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 2024-01-12.

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	8% 10% 4 8% 7% 4 6% 6% 6% 7% 32%	74573
	Night	6% 10% 4 9% 8% 4 6% 6% 6% 7% 33%	75128
Transparency	Day	47% 8% 14% 20% 9% 1	74264
	Night	47% 8% 16% 20% 8% 1	74955
Seeing	Day	16% 18% 26% 23% 14% 4	14444
	Night	13% 16% 26% 27% 16% 3	22967

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	6% 14% 5% 12% 11% 6% 7% 8% 7% 6% 18%	1329
	11th to 20th	Day	5% 10% 4 11% 10% 4 7% 6% 8% 10% 25%	1345
	21st to end	Day	4 14% 6% 9% 8% 5% 6% 7% 9% 8% 25%	1584
February	1st to 10th	Day	7% 16% 7% 9% 9% 5% 6% 6% 6% 6% 23%	1495
	11th to 20th	Day	5% 15% 6% 11% 7% 4 6% 7% 6% 7% 28%	1575
	21st to end	Day	3 13% 8% 11% 11% 5% 6% 5% 6% 6% 24%	1419
March	1st to 10th	Day	4 13% 6% 11% 8% 5% 7% 7% 7% 7% 24%	1800
	11th to 20th	Day	5% 10% 6% 10% 9% 5% 9% 6% 7% 7% 26%	1916
	21st to end	Day	5% 15% 6% 11% 10% 6% 7% 7% 8% 8% 19%	2322
April	1st to 10th	Day	7% 12% 6% 12% 10% 5% 8% 6% 6% 7% 19%	2160
	11th to 20th	Day	8% 14% 6% 10% 9% 5% 6% 6% 5% 7% 22%	2135
	21st to end	Day	8% 14% 4 10% 9% 4 6% 6% 6% 7% 25%	2517
May	1st to 10th	Day	10% 14% 5% 9% 8% 4 6% 5% 5% 6% 28%	2520
	11th to 20th	Day	9% 9% 4 9% 8% 4 7% 6% 7% 9% 27%	2477
	21st to end	Day	16% 11% 4 9% 8% 4 5% 5% 5% 6% 26%	2723
June	1st to 10th	Day	18% 9% 4 8% 7% 4 6% 6% 6% 7% 25%	2852
	11th to 20th	Day	17% 10% 6% 10% 9% 4 7% 5% 5% 6% 22%	2880
	21st to end	Day	11% 7% 3 8% 7% 4 6% 6% 7% 7% 35%	2880
July	1st to 10th	Day	8% 6% 3 7% 6% 3 6% 6% 6% 7% 43%	2731
	11th to 20th	Day	7% 4 2 5% 5% 4 6% 6% 6% 8% 46%	2676
	21st to end	Day	6% 3 2 4 4 3 4 5% 6% 8% 55%	2741
August	1st to 10th	Day	6% 4 2 5% 5% 4 5% 5% 7% 6% 51%	2507
	11th to 20th	Day	5% 4 1 5% 5% 4 5% 5% 5% 7% 54%	2441
	21st to end	Day	9% 5% 3 7% 5% 4 6% 6% 8% 8% 39%	2449
September	1st to 10th	Day	10% 6% 3 6% 5% 3 4 4 5% 6% 48%	2160
	11th to 20th	Day	12% 9% 3 6% 6% 4 5% 4 5% 6% 40%	2160
	21st to end	Day	12% 8% 3 6% 6% 4 5% 5% 6% 8% 36%	1930
October	1st to 10th	Day	10% 9% 4 6% 7% 3 5% 4 6% 6% 39%	1800
	11th to 20th	Day	8% 10% 4 7% 7% 4 7% 6% 6% 8% 33%	1769
	21st to end	Day	9% 11% 4 8% 6% 4 6% 5% 7% 9% 32%	1782
November	1st to 10th	Day	11% 11% 6% 11% 9% 5% 6% 5% 5% 6% 26%	1525
	11th to 20th	Day	6% 13% 5% 8% 8% 5% 6% 6% 7% 6% 29%	1520
	21st to end	Day	8% 12% 4 9% 8% 6% 6% 7% 7% 8% 25%	1425
December	1st to 10th	Day	5% 9% 6% 12% 8% 7% 8% 7% 6% 7% 24%	1330
	11th to 20th	Day	3 12% 6% 11% 10% 5% 7% 7% 8% 8% 23%	1503
	21st to end	Day	4 11% 6% 10% 9% 6% 8% 9% 8% 8% 22%	2195
January	1st to 10th	Night	8% 14% 7% 11% 9% 5% 6% 7% 6% 7% 20%	2836
	11th to 20th	Night	5% 12% 5% 13% 11% 6% 8% 6% 7% 9% 19%	2640
	21st to end	Night	5% 11% 5% 10% 8% 5% 8% 6% 8% 8% 26%	2772
February	1st to 10th	Night	7% 15% 6% 10% 8% 5% 6% 6% 6% 6% 25%	2462
	11th to 20th	Night	5% 11% 5% 11% 7% 4 7% 6% 6% 8% 31%	2272
