Big Stump
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Stump clear sky chart from 2005-11-06 to 2026-09-29.

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% 1 3 2 2 3 2 4 5% 68%	157098
	Night	6% 5% 1 3 2 2 3 2 4 5% 66%	151882
Transparency	Day	20% 4 7% 21% 29% 19%	152167
	Night	21% 4 8% 22% 27% 18%	147789
Seeing	Day	10% 13% 33% 32% 11% 2	17794
	Night	9% 4 23% 41% 22% 1	27304

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	11% 8% 3 5% 3 3 4 3 5% 7% 50%	4722
	11th to 20th	Day	6% 4 2 4 3 3 3 3 4 5% 63%	5017
	21st to end	Day	9% 7% 2 7% 3 3 4 4 6% 8% 47%	5843
February	1st to 10th	Day	14% 10% 2 4 3 2 3 3 6% 5% 49%	2065
	11th to 20th	Day	10% 14% 3 6% 3 4 3 3 4 5% 46%	2050
	21st to end	Day	14% 12% 3 5% 3 4 3 3 4 6% 43%	1730
March	1st to 10th	Day	11% 16% 3 7% 4 4 4 4 5% 7% 35%	4618
	11th to 20th	Day	10% 10% 2 5% 3 3 3 3 5% 7% 47%	4883
	21st to end	Day	14% 14% 3 6% 4 3 3 3 4 6% 40%	6069
April	1st to 10th	Day	9% 10% 2 4 4 3 3 3 5% 7% 50%	2520
	11th to 20th	Day	10% 9% 2 5% 4 3 4 3 5% 6% 49%	2489
	21st to end	Day	10% 9% 1 4 3 4 3 3 5% 7% 52%	2520
May	1st to 10th	Day	5% 6% 2 4 3 2 2 2 3 4 66%	7337
	11th to 20th	Day	2 3 1 3 2 2 2 2 3 5% 76%	7596
	21st to end	Day	4 2 1 3 2 2 2 2 3 4 74%	8765
June	1st to 10th	Day	3 3 1 3 2 3 3 3 4 6% 67%	2940
	11th to 20th	Day	2 1 1 2 1 1 1 1 3 4 83%	2940
	21st to end	Day	1 1 1 2 1 1 2 1 2 3 84%	2940
July	1st to 10th	Day	1 1 1 1 1 1 2 92%	8219
	11th to 20th	Day	1 1 2 2 2 2 1 3 4 82%	8220
	21st to end	Day	1 1 1 1 1 2 2 3 3 86%	8587
August	1st to 10th	Day	1 1 1 1 1 1 1 2 2 88%	6990
	11th to 20th	Day	1 1 2 1 1 2 1 3 3 85%	7001
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 78%	7750
September	1st to 10th	Day	1 1 2 1 1 1 2 3 4 84%	2520
	11th to 20th	Day	2 2 1 2 1 2 2 2 2 4 80%	2426
	21st to end	Day	2 2 1 2 2 2 2 2 2 4 79%	2289
October	1st to 10th	Day	3 2 1 2 2 2 3 2 4 4 74%	3509
	11th to 20th	Day	3 2 1 3 2 3 3 1 2 3 76%	3261
	21st to end	Day	4 3 2 5% 3 3 3 3 4 7% 63%	3471
November	1st to 10th	Day	5% 8% 2 4 2 3 3 2 5% 6% 60%	1854
	11th to 20th	Day	8% 5% 2 3 3 3 3 2 5% 6% 61%	1890
	21st to end	Day	8% 6% 2 4 3 4 3 3 5% 6% 57%	1890
December	1st to 10th	Day	7% 3 2 4 3 2 4 3 4 6% 62%	2977
	11th to 20th	Day	14% 7% 2 5% 3 3 4 3 5% 8% 45%	3108
	21st to end	Day	13% 9% 3 5% 4 3 4 4 5% 9% 40%	4092
January	1st to 10th	Night	12% 9% 2 4 2 2 3 3 5% 6% 51%	8172
	11th to 20th	Night	8% 5% 2 4 2 2 3 3 4 6% 61%	7952
	21st to end	Night	9% 8% 3 6% 3 4 5% 4 6% 7% 46%	8371
February	1st to 10th	Night	13% 9% 2 4 3 3 4 2 5% 5% 50%	2551
	11th to 20th	Night	12% 9% 2 4 3 3 4 3 4 6% 51%	2458
	21st to end	Night	13% 8% 3 4 3 3 3 3 5% 6% 49%	2076
March	1st to 10th	Night	11% 13% 3 5% 3 3 4 4 5% 6% 41%	5496
	11th to 20th	Night	7% 8% 2 6% 4 4 3 4 5% 6% 52%	5321
