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 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	5% 5% 2 4 5% 3 5% 5% 6% 8% 53%	74751
	Night	5% 6% 2 5% 5% 3 5% 5% 6% 8% 51%	74978
Transparency	Day	29% 8% 14% 24% 19% 7%	74454
	Night	30% 8% 16% 24% 17% 5%	74796
Seeing	Day	10% 18% 29% 26% 13% 4	13541
	Night	8% 12% 25% 34% 19% 3	22705

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	6% 8% 3 6% 7% 5% 8% 7% 9% 12% 30%	1519
	11th to 20th	Day	4 8% 4 5% 5% 4 8% 6% 6% 12% 37%	1448
	21st to end	Day	4 10% 4 6% 6% 3 6% 5% 6% 10% 41%	1714
February	1st to 10th	Day	5% 8% 3 6% 7% 4 8% 6% 7% 10% 37%	1620
	11th to 20th	Day	4 7% 3 4 6% 4 6% 7% 8% 9% 43%	1575
	21st to end	Day	3 8% 5% 7% 5% 4 6% 7% 7% 9% 39%	1471
March	1st to 10th	Day	4 8% 3 7% 7% 4 7% 6% 7% 9% 38%	1831
	11th to 20th	Day	4 8% 2 6% 6% 4 6% 7% 8% 10% 39%	1980
	21st to end	Day	5% 8% 3 6% 8% 5% 7% 7% 7% 9% 34%	2264
April	1st to 10th	Day	7% 8% 3 6% 7% 4 6% 7% 7% 7% 37%	2160
	11th to 20th	Day	5% 8% 3 7% 6% 4 5% 5% 7% 8% 43%	2129
	21st to end	Day	4 5% 2 5% 5% 3 4 6% 7% 10% 50%	2232
May	1st to 10th	Day	5% 6% 2 5% 4 3 4 5% 6% 8% 52%	2506
	11th to 20th	Day	8% 5% 2 5% 5% 4 6% 5% 6% 8% 45%	2477
	21st to end	Day	9% 6% 2 5% 5% 3 5% 6% 5% 7% 47%	2723
June	1st to 10th	Day	9% 4 2 5% 5% 3 4 5% 6% 8% 48%	2520
	11th to 20th	Day	6% 3 2 4 4 3 5% 5% 5% 7% 57%	2520
	21st to end	Day	4 2 1 3 3 2 3 4 4 6% 68%	2520
July	1st to 10th	Day	2 1 2 2 1 3 3 3 4 78%	2520
	11th to 20th	Day	2 1 1 1 2 2 3 3 3 4 78%	2520
	21st to end	Day	1 2 1 1 2 1 2 2 3 3 82%	2741
August	1st to 10th	Day	1 1 1 2 3 2 3 4 3 6% 75%	2380
	11th to 20th	Day	2 1 2 3 2 3 3 4 4 77%	2298
	21st to end	Day	2 2 1 1 3 1 2 2 2 5% 78%	2376
September	1st to 10th	Day	3 1 1 1 1 2 2 2 5% 81%	2160
	11th to 20th	Day	3 3 1 3 4 3 4 5% 5% 7% 62%	2160
	21st to end	Day	7% 4 1 3 4 3 3 4 5% 6% 60%	1923
October	1st to 10th	Day	7% 5% 2 5% 5% 4 4 4 5% 5% 55%	1800
	11th to 20th	Day	6% 5% 2 3 4 3 6% 6% 5% 8% 53%	1800
	21st to end	Day	5% 6% 2 6% 6% 4 6% 6% 6% 6% 48%	1907
November	1st to 10th	Day	4 7% 3 7% 6% 3 6% 6% 8% 9% 41%	1674
	11th to 20th	Day	5% 7% 3 7% 6% 4 7% 9% 10% 9% 34%	1521
	21st to end	Day	6% 7% 3 4 5% 4 7% 6% 8% 10% 39%	1520
December	1st to 10th	Day	4 9% 3 5% 5% 4 6% 6% 8% 8% 42%	1520
	11th to 20th	Day	6% 8% 4 8% 7% 4 6% 7% 8% 11% 30%	1928
	21st to end	Day	7% 10% 4 7% 7% 4 6% 6% 8% 9% 33%	2794
January	1st to 10th	Night	5% 8% 3 7% 7% 5% 6% 7% 8% 11% 32%	2646
	11th to 20th	Night	4 7% 3 7% 6% 4 6% 6% 7% 11% 39%	2540
	21st to end	Night	5% 9% 3 7% 6% 3 6% 7% 7% 8% 40%	2640
February	1st to 10th	Night	5% 8% 3 7% 6% 3 7% 6% 7% 9% 38%	2340
	11th to 20th	Night	4 9% 3 5% 6% 4 5% 6% 6% 8% 44%	2272
	21st to end	Night	4 8% 4 7% 7% 4 4 5% 8% 9% 40%	1784
March	1st to 10th	Night	6% 8% 3 5% 6% 4 6% 6% 7% 10% 39%	2127
