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 2024-11-05.

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% 6% 2 5% 4 4 4 4 6% 7% 56%	105394
	Night	5% 6% 2 5% 4 4 4 4 6% 7% 52%	102314
Transparency	Day	26% 7% 13% 24% 20% 10%	105137
	Night	28% 8% 15% 25% 17% 6%	102160
Seeing	Day	10% 18% 29% 26% 13% 4	14670
	Night	8% 12% 25% 34% 19% 3	23687

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% 8% 3 6% 6% 7% 6% 6% 8% 11% 31%	2539
	11th to 20th	Day	10% 11% 4 9% 6% 6% 7% 4 6% 9% 30%	2540
	21st to end	Day	12% 14% 3 7% 5% 4 5% 3 6% 8% 32%	3022
February	1st to 10th	Day	5% 8% 3 8% 7% 7% 6% 5% 7% 10% 36%	1710
	11th to 20th	Day	5% 8% 2 5% 5% 5% 6% 6% 8% 9% 42%	1665
	21st to end	Day	3 9% 4 8% 4 5% 7% 5% 8% 9% 38%	1560
March	1st to 10th	Day	3 9% 3 7% 5% 4 6% 5% 9% 9% 40%	3195
	11th to 20th	Day	3 7% 1 4 4 4 5% 4 6% 7% 55%	3490
	21st to end	Day	5% 10% 2 6% 6% 6% 6% 5% 8% 9% 35%	3975
April	1st to 10th	Day	6% 10% 2 7% 7% 5% 6% 5% 7% 7% 38%	2280
	11th to 20th	Day	5% 8% 2 7% 6% 5% 4 4 7% 8% 43%	2249
	21st to end	Day	4 5% 1 5% 5% 4 5% 5% 7% 10% 49%	2357
May	1st to 10th	Day	9% 8% 2 4 3 3 4 4 5% 6% 52%	4406
	11th to 20th	Day	5% 3 1 3 3 3 4 3 4 5% 66%	4377
	21st to end	Day	7% 5% 1 5% 4 4 4 4 6% 7% 53%	4813
June	1st to 10th	Day	8% 6% 2 5% 4 4 4 4 7% 7% 49%	2660
	11th to 20th	Day	5% 3 1 4 3 4 4 4 6% 7% 58%	2660
	21st to end	Day	3 2 1 3 3 3 3 3 5% 6% 69%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 2 2 87%	4420
	11th to 20th	Day	2 1 1 2 2 2 2 3 3 84%	4420
	21st to end	Day	1 1 1 1 1 2 2 3 4 84%	4790
August	1st to 10th	Day	2 2 1 2 2 2 3 3 3 5% 74%	4169
	11th to 20th	Day	2 1 3 2 3 3 3 4 6% 73%	4026
	21st to end	Day	3 3 1 2 2 2 2 2 3 5% 74%	4180
September	1st to 10th	Day	3 1 1 1 1 2 2 3 5% 80%	2280
	11th to 20th	Day	3 3 1 4 3 4 5% 3 5% 7% 61%	2280
	21st to end	Day	7% 4 1 4 3 3 2 3 5% 6% 61%	2029
October	1st to 10th	Day	4 4 1 4 2 3 3 3 5% 5% 66%	3180
	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% 6% 6% 4 8% 8% 40%	1710
	11th to 20th	Day	5% 8% 2 7% 5% 6% 7% 8% 10% 9% 33%	1521
	21st to end	Day	6% 7% 3 5% 5% 6% 6% 5% 8% 10% 39%	1520
December	1st to 10th	Day	4 9% 3 6% 4 5% 6% 5% 8% 8% 42%	1520
	11th to 20th	Day	6% 9% 3 9% 6% 5% 6% 6% 9% 10% 30%	1928
	21st to end	Day	7% 10% 3 8% 7% 5% 6% 5% 7% 9% 32%	2794
January	1st to 10th	Night	5% 10% 3 10% 6% 6% 6% 6% 9% 11% 29%	4406
	11th to 20th	Night	10% 12% 4 8% 6% 6% 5% 4 5% 7% 32%	4420
	21st to end	Night	13% 12% 4 9% 5% 5% 5% 5% 6% 7% 30%	4639
