Sitkum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sitkum clear sky chart from 2005-11-06 to 2024-11-01.

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	18% 10% 2 6% 4 3 3 3 4 6% 40%	101416
	Night	21% 11% 3 6% 4 3 3 3 4 5% 36%	97721
Transparency	Day	43% 7% 12% 18% 14% 6%	101169
	Night	48% 8% 12% 16% 11% 5%	97558
Seeing	Day	24% 11% 26% 24% 12% 3	14917
	Night	25% 6% 14% 29% 23% 3	23872

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	32% 24% 4 7% 4 3 2 2 4 5% 13%	2539
	11th to 20th	Day	38% 16% 4 6% 4 3 2 2 3 4 18%	2736
	21st to end	Day	37% 13% 3 6% 4 3 4 3 4 4 18%	3146
February	1st to 10th	Day	28% 16% 3 7% 4 5% 4 4 4 6% 18%	1710
	11th to 20th	Day	28% 17% 3 7% 4 3 4 3 4 4 21%	1665
	21st to end	Day	29% 22% 3 6% 4 4 3 3 3 5% 18%	1470
March	1st to 10th	Day	23% 24% 3 6% 4 4 4 3 4 5% 21%	3437
	11th to 20th	Day	21% 13% 2 7% 4 3 3 2 4 4 37%	3490
	21st to end	Day	30% 18% 3 9% 6% 4 5% 3 4 4 14%	3825
April	1st to 10th	Day	30% 17% 4 9% 5% 4 4 3 3 4 17%	2267
	11th to 20th	Day	26% 18% 3 8% 6% 3 3 3 4 5% 22%	2253
	21st to end	Day	28% 11% 3 6% 4 4 4 4 5% 6% 24%	2470
May	1st to 10th	Day	22% 12% 3 7% 4 4 4 3 4 5% 32%	4110
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 5% 7% 51%	4376
	21st to end	Day	16% 8% 3 6% 5% 4 5% 4 6% 8% 35%	4813
June	1st to 10th	Day	23% 11% 2 6% 4 4 3 4 5% 6% 31%	2660
	11th to 20th	Day	17% 9% 2 6% 5% 4 4 4 5% 8% 35%	2660
	21st to end	Day	10% 5% 2 4 3 3 3 3 5% 6% 56%	2660
July	1st to 10th	Day	3 2 1 3 2 2 2 2 3 4 78%	4420
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 4 73%	4420
	21st to end	Day	3 3 1 3 2 3 2 3 4 5% 71%	4500
August	1st to 10th	Day	2 3 2 4 3 3 3 3 5% 7% 65%	4091
	11th to 20th	Day	4 4 1 5% 3 2 3 3 5% 6% 64%	4026
	21st to end	Day	7% 4 1 4 2 2 3 2 4 7% 64%	4314
September	1st to 10th	Day	6% 3 1 3 3 2 3 2 4 7% 66%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 6% 6% 48%	2203
	21st to end	Day	13% 5% 2 5% 3 3 3 3 5% 8% 50%	2043
October	1st to 10th	Day	17% 6% 2 6% 4 3 3 2 5% 5% 48%	1900
	11th to 20th	Day	19% 7% 2 6% 4 4 4 4 5% 7% 39%	1900
	21st to end	Day	23% 9% 2 6% 4 4 4 3 5% 7% 33%	2090
November	1st to 10th	Day	30% 12% 3 9% 4 4 4 3 5% 5% 21%	1660
	11th to 20th	Day	31% 12% 3 9% 4 4 4 2 5% 6% 21%	1520
	21st to end	Day	29% 10% 3 6% 4 3 3 3 5% 6% 27%	1520
December	1st to 10th	Day	23% 13% 3 7% 4 3 4 3 6% 6% 28%	1520
	11th to 20th	Day	29% 13% 3 8% 5% 4 4 4 5% 6% 18%	1928
	21st to end	Day	34% 18% 4 7% 4 3 4 2 4 4 16%	2794
January	1st to 10th	Night	36% 20% 4 7% 5% 4 4 3 3 3 12%	4406
	11th to 20th	Night	38% 16% 3 7% 3 3 3 2 3 4 18%	4229
	21st to end	Night	44% 13% 3 5% 3 3 3 2 3 5% 18%	4521
February	1st to 10th	Night	27% 14% 3 8% 5% 4 4 4 5% 5% 20%	2470
	11th to 20th	Night	26% 15% 4 7% 4 4 3 3 4 5% 26%	2402
	21st to end	Night	27% 18% 4 6% 4 4 3 3 4 4 23%	1984
