Stake Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stake Lake Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	7% 10% 4 9% 6% 6% 6% 5% 7% 7% 34%	129915
	Night	5% 10% 4 10% 6% 6% 6% 5% 7% 8% 34%	127809
Transparency	Day	43% 8% 15% 22% 10% 2	129628
	Night	44% 7% 17% 21% 9% 1	127673
Seeing	Day	16% 17% 26% 24% 13% 4	16770
	Night	13% 16% 26% 27% 15% 3	25005

By Month

Percentage Cloud Cover NW (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	5% 14% 6% 17% 8% 7% 6% 6% 7% 8% 16%	3179
	11th to 20th	Day	4 11% 4 8% 6% 5% 7% 7% 10% 9% 28%	3362
	21st to end	Day	2 14% 5% 11% 7% 6% 7% 5% 7% 7% 28%	4004
February	1st to 10th	Day	6% 17% 5% 11% 7% 6% 7% 5% 7% 6% 23%	1662
	11th to 20th	Day	4 15% 5% 12% 6% 5% 6% 6% 6% 6% 29%	1755
	21st to end	Day	3 14% 7% 13% 9% 8% 5% 5% 7% 6% 22%	1582
March	1st to 10th	Day	2 8% 4 10% 8% 8% 9% 6% 9% 10% 26%	4518
	11th to 20th	Day	2 9% 4 11% 8% 7% 7% 5% 7% 7% 33%	4858
	21st to end	Day	6% 15% 5% 12% 9% 7% 7% 5% 7% 7% 22%	5837
April	1st to 10th	Day	6% 13% 5% 13% 9% 7% 8% 4 6% 7% 20%	2400
	11th to 20th	Day	7% 15% 5% 11% 8% 6% 6% 5% 5% 7% 24%	2376
	21st to end	Day	7% 15% 4 11% 8% 6% 6% 5% 7% 7% 25%	2797
May	1st to 10th	Day	6% 11% 4 8% 5% 4 4 4 5% 6% 43%	6320
	11th to 20th	Day	9% 13% 5% 12% 7% 7% 6% 5% 6% 9% 21%	6277
	21st to end	Day	10% 13% 5% 12% 6% 6% 5% 4 6% 8% 26%	6903
June	1st to 10th	Day	17% 9% 3 8% 6% 6% 6% 5% 6% 7% 27%	3170
	11th to 20th	Day	16% 11% 4 10% 8% 6% 6% 4 6% 6% 23%	3200
	21st to end	Day	11% 8% 2 9% 6% 5% 5% 5% 7% 7% 34%	3200
July	1st to 10th	Day	6% 6% 2 7% 4 4 5% 4 5% 6% 50%	6839
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 6% 7% 54%	6700
	21st to end	Day	6% 4 2 5% 4 5% 5% 4 7% 9% 50%	6874
August	1st to 10th	Day	4 3 2 5% 4 5% 6% 4 7% 7% 54%	4541
	11th to 20th	Day	4 5% 2 8% 6% 7% 7% 6% 7% 9% 41%	4408
	21st to end	Day	9% 5% 2 7% 4 5% 5% 5% 7% 8% 43%	4441
September	1st to 10th	Day	9% 6% 2 7% 4 4 4 3 6% 7% 49%	2400
	11th to 20th	Day	11% 9% 3 7% 5% 5% 5% 3 5% 6% 41%	2377
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 4 7% 8% 35%	2036
October	1st to 10th	Day	6% 7% 3 8% 6% 5% 5% 4 6% 8% 41%	3180
	11th to 20th	Day	13% 13% 3 7% 5% 5% 5% 4 6% 8% 33%	3103
	21st to end	Day	7% 13% 4 11% 6% 5% 6% 4 6% 9% 30%	3101
November	1st to 10th	Day	10% 13% 4 13% 8% 6% 6% 4 5% 6% 26%	1607
	11th to 20th	Day	6% 14% 4 9% 8% 7% 6% 5% 7% 6% 29%	1600
	21st to end	Day	8% 15% 3 10% 7% 7% 6% 6% 7% 8% 24%	1500
December	1st to 10th	Day	3 9% 4 11% 7% 7% 7% 5% 6% 8% 32%	2290
	11th to 20th	Day	4 14% 6% 13% 8% 8% 6% 6% 8% 7% 21%	2365
	21st to end	Day	4 13% 5% 12% 8% 8% 8% 6% 8% 8% 19%	3153
January	1st to 10th	Night	5% 14% 6% 15% 8% 8% 7% 5% 6% 7% 19%	6766
	11th to 20th	Night	3 9% 4 10% 7% 6% 6% 5% 9% 9% 31%	6533
	21st to end	Night	3 11% 4 11% 7% 7% 7% 5% 7% 8% 30%	6952
February	1st to 10th	Night	6% 15% 5% 12% 7% 7% 6% 4 6% 6% 25%	2735
	11th to 20th	Night	4 11% 5% 12% 6% 5% 6% 4 7% 7% 32%	2532