	21st to end	Night	15% 9% 2 5% 3 3 3 4 5% 7% 43%	5146
April	1st to 10th	Night	10% 6% 2 3 3 4 3 3 4 6% 57%	2100
	11th to 20th	Night	8% 5% 2 4 4 3 4 3 4 6% 57%	2079
	21st to end	Night	6% 6% 1 4 3 3 3 2 5% 7% 61%	2100
May	1st to 10th	Night	5% 4 2 3 2 2 3 2 4 5% 69%	5621
	11th to 20th	Night	2 2 1 2 1 1 2 2 2 4 80%	5296
	21st to end	Night	2 2 1 2 1 2 1 1 2 4 82%	5402
June	1st to 10th	Night	4 2 1 3 2 1 2 2 4 6% 74%	1680
	11th to 20th	Night	1 1 1 1 2 1 2 1 2 4 84%	1680
	21st to end	Night	1 1 1 1 1 1 2 2 2 88%	1680
July	1st to 10th	Night	1 1 1 1 1 1 1 94%	4740
	11th to 20th	Night	1 2 1 3 2 1 2 1 2 3 82%	4740
	21st to end	Night	1 1 1 1 1 1 1 2 4 87%	5502
August	1st to 10th	Night	1 1 1 1 1 1 2 3 89%	5861
	11th to 20th	Night	1 1 1 2 2 2 4 86%	5886
	21st to end	Night	1 2 1 2 2 2 2 2 4 4 78%	6499
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 87%	2100
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 3 82%	2193
	21st to end	Night	1 1 1 2 2 2 2 2 3 3 81%	2288
October	1st to 10th	Night	2 2 1 1 1 1 2 1 2 4 82%	3686
	11th to 20th	Night	2 2 1 2 2 2 3 2 3 3 79%	3897
	21st to end	Night	3 3 1 3 2 2 2 2 3 5% 75%	4336
November	1st to 10th	Night	7% 5% 1 2 3 2 2 3 4 5% 66%	2665
	11th to 20th	Night	7% 5% 1 4 2 2 3 3 5% 7% 61%	2730
	21st to end	Night	8% 6% 2 4 3 2 3 3 4 6% 58%	2730
December	1st to 10th	Night	7% 3 1 4 3 3 4 3 4 6% 62%	4426
	11th to 20th	Night	14% 6% 2 4 3 3 4 4 7% 9% 44%	5316
	21st to end	Night	10% 7% 3 7% 5% 4 5% 4 7% 9% 38%	7106

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	33% 4 12% 28% 17% 5%	4723
	11th to 20th	Day	23% 3 8% 15% 24% 27%	5017
	21st to end	Day	32% 5% 9% 19% 25% 9%	5843
February	1st to 10th	Day	35% 4 8% 23% 21% 9%	2065
	11th to 20th	Day	39% 4 8% 20% 22% 7%	2060
	21st to end	Day	42% 4 9% 26% 14% 4	1730
March	1st to 10th	Day	44% 6% 10% 15% 20% 6%	4618
	11th to 20th	Day	32% 8% 16% 28% 10% 7%	4883
	21st to end	Day	41% 8% 12% 24% 13% 2	6069
April	1st to 10th	Day	34% 6% 12% 25% 20% 3	2520
	11th to 20th	Day	34% 6% 12% 25% 20% 3	2492
	21st to end	Day	31% 6% 12% 33% 17% 1	2520
May	1st to 10th	Day	22% 5% 9% 32% 27% 5%	7337
	11th to 20th	Day	14% 4 6% 36% 38% 3	7599
	21st to end	Day	15% 4 5% 36% 37% 2	8766
June	1st to 10th	Day	17% 4 11% 26% 33% 8%	2940
	11th to 20th	Day	9% 2 6% 21% 42% 20%	2940
	21st to end	Day	8% 2 3 14% 43% 30%	2940
July	1st to 10th	Day	4 1 1 6% 26% 61%	8151
	11th to 20th	Day	9% 2 6% 9% 31% 43%	8145
	21st to end	Day	6% 2 4 16% 36% 36%	8395
August	1st to 10th	Day	8% 2 2 16% 41% 31%	5533
	11th to 20th	Day	10% 3 3 18% 31% 36%	5517
	21st to end	Day	11% 2 2 21% 34% 30%	6077
September	1st to 10th	Day	7% 2 4 13% 38% 37%	2520
	11th to 20th	Day	11% 3 4 19% 39% 25%	2426
	21st to end	Day	11% 3 4 19% 40% 23%	2289
