Oak Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Lake Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	16% 10% 3 8% 5% 4 4 4 5% 7% 33%	127292
	Night	16% 11% 4 8% 5% 5% 4 4 5% 6% 32%	123063
Transparency	Day	50% 6% 12% 21% 11% 1	130846
	Night	50% 6% 12% 19% 11% 1	127596
Seeing	Day	23% 16% 31% 19% 8% 3	15737
	Night	22% 13% 31% 24% 9% 1	24965

By Month

Percentage Cloud Cover NE (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	13% 14% 5% 10% 5% 4 4 4 6% 7% 28%	3597
	11th to 20th	Day	10% 19% 8% 15% 7% 5% 5% 3 5% 4 18%	3592
	21st to end	Day	15% 18% 8% 14% 6% 5% 4 3 4 4 20%	4466
February	1st to 10th	Day	13% 18% 4 14% 6% 5% 5% 5% 3 5% 22%	1800
	11th to 20th	Day	13% 15% 5% 13% 7% 5% 4 3 3 4 27%	1753
	21st to end	Day	18% 16% 5% 11% 6% 5% 4 3 4 4 25%	1567
March	1st to 10th	Day	20% 13% 5% 9% 5% 4 3 3 4 5% 29%	4788
	11th to 20th	Day	13% 14% 4 8% 6% 5% 5% 4 6% 7% 28%	4980
	21st to end	Day	19% 11% 3 7% 4 3 3 3 4 4 39%	5552
April	1st to 10th	Day	21% 13% 3 7% 4 3 4 3 5% 6% 31%	2400
	11th to 20th	Day	23% 9% 2 6% 3 4 4 3 5% 5% 37%	2374
	21st to end	Day	21% 10% 3 8% 5% 4 4 3 4 6% 31%	2597
May	1st to 10th	Day	23% 7% 3 7% 5% 4 4 3 5% 6% 32%	6159
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 4 5% 6% 34%	6318
	21st to end	Day	19% 8% 3 6% 4 4 4 4 5% 7% 35%	6949
June	1st to 10th	Day	18% 8% 3 7% 6% 5% 5% 4 5% 7% 33%	2800
	11th to 20th	Day	14% 7% 2 6% 4 5% 6% 5% 6% 9% 37%	2800
	21st to end	Day	17% 8% 2 6% 5% 5% 5% 4 6% 7% 36%	2800
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	6360
	11th to 20th	Day	11% 5% 2 5% 4 5% 4 4 6% 8% 46%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 6% 9% 46%	6866
August	1st to 10th	Day	16% 9% 3 6% 4 5% 5% 4 6% 8% 36%	4234
	11th to 20th	Day	16% 6% 3 7% 5% 4 5% 4 7% 9% 34%	4174
	21st to end	Day	11% 5% 2 7% 5% 5% 5% 4 7% 9% 41%	4393
September	1st to 10th	Day	14% 7% 2 6% 5% 4 6% 4 5% 6% 42%	2400
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 5% 6% 50%	2395
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 7% 39%	2090
October	1st to 10th	Day	20% 8% 2 7% 4 5% 4 3 5% 8% 35%	2000
	11th to 20th	Day	21% 10% 4 9% 5% 5% 4 3 6% 7% 28%	2000
	21st to end	Day	26% 12% 3 8% 5% 5% 4 3 4 5% 25%	2165
November	1st to 10th	Day	17% 10% 3 7% 5% 5% 5% 4 5% 6% 32%	1761
	11th to 20th	Day	16% 14% 4 9% 5% 4 4 4 5% 7% 28%	1656
	21st to end	Day	21% 17% 5% 9% 4 4 5% 4 5% 6% 21%	1600
December	1st to 10th	Day	19% 22% 6% 12% 5% 4 4 2 4 4 18%	2600
	11th to 20th	Day	18% 18% 7% 11% 5% 4 3 3 4 4 24%	2986
	21st to end	Day	24% 18% 4 11% 5% 4 3 2 3 5% 21%	3960
January	1st to 10th	Night	15% 16% 7% 12% 6% 5% 5% 3 4 5% 23%	6349
	11th to 20th	Night	12% 15% 6% 15% 8% 6% 6% 4 6% 6% 16%	6292
	21st to end	Night	15% 14% 7% 14% 6% 5% 4 4 4 5% 22%	6490
February	1st to 10th	Night	13% 16% 5% 11% 7% 5% 6% 4 5% 6% 24%	2600
	11th to 20th	Night	13% 14% 6% 10% 6% 5% 5% 4 5% 5% 28%	2533