	21st to end	Night	3 12% 6% 15% 8% 6% 6% 4 6% 9% 26%	2047
March	1st to 10th	Night	2 8% 4 9% 6% 6% 8% 6% 10% 12% 29%	5376
	11th to 20th	Night	1 6% 3 9% 6% 5% 7% 6% 8% 9% 39%	5137
	21st to end	Night	6% 12% 5% 11% 8% 7% 8% 6% 7% 7% 24%	5138
April	1st to 10th	Night	5% 11% 4 11% 8% 6% 6% 5% 7% 8% 30%	2000
	11th to 20th	Night	3 12% 5% 10% 8% 7% 6% 6% 6% 6% 31%	1973
	21st to end	Night	4 10% 3 8% 6% 6% 5% 5% 7% 8% 37%	1602
May	1st to 10th	Night	5% 11% 4 10% 5% 5% 5% 3 5% 6% 42%	3640
	11th to 20th	Night	5% 10% 4 11% 6% 6% 6% 5% 7% 9% 32%	3616
	21st to end	Night	7% 8% 3 8% 5% 5% 5% 4 6% 9% 38%	3972
June	1st to 10th	Night	13% 7% 3 9% 5% 5% 5% 5% 8% 8% 33%	1221
	11th to 20th	Night	11% 8% 4 9% 8% 4 7% 5% 7% 8% 31%	1200
	21st to end	Night	7% 7% 3 8% 5% 5% 4 3 7% 7% 44%	1200
July	1st to 10th	Night	8% 5% 2 6% 3 4 4 3 6% 7% 50%	3103
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 7% 61%	3244
	21st to end	Night	4 3 2 5% 4 5% 5% 4 6% 8% 55%	3969
August	1st to 10th	Night	4 5% 2 5% 4 5% 4 4 6% 8% 54%	2609
	11th to 20th	Night	5% 4 2 6% 6% 5% 5% 4 8% 9% 45%	2638
	21st to end	Night	8% 6% 2 6% 5% 5% 5% 4 6% 8% 46%	3474
September	1st to 10th	Night	6% 5% 2 6% 3 3 5% 3 7% 7% 54%	2000
	11th to 20th	Night	8% 6% 3 6% 4 4 5% 3 6% 7% 47%	1980
	21st to end	Night	9% 9% 3 7% 4 5% 4 4 5% 9% 42%	2139
October	1st to 10th	Night	8% 8% 3 7% 5% 5% 5% 4 6% 7% 42%	3800
	11th to 20th	Night	11% 11% 3 7% 5% 5% 5% 4 6% 8% 34%	3832
	21st to end	Night	7% 10% 4 11% 6% 5% 5% 4 6% 7% 36%	4457
November	1st to 10th	Night	9% 14% 4 10% 7% 7% 6% 5% 6% 6% 26%	2687
	11th to 20th	Night	5% 13% 4 11% 9% 7% 6% 4 6% 7% 28%	2800
	21st to end	Night	6% 15% 4 11% 8% 6% 7% 5% 6% 8% 24%	2878
December	1st to 10th	Night	4 9% 4 9% 6% 6% 6% 5% 8% 9% 35%	4890
	11th to 20th	Night	5% 14% 7% 15% 9% 8% 7% 4 5% 5% 21%	5824
	21st to end	Night	3 13% 5% 12% 7% 7% 7% 5% 7% 7% 26%	7845

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	61% 8% 15% 15% 2	3179
	11th to 20th	Day	41% 7% 23% 19% 6% 4	3384
	21st to end	Day	49% 10% 20% 15% 5%	4004
February	1st to 10th	Day	57% 8% 13% 13% 8% 1	1662
	11th to 20th	Day	51% 7% 16% 16% 8% 1	1764
	21st to end	Day	56% 7% 15% 16% 6%	1582
March	1st to 10th	Day	43% 9% 15% 22% 10%	4518
	11th to 20th	Day	45% 6% 17% 24% 8%	4858
	21st to end	Day	57% 7% 14% 14% 8%	5837
April	1st to 10th	Day	57% 10% 13% 16% 5%	2400
	11th to 20th	Day	52% 10% 10% 17% 10% 2	2376
	21st to end	Day	52% 8% 14% 17% 9%	2797
May	1st to 10th	Day	39% 5% 9% 30% 15% 2	6320
	11th to 20th	Day	50% 10% 14% 21% 4	6280
	21st to end	Day	49% 8% 15% 22% 4 1	6904
June	1st to 10th	Day	51% 9% 11% 16% 10% 4	3170
	11th to 20th	Day	57% 8% 10% 16% 8%	3200
	21st to end	Day	43% 8% 14% 21% 11% 2	3200
July	1st to 10th	Day	29% 6% 12% 22% 19% 13%	6765
	11th to 20th	Day	24% 6% 15% 24% 22% 8%	6619
	21st to end	Day	23% 6% 20% 30% 15% 5%	6681
August	1st to 10th	Day	23% 6% 12% 31% 24% 3	4524
