Lily Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lily Lake Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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% 9% 3 8% 5% 4 5% 4 6% 7% 37%	126834
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 39%	123470
Transparency	Day	45% 5% 12% 23% 14% 2	130355
	Night	45% 5% 11% 23% 15% 2	128023
Seeing	Day	16% 12% 30% 25% 13% 4	17205
	Night	17% 10% 31% 29% 11% 1	27004

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	11% 10% 6% 14% 7% 5% 5% 4 5% 6% 26%	3617
	11th to 20th	Day	9% 12% 6% 12% 5% 5% 4 6% 6% 5% 30%	3620
	21st to end	Day	14% 15% 6% 9% 4 3 4 3 5% 7% 30%	4143
February	1st to 10th	Day	13% 14% 5% 9% 7% 5% 5% 3 5% 6% 26%	1815
	11th to 20th	Day	13% 11% 5% 8% 6% 5% 4 3 5% 5% 35%	1948
	21st to end	Day	14% 12% 5% 10% 5% 4 5% 3 5% 6% 32%	1650
March	1st to 10th	Day	16% 10% 4 6% 4 3 3 3 4 6% 41%	4480
	11th to 20th	Day	10% 6% 3 7% 5% 5% 4 4 6% 8% 43%	4707
	21st to end	Day	19% 13% 6% 13% 6% 5% 5% 3 5% 5% 20%	5967
April	1st to 10th	Day	17% 13% 4 8% 5% 4 4 3 5% 6% 30%	2400
	11th to 20th	Day	15% 10% 3 7% 5% 5% 5% 4 5% 6% 34%	2374
	21st to end	Day	19% 9% 3 8% 5% 5% 4 3 6% 6% 33%	2490
May	1st to 10th	Day	16% 8% 3 6% 4 4 5% 4 6% 8% 35%	5862
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 7% 41%	6260
	21st to end	Day	13% 7% 3 7% 4 4 5% 4 7% 9% 37%	6935
June	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 6% 7% 46%	2800
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 6% 9% 45%	2800
	21st to end	Day	10% 7% 2 6% 6% 4 5% 5% 7% 8% 39%	2800
July	1st to 10th	Day	10% 6% 3 8% 5% 6% 6% 5% 7% 9% 36%	6360
	11th to 20th	Day	8% 6% 2 6% 4 5% 6% 4 8% 10% 41%	6360
	21st to end	Day	11% 6% 3 8% 5% 5% 5% 4 6% 9% 38%	6924
August	1st to 10th	Day	9% 6% 2 7% 4 5% 5% 4 7% 8% 44%	4483
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 7% 10% 42%	3849
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 5% 4 4 5% 7% 47%	2400
	11th to 20th	Day	12% 3 1 4 3 3 4 3 6% 7% 54%	2290
	21st to end	Day	10% 7% 2 6% 4 4 5% 4 5% 7% 46%	2200
October	1st to 10th	Day	12% 6% 2 6% 5% 4 4 3 5% 7% 48%	2120
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 4 5% 7% 36%	2000
	21st to end	Day	18% 9% 3 6% 3 4 4 3 4 5% 39%	1985
November	1st to 10th	Day	12% 10% 3 7% 3 3 5% 3 5% 6% 44%	1780
	11th to 20th	Day	15% 13% 4 8% 5% 4 4 4 5% 7% 32%	1800
	21st to end	Day	18% 11% 3 10% 5% 4 4 3 4 5% 34%	1703
December	1st to 10th	Day	9% 8% 5% 8% 5% 3 5% 3 4 6% 44%	2610
	11th to 20th	Day	12% 14% 5% 11% 6% 5% 6% 5% 5% 7% 24%	3008
	21st to end	Day	22% 24% 8% 12% 5% 4 4 3 3 4 12%	3982
January	1st to 10th	Night	11% 12% 5% 14% 6% 5% 4 3 4 4 32%	6348
	11th to 20th	Night	12% 14% 6% 13% 6% 5% 4 5% 6% 4 23%	6360
	21st to end	Night	16% 14% 7% 9% 4 4 3 3 4 5% 32%	6830
February	1st to 10th	Night	14% 13% 5% 9% 5% 5% 4 3 4 6% 32%	2583
	11th to 20th	Night	13% 11% 4 7% 5% 4 4 3 5% 6% 37%	2338