February	1st to 10th	Night	5% 9% 3 8% 6% 5% 6% 5% 7% 8% 38%	2470
	11th to 20th	Night	5% 11% 3 6% 5% 5% 5% 5% 6% 7% 43%	2402
	21st to end	Night	4 8% 4 8% 6% 5% 4 4 8% 9% 40%	1893
March	1st to 10th	Night	3 9% 2 5% 5% 5% 5% 6% 10% 10% 40%	3677
	11th to 20th	Night	2 6% 1 4 4 3 3 4 7% 7% 59%	3490
	21st to end	Night	6% 10% 2 7% 5% 6% 6% 5% 8% 9% 34%	3677
April	1st to 10th	Night	7% 8% 3 6% 4 4 6% 4 6% 8% 44%	1900
	11th to 20th	Night	5% 8% 2 6% 4 5% 5% 5% 7% 7% 46%	1879
	21st to end	Night	6% 7% 2 5% 4 4 5% 5% 6% 9% 47%	1819
May	1st to 10th	Night	11% 7% 1 4 3 3 3 2 4 6% 55%	2554
	11th to 20th	Night	3 3 1 3 3 3 3 3 4 6% 68%	2516
	21st to end	Night	5% 5% 1 3 3 3 4 3 5% 7% 60%	2762
June	1st to 10th	Night	11% 5% 2 5% 5% 4 4 3 6% 6% 49%	1520
	11th to 20th	Night	5% 4 1 4 4 3 3 3 5% 7% 61%	1520
	21st to end	Night	3 2 2 2 2 4 3 4 5% 71%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	2540
	11th to 20th	Night	2 1 2 1 1 1 2 3 3 83%	2540
	21st to end	Night	1 1 1 2 1 1 1 1 2 3 85%	2761
August	1st to 10th	Night	2 2 1 2 1 3 3 2 4 5% 75%	2751
	11th to 20th	Night	1 1 1 2 2 2 3 2 4 5% 77%	2812
	21st to end	Night	4 2 2 1 2 2 2 4 4 78%	3497
September	1st to 10th	Night	2 1 1 1 2 2 3 5% 83%	1900
	11th to 20th	Night	4 3 1 2 3 4 3 4 5% 7% 65%	1900
	21st to end	Night	6% 4 1 3 3 2 3 3 5% 6% 65%	2148
October	1st to 10th	Night	3 3 1 3 2 3 3 2 4 4 71%	3800
	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	7% 8% 2 6% 6% 5% 6% 5% 7% 7% 41%	2469
	11th to 20th	Night	6% 7% 2 8% 6% 5% 6% 5% 8% 9% 39%	2655
	21st to end	Night	4 7% 2 5% 5% 4 6% 5% 7% 9% 46%	2660
December	1st to 10th	Night	6% 9% 3 8% 6% 4 5% 4 7% 9% 39%	2660
	11th to 20th	Night	6% 11% 4 9% 7% 6% 6% 6% 7% 9% 30%	3276
	21st to end	Night	6% 9% 3 7% 7% 5% 5% 5% 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	41% 11% 18% 20% 9%	2539
	11th to 20th	Day	45% 9% 24% 13% 5% 3	2540
	21st to end	Day	44% 10% 17% 20% 7% 2	3022
February	1st to 10th	Day	42% 11% 16% 22% 9% 1	1710
	11th to 20th	Day	33% 12% 18% 23% 12% 3	1674
	21st to end	Day	37% 13% 18% 18% 12% 1	1560
March	1st to 10th	Day	31% 8% 13% 25% 22% 1	3195
	11th to 20th	Day	27% 8% 15% 32% 18%	3490
	21st to end	Day	37% 10% 20% 25% 7%	3975
April	1st to 10th	Day	42% 8% 18% 20% 10% 2	2280
	11th to 20th	Day	38% 9% 16% 21% 14% 3	2252
	21st to end	Day	30% 9% 19% 27% 14% 2	2357
May	1st to 10th	Day	31% 6% 11% 30% 22% 1	4406
	11th to 20th	Day	21% 6% 8% 29% 31% 5%	4380
	21st to end	Day	30% 7% 20% 32% 11%	4814
June	1st to 10th	Day	34% 7% 14% 31% 12% 1	2660