	11th to 20th	Night	4 7% 2 5% 6% 3 6% 6% 9% 10% 41%	1980
	21st to end	Night	5% 8% 3 6% 7% 4 7% 7% 8% 10% 36%	2091
April	1st to 10th	Night	7% 8% 3 6% 4 3 6% 6% 6% 8% 43%	1800
	11th to 20th	Night	5% 8% 2 5% 5% 2 6% 6% 6% 7% 46%	1779
	21st to end	Night	6% 7% 2 4 4 3 5% 5% 7% 9% 47%	1724
May	1st to 10th	Night	5% 6% 1 4 5% 2 5% 4 5% 9% 55%	1454
	11th to 20th	Night	6% 4 2 4 5% 3 5% 6% 5% 9% 52%	1416
	21st to end	Night	8% 4 2 4 5% 3 5% 5% 6% 8% 50%	1552
June	1st to 10th	Night	11% 4 2 5% 5% 2 4 4 5% 7% 50%	1440
	11th to 20th	Night	5% 3 1 3 4 2 3 4 5% 8% 61%	1440
	21st to end	Night	3 2 2 2 2 5% 4 4 5% 71%	1440
July	1st to 10th	Night	2 1 1 1 2 1 2 3 3 5% 79%	1440
	11th to 20th	Night	1 1 1 2 2 1 1 3 3 4 82%	1440
	21st to end	Night	1 1 1 1 1 1 1 2 2 4 85%	1576
August	1st to 10th	Night	2 1 1 1 1 3 2 4 6% 78%	1556
	11th to 20th	Night	1 1 1 2 2 3 2 3 4 79%	1602
	21st to end	Night	2 1 1 2 1 2 2 4 4 80%	1979
September	1st to 10th	Night	2 1 1 1 1 2 2 3 5% 83%	1800
	11th to 20th	Night	4 2 1 2 3 2 4 4 5% 6% 67%	1800
	21st to end	Night	6% 4 1 2 3 2 3 4 5% 6% 64%	2034
October	1st to 10th	Night	6% 5% 2 4 3 2 4 4 6% 6% 59%	2160
	11th to 20th	Night	5% 4 1 3 4 3 4 4 5% 7% 58%	2160
	21st to end	Night	7% 7% 1 5% 5% 3 4 5% 6% 6% 50%	2448
November	1st to 10th	Night	7% 7% 2 5% 6% 3 7% 6% 7% 7% 41%	2405
	11th to 20th	Night	6% 6% 3 6% 6% 4 6% 6% 7% 9% 40%	2655
	21st to end	Night	4 7% 2 4 5% 2 6% 6% 7% 9% 47%	2660
December	1st to 10th	Night	6% 8% 4 7% 6% 3 6% 5% 7% 9% 39%	2660
	11th to 20th	Night	6% 10% 4 8% 8% 4 7% 7% 7% 9% 30%	3276
	21st to end	Night	6% 9% 3 6% 7% 4 5% 6% 7% 8% 38%	4862

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	40% 14% 19% 18% 8% 1	1519
	11th to 20th	Day	38% 10% 17% 20% 8% 6%	1448
	21st to end	Day	37% 9% 15% 25% 11% 3	1714
February	1st to 10th	Day	41% 11% 16% 22% 9% 1	1620
	11th to 20th	Day	32% 12% 17% 23% 13% 3	1584
	21st to end	Day	38% 13% 18% 17% 12% 1	1471
March	1st to 10th	Day	41% 8% 18% 21% 11% 1	1831
	11th to 20th	Day	36% 11% 19% 25% 9%	1980
	21st to end	Day	42% 11% 19% 21% 7%	2264
April	1st to 10th	Day	43% 8% 18% 19% 10% 2	2160
	11th to 20th	Day	38% 9% 16% 20% 14% 3	2132
	21st to end	Day	30% 9% 19% 26% 14% 2	2232
May	1st to 10th	Day	30% 6% 15% 31% 17% 1	2506
	11th to 20th	Day	36% 9% 13% 28% 12% 1	2480
	21st to end	Day	38% 8% 16% 26% 11% 1	2724
June	1st to 10th	Day	35% 7% 14% 32% 11% 1	2520
	11th to 20th	Day	25% 7% 15% 34% 17% 3	2520
	21st to end	Day	18% 5% 11% 34% 25% 8%	2520
July	1st to 10th	Day	12% 4 9% 23% 29% 23%	2451
	11th to 20th	Day	12% 4 7% 14% 34% 29%	2445
	21st to end	Day	10% 2 3 15% 42% 27%	2589
August	1st to 10th	Day	13% 5% 7% 27% 33% 17%	2363
	11th to 20th	Day	12% 4 6% 25% 35% 19%	2298
	21st to end	Day	12% 3 6% 25% 41% 14%	2376
September	1st to 10th	Day	10% 4 5% 28% 33% 19%	2160
	11th to 20th	Day	22% 7% 10% 26% 25% 11%	2160
