Sisters Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sisters Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	4 6% 2 5% 4 4 4 4 6% 7% 56%	135666
	Night	5% 6% 2 5% 4 4 4 4 6% 7% 53%	131034
Transparency	Day	26% 7% 12% 25% 20% 9%	130460
	Night	28% 8% 15% 26% 18% 6%	127235
Seeing	Day	10% 17% 29% 26% 13% 4	15821
	Night	7% 11% 26% 34% 19% 3	24805

By Month

Percentage Cloud Cover SW (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	8% 12% 4 10% 7% 7% 6% 5% 8% 10% 24%	3639
	11th to 20th	Day	7% 8% 2 6% 5% 5% 6% 4 6% 8% 44%	3640
	21st to end	Day	8% 10% 3 6% 4 3 4 3 5% 8% 45%	4344
February	1st to 10th	Day	5% 8% 3 8% 7% 7% 6% 5% 7% 9% 35%	1800
	11th to 20th	Day	5% 8% 2 5% 6% 5% 6% 6% 8% 9% 40%	1755
	21st to end	Day	3 8% 4 8% 4 5% 6% 5% 7% 9% 39%	1640
March	1st to 10th	Day	5% 9% 3 7% 4 4 6% 5% 9% 11% 38%	4601
	11th to 20th	Day	6% 11% 3 7% 4 5% 5% 4 6% 7% 42%	5000
	21st to end	Day	5% 10% 3 7% 6% 7% 7% 6% 8% 9% 32%	5701
April	1st to 10th	Day	6% 9% 2 7% 7% 6% 6% 5% 7% 7% 38%	2400
	11th to 20th	Day	5% 8% 2 7% 6% 5% 4 4 7% 8% 44%	2369
	21st to end	Day	4 5% 1 5% 4 4 5% 5% 7% 9% 50%	2481
May	1st to 10th	Day	7% 6% 2 4 3 3 4 3 5% 6% 58%	6293
	11th to 20th	Day	4 4 2 5% 4 4 5% 4 6% 6% 56%	6277
	21st to end	Day	5% 4 1 4 3 3 4 4 5% 9% 58%	6902
June	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 6% 7% 51%	2800
	11th to 20th	Day	5% 3 1 4 3 4 4 4 6% 8% 58%	2800
	21st to end	Day	4 2 1 3 3 3 3 3 5% 6% 69%	2800
July	1st to 10th	Day	1 1 1 1 2 2 2 3 3 84%	6319
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 86%	6320
	21st to end	Day	2 1 1 1 1 1 2 2 3 5% 81%	6880
August	1st to 10th	Day	2 2 1 2 2 2 3 3 4 6% 73%	5893
	11th to 20th	Day	2 1 1 2 2 3 3 2 4 5% 74%	5796
	21st to end	Day	4 3 1 3 2 2 3 3 4 6% 68%	5984
September	1st to 10th	Day	3 1 1 2 2 2 2 2 3 5% 78%	2400
	11th to 20th	Day	3 3 1 4 3 4 5% 3 5% 7% 62%	2400
	21st to end	Day	7% 4 1 4 3 3 2 3 5% 6% 62%	2135
October	1st to 10th	Day	4 4 1 4 3 3 3 3 5% 5% 66%	3211
	11th to 20th	Day	4 5% 1 4 3 4 5% 5% 7% 10% 52%	3161
	21st to end	Day	6% 8% 2 6% 5% 5% 5% 4 6% 8% 45%	3308
November	1st to 10th	Day	4 8% 3 8% 5% 5% 6% 5% 8% 8% 40%	1764
	11th to 20th	Day	4 8% 3 8% 5% 6% 7% 8% 9% 9% 33%	1601
	21st to end	Day	5% 7% 3 5% 5% 6% 6% 5% 8% 10% 40%	1600
December	1st to 10th	Day	3 6% 3 5% 3 4 5% 4 7% 7% 53%	2620
	11th to 20th	Day	8% 12% 4 10% 6% 6% 6% 5% 7% 9% 26%	3028
	21st to end	Day	6% 11% 4 9% 6% 6% 6% 5% 7% 9% 30%	4004
January	1st to 10th	Night	8% 12% 4 11% 7% 6% 6% 5% 8% 9% 24%	6306
	11th to 20th	Night	7% 9% 3 6% 5% 5% 4 3 4 7% 47%	6320
	21st to end	Night	10% 10% 3 7% 4 4 4 4 4 5% 45%	6625
February	1st to 10th	Night	5% 10% 3 9% 6% 5% 6% 5% 7% 9% 37%	2600
	11th to 20th	Night	5% 11% 4 6% 5% 5% 5% 5% 6% 7% 41%	2532
	21st to end	Night	4 9% 4 8% 6% 5% 4 4 8% 8% 41%	1989
March	1st to 10th	Night	4 8% 2 5% 4 5% 5% 6% 9% 11% 40%	5291
