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 2025-05-26.

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	6% 5% 1 3 2 2 3 2 4 5% 64%	117900
	Night	7% 5% 2 3 2 2 3 3 4 5% 63%	120241
Transparency	Day	22% 4 7% 21% 27% 19%	117647
	Night	22% 4 8% 22% 26% 18%	120084
Seeing	Day	10% 13% 33% 32% 11% 2	15964
	Night	9% 4 23% 41% 23% 1	25618

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	7% 8% 3 4 2 3 3 3 5% 8% 54%	3639
	11th to 20th	Day	8% 5% 2 5% 3 3 4 3 5% 5% 57%	3847
	21st to end	Day	11% 8% 2 6% 4 3 4 4 6% 7% 45%	4510
February	1st to 10th	Day	14% 10% 2 4 3 2 3 3 6% 5% 49%	1966
	11th to 20th	Day	10% 13% 2 6% 3 4 3 3 5% 5% 46%	1950
	21st to end	Day	15% 12% 3 6% 3 4 3 3 4 5% 42%	1650
March	1st to 10th	Day	11% 16% 3 7% 4 4 4 4 5% 7% 35%	4518
	11th to 20th	Day	10% 11% 2 5% 3 3 3 3 5% 7% 46%	4778
	21st to end	Day	14% 14% 3 6% 4 3 3 3 4 6% 40%	5938
April	1st to 10th	Day	10% 10% 2 4 4 3 3 2 5% 7% 49%	2400
	11th to 20th	Day	11% 9% 2 5% 4 3 4 3 5% 6% 50%	2369
	21st to end	Day	9% 8% 1 4 3 4 3 3 5% 7% 53%	2400
May	1st to 10th	Day	6% 5% 2 3 2 2 2 2 4 5% 67%	4076
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 6% 72%	4201
	21st to end	Day	5% 3 1 2 2 2 2 2 3 5% 74%	4759
June	1st to 10th	Day	3 3 1 4 3 3 3 3 5% 7% 65%	2660
	11th to 20th	Day	2 1 1 2 1 1 1 1 3 4 82%	2660
	21st to end	Day	1 1 1 2 1 2 2 1 2 3 84%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 2 90%	4420
	11th to 20th	Day	1 1 1 2 2 2 3 1 3 4 80%	4420
	21st to end	Day	1 1 2 1 2 2 2 2 3 84%	4611
August	1st to 10th	Day	1 2 2 1 1 2 1 2 3 85%	3787
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4180
September	1st to 10th	Day	1 1 2 1 1 1 2 3 4 84%	2280
	11th to 20th	Day	2 2 1 2 2 2 2 2 2 4 81%	2195
	21st to end	Day	2 2 1 2 1 2 2 2 2 4 80%	2090
October	1st to 10th	Day	3 2 1 2 2 2 3 2 4 4 74%	3402
	11th to 20th	Day	2 2 1 3 2 3 3 1 2 3 77%	3161
	21st to end	Day	4 4 2 5% 3 3 3 3 4 7% 63%	3364
November	1st to 10th	Day	6% 9% 2 4 2 3 3 2 5% 6% 59%	1764
	11th to 20th	Day	7% 4 2 3 3 2 3 2 5% 6% 63%	1800
	21st to end	Day	9% 7% 2 4 3 4 3 3 5% 6% 56%	1800
December	1st to 10th	Day	7% 3 2 4 3 2 4 4 4 6% 61%	2889
	11th to 20th	Day	15% 8% 2 5% 3 3 4 3 5% 8% 44%	3028
	21st to end	Day	13% 9% 3 5% 4 3 4 4 5% 9% 41%	4004
January	1st to 10th	Night	10% 7% 2 4 2 2 3 3 5% 7% 55%	6306
	11th to 20th	Night	10% 7% 2 5% 3 2 3 2 4 5% 57%	6117
	21st to end	Night	11% 8% 3 5% 3 4 4 3 5% 7% 47%	6468
February	1st to 10th	Night	14% 9% 2 4 3 3 3 2 5% 5% 50%	2430
	11th to 20th	Night	12% 8% 2 4 3 2 4 3 4 6% 52%	2338
	21st to end	Night	13% 9% 3 4 3 3 3 3 4 6% 50%	1980
March	1st to 10th	Night	11% 13% 3 6% 3 3 4 4 5% 6% 41%	5376
	11th to 20th	Night	7% 8% 2 6% 4 4 3 4 5% 6% 51%	5206
	21st to end	Night	15% 9% 2 5% 3 3 3 4 5% 7% 43%	5035
April	1st to 10th	Night	9% 7% 2 3 3 4 3 3 4 6% 57%	2000
	11th to 20th	Night	8% 5% 2 4 4 3 4 3 4 6% 58%	1979
	21st to end	Night	6% 5% 1 4 3 3 3 2 5% 7% 61%	2000
May	1st to 10th	Night	6% 4 1 2 2 2 2 2 3 5% 72%	3115