	21st to end	Night	4 10% 6% 13% 9% 4 7% 5% 5% 9% 27%	1836
March	1st to 10th	Night	4 11% 4 10% 8% 5% 7% 6% 7% 8% 31%	2160
	11th to 20th	Night	3 9% 5% 9% 9% 4 7% 6% 8% 8% 33%	2039
	21st to end	Night	4 11% 5% 9% 10% 5% 8% 7% 7% 8% 26%	2034
April	1st to 10th	Night	5% 9% 4 10% 9% 5% 7% 6% 7% 8% 31%	1800
	11th to 20th	Night	3 11% 6% 9% 10% 5% 7% 8% 6% 6% 29%	1775
	21st to end	Night	5% 9% 4 8% 6% 4 6% 6% 7% 8% 37%	1442
May	1st to 10th	Night	8% 13% 5% 8% 6% 4 6% 5% 5% 6% 35%	1440
	11th to 20th	Night	6% 8% 3 9% 8% 4 6% 6% 8% 8% 35%	1416
	21st to end	Night	13% 8% 3 7% 7% 4 5% 6% 7% 8% 34%	1552
June	1st to 10th	Night	13% 6% 4 8% 7% 4 5% 6% 8% 7% 32%	1099
	11th to 20th	Night	12% 7% 4 8% 9% 3 6% 6% 7% 8% 30%	1080
	21st to end	Night	7% 6% 4 7% 5% 4 4 4 6% 8% 45%	1080
July	1st to 10th	Night	7% 5% 2 7% 5% 4 5% 6% 6% 7% 46%	1225
	11th to 20th	Night	6% 3 2 4 4 3 4 6% 7% 8% 52%	1282
	21st to end	Night	5% 3 1 4 4 3 4 4 6% 7% 60%	1576
August	1st to 10th	Night	5% 4 2 3 4 2 4 4 6% 7% 59%	1432
	11th to 20th	Night	5% 3 2 4 5% 3 4 4 6% 8% 56%	1455
	21st to end	Night	7% 6% 3 5% 6% 4 5% 5% 6% 8% 45%	1906
September	1st to 10th	Night	6% 6% 2 5% 4 2 5% 3 6% 7% 53%	1800
	11th to 20th	Night	8% 6% 3 5% 6% 3 5% 5% 6% 8% 46%	1800
	21st to end	Night	9% 8% 3 6% 5% 4 4 5% 5% 9% 42%	2025
October	1st to 10th	Night	7% 7% 3 7% 6% 4 6% 4 7% 7% 41%	2160
	11th to 20th	Night	9% 10% 4 7% 6% 4 5% 5% 5% 7% 37%	2190
	21st to end	Night	8% 9% 4 9% 7% 4 5% 5% 7% 7% 34%	2574
November	1st to 10th	Night	10% 13% 5% 9% 8% 5% 7% 6% 6% 6% 26%	2549
	11th to 20th	Night	5% 12% 6% 9% 9% 5% 7% 6% 6% 7% 29%	2660
	21st to end	Night	7% 13% 6% 9% 8% 5% 7% 7% 5% 8% 25%	2734
December	1st to 10th	Night	5% 11% 6% 9% 8% 6% 7% 6% 7% 8% 27%	2850
	11th to 20th	Night	6% 15% 7% 13% 10% 6% 7% 6% 5% 5% 19%	3686
	21st to end	Night	4 12% 5% 11% 9% 5% 7% 8% 7% 7% 25%	5489

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	63% 9% 14% 10% 3 1	1329
	11th to 20th	Day	52% 9% 21% 13% 3 1	1345
	21st to end	Day	53% 8% 16% 17% 5% 1	1584
February	1st to 10th	Day	58% 8% 13% 14% 6% 1	1495
	11th to 20th	Day	53% 8% 16% 15% 7%	1584
	21st to end	Day	57% 7% 14% 16% 6%	1419
March	1st to 10th	Day	53% 9% 13% 16% 8% 1	1800
	11th to 20th	Day	53% 8% 15% 17% 6%	1916
	21st to end	Day	57% 9% 15% 15% 4	2322
April	1st to 10th	Day	58% 10% 12% 15% 4	2160
	11th to 20th	Day	55% 9% 10% 16% 9% 1	2135
	21st to end	Day	53% 8% 14% 16% 8%	2517
May	1st to 10th	Day	54% 7% 13% 17% 9% 1	2520
	11th to 20th	Day	46% 10% 14% 21% 8%	2480
	21st to end	Day	54% 7% 12% 20% 6% 1	2724
June	1st to 10th	Day	53% 9% 11% 15% 8% 4	2852
	11th to 20th	Day	59% 8% 10% 15% 7%	2880
	21st to end	Day	43% 8% 14% 22% 12% 2	2880
July	1st to 10th	Day	36% 7% 12% 24% 16% 4	2657
	11th to 20th	Day	31% 8% 13% 29% 16% 3	2595
	21st to end	Day	23% 7% 10% 32% 23% 5%	2589
August	1st to 10th	Day	29% 7% 15% 28% 18% 4	2490
	11th to 20th	Day	27% 7% 17% 29% 18% 3	2441