	11th to 20th	Night	4 8% 3 6% 5% 4 4 4 7% 7% 49%	5000
	21st to end	Night	5% 9% 3 8% 5% 6% 6% 5% 8% 10% 34%	5273
April	1st to 10th	Night	6% 8% 3 6% 4 4 6% 5% 6% 8% 44%	2000
	11th to 20th	Night	4 8% 2 6% 4 4 5% 5% 6% 7% 47%	1979
	21st to end	Night	6% 7% 2 5% 4 4 5% 4 6% 9% 49%	1915
May	1st to 10th	Night	9% 5% 1 4 3 2 3 2 4 5% 62%	3668
	11th to 20th	Night	4 4 2 5% 3 4 4 4 5% 7% 59%	3616
	21st to end	Night	6% 5% 1 3 3 3 3 3 4 7% 62%	3972
June	1st to 10th	Night	11% 5% 2 5% 5% 4 4 3 5% 6% 52%	1600
	11th to 20th	Night	5% 4 1 4 4 3 3 3 5% 8% 61%	1600
	21st to end	Night	3 2 1 2 2 2 4 3 4 5% 71%	1600
July	1st to 10th	Night	1 1 1 2 1 2 2 2 3 3 81%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 87%	3640
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 79%	3970
August	1st to 10th	Night	2 2 1 3 2 3 3 3 5% 7% 70%	3897
	11th to 20th	Night	1 2 1 3 2 3 3 2 4 5% 75%	4052
	21st to end	Night	4 3 1 2 2 2 2 2 4 6% 71%	5014
September	1st to 10th	Night	2 1 2 1 1 3 2 3 5% 80%	2000
	11th to 20th	Night	4 3 1 2 3 4 3 4 5% 6% 65%	2000
	21st to end	Night	6% 4 1 3 3 2 3 3 5% 5% 66%	2262
October	1st to 10th	Night	3 3 1 3 2 3 3 2 4 4 71%	3836
	11th to 20th	Night	4 4 2 4 3 4 4 4 6% 9% 57%	3777
	21st to end	Night	5% 8% 3 7% 4 4 5% 4 7% 8% 45%	4242
November	1st to 10th	Night	6% 8% 2 6% 6% 5% 6% 5% 8% 7% 41%	2535
	11th to 20th	Night	6% 7% 3 8% 6% 5% 6% 5% 8% 8% 39%	2795
	21st to end	Night	4 7% 2 5% 5% 4 6% 5% 7% 9% 46%	2800
December	1st to 10th	Night	3 6% 2 5% 4 3 4 3 5% 8% 56%	4560
	11th to 20th	Night	8% 12% 4 9% 7% 6% 6% 5% 7% 8% 26%	5176
	21st to end	Night	10% 10% 3 8% 6% 5% 5% 4 6% 7% 36%	6952

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	48% 11% 16% 18% 8%	3639
	11th to 20th	Day	34% 8% 20% 25% 11% 2	3640
	21st to end	Day	34% 9% 12% 24% 16% 5%	4344
February	1st to 10th	Day	42% 11% 15% 21% 9% 1	1800
	11th to 20th	Day	34% 12% 17% 23% 12% 2	1764
	21st to end	Day	37% 13% 18% 18% 13% 2	1640
March	1st to 10th	Day	32% 7% 11% 28% 21% 1	4601
	11th to 20th	Day	36% 9% 13% 25% 15%	5000
	21st to end	Day	38% 12% 20% 23% 7%	5701
April	1st to 10th	Day	41% 8% 18% 20% 10% 2	2400
	11th to 20th	Day	36% 8% 15% 22% 16% 3	2372
	21st to end	Day	30% 8% 19% 27% 14% 2	2481
May	1st to 10th	Day	25% 6% 9% 32% 26% 3	6293
	11th to 20th	Day	24% 7% 14% 28% 23% 4	6280
	21st to end	Day	23% 6% 20% 37% 13%	6904
June	1st to 10th	Day	33% 7% 14% 32% 13% 2	2800
	11th to 20th	Day	24% 7% 13% 33% 20% 3	2800
	21st to end	Day	17% 5% 11% 34% 25% 8%	2800
July	1st to 10th	Day	9% 3 4 18% 29% 38%	6251
	11th to 20th	Day	6% 2 4 9% 25% 53%	6245
	21st to end	Day	9% 3 4 29% 36% 20%	6681
August	1st to 10th	Day	14% 4 5% 18% 39% 20%	4286
	11th to 20th	Day	13% 5% 7% 33% 31% 13%	4195
	21st to end	Day	15% 3 7% 28% 32% 16%	4311
September	1st to 10th	Day	11% 4 5% 27% 34% 18%	2400
