February	1st to 10th	Day	42% 5% 9% 18% 21% 5%	2100
	11th to 20th	Day	46% 4 5% 19% 21% 6%	2058
	21st to end	Day	43% 6% 8% 16% 18% 9%	1730
March	1st to 10th	Day	49% 5% 8% 12% 21% 6%	4739
	11th to 20th	Day	31% 6% 8% 14% 23% 17%	5090
	21st to end	Day	41% 5% 7% 21% 21% 6%	5586
April	1st to 10th	Day	36% 7% 9% 22% 20% 6%	2352
	11th to 20th	Day	33% 6% 8% 19% 25% 8%	2494
	21st to end	Day	32% 6% 11% 25% 23% 2	2683
May	1st to 10th	Day	39% 9% 15% 20% 15% 1	5910
	11th to 20th	Day	47% 9% 16% 19% 8% 1	5865
	21st to end	Day	36% 9% 12% 23% 18% 1	6447
June	1st to 10th	Day	40% 12% 16% 21% 11% 1	2600
	11th to 20th	Day	29% 11% 22% 23% 15% 1	2600
	21st to end	Day	34% 12% 18% 21% 12% 3	2600
July	1st to 10th	Day	41% 14% 18% 21% 6% 1	5835
	11th to 20th	Day	32% 15% 27% 22% 3 1	5831
	21st to end	Day	45% 16% 18% 18% 3	6240
August	1st to 10th	Day	37% 10% 16% 30% 6%	4221
	11th to 20th	Day	31% 10% 16% 28% 13% 2	3927
	21st to end	Day	30% 10% 16% 23% 18% 4	4296
September	1st to 10th	Day	39% 9% 16% 16% 16% 5%	2200
	11th to 20th	Day	39% 8% 11% 19% 21% 2	2200
	21st to end	Day	36% 5% 11% 20% 21% 6%	2200
October	1st to 10th	Day	26% 4 7% 14% 36% 13%	3590
	11th to 20th	Day	18% 3 5% 16% 31% 27%	3382
	21st to end	Day	26% 4 5% 20% 34% 12%	3538
November	1st to 10th	Day	33% 5% 8% 15% 29% 10%	1890
	11th to 20th	Day	32% 4 7% 19% 27% 11%	1890
	21st to end	Day	35% 6% 9% 18% 25% 7%	1890
December	1st to 10th	Day	40% 4 5% 18% 26% 7%	3020
	11th to 20th	Day	43% 5% 8% 17% 21% 7%	3460
	21st to end	Day	41% 6% 10% 22% 19% 2	4565
January	1st to 10th	Night	37% 6% 8% 22% 21% 5%	7636
	11th to 20th	Night	34% 4 6% 18% 28% 9%	7680
	21st to end	Night	47% 3 4 15% 23% 7%	8122
February	1st to 10th	Night	40% 4 8% 18% 20% 9%	2520
	11th to 20th	Night	44% 4 6% 18% 20% 8%	2459
	21st to end	Night	43% 5% 8% 16% 18% 11%	2076
March	1st to 10th	Night	46% 6% 9% 14% 16% 9%	5320
	11th to 20th	Night	29% 5% 9% 15% 24% 18%	5090
	21st to end	Night	37% 5% 7% 20% 21% 10%	5588
April	1st to 10th	Night	33% 7% 10% 20% 21% 9%	2284
	11th to 20th	Night	31% 5% 7% 21% 25% 10%	2075
	21st to end	Night	31% 6% 12% 24% 22% 5%	1936
May	1st to 10th	Night	37% 11% 17% 19% 13% 2	4069
	11th to 20th	Night	46% 10% 14% 20% 9% 1	4033
	21st to end	Night	32% 9% 16% 23% 18% 2	4429
June	1st to 10th	Night	35% 14% 17% 19% 13% 1	1800
	11th to 20th	Night	27% 11% 21% 27% 12% 2	1800
	21st to end	Night	30% 12% 21% 19% 15% 4	1800
July	1st to 10th	Night	41% 14% 19% 20% 5% 1	4015
	11th to 20th	Night	32% 15% 29% 17% 5% 1	4006
	21st to end	Night	45% 14% 21% 16% 3 1	4297
August	1st to 10th	Night	37% 13% 16% 28% 6%	2904
	11th to 20th	Night	31% 10% 15% 28% 14% 1	3182
	21st to end	Night	29% 10% 14% 22% 23% 3	3586
September	1st to 10th	Night	35% 11% 17% 17% 15% 5%	2181
	11th to 20th	Night	36% 10% 11% 21% 18% 3	2200
	21st to end	Night	34% 8% 13% 18% 20% 7%	2200
October	1st to 10th	Night	27% 6% 9% 12% 32% 14%	3590
	11th to 20th	Night	17% 3 4 16% 31% 29%	3755
	21st to end	Night	23% 3 4 21% 31% 18%	4252
November	1st to 10th	Night	32% 3 8% 17% 27% 13%	2625
	11th to 20th	Night	32% 3 6% 18% 28% 13%	2730
	21st to end	Night	35% 5% 7% 18% 26% 8%	2730
December	1st to 10th	Night	35% 4 6% 18% 28% 8%	4380
	11th to 20th	Night	38% 4 11% 16% 24% 7%	4954
	21st to end	Night	42% 5% 7% 25% 18% 2	6633