	21st to end	Night	16% 13% 3 8% 4 3 4 3 4 6% 37%	1980
March	1st to 10th	Night	16% 9% 4 7% 4 4 3 2 4 5% 42%	5373
	11th to 20th	Night	10% 7% 2 6% 4 4 3 3 5% 7% 48%	5294
	21st to end	Night	19% 10% 5% 11% 6% 5% 4 3 4 5% 28%	4962
April	1st to 10th	Night	16% 9% 3 7% 4 3 4 3 5% 6% 39%	2000
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 3 4 6% 38%	1975
	21st to end	Night	17% 8% 4 7% 4 4 4 4 5% 8% 37%	1910
May	1st to 10th	Night	16% 9% 3 6% 4 4 4 4 6% 6% 37%	4033
	11th to 20th	Night	12% 6% 2 5% 4 4 4 5% 6% 9% 44%	3650
	21st to end	Night	15% 8% 3 7% 4 4 4 3 5% 7% 40%	3942
June	1st to 10th	Night	9% 5% 2 6% 5% 4 3 5% 6% 7% 49%	1600
	11th to 20th	Night	9% 4 2 6% 5% 5% 4 4 7% 8% 47%	1600
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 5% 6% 8% 44%	1600
July	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 5% 6% 9% 41%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 3 6% 8% 50%	3617
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 5% 8% 44%	3947
August	1st to 10th	Night	5% 4 2 6% 5% 4 6% 5% 6% 9% 49%	2670
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 7% 8% 49%	3200
	21st to end	Night	8% 4 2 5% 3 3 4 3 6% 9% 53%	3586
September	1st to 10th	Night	10% 5% 1 5% 3 4 3 3 4 7% 56%	2000
	11th to 20th	Night	9% 4 2 6% 3 3 3 3 6% 7% 55%	2090
	21st to end	Night	10% 7% 2 5% 5% 4 4 4 5% 7% 48%	2200
October	1st to 10th	Night	10% 5% 3 4 4 4 3 4 5% 6% 52%	2277
	11th to 20th	Night	17% 7% 2 6% 3 4 3 3 4 6% 45%	2400
	21st to end	Night	19% 9% 3 6% 4 4 4 3 4 6% 38%	2854
November	1st to 10th	Night	13% 8% 2 6% 4 3 4 3 4 6% 48%	2563
	11th to 20th	Night	15% 10% 3 8% 5% 4 3 3 6% 6% 37%	2600
	21st to end	Night	16% 10% 4 9% 4 4 4 4 4 5% 36%	2695
December	1st to 10th	Night	11% 8% 3 7% 4 4 4 3 5% 7% 43%	4580
	11th to 20th	Night	16% 16% 4 9% 5% 4 4 3 4 5% 30%	5199
	21st to end	Night	22% 19% 6% 10% 5% 5% 5% 3 4 5% 17%	6996

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	59% 4 11% 17% 8%	3618
	11th to 20th	Day	52% 5% 11% 11% 15% 5%	3620
	21st to end	Day	54% 4 13% 19% 9%	4143
February	1st to 10th	Day	58% 5% 13% 14% 9% 1	1815
	11th to 20th	Day	49% 5% 11% 23% 9% 2	1958
	21st to end	Day	53% 7% 11% 18% 9% 2	1650
March	1st to 10th	Day	45% 6% 9% 25% 11% 4	4480
	11th to 20th	Day	39% 5% 12% 32% 11% 1	4707
	21st to end	Day	65% 6% 10% 12% 6% 1	5969
April	1st to 10th	Day	55% 6% 10% 20% 10%	2400
	11th to 20th	Day	48% 7% 10% 21% 11% 3	2374
	21st to end	Day	50% 7% 12% 18% 11% 1	2490
May	1st to 10th	Day	45% 7% 11% 22% 14% 1	5897
	11th to 20th	Day	42% 5% 10% 25% 16% 1	6264
	21st to end	Day	42% 6% 11% 25% 13% 3	6949
June	1st to 10th	Day	35% 7% 13% 27% 17% 2	2800
	11th to 20th	Day	35% 7% 13% 24% 19% 3	2800
	21st to end	Day	40% 8% 13% 22% 14% 3	2800
July	1st to 10th	Day	45% 4 11% 25% 14% 1	6291
	11th to 20th	Day	38% 4 14% 27% 16% 1	6285
	21st to end	Day	42% 4 16% 21% 15% 2	6728