	21st to end	Night	16% 13% 4 8% 5% 5% 5% 4 5% 7% 28%	2062
March	1st to 10th	Night	18% 12% 4 10% 5% 5% 5% 3 4 5% 29%	5047
	11th to 20th	Night	11% 11% 4 8% 5% 6% 6% 5% 6% 7% 31%	4979
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 6% 35%	5378
April	1st to 10th	Night	20% 11% 3 7% 5% 4 3 3 5% 7% 32%	2000
	11th to 20th	Night	18% 9% 3 7% 5% 3 4 3 5% 6% 37%	1975
	21st to end	Night	18% 10% 3 6% 5% 5% 4 4 5% 7% 33%	1820
May	1st to 10th	Night	24% 8% 3 7% 4 4 4 3 5% 6% 32%	3781
	11th to 20th	Night	15% 7% 2 7% 4 4 3 3 5% 7% 41%	3575
	21st to end	Night	18% 7% 3 7% 4 5% 4 3 5% 7% 38%	3926
June	1st to 10th	Night	14% 7% 2 7% 5% 5% 4 5% 5% 9% 38%	1600
	11th to 20th	Night	12% 6% 3 6% 4 5% 4 4 5% 8% 44%	1600
	21st to end	Night	14% 7% 2 7% 4 4 5% 4 5% 7% 40%	1600
July	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 5% 6% 8% 41%	3600
	11th to 20th	Night	12% 5% 2 5% 4 4 3 3 5% 7% 50%	3600
	21st to end	Night	10% 5% 2 5% 4 4 4 4 5% 8% 50%	3970
August	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 5% 8% 47%	2915
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 5% 7% 46%	2875
	21st to end	Night	10% 5% 2 6% 3 4 5% 3 7% 8% 47%	3472
September	1st to 10th	Night	10% 6% 3 5% 4 4 4 3 7% 8% 46%	2000
	11th to 20th	Night	12% 4 1 5% 4 3 3 4 5% 7% 53%	2001
	21st to end	Night	18% 7% 3 5% 4 4 4 3 5% 5% 42%	2310
October	1st to 10th	Night	17% 6% 3 6% 4 4 4 3 5% 7% 40%	2400
	11th to 20th	Night	22% 10% 3 7% 4 4 4 3 4 6% 32%	2400
	21st to end	Night	21% 12% 3 7% 5% 5% 5% 3 5% 5% 29%	2674
November	1st to 10th	Night	18% 10% 3 6% 4 5% 4 3 5% 6% 36%	2756
	11th to 20th	Night	18% 12% 3 7% 6% 5% 4 3 5% 6% 30%	2899
	21st to end	Night	20% 12% 3 8% 6% 5% 5% 4 6% 6% 24%	2800
December	1st to 10th	Night	20% 16% 6% 10% 5% 5% 5% 4 6% 6% 18%	4580
	11th to 20th	Night	18% 17% 6% 11% 6% 4 5% 4 5% 5% 20%	5208
	21st to end	Night	25% 18% 4 12% 5% 4 4 3 4 4 19%	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	52% 7% 10% 20% 9% 1	3598
	11th to 20th	Day	69% 6% 9% 9% 6% 1	3622
	21st to end	Day	68% 4 8% 10% 8% 1	4466
February	1st to 10th	Day	64% 6% 10% 15% 5% 1	1800
	11th to 20th	Day	61% 4 9% 14% 10% 1	1762
	21st to end	Day	63% 4 9% 13% 10% 1	1567
March	1st to 10th	Day	57% 4 11% 12% 14% 1	4791
	11th to 20th	Day	53% 6% 16% 16% 8% 2	4980
	21st to end	Day	49% 4 8% 21% 16% 3	5552
April	1st to 10th	Day	54% 4 8% 19% 11% 3	2400
	11th to 20th	Day	50% 4 9% 20% 16% 1	2374
	21st to end	Day	54% 4 9% 18% 12% 3	2597
May	1st to 10th	Day	51% 5% 9% 20% 11% 4	6168
	11th to 20th	Day	49% 7% 10% 15% 18% 1	6322
	21st to end	Day	48% 6% 11% 21% 12% 2	6949
June	1st to 10th	Day	50% 7% 11% 17% 14% 1	2800
	11th to 20th	Day	42% 8% 12% 24% 13% 2	2800
	21st to end	Day	46% 7% 12% 21% 12% 1	2800
July	1st to 10th	Day	40% 7% 15% 25% 13% 1	6290
	11th to 20th	Day	34% 7% 13% 29% 17%	6285
	21st to end	Day	32% 8% 13% 35% 12%	6669