	11th to 20th	Day	24% 7% 14% 33% 18% 3	2660
	21st to end	Day	17% 5% 11% 34% 25% 8%	2660
July	1st to 10th	Day	7% 3 5% 13% 20% 52%	4351
	11th to 20th	Day	8% 3 4 8% 24% 52%	4345
	21st to end	Day	7% 3 4 17% 40% 29%	4639
August	1st to 10th	Day	14% 4 5% 17% 40% 21%	4153
	11th to 20th	Day	12% 5% 7% 33% 31% 13%	4065
	21st to end	Day	15% 3 7% 27% 32% 16%	4179
September	1st to 10th	Day	10% 4 5% 28% 34% 19%	2280
	11th to 20th	Day	22% 7% 10% 27% 24% 10%	2280
	21st to end	Day	23% 5% 11% 27% 23% 10%	2029
October	1st to 10th	Day	20% 5% 12% 22% 27% 14%	3180
	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	1710
	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	38% 13% 22% 19% 7%	4406
	11th to 20th	Night	44% 10% 20% 17% 6% 4	4420
	21st to end	Night	50% 9% 15% 17% 7% 2	4639
February	1st to 10th	Night	40% 9% 21% 18% 11% 1	2470
	11th to 20th	Night	36% 8% 21% 22% 11% 2	2404
	21st to end	Night	37% 10% 19% 22% 11% 2	1893
March	1st to 10th	Night	30% 12% 14% 26% 17% 1	3677
	11th to 20th	Night	21% 8% 17% 38% 16%	3490
	21st to end	Night	40% 10% 20% 23% 7%	3677
April	1st to 10th	Night	36% 9% 22% 21% 11% 1	1900
	11th to 20th	Night	33% 10% 19% 25% 12% 2	1879
	21st to end	Night	32% 9% 18% 27% 12% 2	1819
May	1st to 10th	Night	31% 6% 11% 37% 16% 1	2554
	11th to 20th	Night	17% 6% 12% 34% 30% 2	2516
	21st to end	Night	23% 8% 31% 31% 7%	2762
June	1st to 10th	Night	36% 8% 14% 30% 12% 1	1520
	11th to 20th	Night	22% 5% 17% 37% 16% 2	1520
	21st to end	Night	16% 4 11% 37% 26% 7%	1520
July	1st to 10th	Night	6% 2 6% 15% 24% 47%	2500
	11th to 20th	Night	9% 2 5% 14% 22% 49%	2492
	21st to end	Night	7% 2 4 19% 41% 27%	2674
August	1st to 10th	Night	12% 3 4 20% 43% 18%	2743
	11th to 20th	Night	11% 4 8% 38% 31% 8%	2839
	21st to end	Night	12% 3 9% 30% 33% 13%	3497
September	1st to 10th	Night	8% 3 5% 30% 37% 16%	1900
	11th to 20th	Night	19% 6% 11% 27% 27% 10%	1900
	21st to end	Night	20% 4 11% 29% 26% 10%	2148
October	1st to 10th	Night	18% 4 9% 26% 33% 9%	3800
	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% 13% 3	2469
	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	21% 17% 36% 17% 8% 2	180
	11th to 20th	Day	19% 14% 27% 25% 11% 4	180
	21st to end	Day	13% 25% 24% 21% 14% 3	335
February	1st to 10th	Day	16% 24% 28% 22% 8% 2	370
	11th to 20th	Day	15% 32% 30% 16% 7% 1	365
	21st to end	Day	17% 25% 30% 19% 8% 1	377
March	1st to 10th	Day	14% 26% 31% 23% 6%	460
	11th to 20th	Day	15% 23% 28% 24% 9% 1	460
	21st to end	Day	19% 27% 31% 19% 4	506