August	1st to 10th	Day	36% 6% 16% 28% 13% 1	4468
	11th to 20th	Day	37% 5% 12% 31% 15% 1	3885
	21st to end	Day	33% 5% 12% 29% 19% 2	4311
September	1st to 10th	Day	36% 6% 11% 26% 17% 4	2400
	11th to 20th	Day	29% 6% 11% 30% 22% 2	2290
	21st to end	Day	37% 6% 11% 26% 20% 1	2200
October	1st to 10th	Day	25% 4 7% 31% 31% 1	3458
	11th to 20th	Day	33% 3 12% 26% 25% 1	3240
	21st to end	Day	43% 4 8% 28% 14% 3	3185
November	1st to 10th	Day	41% 5% 14% 23% 15% 2	1780
	11th to 20th	Day	52% 5% 12% 20% 10% 1	1800
	21st to end	Day	54% 5% 11% 23% 8%	1703
December	1st to 10th	Day	43% 3 18% 28% 8% 1	2610
	11th to 20th	Day	59% 4 12% 17% 7% 1	3005
	21st to end	Day	78% 3 6% 9% 4	3982
January	1st to 10th	Night	55% 5% 9% 20% 11% 1	6348
	11th to 20th	Night	62% 4 9% 9% 14% 1	6360
	21st to end	Night	56% 4 13% 16% 11% 1	6830
February	1st to 10th	Night	54% 5% 11% 17% 12% 1	2583
	11th to 20th	Night	47% 5% 11% 22% 13% 1	2339
	21st to end	Night	50% 5% 12% 19% 11% 4	1980
March	1st to 10th	Night	45% 4 11% 24% 11% 5%	5373
	11th to 20th	Night	36% 4 11% 32% 14% 3	5294
	21st to end	Night	58% 5% 8% 18% 9% 1	4962
April	1st to 10th	Night	45% 6% 12% 23% 13% 2	2000
	11th to 20th	Night	47% 5% 7% 21% 16% 4	1975
	21st to end	Night	46% 6% 11% 23% 11% 3	1910
May	1st to 10th	Night	46% 6% 13% 17% 16% 3	4060
	11th to 20th	Night	34% 8% 16% 24% 14% 4	3651
	21st to end	Night	43% 6% 10% 22% 16% 3	3950
June	1st to 10th	Night	32% 7% 13% 25% 20% 3	1600
	11th to 20th	Night	35% 6% 11% 25% 21% 2	1600
	21st to end	Night	36% 10% 14% 21% 17% 2	1600
July	1st to 10th	Night	41% 4 13% 25% 14% 3	3580
	11th to 20th	Night	32% 4 14% 31% 18% 1	3572
	21st to end	Night	38% 5% 14% 23% 18% 2	3832
August	1st to 10th	Night	31% 6% 15% 29% 18% 1	2661
	11th to 20th	Night	32% 4 15% 31% 18% 1	3230
	21st to end	Night	29% 4 12% 34% 17% 3	3586
September	1st to 10th	Night	29% 6% 11% 27% 24% 3	2000
	11th to 20th	Night	28% 5% 8% 33% 23% 2	2090
	21st to end	Night	35% 6% 11% 29% 18% 2	2200
October	1st to 10th	Night	23% 3 7% 37% 28% 2	3719
	11th to 20th	Night	35% 3 8% 24% 28% 3	3899
	21st to end	Night	42% 3 8% 26% 18% 3	4604
November	1st to 10th	Night	39% 5% 8% 27% 18% 3	2563
	11th to 20th	Night	47% 5% 12% 22% 12% 2	2600
	21st to end	Night	49% 5% 12% 22% 11%	2695
December	1st to 10th	Night	42% 4 14% 28% 11% 2	4580
	11th to 20th	Night	57% 4 12% 17% 10% 1	5201
	21st to end	Night	69% 5% 9% 11% 6%	6996

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	21% 14% 37% 23% 4 1	300
	11th to 20th	Day	20% 20% 39% 18% 3	300
	21st to end	Day	22% 21% 35% 18% 5% 1	330
February	1st to 10th	Day	19% 20% 41% 16% 4	300
	11th to 20th	Day	23% 22% 34% 17% 3	300
	21st to end	Day	22% 20% 41% 12% 4	249
March	1st to 10th	Day	23% 22% 36% 17% 1	300
	11th to 20th	Day	27% 20% 33% 16% 4 1	309
	21st to end	Day	27% 18% 32% 16% 5% 1	550
April	1st to 10th	Day	25% 15% 33% 22% 5%	500