	21st to end	Day	36% 8% 13% 29% 11% 2	2449
September	1st to 10th	Day	35% 6% 14% 29% 13% 2	2160
	11th to 20th	Day	43% 6% 12% 24% 13% 3	2160
	21st to end	Day	42% 9% 18% 20% 10% 2	1930
October	1st to 10th	Day	44% 7% 17% 26% 7%	1800
	11th to 20th	Day	46% 7% 18% 22% 6% 1	1769
	21st to end	Day	47% 8% 18% 21% 6%	1782
November	1st to 10th	Day	57% 8% 13% 17% 5% 1	1525
	11th to 20th	Day	51% 8% 17% 18% 6% 1	1520
	21st to end	Day	53% 9% 16% 16% 6%	1425
December	1st to 10th	Day	54% 12% 16% 15% 3	1330
	11th to 20th	Day	57% 10% 16% 12% 4	1505
	21st to end	Day	55% 10% 17% 17% 2	2195
January	1st to 10th	Night	61% 8% 15% 13% 3	2836
	11th to 20th	Night	60% 9% 17% 12% 1 1	2640
	21st to end	Night	54% 8% 15% 17% 5%	2772
February	1st to 10th	Night	57% 6% 14% 14% 7% 1	2462
	11th to 20th	Night	50% 7% 17% 18% 8%	2274
	21st to end	Night	53% 7% 14% 18% 8%	1836
March	1st to 10th	Night	49% 9% 16% 17% 9% 1	2160
	11th to 20th	Night	45% 8% 17% 23% 7%	2040
	21st to end	Night	51% 9% 17% 17% 5%	2034
April	1st to 10th	Night	47% 9% 18% 20% 7%	1800
	11th to 20th	Night	49% 9% 14% 18% 10%	1775
	21st to end	Night	42% 7% 15% 25% 10% 1	1442
May	1st to 10th	Night	48% 8% 14% 19% 10% 1	1440
	11th to 20th	Night	41% 10% 16% 24% 9%	1416
	21st to end	Night	47% 5% 18% 20% 10%	1552
June	1st to 10th	Night	46% 8% 12% 23% 7% 4	1099
	11th to 20th	Night	46% 10% 16% 22% 6%	1080
	21st to end	Night	36% 5% 19% 24% 13% 3	1080
July	1st to 10th	Night	35% 7% 13% 27% 15% 4	1190
	11th to 20th	Night	24% 8% 14% 34% 17% 2	1240
	21st to end	Night	20% 7% 13% 30% 23% 7%	1488
August	1st to 10th	Night	23% 6% 16% 27% 24% 4	1424
	11th to 20th	Night	25% 5% 15% 30% 21% 4	1455
	21st to end	Night	33% 8% 16% 27% 14% 2	1906
September	1st to 10th	Night	28% 6% 17% 31% 15% 3	1800
	11th to 20th	Night	35% 6% 15% 25% 14% 5%	1800
	21st to end	Night	39% 6% 20% 20% 12% 2	2025
October	1st to 10th	Night	38% 7% 18% 27% 10%	2160
	11th to 20th	Night	44% 7% 15% 25% 8% 1	2190
	21st to end	Night	47% 7% 15% 25% 6%	2574
November	1st to 10th	Night	55% 8% 15% 15% 6%	2549
	11th to 20th	Night	53% 8% 16% 15% 7% 1	2660
	21st to end	Night	56% 8% 17% 13% 6%	2734
December	1st to 10th	Night	52% 9% 17% 17% 5%	2850
	11th to 20th	Night	66% 8% 12% 11% 3	3683
	21st to end	Night	55% 8% 18% 15% 4	5489

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	13% 27% 26% 18% 8% 7%	180
	11th to 20th	Day	12% 20% 28% 22% 13% 4	162
	21st to end	Day	16% 23% 27% 20% 12% 2	176
February	1st to 10th	Day	20% 25% 34% 17% 4	215
	11th to 20th	Day	17% 25% 28% 22% 7% 1	335
	21st to end	Day	12% 21% 34% 19% 11% 2	320
March	1st to 10th	Day	20% 26% 24% 17% 11%	420
	11th to 20th	Day	20% 25% 25% 18% 8% 3	420
	21st to end	Day	20% 27% 24% 15% 10% 3	462
April	1st to 10th	Day	21% 22% 28% 21% 6% 2	420
	11th to 20th	Day	17% 27% 24% 18% 12% 3	412
	21st to end	Day	19% 21% 30% 17% 10% 3	420
