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 2025-01-07.

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%	105042
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	103949
Transparency	Day	37% 8% 11% 20% 18% 5%	108873
	Night	35% 7% 11% 19% 20% 7%	108257
Seeing	Day	15% 9% 20% 23% 20% 13%	15099
	Night	15% 7% 22% 30% 22% 4	25162

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% 3 5% 4 3 4 3 6% 6% 40%	2890
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 5% 6% 53%	2840
	21st to end	Day	25% 8% 2 5% 3 3 3 3 4 4 40%	3261
February	1st to 10th	Day	15% 7% 3 7% 5% 4 4 4 5% 6% 39%	1900
	11th to 20th	Day	20% 7% 2 5% 5% 4 4 3 4 5% 42%	1848
	21st to end	Day	19% 7% 3 5% 4 4 4 4 5% 5% 39%	1570
March	1st to 10th	Day	23% 11% 3 7% 4 3 3 3 4 5% 35%	3234
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 6% 7% 47%	3480
	21st to end	Day	18% 6% 2 6% 4 3 4 3 5% 7% 41%	3815
April	1st to 10th	Day	11% 6% 2 5% 5% 4 4 3 5% 7% 46%	2128
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 3 5% 6% 48%	2254
	21st to end	Day	8% 5% 2 5% 4 4 4 3 6% 8% 51%	2427
May	1st to 10th	Day	9% 6% 2 6% 5% 4 4 4 6% 8% 45%	4082
	11th to 20th	Day	17% 8% 2 7% 4 3 5% 4 6% 7% 37%	4081
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 7% 10% 46%	4476
June	1st to 10th	Day	9% 6% 2 8% 6% 6% 6% 5% 7% 10% 35%	2470
	11th to 20th	Day	4 4 2 6% 4 5% 5% 5% 7% 12% 45%	2470
	21st to end	Day	6% 5% 2 5% 5% 6% 6% 6% 7% 11% 40%	2470
July	1st to 10th	Day	9% 6% 3 9% 7% 7% 8% 6% 10% 11% 25%	4120
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 6% 8% 11% 36%	4120
	21st to end	Day	8% 9% 3 9% 7% 7% 8% 7% 10% 11% 21%	4466
August	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 9% 12% 37%	4104
	11th to 20th	Day	7% 4 2 5% 5% 5% 5% 4 6% 9% 47%	3769
	21st to end	Day	8% 4 2 4 4 4 5% 4 7% 10% 49%	4164
September	1st to 10th	Day	11% 7% 2 7% 4 4 5% 4 7% 8% 40%	2090
	11th to 20th	Day	15% 7% 2 5% 5% 4 5% 4 6% 7% 41%	2090
	21st to end	Day	14% 5% 2 5% 4 4 4 3 4 6% 49%	2090
October	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 4 6% 56%	2090
	11th to 20th	Day	11% 5% 1 4 3 3 3 3 4 7% 57%	1986
	21st to end	Day	12% 4 2 5% 3 4 4 3 4 5% 55%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 3 3 3 4 5% 54%	1798
	11th to 20th	Day	14% 5% 1 5% 4 3 4 3 5% 6% 52%	1800
	21st to end	Day	12% 6% 2 5% 3 4 5% 4 5% 7% 47%	1800
December	1st to 10th	Day	16% 7% 2 5% 4 3 4 2 6% 5% 47%	2930
	11th to 20th	Day	15% 9% 3 6% 4 5% 4 3 5% 5% 42%	3373
	21st to end	Day	21% 7% 2 5% 3 3 4 3 5% 6% 41%	4466
January	1st to 10th	Night	17% 6% 2 6% 4 4 4 3 6% 7% 41%	4188
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 5% 6% 47%	4120
	21st to end	Night	23% 10% 3 6% 4 3 3 2 3 4 40%	4392
February	1st to 10th	Night	15% 7% 2 6% 4 5% 4 3 5% 6% 43%	2280
	11th to 20th	Night	19% 7% 2 6% 4 3 2 3 4 6% 43%	2218
	21st to end	Night	18% 7% 2 6% 4 4 4 3 4 6% 41%	1884