	21st to end	Day	24% 5% 11% 26% 23% 10%	1923
October	1st to 10th	Day	32% 6% 12% 20% 23% 8%	1800
	11th to 20th	Day	29% 7% 15% 25% 20% 5%	1800
	21st to end	Day	34% 8% 14% 22% 21% 2	1907
November	1st to 10th	Day	35% 8% 18% 23% 14% 1	1674
	11th to 20th	Day	35% 14% 23% 17% 10% 1	1521
	21st to end	Day	35% 12% 18% 18% 16% 2	1520
December	1st to 10th	Day	35% 13% 16% 24% 11% 1	1520
	11th to 20th	Day	42% 11% 21% 18% 8%	1928
	21st to end	Day	44% 11% 14% 24% 6% 1	2794
January	1st to 10th	Night	39% 14% 22% 17% 7%	2646
	11th to 20th	Night	36% 12% 16% 22% 8% 6%	2540
	21st to end	Night	38% 10% 16% 21% 12% 4	2640
February	1st to 10th	Night	39% 10% 21% 18% 10% 1	2340
	11th to 20th	Night	35% 9% 21% 23% 10% 2	2274
	21st to end	Night	38% 10% 18% 22% 11% 2	1784
March	1st to 10th	Night	37% 11% 18% 21% 11% 1	2127
	11th to 20th	Night	33% 10% 22% 27% 8% 1	1980
	21st to end	Night	40% 12% 20% 21% 7%	2091
April	1st to 10th	Night	37% 9% 21% 21% 11% 1	1800
	11th to 20th	Night	33% 10% 19% 24% 12% 2	1779
	21st to end	Night	33% 9% 18% 27% 12% 2	1724
May	1st to 10th	Night	28% 7% 15% 35% 15% 1	1454
	11th to 20th	Night	28% 9% 18% 31% 11% 1	1416
	21st to end	Night	32% 8% 21% 28% 10%	1552
June	1st to 10th	Night	35% 8% 15% 30% 11% 1	1440
	11th to 20th	Night	22% 6% 17% 38% 16% 2	1440
	21st to end	Night	16% 4 11% 37% 25% 7%	1440
July	1st to 10th	Night	11% 3 10% 27% 29% 21%	1400
	11th to 20th	Night	9% 3 7% 20% 33% 28%	1392
	21st to end	Night	8% 2 2 17% 45% 26%	1488
August	1st to 10th	Night	10% 3 6% 31% 36% 13%	1548
	11th to 20th	Night	10% 4 7% 28% 38% 14%	1602
	21st to end	Night	9% 3 7% 27% 41% 13%	1979
September	1st to 10th	Night	8% 3 5% 30% 37% 17%	1800
	11th to 20th	Night	19% 6% 10% 26% 29% 10%	1800
	21st to end	Night	21% 5% 11% 27% 26% 11%	2034
October	1st to 10th	Night	27% 6% 13% 24% 24% 7%	2160
	11th to 20th	Night	24% 6% 17% 26% 20% 6%	2160
	21st to end	Night	32% 9% 14% 21% 20% 3	2448
November	1st to 10th	Night	37% 9% 17% 21% 14% 3	2405
	11th to 20th	Night	36% 12% 20% 22% 9% 1	2655
	21st to end	Night	30% 10% 20% 19% 18% 3	2660
December	1st to 10th	Night	37% 10% 18% 22% 12% 1	2660
	11th to 20th	Night	43% 11% 18% 18% 9% 1	3276
	21st to end	Night	38% 10% 18% 24% 8% 1	4862

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	20% 18% 36% 17% 8% 2	180
	11th to 20th	Day	17% 14% 30% 24% 11% 4	162
	21st to end	Day	13% 25% 22% 22% 15% 3	306
February	1st to 10th	Day	17% 24% 28% 22% 8% 1	340
	11th to 20th	Day	15% 32% 29% 17% 7% 1	335
	21st to end	Day	17% 26% 30% 18% 8% 1	342
March	1st to 10th	Day	14% 26% 30% 23% 5%	420
	11th to 20th	Day	15% 24% 28% 23% 8% 1	420
	21st to end	Day	20% 26% 29% 20% 5%	462
April	1st to 10th	Day	19% 22% 29% 18% 10% 2	420
	11th to 20th	Day	16% 27% 30% 18% 6% 3	412
	21st to end	Day	11% 27% 30% 20% 9% 3	420