	11th to 20th	Day	22% 21% 28% 18% 9% 2	492
	21st to end	Day	22% 17% 30% 22% 8% 1	500
May	1st to 10th	Day	20% 13% 33% 24% 9% 2	503
	11th to 20th	Day	19% 13% 29% 25% 11% 3	692
	21st to end	Day	17% 10% 31% 23% 15% 5%	789
June	1st to 10th	Day	10% 10% 32% 29% 15% 5%	730
	11th to 20th	Day	11% 7% 33% 30% 15% 3	730
	21st to end	Day	12% 10% 31% 30% 12% 5%	730
July	1st to 10th	Day	10% 4 21% 33% 19% 13%	721
	11th to 20th	Day	11% 6% 26% 32% 17% 8%	718
	21st to end	Day	9% 4 26% 37% 18% 7%	794
August	1st to 10th	Day	8% 4 21% 35% 22% 10%	697
	11th to 20th	Day	11% 4 19% 29% 23% 13%	519
	21st to end	Day	7% 4 20% 33% 26% 10%	583
September	1st to 10th	Day	15% 6% 20% 28% 25% 7%	530
	11th to 20th	Day	9% 4 18% 32% 26% 11%	530
	21st to end	Day	11% 14% 25% 25% 18% 7%	530
October	1st to 10th	Day	14% 9% 28% 28% 17% 4	450
	11th to 20th	Day	21% 19% 32% 21% 7%	330
	21st to end	Day	28% 15% 34% 17% 6% 1	363
November	1st to 10th	Day	18% 13% 38% 19% 9% 3	306
	11th to 20th	Day	25% 18% 34% 16% 6% 1	300
	21st to end	Day	25% 13% 38% 21% 3	300
December	1st to 10th	Day	15% 24% 39% 17% 5%	300
	11th to 20th	Day	19% 18% 37% 20% 4 1	300
	21st to end	Day	27% 21% 35% 14% 3	330
January	1st to 10th	Night	18% 15% 42% 21% 5%	1000
	11th to 20th	Night	20% 17% 37% 21% 4 1	1000
	21st to end	Night	20% 13% 37% 26% 4	1100
February	1st to 10th	Night	22% 14% 37% 23% 4	983
	11th to 20th	Night	19% 15% 38% 23% 4	783
	21st to end	Night	21% 16% 39% 20% 3	660
March	1st to 10th	Night	21% 15% 37% 21% 6%	800
	11th to 20th	Night	16% 11% 40% 25% 7%	755
	21st to end	Night	22% 13% 32% 26% 6% 1	660
April	1st to 10th	Night	19% 15% 34% 26% 5%	600
	11th to 20th	Night	22% 14% 31% 26% 6%	593
	21st to end	Night	19% 11% 34% 28% 7%	600
May	1st to 10th	Night	18% 10% 35% 29% 8% 1	600
	11th to 20th	Night	18% 7% 31% 31% 13%	591
	21st to end	Night	15% 6% 27% 36% 17%	650
June	1st to 10th	Night	11% 5% 29% 40% 15% 1	600
	11th to 20th	Night	10% 4 32% 42% 11% 1	600
	21st to end	Night	10% 5% 32% 37% 17% 1	600
July	1st to 10th	Night	8% 2 18% 42% 27% 3	585
	11th to 20th	Night	9% 2 23% 39% 24% 3	582
	21st to end	Night	5% 2 20% 46% 25% 2	657
August	1st to 10th	Night	5% 2 15% 46% 28% 4	597
	11th to 20th	Night	7% 3 15% 41% 30% 4	594
	21st to end	Night	9% 1 18% 42% 29% 2	660
September	1st to 10th	Night	11% 4 22% 35% 25% 4	600
	11th to 20th	Night	10% 3 19% 36% 28% 4	690
	21st to end	Night	12% 8% 24% 33% 21% 3	800
October	1st to 10th	Night	12% 7% 27% 35% 17% 2	800
	11th to 20th	Night	21% 12% 31% 27% 10%	800
	21st to end	Night	24% 13% 32% 22% 9%	880
November	1st to 10th	Night	18% 11% 36% 25% 9% 1	789
	11th to 20th	Night	21% 17% 33% 24% 6%	800
	21st to end	Night	20% 15% 38% 23% 6%	895
December	1st to 10th	Night	21% 14% 38% 23% 4	1000
	11th to 20th	Night	22% 13% 36% 26% 4	1000
	21st to end	Night	29% 15% 34% 20% 3	1100

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