March	1st to 10th	Night	22% 22% 4 7% 4 4 3 3 4 5% 22%	3436
	11th to 20th	Night	19% 14% 4 6% 3 3 3 2 3 3 39%	3490
	21st to end	Night	28% 21% 4 7% 4 3 4 3 3 5% 17%	3828
April	1st to 10th	Night	28% 17% 3 7% 4 4 5% 3 4 5% 21%	1912
	11th to 20th	Night	23% 16% 4 7% 3 3 3 3 5% 5% 28%	1873
	21st to end	Night	23% 13% 3 6% 5% 5% 4 5% 4 6% 26%	1710
May	1st to 10th	Night	20% 15% 3 7% 3 2 3 3 4 4 36%	2860
	11th to 20th	Night	10% 8% 2 6% 4 4 4 3 4 6% 49%	2516
	21st to end	Night	16% 11% 3 6% 4 3 3 2 4 7% 41%	2762
June	1st to 10th	Night	20% 9% 2 6% 5% 4 3 3 4 7% 37%	1520
	11th to 20th	Night	11% 8% 3 6% 4 3 3 4 6% 7% 46%	1520
	21st to end	Night	8% 5% 2 4 3 3 4 3 4 6% 57%	1520
July	1st to 10th	Night	2 2 1 3 2 2 2 2 2 4 77%	2540
	11th to 20th	Night	3 2 1 3 3 4 3 2 3 5% 70%	2540
	21st to end	Night	4 5% 3 7% 4 2 3 3 3 4 62%	3045
August	1st to 10th	Night	1 3 3 7% 5% 6% 6% 4 5% 6% 53%	2848
	11th to 20th	Night	3 4 3 9% 5% 5% 4 4 5% 6% 52%	2812
	21st to end	Night	6% 5% 1 3 2 3 2 2 3 4 68%	3359
September	1st to 10th	Night	4 4 1 3 2 2 3 3 4 5% 68%	1900
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 5% 6% 55%	1974
	21st to end	Night	12% 6% 2 5% 4 3 3 3 4 6% 54%	2136
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 5% 52%	2280
	11th to 20th	Night	18% 7% 2 6% 4 4 4 4 5% 6% 41%	2280
	21st to end	Night	24% 10% 3 7% 4 4 3 3 4 4 36%	2508
November	1st to 10th	Night	29% 10% 3 7% 3 4 4 3 5% 7% 26%	2422
	11th to 20th	Night	32% 11% 4 7% 4 4 3 4 4 6% 21%	2660
	21st to end	Night	26% 10% 4 6% 4 4 4 4 4 5% 31%	2660
December	1st to 10th	Night	26% 11% 3 6% 4 4 4 4 5% 5% 28%	2660
	11th to 20th	Night	32% 12% 3 7% 4 4 5% 4 5% 7% 18%	3276
	21st to end	Night	31% 16% 4 8% 5% 4 3 3 4 4 20%	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	72% 8% 10% 6% 3 1	2539
	11th to 20th	Day	71% 5% 6% 8% 5% 5%	2736
	21st to end	Day	65% 9% 9% 8% 6% 3	3146
February	1st to 10th	Day	64% 8% 12% 10% 5% 1	1710
	11th to 20th	Day	65% 7% 11% 10% 5% 2	1674
	21st to end	Day	68% 6% 9% 10% 5% 1	1470
March	1st to 10th	Day	61% 9% 12% 16% 3	3437
	11th to 20th	Day	48% 9% 9% 18% 14% 2	3490
	21st to end	Day	67% 13% 9% 8% 3	3825
April	1st to 10th	Day	69% 8% 9% 10% 5%	2267
	11th to 20th	Day	64% 7% 10% 10% 7% 2	2256
	21st to end	Day	57% 9% 12% 14% 7% 1	2470
May	1st to 10th	Day	49% 10% 13% 17% 9% 2	4110
	11th to 20th	Day	30% 7% 17% 29% 17% 1	4379
	21st to end	Day	41% 12% 28% 17% 2	4814
June	1st to 10th	Day	52% 7% 15% 20% 5%	2660
	11th to 20th	Day	45% 9% 17% 21% 7% 1	2660
	21st to end	Day	28% 6% 17% 32% 15% 3	2660
July	1st to 10th	Day	12% 3 8% 17% 23% 36%	4351
	11th to 20th	Day	16% 2 6% 18% 27% 31%	4345
	21st to end	Day	14% 4 11% 23% 38% 10%	4358
August	1st to 10th	Day	18% 4 12% 23% 34% 8%	4076
	11th to 20th	Day	21% 4 14% 37% 22% 3	4065
	21st to end	Day	20% 4 15% 27% 27% 6%	4313
September	1st to 10th	Day	19% 3 9% 28% 28% 13%	2280