	11th to 20th	Day	21% 7% 10% 27% 24% 10%	2400
	21st to end	Day	23% 5% 11% 27% 24% 10%	2135
October	1st to 10th	Day	21% 5% 12% 22% 27% 14%	3211
	11th to 20th	Day	25% 7% 19% 27% 20% 3	3161
	21st to end	Day	32% 9% 10% 26% 20% 2	3308
November	1st to 10th	Day	35% 9% 18% 23% 14% 1	1764
	11th to 20th	Day	36% 14% 22% 16% 10% 1	1601
	21st to end	Day	34% 12% 17% 20% 15% 2	1600
December	1st to 10th	Day	25% 10% 14% 32% 19% 1	2620
	11th to 20th	Day	49% 9% 19% 16% 6%	3028
	21st to end	Day	45% 10% 14% 23% 7% 1	4004
January	1st to 10th	Night	46% 12% 19% 17% 6%	6306
	11th to 20th	Night	32% 8% 15% 30% 12% 2	6320
	21st to end	Night	39% 7% 11% 21% 16% 6%	6625
February	1st to 10th	Night	41% 10% 20% 17% 11% 1	2600
	11th to 20th	Night	37% 9% 20% 21% 11% 2	2534
	21st to end	Night	37% 10% 19% 22% 11% 2	1989
March	1st to 10th	Night	30% 10% 13% 28% 18% 1	5291
	11th to 20th	Night	27% 9% 17% 30% 16%	5000
	21st to end	Night	37% 13% 21% 23% 6%	5273
April	1st to 10th	Night	35% 9% 22% 22% 10% 1	2000
	11th to 20th	Night	31% 10% 19% 26% 12% 2	1979
	21st to end	Night	31% 9% 19% 27% 12% 2	1915
May	1st to 10th	Night	25% 5% 9% 33% 28% 1	3665
	11th to 20th	Night	21% 7% 18% 30% 22% 2	3616
	21st to end	Night	21% 6% 30% 35% 8%	3972
June	1st to 10th	Night	34% 7% 14% 30% 14% 1	1600
	11th to 20th	Night	22% 5% 16% 37% 17% 2	1600
	21st to end	Night	16% 4 11% 37% 25% 6%	1600
July	1st to 10th	Night	11% 2 5% 24% 25% 34%	3600
	11th to 20th	Night	6% 2 4 14% 25% 49%	3592
	21st to end	Night	10% 3 4 32% 33% 19%	3857
August	1st to 10th	Night	12% 3 4 21% 42% 17%	2830
	11th to 20th	Night	11% 4 8% 38% 31% 8%	2929
	21st to end	Night	12% 3 9% 30% 33% 13%	3607
September	1st to 10th	Night	11% 3 6% 29% 36% 16%	2000
	11th to 20th	Night	19% 6% 11% 28% 27% 9%	2000
	21st to end	Night	20% 4 10% 28% 26% 11%	2262
October	1st to 10th	Night	18% 4 9% 26% 33% 9%	3836
	11th to 20th	Night	23% 6% 22% 28% 17% 3	3777
	21st to end	Night	32% 10% 12% 30% 14% 2	4242
November	1st to 10th	Night	37% 9% 17% 21% 14% 3	2535
	11th to 20th	Night	37% 12% 19% 21% 10% 1	2795
	21st to end	Night	31% 10% 19% 20% 18% 3	2800
December	1st to 10th	Night	24% 7% 13% 34% 22% 1	4560
	11th to 20th	Night	47% 10% 18% 17% 7%	5176
	21st to end	Night	41% 10% 18% 22% 8% 1	6952

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	19% 17% 36% 20% 7% 2	200
	11th to 20th	Day	17% 14% 27% 27% 11% 6%	200
	21st to end	Day	13% 23% 24% 22% 14% 4	365
February	1st to 10th	Day	16% 25% 29% 22% 8% 2	400
	11th to 20th	Day	14% 29% 31% 18% 7% 1	395
	21st to end	Day	16% 24% 30% 20% 8% 1	409
March	1st to 10th	Day	14% 25% 31% 23% 6%	500
	11th to 20th	Day	15% 23% 30% 23% 9% 1	500
	21st to end	Day	18% 27% 31% 19% 5%	550
April	1st to 10th	Day	18% 21% 31% 18% 10% 2	500
	11th to 20th	Day	14% 26% 30% 20% 6% 3	492
	21st to end	Day	10% 27% 30% 22% 8% 3	500
May	1st to 10th	Day	14% 20% 30% 24% 9% 4	503