March	1st to 10th	Night	23% 8% 3 6% 4 4 4 2 4 6% 36%	3619
	11th to 20th	Night	11% 5% 1 4 3 3 4 5% 6% 8% 50%	3480
	21st to end	Night	17% 6% 2 5% 4 4 4 3 5% 6% 43%	3817
April	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 49%	2066
	11th to 20th	Night	13% 5% 2 5% 3 3 3 4 5% 7% 50%	1875
	21st to end	Night	8% 4 2 6% 4 3 4 4 5% 7% 54%	1752
May	1st to 10th	Night	7% 5% 2 7% 6% 5% 5% 4 8% 9% 42%	2813
	11th to 20th	Night	14% 6% 2 7% 4 5% 5% 4 6% 7% 41%	2812
	21st to end	Night	7% 4 2 4 3 3 4 4 7% 12% 50%	3079
June	1st to 10th	Night	7% 6% 2 6% 5% 5% 6% 5% 9% 11% 37%	1710
	11th to 20th	Night	4 4 1 5% 4 5% 5% 4 9% 13% 46%	1710
	21st to end	Night	5% 3 2 6% 5% 5% 6% 6% 9% 12% 42%	1710
July	1st to 10th	Night	9% 7% 3 9% 6% 6% 7% 6% 7% 11% 30%	2840
	11th to 20th	Night	6% 5% 2 7% 5% 5% 7% 6% 11% 13% 33%	2840
	21st to end	Night	9% 8% 3 9% 7% 7% 7% 6% 9% 11% 23%	3087
August	1st to 10th	Night	7% 5% 3 8% 5% 6% 7% 6% 8% 13% 33%	2825
	11th to 20th	Night	7% 5% 2 5% 5% 4 5% 5% 6% 10% 47%	3055
	21st to end	Night	7% 5% 2 5% 3 4 4 4 6% 8% 52%	3476
September	1st to 10th	Night	8% 5% 2 7% 5% 4 5% 5% 7% 10% 42%	2072
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 7% 7% 44%	2090
	21st to end	Night	11% 5% 1 6% 4 4 4 3 6% 6% 48%	2090
October	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 5% 54%	2090
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 3 5% 59%	2192
	21st to end	Night	11% 4 2 4 3 3 3 3 4 5% 58%	2508
November	1st to 10th	Night	14% 5% 2 4 3 4 2 2 4 5% 56%	2497
	11th to 20th	Night	14% 5% 2 5% 3 2 4 3 3 6% 53%	2600
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2600
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 52%	4250
	11th to 20th	Night	16% 6% 2 6% 3 4 4 3 4 6% 47%	4822
	21st to end	Night	20% 7% 2 6% 4 3 4 3 5% 5% 42%	6490

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	41% 7% 8% 17% 21% 6%	2891
	11th to 20th	Day	31% 5% 7% 26% 24% 8%	2840
	21st to end	Day	47% 4 5% 16% 21% 7%	3261
February	1st to 10th	Day	45% 5% 9% 17% 20% 5%	1900
	11th to 20th	Day	45% 4 5% 17% 21% 6%	1858
	21st to end	Day	45% 6% 8% 15% 18% 8%	1570
March	1st to 10th	Day	51% 5% 9% 13% 15% 7%	3234
	11th to 20th	Day	34% 7% 10% 16% 24% 8%	3480
	21st to end	Day	42% 6% 9% 20% 18% 6%	3815
April	1st to 10th	Day	36% 7% 9% 21% 20% 7%	2128
	11th to 20th	Day	35% 6% 8% 18% 24% 9%	2254
	21st to end	Day	30% 7% 11% 26% 24% 2	2427
May	1st to 10th	Day	34% 9% 14% 22% 19% 2	4120
	11th to 20th	Day	43% 9% 13% 23% 11% 1	4085
	21st to end	Day	29% 10% 15% 25% 20% 1	4489
June	1st to 10th	Day	40% 12% 15% 20% 11% 2	2470
	11th to 20th	Day	28% 10% 21% 24% 15% 1	2470
	21st to end	Day	35% 12% 18% 20% 12% 3	2470
July	1st to 10th	Day	46% 15% 19% 15% 4 1	4055
	11th to 20th	Day	37% 14% 20% 23% 4 1	4051
	21st to end	Day	51% 17% 15% 12% 4	4324
August	1st to 10th	Day	35% 10% 16% 31% 7%	4091
	11th to 20th	Day	30% 10% 16% 28% 14% 2	3806