October	1st to 10th	Day	15% 2 4 13% 31% 36%	3509
	11th to 20th	Day	16% 2 3 19% 40% 21%	3261
	21st to end	Day	20% 4 4 19% 34% 18%	3471
November	1st to 10th	Day	24% 3 8% 22% 28% 15%	1854
	11th to 20th	Day	24% 3 8% 23% 30% 12%	1890
	21st to end	Day	28% 4 8% 18% 27% 15%	1890
December	1st to 10th	Day	23% 5% 7% 23% 27% 15%	2977
	11th to 20th	Day	36% 6% 10% 23% 18% 7%	3108
	21st to end	Day	39% 5% 11% 24% 15% 6%	4092
January	1st to 10th	Night	32% 3 14% 23% 20% 7%	8172
	11th to 20th	Night	24% 3 6% 16% 26% 25%	7952
	21st to end	Night	35% 5% 7% 16% 23% 14%	8371
February	1st to 10th	Night	36% 3 6% 24% 20% 10%	2551
	11th to 20th	Night	33% 3 10% 22% 25% 7%	2459
	21st to end	Night	35% 4 11% 27% 18% 6%	2076
March	1st to 10th	Night	39% 7% 7% 18% 21% 9%	5496
	11th to 20th	Night	29% 7% 12% 30% 15% 7%	5321
	21st to end	Night	38% 7% 17% 24% 12% 3	5146
April	1st to 10th	Night	28% 6% 13% 30% 18% 6%	2100
	11th to 20th	Night	27% 6% 13% 29% 20% 5%	2079
	21st to end	Night	22% 5% 14% 39% 19% 1	2100
May	1st to 10th	Night	18% 5% 11% 39% 24% 2	5619
	11th to 20th	Night	9% 4 9% 40% 36% 2	5297
	21st to end	Night	9% 3 6% 43% 36% 2	5403
June	1st to 10th	Night	14% 3 12% 30% 34% 7%	1680
	11th to 20th	Night	7% 3 7% 22% 41% 20%	1680
	21st to end	Night	6% 1 3 15% 45% 30%	1680
July	1st to 10th	Night	3 1 1 6% 27% 62%	4700
	11th to 20th	Night	11% 2 4 11% 29% 43%	4692
	21st to end	Night	5% 2 5% 19% 33% 37%	5382
August	1st to 10th	Night	6% 1 2 21% 39% 31%	4632
	11th to 20th	Night	7% 3 3 23% 24% 39%	4636
	21st to end	Night	11% 2 2 17% 36% 31%	5092
September	1st to 10th	Night	6% 2 2 13% 36% 41%	2100
	11th to 20th	Night	9% 2 4 20% 41% 24%	2193
	21st to end	Night	10% 3 4 20% 39% 25%	2288
October	1st to 10th	Night	9% 2 4 12% 31% 41%	3686
	11th to 20th	Night	12% 3 3 14% 37% 31%	3897
	21st to end	Night	13% 2 4 17% 42% 21%	4336
November	1st to 10th	Night	21% 3 8% 23% 28% 17%	2665
	11th to 20th	Night	23% 3 9% 20% 31% 14%	2730
	21st to end	Night	27% 4 9% 18% 30% 13%	2730
December	1st to 10th	Night	24% 4 7% 22% 26% 18%	4426
	11th to 20th	Night	34% 6% 10% 22% 21% 7%	5316
	21st to end	Night	39% 6% 11% 22% 17% 5%	7106

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	26% 11% 33% 20% 9% 2	220
	11th to 20th	Day	15% 14% 30% 27% 13% 1	315
	21st to end	Day	17% 12% 33% 27% 10% 1	473
February	1st to 10th	Day	20% 15% 31% 24% 8% 1	523
	11th to 20th	Day	20% 19% 36% 20% 5%	535
	21st to end	Day	18% 19% 40% 19% 4	446
March	1st to 10th	Day	23% 19% 36% 19% 4	540
	11th to 20th	Day	17% 21% 31% 23% 8%	540
	21st to end	Day	21% 23% 30% 22% 4	594
April	1st to 10th	Day	16% 19% 31% 26% 7%	540
	11th to 20th	Day	19% 20% 31% 25% 5%	532
	21st to end	Day	14% 16% 32% 25% 10% 2	540
May	1st to 10th	Day	13% 19% 34% 24% 8% 1	543