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% 36% 17% 3	330
	11th to 20th	Day	24% 23% 29% 17% 6% 2	330
	21st to end	Day	20% 25% 35% 15% 4 1	363
February	1st to 10th	Day	20% 17% 38% 20% 5%	330
	11th to 20th	Day	34% 18% 31% 15% 3	330
	21st to end	Day	26% 20% 34% 16% 4	273
March	1st to 10th	Day	21% 16% 32% 22% 7% 1	398
	11th to 20th	Day	20% 13% 33% 25% 8% 1	540
	21st to end	Day	18% 16% 31% 26% 8% 1	594
April	1st to 10th	Day	16% 17% 26% 24% 15% 2	540
	11th to 20th	Day	11% 12% 31% 25% 18% 3	532
	21st to end	Day	13% 9% 24% 29% 20% 6%	540
May	1st to 10th	Day	12% 11% 27% 25% 18% 7%	507
	11th to 20th	Day	14% 4 23% 24% 22% 14%	527
	21st to end	Day	14% 6% 16% 24% 24% 16%	573
June	1st to 10th	Day	11% 3 14% 29% 26% 17%	530
	11th to 20th	Day	6% 2 14% 25% 25% 28%	530
	21st to end	Day	8% 2 10% 25% 32% 23%	530
July	1st to 10th	Day	8% 2 6% 20% 32% 32%	525
	11th to 20th	Day	7% 2 5% 16% 32% 38%	524
	21st to end	Day	6% 2 6% 20% 38% 28%	576
August	1st to 10th	Day	13% 2 12% 22% 25% 26%	526
	11th to 20th	Day	10% 2 8% 18% 34% 27%	519
	21st to end	Day	15% 2 7% 23% 31% 22%	583
September	1st to 10th	Day	9% 4 13% 25% 28% 21%	530
	11th to 20th	Day	16% 4 9% 22% 34% 15%	530
	21st to end	Day	15% 3 16% 24% 33% 10%	530
October	1st to 10th	Day	14% 5% 19% 32% 20% 10%	530
	11th to 20th	Day	12% 9% 20% 27% 24% 8%	530
	21st to end	Day	18% 7% 22% 28% 20% 5%	583
November	1st to 10th	Day	20% 9% 24% 25% 18% 4	368
	11th to 20th	Day	15% 12% 37% 25% 10% 2	330
	21st to end	Day	12% 18% 32% 25% 11% 2	330
December	1st to 10th	Day	29% 16% 32% 14% 8% 1	330
	11th to 20th	Day	25% 14% 29% 24% 8% 1	330
	21st to end	Day	28% 15% 31% 20% 6% 1	363
January	1st to 10th	Night	22% 15% 33% 25% 5% 1	840
	11th to 20th	Night	19% 15% 32% 26% 8%	840
	21st to end	Night	21% 13% 37% 25% 4	924
February	1st to 10th	Night	17% 13% 31% 34% 6%	840
	11th to 20th	Night	23% 14% 35% 25% 3	823
	21st to end	Night	22% 14% 36% 25% 4	692
March	1st to 10th	Night	18% 10% 32% 33% 6%	840
	11th to 20th	Night	16% 11% 35% 27% 10%	840
	21st to end	Night	16% 10% 33% 30% 11%	924
April	1st to 10th	Night	13% 11% 31% 31% 13%	814
	11th to 20th	Night	14% 10% 27% 32% 17% 1	623
	21st to end	Night	10% 7% 26% 36% 21% 1	630
May	1st to 10th	Night	9% 7% 25% 36% 22% 2	603
	11th to 20th	Night	11% 3 23% 34% 25% 5%	591
	21st to end	Night	12% 2 12% 32% 35% 6%	650
June	1st to 10th	Night	8% 2 10% 38% 37% 6%	600
	11th to 20th	Night	4 2 12% 32% 37% 14%	600
	21st to end	Night	6% 1 7% 33% 43% 10%	600
July	1st to 10th	Night	8% 3 5% 24% 46% 15%	585
	11th to 20th	Night	7% 2 4 20% 52% 16%	582
	21st to end	Night	6% 1 5% 27% 48% 13%	657
August	1st to 10th	Night	10% 2 5% 27% 43% 13%	597
	11th to 20th	Night	9% 2 7% 28% 43% 10%	594
	21st to end	Night	14% 3 7% 25% 42% 10%	660
September	1st to 10th	Night	8% 3 9% 28% 41% 10%	781
	11th to 20th	Night	15% 2 10% 25% 44% 5%	800
	21st to end	Night	14% 3 12% 31% 35% 4	800
October	1st to 10th	Night	12% 3 17% 35% 27% 6%	800
	11th to 20th	Night	12% 6% 23% 34% 23% 3	800
	21st to end	Night	14% 7% 20% 35% 21% 3	880
November	1st to 10th	Night	16% 5% 21% 35% 21% 1	820
	11th to 20th	Night	16% 9% 30% 29% 15% 1	840
	21st to end	Night	15% 10% 28% 33% 13% 1	840
December	1st to 10th	Night	18% 12% 30% 31% 9% 1	840
	11th to 20th	Night	20% 8% 30% 31% 11%	840
	21st to end	Night	25% 11% 27% 30% 7%	924

Page generated at 2026-04-29 02:21:46. Version ac07

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