	11th to 20th	Day	32% 7% 14% 24% 17% 5%	2203
	21st to end	Day	31% 6% 14% 23% 19% 6%	2043
October	1st to 10th	Day	39% 6% 9% 20% 17% 9%	1900
	11th to 20th	Day	42% 7% 13% 17% 16% 4	1900
	21st to end	Day	49% 8% 11% 13% 13% 5%	2090
November	1st to 10th	Day	63% 7% 9% 12% 8% 1	1660
	11th to 20th	Day	65% 6% 10% 8% 7% 4	1520
	21st to end	Day	57% 6% 10% 9% 9% 8%	1520
December	1st to 10th	Day	54% 8% 11% 14% 12% 2	1520
	11th to 20th	Day	63% 10% 11% 10% 5% 1	1928
	21st to end	Day	70% 9% 8% 9% 2 2	2794
January	1st to 10th	Night	74% 9% 7% 6% 3 2	4406
	11th to 20th	Night	68% 8% 8% 7% 4 5%	4229
	21st to end	Night	70% 5% 6% 9% 7% 2	4521
February	1st to 10th	Night	62% 9% 11% 11% 6% 1	2470
	11th to 20th	Night	60% 8% 10% 14% 7% 2	2404
	21st to end	Night	63% 8% 9% 12% 6% 2	1984
March	1st to 10th	Night	59% 11% 13% 14% 3	3436
	11th to 20th	Night	44% 12% 12% 18% 12% 2	3490
	21st to end	Night	64% 12% 13% 8% 2	3828
April	1st to 10th	Night	62% 11% 10% 13% 4	1912
	11th to 20th	Night	56% 10% 14% 13% 5% 3	1873
	21st to end	Night	54% 9% 15% 16% 5% 1	1710
May	1st to 10th	Night	45% 11% 16% 17% 9% 1	2860
	11th to 20th	Night	31% 9% 18% 28% 12% 2	2516
	21st to end	Night	38% 11% 34% 15% 1	2762
June	1st to 10th	Night	45% 9% 17% 25% 4	1520
	11th to 20th	Night	33% 10% 22% 27% 8% 1	1520
	21st to end	Night	23% 8% 18% 32% 15% 4	1520
July	1st to 10th	Night	11% 4 11% 16% 24% 34%	2500
	11th to 20th	Night	15% 2 9% 18% 24% 31%	2492
	21st to end	Night	25% 2 11% 21% 31% 10%	2950
August	1st to 10th	Night	27% 4 14% 22% 25% 7%	2839
	11th to 20th	Night	30% 7% 15% 28% 16% 4	2839
	21st to end	Night	19% 5% 11% 32% 27% 6%	3359
September	1st to 10th	Night	17% 5% 10% 27% 26% 14%	1900
	11th to 20th	Night	28% 8% 15% 26% 18% 6%	1974
	21st to end	Night	31% 6% 14% 22% 18% 10%	2136
October	1st to 10th	Night	32% 8% 12% 20% 18% 10%	2280
	11th to 20th	Night	41% 7% 14% 19% 16% 3	2280
	21st to end	Night	50% 7% 10% 14% 12% 7%	2508
November	1st to 10th	Night	56% 8% 12% 12% 11% 2	2422
	11th to 20th	Night	63% 7% 11% 9% 6% 4	2660
	21st to end	Night	54% 7% 9% 11% 11% 7%	2660
December	1st to 10th	Night	55% 8% 10% 14% 12% 2	2660
	11th to 20th	Night	64% 8% 9% 10% 8% 1	3276
	21st to end	Night	63% 10% 11% 11% 3 2	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	41% 21% 16% 13% 8% 2	180
	11th to 20th	Day	41% 9% 16% 16% 14% 4	298
	21st to end	Day	36% 9% 19% 19% 14% 2	407
February	1st to 10th	Day	35% 8% 23% 22% 11% 1	370
	11th to 20th	Day	40% 16% 20% 18% 5% 1	365
	21st to end	Day	43% 16% 17% 18% 5% 1	334
March	1st to 10th	Day	41% 15% 21% 16% 6%	460
	11th to 20th	Day	34% 12% 22% 21% 8% 2	460
	21st to end	Day	39% 15% 23% 17% 5% 1	506
April	1st to 10th	Day	38% 16% 22% 16% 7% 2	460
	11th to 20th	Day	34% 18% 24% 17% 6% 2	452
	21st to end	Day	34% 15% 25% 17% 7% 2	460
May	1st to 10th	Day	24% 17% 28% 20% 8% 3	463