	21st to end	Day	29% 10% 16% 23% 18% 4	4164
September	1st to 10th	Day	40% 9% 15% 15% 16% 5%	2090
	11th to 20th	Day	41% 8% 11% 18% 21% 2	2090
	21st to end	Day	36% 5% 10% 20% 22% 6%	2090
October	1st to 10th	Day	25% 4 7% 14% 36% 13%	3480
	11th to 20th	Day	18% 3 6% 16% 30% 27%	3278
	21st to end	Day	25% 4 5% 20% 34% 12%	3428
November	1st to 10th	Day	34% 5% 8% 15% 29% 10%	1798
	11th to 20th	Day	33% 4 7% 18% 27% 11%	1800
	21st to end	Day	36% 6% 9% 18% 25% 7%	1800
December	1st to 10th	Day	39% 4 5% 18% 27% 7%	2930
	11th to 20th	Day	44% 5% 8% 17% 20% 6%	3370
	21st to end	Day	42% 6% 9% 22% 19% 2	4466
January	1st to 10th	Night	39% 7% 7% 16% 24% 6%	4188
	11th to 20th	Night	35% 6% 8% 22% 22% 8%	4120
	21st to end	Night	48% 4 5% 12% 24% 7%	4392
February	1st to 10th	Night	42% 4 8% 17% 20% 9%	2280
	11th to 20th	Night	43% 4 6% 17% 21% 9%	2219
	21st to end	Night	43% 5% 9% 15% 17% 11%	1884
March	1st to 10th	Night	48% 6% 10% 15% 13% 8%	3619
	11th to 20th	Night	30% 6% 10% 16% 26% 12%	3480
	21st to end	Night	40% 6% 8% 17% 20% 9%	3817
April	1st to 10th	Night	33% 6% 10% 20% 22% 9%	2066
	11th to 20th	Night	33% 5% 7% 20% 24% 11%	1875
	21st to end	Night	29% 6% 12% 25% 22% 6%	1752
May	1st to 10th	Night	31% 13% 16% 20% 18% 3	2840
	11th to 20th	Night	39% 9% 13% 24% 13% 1	2813
	21st to end	Night	23% 10% 18% 25% 20% 3	3087
June	1st to 10th	Night	35% 14% 17% 19% 13% 2	1710
	11th to 20th	Night	25% 11% 21% 28% 12% 2	1710
	21st to end	Night	31% 12% 21% 19% 14% 4	1710
July	1st to 10th	Night	44% 15% 20% 15% 5% 1	2795
	11th to 20th	Night	35% 16% 23% 18% 7% 1	2786
	21st to end	Night	49% 16% 18% 12% 4 1	2988
August	1st to 10th	Night	36% 13% 16% 29% 6%	2814
	11th to 20th	Night	30% 10% 15% 29% 14% 1	3084
	21st to end	Night	28% 10% 14% 22% 23% 3	3476
September	1st to 10th	Night	35% 11% 17% 17% 16% 5%	2072
	11th to 20th	Night	37% 10% 11% 20% 18% 4	2090
	21st to end	Night	34% 8% 12% 18% 21% 8%	2090
October	1st to 10th	Night	26% 6% 9% 12% 33% 15%	3480
	11th to 20th	Night	18% 3 4 16% 31% 29%	3639
	21st to end	Night	22% 3 4 21% 31% 18%	4120
November	1st to 10th	Night	32% 3 8% 17% 26% 13%	2497
	11th to 20th	Night	33% 3 6% 17% 28% 13%	2600
	21st to end	Night	35% 5% 7% 18% 26% 8%	2600
December	1st to 10th	Night	35% 5% 6% 18% 28% 8%	4250
	11th to 20th	Night	38% 4 11% 15% 24% 6%	4824
	21st to end	Night	42% 6% 7% 25% 18% 2	6490

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% 20% 37% 16% 3	288
	11th to 20th	Day	23% 20% 30% 18% 7% 2	270
	21st to end	Day	19% 25% 35% 16% 4 1	297
February	1st to 10th	Day	23% 17% 36% 19% 6%	270
	11th to 20th	Day	33% 17% 31% 16% 3	270
	21st to end	Day	26% 20% 36% 15% 3	225
March	1st to 10th	Day	21% 17% 33% 22% 7% 1	332
	11th to 20th	Day	20% 13% 33% 25% 8% 1	460
	21st to end	Day	19% 15% 32% 25% 8% 1	506
April	1st to 10th	Day	16% 18% 26% 23% 15% 2	460
	11th to 20th	Day	13% 12% 32% 23% 17% 3	452