May	1st to 10th	Day	16% 22% 22% 29% 8% 3	423
	11th to 20th	Day	17% 13% 20% 28% 16% 5%	448
	21st to end	Day	16% 14% 25% 24% 16% 5%	485
June	1st to 10th	Day	20% 21% 25% 21% 11% 2	782
	11th to 20th	Day	19% 14% 28% 24% 12% 2	810
	21st to end	Day	14% 12% 26% 26% 18% 5%	810
July	1st to 10th	Day	12% 13% 25% 30% 15% 4	652
	11th to 20th	Day	10% 15% 28% 27% 15% 5%	594
	21st to end	Day	6% 7% 21% 38% 21% 7%	487
August	1st to 10th	Day	7% 6% 23% 29% 27% 8%	447
	11th to 20th	Day	7% 11% 24% 31% 20% 7%	437
	21st to end	Day	12% 13% 28% 28% 14% 6%	495
September	1st to 10th	Day	14% 15% 27% 25% 16% 4	450
	11th to 20th	Day	18% 16% 23% 22% 17% 4	450
	21st to end	Day	19% 18% 26% 23% 13% 2	450
October	1st to 10th	Day	21% 14% 24% 21% 17% 4	450
	11th to 20th	Day	21% 20% 23% 19% 15% 2	419
	21st to end	Day	23% 18% 24% 22% 11% 2	297
November	1st to 10th	Day	21% 22% 26% 19% 11% 1	198
	11th to 20th	Day	22% 21% 27% 12% 14% 4	180
	21st to end	Day	20% 19% 29% 15% 11% 6%	180
December	1st to 10th	Day	13% 14% 24% 24% 18% 7%	180
	11th to 20th	Day	12% 27% 29% 15% 12% 5%	180
	21st to end	Day	16% 13% 32% 27% 9% 3	198
January	1st to 10th	Night	22% 20% 24% 23% 10% 1	850
	11th to 20th	Night	13% 15% 27% 30% 13% 2	771
	21st to end	Night	11% 17% 27% 29% 15% 2	792
February	1st to 10th	Night	16% 17% 25% 29% 11% 2	720
	11th to 20th	Night	13% 17% 24% 31% 14% 1	704
	21st to end	Night	11% 15% 29% 24% 19% 2	592
March	1st to 10th	Night	12% 23% 23% 25% 14% 3	720
	11th to 20th	Night	13% 24% 27% 22% 13% 2	720
	21st to end	Night	13% 18% 29% 21% 13% 4	792
April	1st to 10th	Night	12% 17% 25% 27% 15% 3	720
	11th to 20th	Night	12% 20% 28% 25% 15% 1	707
	21st to end	Night	9% 19% 32% 27% 10% 2	362
May	1st to 10th	Night	13% 14% 28% 28% 15% 3	360
	11th to 20th	Night	10% 10% 25% 36% 15% 4	354
	21st to end	Night	14% 8% 27% 27% 22% 2	389
June	1st to 10th	Night	15% 14% 30% 28% 9% 3	360
	11th to 20th	Night	14% 12% 32% 26% 13% 3	360
	21st to end	Night	7% 13% 30% 27% 20% 3	360
July	1st to 10th	Night	7% 9% 28% 36% 17% 2	350
	11th to 20th	Night	8% 10% 30% 30% 21% 1	348
	21st to end	Night	5% 4 27% 35% 24% 5%	394
August	1st to 10th	Night	8% 4 20% 32% 29% 7%	358
	11th to 20th	Night	6% 5% 25% 28% 29% 5%	387
	21st to end	Night	10% 9% 25% 30% 21% 4	718
September	1st to 10th	Night	11% 10% 22% 28% 25% 5%	720
	11th to 20th	Night	12% 9% 20% 23% 28% 8%	720
	21st to end	Night	16% 16% 22% 24% 18% 4	720
October	1st to 10th	Night	12% 13% 23% 28% 21% 4	720
	11th to 20th	Night	16% 17% 22% 24% 17% 3	720
	21st to end	Night	18% 18% 27% 22% 12% 3	792
November	1st to 10th	Night	20% 17% 28% 26% 8% 1	740
	11th to 20th	Night	17% 24% 24% 21% 13% 1	760
	21st to end	Night	16% 19% 25% 25% 12% 3	834
December	1st to 10th	Night	14% 14% 27% 28% 15% 2	950
	11th to 20th	Night	14% 21% 27% 27% 10% 1	1010
	21st to end	Night	16% 18% 28% 27% 10% 2	1093

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