	11th to 20th	Day	17% 18% 27% 23% 12% 4	528
	21st to end	Day	11% 15% 29% 26% 14% 5%	573
June	1st to 10th	Day	10% 19% 26% 26% 13% 6%	530
	11th to 20th	Day	10% 23% 33% 23% 10% 2	530
	21st to end	Day	4 13% 30% 29% 18% 6%	530
July	1st to 10th	Day	3 10% 34% 33% 15% 6%	526
	11th to 20th	Day	1 10% 38% 26% 18% 7%	524
	21st to end	Day	2 7% 33% 34% 16% 9%	575
August	1st to 10th	Day	2 8% 29% 36% 17% 9%	527
	11th to 20th	Day	3 8% 26% 34% 19% 10%	517
	21st to end	Day	4 9% 30% 33% 14% 9%	583
September	1st to 10th	Day	4 8% 27% 33% 21% 7%	530
	11th to 20th	Day	6% 12% 23% 37% 17% 5%	530
	21st to end	Day	10% 14% 23% 31% 18% 4	530
October	1st to 10th	Day	8% 17% 27% 27% 18% 3	503
	11th to 20th	Day	9% 18% 23% 28% 19% 4	490
	21st to end	Day	12% 21% 29% 18% 16% 3	461
November	1st to 10th	Day	14% 15% 30% 27% 10% 4	300
	11th to 20th	Day	14% 22% 29% 18% 14% 4	200
	21st to end	Day	14% 17% 26% 23% 15% 6%	200
December	1st to 10th	Day	12% 19% 21% 23% 16% 11%	200
	11th to 20th	Day	11% 22% 34% 25% 9% 1	200
	21st to end	Day	14% 22% 28% 24% 10% 2	220
January	1st to 10th	Night	13% 17% 31% 24% 13% 2	800
	11th to 20th	Night	12% 15% 28% 27% 16% 3	800
	21st to end	Night	12% 11% 27% 31% 14% 4	880
February	1st to 10th	Night	9% 18% 30% 31% 11% 2	800
	11th to 20th	Night	9% 15% 32% 31% 12% 1	784
	21st to end	Night	11% 17% 32% 32% 8% 1	660
March	1st to 10th	Night	10% 19% 31% 27% 12% 1	800
	11th to 20th	Night	7% 18% 30% 32% 13% 1	800
	21st to end	Night	8% 20% 34% 27% 10% 1	880
April	1st to 10th	Night	9% 17% 29% 31% 13% 2	800
	11th to 20th	Night	7% 15% 34% 27% 15% 3	791
	21st to end	Night	7% 12% 31% 35% 14% 2	715
May	1st to 10th	Night	7% 11% 27% 34% 20% 2	415
	11th to 20th	Night	8% 9% 23% 38% 21% 1	394
	21st to end	Night	7% 7% 21% 40% 23% 2	433
June	1st to 10th	Night	12% 7% 22% 38% 20% 2	400
	11th to 20th	Night	6% 7% 25% 43% 19% 1	400
	21st to end	Night	3 3 21% 49% 24% 1	400
July	1st to 10th	Night	2 2 14% 54% 27% 2	390
	11th to 20th	Night	1 1 16% 58% 23% 1	388
	21st to end	Night	1 1 11% 52% 34% 1	438
August	1st to 10th	Night	1 1 10% 46% 37% 5%	536
	11th to 20th	Night	2 2 8% 39% 42% 8%	594
	21st to end	Night	2 3 19% 45% 29% 2	879
September	1st to 10th	Night	2 2 18% 43% 32% 3	800
	11th to 20th	Night	4 3 17% 39% 32% 5%	800
	21st to end	Night	7% 5% 20% 35% 28% 5%	800
October	1st to 10th	Night	6% 5% 25% 36% 23% 4	772
	11th to 20th	Night	6% 12% 21% 34% 23% 5%	760
	21st to end	Night	8% 11% 29% 30% 14% 6%	836
November	1st to 10th	Night	8% 11% 31% 36% 13% 1	780
	11th to 20th	Night	11% 17% 31% 28% 12% 2	800
	21st to end	Night	8% 15% 27% 32% 15% 3	800
December	1st to 10th	Night	8% 12% 28% 32% 17% 3	800
	11th to 20th	Night	10% 16% 32% 28% 13% 2	800
	21st to end	Night	12% 22% 31% 25% 10% 1	880

Page generated at 2025-09-30 22:39:03. Version ac07

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