August	1st to 10th	Day	45% 6% 15% 22% 11% 1	4221
	11th to 20th	Day	44% 7% 14% 26% 8%	4213
	21st to end	Day	36% 7% 13% 32% 10% 1	4393
September	1st to 10th	Day	41% 7% 12% 28% 12% 1	2400
	11th to 20th	Day	33% 6% 15% 31% 15% 1	2395
	21st to end	Day	44% 7% 14% 24% 11%	2090
October	1st to 10th	Day	43% 6% 12% 29% 11% 1	3260
	11th to 20th	Day	47% 5% 15% 20% 12% 1	3240
	21st to end	Day	56% 5% 12% 19% 8%	3479
November	1st to 10th	Day	51% 7% 14% 20% 8% 1	1761
	11th to 20th	Day	55% 6% 14% 15% 9% 1	1656
	21st to end	Day	64% 7% 12% 12% 6%	1600
December	1st to 10th	Day	71% 4 14% 8% 3	2600
	11th to 20th	Day	65% 6% 9% 14% 6%	2986
	21st to end	Day	66% 5% 7% 14% 8%	3960
January	1st to 10th	Night	62% 5% 9% 15% 8%	6349
	11th to 20th	Night	66% 7% 12% 9% 5% 1	6337
	21st to end	Night	64% 5% 9% 9% 12% 1	6490
February	1st to 10th	Night	60% 7% 12% 15% 6% 1	2600
	11th to 20th	Night	57% 5% 10% 18% 9% 1	2535
	21st to end	Night	54% 5% 11% 20% 9% 1	2062
March	1st to 10th	Night	56% 5% 7% 16% 11% 5%	5057
	11th to 20th	Night	48% 7% 18% 17% 8% 1	4980
	21st to end	Night	46% 6% 11% 21% 14% 2	5378
April	1st to 10th	Night	52% 5% 8% 20% 12% 2	2000
	11th to 20th	Night	47% 6% 9% 22% 15% 1	1975
	21st to end	Night	49% 6% 10% 16% 16% 2	1820
May	1st to 10th	Night	52% 5% 10% 15% 12% 4	3788
	11th to 20th	Night	40% 7% 14% 20% 16% 2	3576
	21st to end	Night	45% 7% 13% 20% 14% 2	3927
June	1st to 10th	Night	42% 7% 14% 22% 14% 1	1600
	11th to 20th	Night	38% 7% 12% 23% 19% 2	1600
	21st to end	Night	43% 6% 14% 21% 13% 2	1600
July	1st to 10th	Night	38% 7% 16% 22% 16% 1	3560
	11th to 20th	Night	33% 6% 13% 31% 16%	3552
	21st to end	Night	31% 6% 13% 35% 14%	3853
August	1st to 10th	Night	36% 6% 15% 26% 16% 1	2904
	11th to 20th	Night	37% 7% 18% 25% 12% 1	2902
	21st to end	Night	32% 8% 14% 34% 11% 2	3472
September	1st to 10th	Night	34% 8% 15% 28% 14% 1	2000
	11th to 20th	Night	30% 6% 15% 33% 15% 1	2001
	21st to end	Night	43% 6% 12% 27% 12%	2310
October	1st to 10th	Night	35% 6% 11% 29% 19%	3920
	11th to 20th	Night	44% 6% 15% 26% 9%	3899
	21st to end	Night	48% 7% 14% 23% 8%	4310
November	1st to 10th	Night	48% 6% 14% 22% 10% 1	2756
	11th to 20th	Night	54% 6% 13% 18% 8% 2	2899
	21st to end	Night	58% 9% 15% 14% 4	2800
December	1st to 10th	Night	65% 7% 13% 12% 3	4580
	11th to 20th	Night	65% 6% 10% 12% 6%	5208
	21st to end	Night	69% 5% 7% 11% 8%	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	23% 21% 37% 17% 3 1	200
	11th to 20th	Day	27% 21% 32% 16% 3	205
	21st to end	Day	24% 25% 35% 12% 4	330
February	1st to 10th	Day	30% 25% 32% 10% 3	300
	11th to 20th	Day	32% 24% 37% 5% 1	300
	21st to end	Day	37% 19% 33% 11% 1	331
March	1st to 10th	Day	27% 20% 34% 15% 3	500
	11th to 20th	Day	27% 19% 33% 17% 3 1	500
	21st to end	Day	33% 20% 28% 12% 6% 1	550
April	1st to 10th	Day	30% 20% 33% 11% 6%	500
	11th to 20th	Day	32% 23% 21% 14% 7% 2	492