	11th to 20th	Day	32% 10% 21% 25% 10% 2	4450
	21st to end	Day	34% 7% 19% 30% 9% 2	4440
September	1st to 10th	Day	33% 6% 15% 29% 15% 3	2400
	11th to 20th	Day	41% 6% 13% 26% 12% 3	2377
	21st to end	Day	43% 9% 18% 20% 10% 1	2036
October	1st to 10th	Day	38% 6% 21% 26% 8%	3180
	11th to 20th	Day	47% 6% 15% 27% 6%	3103
	21st to end	Day	47% 10% 19% 20% 4	3101
November	1st to 10th	Day	57% 8% 13% 17% 5% 1	1607
	11th to 20th	Day	52% 8% 17% 17% 6% 1	1600
	21st to end	Day	54% 9% 17% 16% 5%	1500
December	1st to 10th	Day	43% 11% 29% 16% 2	2290
	11th to 20th	Day	56% 10% 17% 12% 4	2367
	21st to end	Day	57% 10% 16% 15% 2	3153
January	1st to 10th	Night	59% 8% 16% 13% 3	6766
	11th to 20th	Night	43% 8% 20% 18% 7% 3	6573
	21st to end	Night	47% 10% 22% 17% 4	6952
February	1st to 10th	Night	58% 6% 14% 13% 7% 2	2735
	11th to 20th	Night	48% 7% 16% 19% 8% 1	2534
	21st to end	Night	54% 7% 15% 17% 7%	2047
March	1st to 10th	Night	38% 10% 16% 26% 9%	5376
	11th to 20th	Night	37% 6% 19% 25% 13%	5138
	21st to end	Night	53% 8% 13% 17% 9%	5138
April	1st to 10th	Night	47% 8% 19% 19% 6%	2000
	11th to 20th	Night	47% 9% 13% 20% 11%	1973
	21st to end	Night	41% 8% 15% 26% 10%	1602
May	1st to 10th	Night	41% 6% 9% 26% 17% 1	3637
	11th to 20th	Night	42% 10% 16% 27% 5%	3616
	21st to end	Night	41% 6% 17% 29% 6% 1	3972
June	1st to 10th	Night	45% 7% 13% 22% 10% 3	1221
	11th to 20th	Night	46% 10% 15% 21% 9%	1200
	21st to end	Night	37% 5% 19% 24% 13% 2	1200
July	1st to 10th	Night	30% 5% 15% 18% 22% 10%	3068
	11th to 20th	Night	20% 5% 15% 28% 26% 5%	3202
	21st to end	Night	22% 7% 19% 28% 15% 9%	3857
August	1st to 10th	Night	25% 6% 12% 26% 27% 3	2601
	11th to 20th	Night	30% 7% 20% 28% 12% 2	2662
	21st to end	Night	32% 7% 22% 27% 11% 2	3474
September	1st to 10th	Night	26% 6% 16% 31% 17% 3	2000
	11th to 20th	Night	34% 6% 16% 26% 13% 4	1980
	21st to end	Night	39% 6% 21% 20% 12% 2	2139
October	1st to 10th	Night	38% 6% 21% 27% 9%	3800
	11th to 20th	Night	45% 6% 14% 27% 7%	3832
	21st to end	Night	45% 6% 16% 28% 5%	4457
November	1st to 10th	Night	55% 8% 15% 15% 6%	2687
	11th to 20th	Night	53% 8% 16% 15% 7% 1	2800
	21st to end	Night	56% 8% 17% 13% 6%	2878
December	1st to 10th	Night	41% 10% 26% 18% 4	4890
	11th to 20th	Night	63% 7% 16% 11% 3	5821
	21st to end	Night	53% 8% 17% 18% 4	7845

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	12% 25% 29% 19% 8% 7%	200
	11th to 20th	Day	13% 21% 29% 24% 12% 4	200
	21st to end	Day	14% 23% 29% 21% 12% 2	220
February	1st to 10th	Day	18% 24% 32% 18% 6% 2	262
	11th to 20th	Day	16% 23% 30% 22% 8% 2	395
	21st to end	Day	17% 21% 32% 18% 11% 2	381
March	1st to 10th	Day	20% 25% 26% 19% 11% 1	500
	11th to 20th	Day	19% 24% 26% 19% 10% 3	500
	21st to end	Day	21% 27% 24% 16% 10% 3	550
April	1st to 10th	Day	22% 21% 27% 22% 6% 2	500
	11th to 20th	Day	17% 26% 25% 19% 11% 2	492
	21st to end	Day	18% 20% 29% 19% 10% 4	500
May	1st to 10th	Day	16% 22% 24% 27% 8% 3	503