	11th to 20th	Day	31% 13% 26% 18% 9% 3	488
	21st to end	Day	25% 13% 28% 23% 9% 3	529
June	1st to 10th	Day	24% 12% 28% 23% 9% 4	490
	11th to 20th	Day	20% 22% 30% 20% 7% 1	490
	21st to end	Day	9% 8% 36% 32% 12% 3	490
July	1st to 10th	Day	7% 7% 35% 34% 15% 2	486
	11th to 20th	Day	5% 7% 36% 33% 15% 4	484
	21st to end	Day	2 5% 35% 40% 14% 5%	531
August	1st to 10th	Day	5% 7% 28% 38% 16% 6%	487
	11th to 20th	Day	4 6% 27% 36% 21% 6%	477
	21st to end	Day	8% 7% 31% 32% 17% 6%	539
September	1st to 10th	Day	8% 7% 29% 31% 19% 6%	490
	11th to 20th	Day	15% 8% 28% 30% 16% 3	490
	21st to end	Day	15% 11% 26% 31% 14% 3	490
October	1st to 10th	Day	19% 11% 22% 28% 17% 4	490
	11th to 20th	Day	21% 11% 22% 23% 19% 4	490
	21st to end	Day	30% 10% 24% 20% 12% 4	539
November	1st to 10th	Day	31% 15% 18% 21% 13% 2	304
	11th to 20th	Day	32% 8% 24% 20% 12% 5%	180
	21st to end	Day	36% 9% 18% 23% 13% 2	180
December	1st to 10th	Day	34% 11% 17% 19% 13% 6%	180
	11th to 20th	Day	42% 9% 16% 23% 8% 3	180
	21st to end	Day	39% 14% 18% 17% 12% 1	198
January	1st to 10th	Night	42% 8% 18% 18% 12% 3	760
	11th to 20th	Night	36% 7% 14% 23% 15% 5%	760
	21st to end	Night	34% 5% 11% 27% 19% 3	836
February	1st to 10th	Night	34% 6% 14% 27% 18% 2	760
	11th to 20th	Night	32% 8% 15% 28% 16% 2	744
	21st to end	Night	34% 10% 15% 26% 13% 1	628
March	1st to 10th	Night	34% 10% 17% 24% 13% 1	760
	11th to 20th	Night	30% 10% 13% 29% 18% 1	760
	21st to end	Night	38% 10% 16% 24% 12% 1	836
April	1st to 10th	Night	32% 11% 17% 21% 18% 1	760
	11th to 20th	Night	28% 10% 17% 25% 17% 2	745
	21st to end	Night	27% 6% 15% 28% 22% 2	570
May	1st to 10th	Night	24% 7% 16% 29% 22% 3	570
	11th to 20th	Night	21% 5% 13% 32% 27% 2	374
	21st to end	Night	21% 5% 16% 28% 27% 1	411
June	1st to 10th	Night	21% 5% 11% 36% 26% 2	380
	11th to 20th	Night	12% 5% 15% 44% 21% 3	380
	21st to end	Night	11% 2 9% 40% 37% 1	380
July	1st to 10th	Night	5% 1 9% 40% 44% 1	370
	11th to 20th	Night	5% 1 11% 48% 33% 1	368
	21st to end	Night	3 1 8% 41% 43% 4	586
August	1st to 10th	Night	4 10% 35% 43% 7%	567
	11th to 20th	Night	4 2 7% 32% 49% 7%	564
	21st to end	Night	6% 2 14% 38% 36% 5%	749
September	1st to 10th	Night	6% 2 12% 35% 40% 4	760
	11th to 20th	Night	9% 3 13% 30% 38% 8%	760
	21st to end	Night	14% 4 12% 34% 31% 4	760
October	1st to 10th	Night	15% 3 17% 31% 30% 4	760
	11th to 20th	Night	19% 4 11% 31% 29% 6%	760
	21st to end	Night	27% 7% 14% 29% 17% 6%	836
November	1st to 10th	Night	30% 6% 14% 29% 18% 1	742
	11th to 20th	Night	35% 7% 14% 28% 14% 2	760
	21st to end	Night	29% 6% 14% 31% 18% 3	760
December	1st to 10th	Night	29% 9% 13% 25% 19% 4	760
	11th to 20th	Night	41% 6% 13% 24% 15% 2	760
	21st to end	Night	42% 10% 14% 23% 10% 1	836

Page generated at 2024-10-29 08:20:48. Version ac07

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