	21st to end	Day	28% 20% 33% 12% 6% 1	500
May	1st to 10th	Day	27% 16% 29% 19% 4 4	503
	11th to 20th	Day	20% 14% 36% 18% 9% 3	527
	21st to end	Day	19% 15% 35% 17% 10% 3	573
June	1st to 10th	Day	22% 14% 33% 24% 7% 1	530
	11th to 20th	Day	16% 13% 36% 22% 11% 3	530
	21st to end	Day	21% 13% 33% 22% 8% 2	530
July	1st to 10th	Day	12% 10% 32% 31% 13% 3	525
	11th to 20th	Day	13% 10% 32% 26% 14% 5%	524
	21st to end	Day	12% 10% 37% 26% 12% 3	576
August	1st to 10th	Day	14% 8% 29% 29% 15% 5%	526
	11th to 20th	Day	13% 8% 27% 30% 17% 6%	519
	21st to end	Day	14% 10% 32% 25% 12% 8%	583
September	1st to 10th	Day	15% 12% 30% 24% 13% 7%	530
	11th to 20th	Day	10% 11% 25% 22% 19% 13%	530
	21st to end	Day	18% 14% 26% 26% 10% 5%	530
October	1st to 10th	Day	24% 15% 24% 17% 14% 5%	530
	11th to 20th	Day	24% 23% 30% 15% 6% 2	530
	21st to end	Day	31% 18% 29% 18% 3 1	564
November	1st to 10th	Day	28% 18% 29% 19% 7%	335
	11th to 20th	Day	30% 26% 29% 10% 4 1	214
	21st to end	Day	29% 21% 35% 12% 4	200
December	1st to 10th	Day	34% 18% 34% 14% 2	200
	11th to 20th	Day	36% 21% 29% 12% 2 1	200
	21st to end	Day	35% 19% 30% 13% 3	220
January	1st to 10th	Night	28% 20% 31% 17% 4	800
	11th to 20th	Night	28% 17% 31% 20% 4	800
	21st to end	Night	20% 16% 35% 22% 6% 1	880
February	1st to 10th	Night	24% 20% 34% 20% 2	800
	11th to 20th	Night	23% 18% 39% 16% 4	783
	21st to end	Night	25% 21% 31% 21% 3	660
March	1st to 10th	Night	24% 18% 35% 20% 3	800
	11th to 20th	Night	23% 19% 33% 20% 4 1	800
	21st to end	Night	22% 15% 33% 22% 8% 1	880
April	1st to 10th	Night	25% 16% 33% 21% 5%	800
	11th to 20th	Night	25% 18% 28% 20% 8% 1	791
	21st to end	Night	23% 10% 32% 24% 10%	620
May	1st to 10th	Night	24% 7% 32% 24% 11% 1	484
	11th to 20th	Night	15% 10% 36% 30% 9%	394
	21st to end	Night	14% 7% 35% 31% 12%	433
June	1st to 10th	Night	15% 8% 37% 30% 11%	400
	11th to 20th	Night	13% 11% 28% 37% 10% 1	400
	21st to end	Night	17% 9% 27% 35% 12%	400
July	1st to 10th	Night	7% 5% 31% 41% 15%	390
	11th to 20th	Night	11% 6% 27% 37% 19% 1	388
	21st to end	Night	12% 3 30% 40% 14% 1	457
August	1st to 10th	Night	12% 4 23% 34% 25% 2	597
	11th to 20th	Night	11% 4 21% 35% 26% 2	594
	21st to end	Night	13% 5% 28% 32% 20% 2	805
September	1st to 10th	Night	13% 6% 32% 31% 18% 2	800
	11th to 20th	Night	12% 5% 23% 31% 24% 5%	800
	21st to end	Night	20% 11% 23% 26% 18% 3	800
October	1st to 10th	Night	21% 10% 27% 25% 16% 2	800
	11th to 20th	Night	26% 14% 30% 22% 7% 1	800
	21st to end	Night	27% 15% 29% 23% 5%	880
November	1st to 10th	Night	24% 14% 28% 25% 8%	820
	11th to 20th	Night	26% 16% 33% 18% 6% 1	829
	21st to end	Night	29% 16% 29% 20% 5%	800
December	1st to 10th	Night	29% 17% 33% 16% 4	800
	11th to 20th	Night	26% 19% 34% 18% 3	800
	21st to end	Night	33% 16% 30% 18% 4	880

Page generated at 2025-11-15 00:56:00. Version ac07

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