	11th to 20th	Day	12% 14% 34% 27% 11% 2	568
	21st to end	Day	12% 12% 33% 29% 11% 2	617
June	1st to 10th	Day	3 12% 32% 36% 14% 3	570
	11th to 20th	Day	4 14% 34% 34% 11% 3	570
	21st to end	Day	3 10% 35% 36% 15% 2	570
July	1st to 10th	Day	1 5% 35% 39% 17% 3	566
	11th to 20th	Day	2 2 26% 47% 16% 8%	564
	21st to end	Day	2 2 28% 46% 15% 7%	619
August	1st to 10th	Day	2 6% 37% 38% 13% 5%	567
	11th to 20th	Day	2 7% 29% 43% 13% 7%	557
	21st to end	Day	2 3 32% 45% 14% 4	627
September	1st to 10th	Day	3 6% 36% 40% 14% 1	570
	11th to 20th	Day	4 12% 36% 36% 11% 1	570
	21st to end	Day	4 13% 35% 36% 12% 1	563
October	1st to 10th	Day	5% 16% 31% 38% 11%	530
	11th to 20th	Day	5% 12% 35% 37% 11%	530
	21st to end	Day	5% 12% 33% 37% 12%	532
November	1st to 10th	Day	8% 12% 32% 33% 14% 1	330
	11th to 20th	Day	9% 16% 32% 31% 12% 1	330
	21st to end	Day	11% 13% 32% 30% 13% 1	330
December	1st to 10th	Day	13% 9% 36% 28% 11% 2	311
	11th to 20th	Day	15% 11% 32% 25% 15% 1	220
	21st to end	Day	23% 14% 31% 21% 10% 1	242
January	1st to 10th	Night	19% 5% 30% 30% 15% 2	840
	11th to 20th	Night	10% 8% 30% 38% 13% 1	840
	21st to end	Night	17% 5% 32% 33% 13% 1	924
February	1st to 10th	Night	17% 5% 31% 33% 13% 1	840
	11th to 20th	Night	16% 7% 30% 32% 15% 1	824
	21st to end	Night	17% 8% 33% 34% 8% 1	692
March	1st to 10th	Night	17% 9% 28% 37% 9% 1	840
	11th to 20th	Night	13% 6% 28% 34% 17% 1	840
	21st to end	Night	16% 6% 29% 38% 11%	924
April	1st to 10th	Night	13% 4 28% 43% 12%	840
	11th to 20th	Night	10% 7% 27% 41% 15%	831
	21st to end	Night	7% 5% 26% 42% 19% 1	840
May	1st to 10th	Night	7% 4 27% 43% 19% 1	735
	11th to 20th	Night	6% 3 22% 45% 22% 1	621
	21st to end	Night	6% 4 25% 41% 23% 1	534
June	1st to 10th	Night	4 1 18% 49% 26% 2	420
	11th to 20th	Night	1 20% 51% 26% 2	420
	21st to end	Night	2 15% 49% 33% 1	420
July	1st to 10th	Night	11% 45% 42% 1	410
	11th to 20th	Night	1 6% 46% 44% 2	408
	21st to end	Night	1 1 10% 41% 44% 3	578
August	1st to 10th	Night	2 1 11% 48% 38% 1	831
	11th to 20th	Night	1 10% 46% 42% 1	832
	21st to end	Night	2 9% 41% 47% 2	924
September	1st to 10th	Night	1 1 12% 45% 39% 1	840
	11th to 20th	Night	1 2 15% 51% 29% 2	840
	21st to end	Night	2 2 19% 46% 29% 1	832
October	1st to 10th	Night	3 2 21% 52% 21% 1	800
	11th to 20th	Night	5% 2 23% 45% 24% 1	800
	21st to end	Night	5% 2 23% 39% 27% 4	880
November	1st to 10th	Night	9% 3 19% 43% 23% 3	820
	11th to 20th	Night	8% 6% 24% 42% 18% 2	840
	21st to end	Night	10% 5% 29% 39% 16% 2	840
December	1st to 10th	Night	12% 5% 30% 38% 13% 2	840
	11th to 20th	Night	18% 4 30% 37% 11% 1	840
	21st to end	Night	18% 7% 34% 30% 10% 1	924

Page generated at 2026-09-26 01:18:43. Version ac07

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