	21st to end	Day	14% 9% 24% 28% 20% 5%	460
May	1st to 10th	Day	11% 11% 26% 25% 19% 8%	463
	11th to 20th	Day	13% 4 21% 24% 23% 15%	487
	21st to end	Day	13% 4 14% 25% 26% 17%	529
June	1st to 10th	Day	11% 2 13% 29% 26% 19%	490
	11th to 20th	Day	6% 2 14% 23% 26% 30%	490
	21st to end	Day	9% 3 11% 26% 29% 23%	490
July	1st to 10th	Day	9% 1 6% 19% 32% 33%	485
	11th to 20th	Day	8% 2 5% 16% 29% 40%	484
	21st to end	Day	6% 3 6% 19% 36% 30%	532
August	1st to 10th	Day	10% 2 12% 22% 26% 28%	486
	11th to 20th	Day	11% 2 8% 19% 34% 27%	479
	21st to end	Day	16% 1 6% 22% 32% 24%	539
September	1st to 10th	Day	10% 5% 12% 23% 29% 22%	490
	11th to 20th	Day	18% 4 10% 21% 32% 15%	490
	21st to end	Day	15% 3 16% 23% 33% 10%	490
October	1st to 10th	Day	14% 4 18% 31% 21% 10%	490
	11th to 20th	Day	12% 9% 20% 27% 23% 8%	490
	21st to end	Day	17% 7% 20% 30% 22% 5%	539
November	1st to 10th	Day	21% 9% 24% 24% 19% 5%	336
	11th to 20th	Day	16% 12% 34% 26% 10% 2	300
	21st to end	Day	12% 16% 32% 26% 12% 2	300
December	1st to 10th	Day	27% 16% 34% 14% 8% 1	300
	11th to 20th	Day	25% 14% 30% 23% 7% 1	300
	21st to end	Day	28% 16% 29% 19% 6% 2	330
January	1st to 10th	Night	22% 15% 33% 24% 5% 1	785
	11th to 20th	Night	19% 15% 30% 28% 9%	760
	21st to end	Night	21% 13% 37% 26% 4	836
February	1st to 10th	Night	17% 12% 31% 34% 6%	760
	11th to 20th	Night	23% 13% 36% 26% 3	743
	21st to end	Night	22% 14% 36% 25% 4	628
March	1st to 10th	Night	18% 10% 33% 33% 6%	760
	11th to 20th	Night	17% 10% 36% 26% 11% 1	760
	21st to end	Night	16% 10% 32% 30% 12%	836
April	1st to 10th	Night	14% 11% 31% 31% 13% 1	736
	11th to 20th	Night	15% 10% 28% 29% 17% 1	563
	21st to end	Night	9% 7% 26% 35% 22% 2	570
May	1st to 10th	Night	9% 7% 24% 35% 23% 2	570
	11th to 20th	Night	10% 3 21% 35% 26% 5%	561
	21st to end	Night	11% 2 11% 32% 37% 6%	617
June	1st to 10th	Night	8% 2 9% 38% 37% 6%	570
	11th to 20th	Night	4 1 12% 31% 38% 15%	570
	21st to end	Night	6% 1 7% 34% 42% 10%	570
July	1st to 10th	Night	8% 2 5% 24% 46% 15%	555
	11th to 20th	Night	7% 2 4 20% 50% 16%	552
	21st to end	Night	7% 1 5% 27% 47% 13%	624
August	1st to 10th	Night	8% 2 5% 27% 45% 14%	567
	11th to 20th	Night	10% 1 7% 29% 43% 9%	564
	21st to end	Night	14% 3 6% 25% 43% 10%	627
September	1st to 10th	Night	9% 3 8% 27% 43% 11%	742
	11th to 20th	Night	15% 2 10% 25% 43% 5%	760
	21st to end	Night	14% 3 13% 30% 36% 4	760
October	1st to 10th	Night	12% 2 17% 34% 28% 7%	760
	11th to 20th	Night	12% 6% 23% 33% 23% 3	760
	21st to end	Night	13% 6% 19% 36% 22% 3	836
November	1st to 10th	Night	16% 5% 21% 35% 22% 1	780
	11th to 20th	Night	17% 10% 29% 29% 15% 1	800
	21st to end	Night	16% 10% 27% 33% 13% 1	800
December	1st to 10th	Night	17% 12% 30% 31% 10% 1	800
	11th to 20th	Night	21% 8% 30% 30% 11%	800
	21st to end	Night	26% 11% 27% 29% 7%	880

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