	11th to 20th	Day	18% 13% 21% 28% 16% 5%	528
	21st to end	Day	17% 15% 26% 23% 15% 4	573
June	1st to 10th	Day	20% 20% 26% 22% 11% 1	900
	11th to 20th	Day	19% 14% 28% 24% 12% 3	930
	21st to end	Day	14% 12% 26% 26% 18% 4	930
July	1st to 10th	Day	11% 13% 27% 30% 15% 4	754
	11th to 20th	Day	10% 13% 29% 28% 15% 5%	690
	21st to end	Day	7% 7% 22% 38% 20% 7%	575
August	1st to 10th	Day	7% 6% 23% 30% 27% 7%	527
	11th to 20th	Day	8% 11% 25% 30% 20% 7%	517
	21st to end	Day	11% 11% 26% 29% 16% 6%	583
September	1st to 10th	Day	12% 13% 25% 25% 18% 6%	530
	11th to 20th	Day	17% 16% 23% 24% 17% 4	523
	21st to end	Day	20% 17% 27% 22% 12% 2	490
October	1st to 10th	Day	21% 14% 24% 21% 16% 3	490
	11th to 20th	Day	23% 19% 23% 18% 14% 2	457
	21st to end	Day	22% 17% 26% 23% 10% 2	330
November	1st to 10th	Day	21% 22% 27% 19% 10% 1	220
	11th to 20th	Day	24% 21% 26% 13% 13% 4	200
	21st to end	Day	19% 19% 30% 16% 11% 6%	200
December	1st to 10th	Day	13% 14% 23% 26% 19% 7%	200
	11th to 20th	Day	13% 26% 30% 16% 11% 5%	200
	21st to end	Day	17% 13% 31% 26% 10% 3	220
January	1st to 10th	Night	21% 19% 24% 24% 10% 2	900
	11th to 20th	Night	12% 15% 28% 31% 13% 2	855
	21st to end	Night	10% 17% 29% 28% 14% 2	880
February	1st to 10th	Night	16% 17% 26% 29% 11% 3	800
	11th to 20th	Night	12% 16% 24% 33% 14% 1	784
	21st to end	Night	14% 16% 28% 23% 17% 2	660
March	1st to 10th	Night	13% 21% 23% 25% 15% 3	800
	11th to 20th	Night	12% 23% 28% 22% 14% 2	800
	21st to end	Night	14% 18% 29% 23% 13% 4	880
April	1st to 10th	Night	13% 18% 26% 27% 14% 3	800
	11th to 20th	Night	12% 20% 28% 25% 14% 1	785
	21st to end	Night	9% 21% 30% 26% 11% 2	402
May	1st to 10th	Night	14% 14% 28% 28% 14% 3	400
	11th to 20th	Night	11% 11% 26% 35% 14% 3	394
	21st to end	Night	14% 9% 29% 26% 20% 2	433
June	1st to 10th	Night	16% 14% 30% 30% 9% 2	400
	11th to 20th	Night	15% 12% 32% 26% 13% 3	400
	21st to end	Night	8% 13% 29% 28% 19% 3	400
July	1st to 10th	Night	8% 9% 29% 36% 17% 2	390
	11th to 20th	Night	7% 9% 29% 33% 22% 1	388
	21st to end	Night	5% 4 28% 35% 24% 4	438
August	1st to 10th	Night	8% 4 20% 33% 29% 6%	398
	11th to 20th	Night	7% 6% 26% 29% 28% 5%	431
	21st to end	Night	10% 9% 25% 30% 22% 4	798
September	1st to 10th	Night	10% 9% 20% 28% 28% 5%	800
	11th to 20th	Night	11% 9% 19% 27% 27% 7%	792
	21st to end	Night	17% 16% 23% 23% 17% 4	760
October	1st to 10th	Night	12% 15% 23% 27% 20% 4	760
	11th to 20th	Night	17% 17% 21% 25% 17% 3	760
	21st to end	Night	17% 18% 28% 22% 12% 2	836
November	1st to 10th	Night	20% 17% 29% 26% 8% 1	780
	11th to 20th	Night	17% 24% 24% 21% 12% 1	800
	21st to end	Night	15% 19% 26% 25% 12% 3	878
December	1st to 10th	Night	13% 14% 26% 29% 15% 3	1000
	11th to 20th	Night	14% 21% 27% 26% 10% 1	1067
	21st to end	Night	16% 18% 28% 26% 10% 2	1156

Page generated at 2025-09-15 22:49:10. Version ac07

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