May	1st to 10th	Day	13% 19% 30% 25% 9% 4	423
	11th to 20th	Day	18% 17% 28% 22% 12% 4	448
	21st to end	Day	11% 14% 28% 27% 14% 5%	485
June	1st to 10th	Day	10% 20% 24% 26% 13% 5%	450
	11th to 20th	Day	10% 21% 33% 23% 11% 2	450
	21st to end	Day	4 12% 28% 31% 18% 6%	450
July	1st to 10th	Day	3 9% 35% 34% 14% 5%	446
	11th to 20th	Day	1 10% 38% 26% 19% 7%	444
	21st to end	Day	1 7% 33% 34% 16% 8%	487
August	1st to 10th	Day	1 7% 30% 38% 15% 9%	447
	11th to 20th	Day	3 7% 24% 35% 19% 11%	437
	21st to end	Day	3 9% 31% 34% 14% 9%	495
September	1st to 10th	Day	4 8% 27% 34% 20% 6%	450
	11th to 20th	Day	6% 11% 24% 38% 16% 4	450
	21st to end	Day	11% 15% 23% 32% 16% 2	450
October	1st to 10th	Day	8% 18% 27% 26% 18% 2	450
	11th to 20th	Day	9% 19% 22% 27% 19% 4	450
	21st to end	Day	12% 21% 29% 17% 18% 4	422
November	1st to 10th	Day	14% 14% 30% 27% 10% 4	270
	11th to 20th	Day	13% 19% 29% 18% 16% 4	180
	21st to end	Day	14% 19% 26% 23% 13% 6%	180
December	1st to 10th	Day	12% 19% 23% 22% 15% 8%	180
	11th to 20th	Day	11% 22% 34% 22% 9% 1	180
	21st to end	Day	14% 23% 28% 22% 11% 3	198
January	1st to 10th	Night	14% 17% 30% 23% 13% 3	760
	11th to 20th	Night	12% 15% 28% 26% 16% 4	726
	21st to end	Night	12% 11% 25% 32% 15% 5%	792
February	1st to 10th	Night	9% 18% 29% 31% 12% 2	720
	11th to 20th	Night	9% 16% 30% 31% 13% 1	704
	21st to end	Night	11% 17% 32% 32% 8% 1	592
March	1st to 10th	Night	11% 19% 32% 26% 12% 1	720
	11th to 20th	Night	7% 18% 28% 33% 13% 1	720
	21st to end	Night	8% 21% 32% 28% 11% 1	792
April	1st to 10th	Night	10% 17% 28% 30% 13% 2	720
	11th to 20th	Night	7% 16% 33% 25% 15% 3	711
	21st to end	Night	7% 12% 30% 34% 15% 2	644
May	1st to 10th	Night	6% 10% 27% 33% 21% 2	374
	11th to 20th	Night	8% 8% 21% 38% 23% 1	354
	21st to end	Night	8% 7% 22% 40% 23% 2	389
June	1st to 10th	Night	12% 7% 21% 38% 21% 2	360
	11th to 20th	Night	6% 6% 24% 42% 20% 1	360
	21st to end	Night	3 3 19% 48% 26% 1	360
July	1st to 10th	Night	2 2 11% 55% 29% 2	350
	11th to 20th	Night	1 1 16% 59% 22% 1	348
	21st to end	Night	1 11% 52% 34% 1	394
August	1st to 10th	Night	2 1 9% 44% 38% 6%	482
	11th to 20th	Night	2 2 7% 37% 44% 8%	534
	21st to end	Night	2 3 19% 45% 30% 2	791
September	1st to 10th	Night	1 3 18% 41% 33% 3	720
	11th to 20th	Night	4 3 16% 38% 33% 6%	720
	21st to end	Night	8% 5% 20% 35% 27% 5%	720
October	1st to 10th	Night	7% 5% 25% 35% 24% 4	720
	11th to 20th	Night	6% 12% 21% 33% 23% 5%	720
	21st to end	Night	9% 12% 28% 29% 15% 6%	792
November	1st to 10th	Night	9% 10% 31% 36% 13% 1	740
	11th to 20th	Night	11% 17% 29% 29% 12% 2	760
	21st to end	Night	8% 16% 27% 31% 15% 3	760
December	1st to 10th	Night	9% 13% 28% 32% 16% 3	760
	11th to 20th	Night	10% 16% 31% 28% 13% 2	760
	21st to end	Night	12% 22% 30% 25% 10% 1	836

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