April	1st to 10th	Day	18% 21% 31% 18% 10% 2	460
	11th to 20th	Day	15% 27% 30% 19% 6% 3	452
	21st to end	Day	10% 28% 30% 21% 8% 3	460
May	1st to 10th	Day	14% 20% 30% 23% 8% 4	463
	11th to 20th	Day	17% 16% 27% 23% 12% 5%	488
	21st to end	Day	12% 14% 28% 27% 14% 5%	529
June	1st to 10th	Day	11% 20% 25% 26% 13% 5%	490
	11th to 20th	Day	10% 22% 32% 23% 11% 2	490
	21st to end	Day	4 13% 30% 30% 18% 6%	490
July	1st to 10th	Day	3 9% 35% 33% 14% 6%	486
	11th to 20th	Day	1 10% 38% 25% 19% 7%	484
	21st to end	Day	1 8% 33% 33% 17% 8%	531
August	1st to 10th	Day	2 7% 30% 37% 15% 9%	487
	11th to 20th	Day	3 8% 26% 34% 19% 11%	477
	21st to end	Day	4 10% 30% 33% 14% 9%	539
September	1st to 10th	Day	4 8% 27% 34% 21% 7%	490
	11th to 20th	Day	7% 11% 24% 37% 17% 4	490
	21st to end	Day	10% 14% 24% 32% 17% 3	490
October	1st to 10th	Day	8% 17% 27% 27% 18% 3	490
	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% 31% 27% 10% 4	282
	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	13% 18% 30% 23% 13% 3	760
	11th to 20th	Night	12% 15% 28% 26% 16% 4	760
	21st to end	Night	12% 12% 26% 31% 14% 4	836
February	1st to 10th	Night	9% 18% 29% 31% 11% 2	760
	11th to 20th	Night	9% 16% 31% 31% 12% 1	744
	21st to end	Night	11% 17% 32% 32% 8% 1	628
March	1st to 10th	Night	10% 20% 31% 26% 12% 1	760
	11th to 20th	Night	7% 17% 28% 33% 13% 1	760
	21st to end	Night	8% 21% 33% 27% 10% 1	836
April	1st to 10th	Night	9% 18% 28% 30% 13% 2	760
	11th to 20th	Night	7% 16% 34% 26% 15% 3	751
	21st to end	Night	7% 12% 30% 34% 14% 2	679
May	1st to 10th	Night	7% 11% 27% 33% 20% 2	394
	11th to 20th	Night	8% 9% 21% 39% 22% 1	374
	21st to end	Night	8% 7% 21% 40% 22% 1	411
June	1st to 10th	Night	12% 7% 22% 38% 19% 2	380
	11th to 20th	Night	6% 7% 24% 42% 19% 1	380
	21st to end	Night	3 3 20% 48% 25% 1	380
July	1st to 10th	Night	2 2 12% 54% 28% 2	370
	11th to 20th	Night	1 1 16% 59% 22% 1	368
	21st to end	Night	1 12% 52% 34% 1	416
August	1st to 10th	Night	2 1 9% 45% 37% 5%	509
	11th to 20th	Night	2 2 8% 38% 43% 8%	564
	21st to end	Night	2 3 19% 44% 29% 3	835
September	1st to 10th	Night	2 2 18% 42% 33% 3	760
	11th to 20th	Night	4 3 17% 39% 32% 5%	760
	21st to end	Night	8% 5% 20% 35% 27% 5%	760
October	1st to 10th	Night	6% 5% 25% 36% 23% 4	760
	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	9% 11% 31% 35% 13% 1	760
	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

Page generated at 2024-11-02 23:11:25. Version ac07

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