	11th to 20th	Night	3 2 1 2 1 1 2 2 3 5% 77%	2924
	21st to end	Night	3 2 1 2 1 2 1 2 2 4 81%	2918
June	1st to 10th	Night	4 2 1 3 2 1 2 2 4 6% 72%	1520
	11th to 20th	Night	1 1 1 2 2 1 2 1 2 4 83%	1520
	21st to end	Night	1 1 1 1 1 2 2 2 2 87%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 92%	2540
	11th to 20th	Night	1 2 1 2 2 2 2 2 2 4 81%	2540
	21st to end	Night	1 1 1 1 1 1 2 2 3 85%	2942
August	1st to 10th	Night	1 1 2 2 1 2 1 3 4 83%	3161
	11th to 20th	Night	1 1 1 1 1 1 2 92%	3126
	21st to end	Night	1 1 1 1 1 1 1 1 2 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	1900
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 3 82%	1984
	21st to end	Night	1 1 1 2 2 2 2 2 3 3 82%	2090
October	1st to 10th	Night	2 2 1 1 1 1 2 2 2 4 82%	3573
	11th to 20th	Night	2 2 1 2 2 2 3 2 3 3 79%	3777
	21st to end	Night	3 3 1 3 1 2 2 2 3 5% 75%	4201
November	1st to 10th	Night	7% 6% 1 2 3 3 3 3 4 5% 64%	2535
	11th to 20th	Night	7% 4 1 3 2 2 3 3 5% 7% 63%	2600
	21st to end	Night	9% 6% 2 4 3 2 3 3 4 6% 57%	2600
December	1st to 10th	Night	7% 4 1 4 3 3 4 3 4 6% 61%	4294
	11th to 20th	Night	15% 6% 2 4 3 3 4 4 7% 9% 43%	5176
	21st to end	Night	10% 7% 3 7% 5% 4 5% 4 7% 9% 39%	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	28% 4 14% 30% 18% 6%	3639
	11th to 20th	Day	28% 4 9% 17% 21% 21%	3847
	21st to end	Day	35% 5% 11% 17% 22% 10%	4510
February	1st to 10th	Day	35% 4 8% 24% 20% 9%	1966
	11th to 20th	Day	38% 4 9% 21% 22% 7%	1960
	21st to end	Day	43% 4 8% 26% 15% 4	1650
March	1st to 10th	Day	44% 6% 9% 15% 19% 6%	4518
	11th to 20th	Day	33% 8% 16% 28% 9% 6%	4778
	21st to end	Day	41% 8% 13% 24% 13% 2	5938
April	1st to 10th	Day	35% 6% 12% 26% 19% 3	2400
	11th to 20th	Day	34% 5% 12% 25% 20% 3	2372
	21st to end	Day	30% 6% 12% 34% 16% 1	2400
May	1st to 10th	Day	18% 5% 10% 32% 30% 5%	4076
	11th to 20th	Day	14% 5% 7% 36% 34% 4	4204
	21st to end	Day	15% 5% 7% 37% 35% 2	4759
June	1st to 10th	Day	18% 4 12% 25% 32% 8%	2660
	11th to 20th	Day	9% 2 7% 22% 40% 18%	2660
	21st to end	Day	9% 2 3 14% 41% 31%	2660
July	1st to 10th	Day	5% 1 2 6% 19% 67%	4351
	11th to 20th	Day	11% 3 3 8% 28% 47%	4345
	21st to end	Day	7% 3 3 20% 27% 40%	4465
August	1st to 10th	Day	9% 2 2 12% 39% 36%	3773
	11th to 20th	Day	4 2 2 8% 33% 51%	3760
	21st to end	Day	6% 2 2 11% 39% 41%	4179
September	1st to 10th	Day	7% 2 3 11% 38% 39%	2280
	11th to 20th	Day	10% 3 4 20% 38% 25%	2195
	21st to end	Day	11% 3 3 19% 40% 24%	2090
October	1st to 10th	Day	15% 2 3 12% 31% 36%	3402
	11th to 20th	Day	15% 2 3 19% 39% 21%	3161
	21st to end	Day	20% 5% 4 19% 34% 18%	3364
November	1st to 10th	Day	25% 3 8% 22% 27% 15%	1764
	11th to 20th	Day	21% 3 9% 24% 31% 12%	1800
	21st to end	Day	28% 4 8% 17% 27% 15%	1800
December	1st to 10th	Day	23% 5% 7% 23% 27% 14%	2889
	11th to 20th	Day	37% 6% 10% 22% 18% 6%	3028
	21st to end	Day	38% 6% 11% 24% 15% 6%	4004
January	1st to 10th	Night	27% 3 16% 25% 22% 7%	6306
	11th to 20th	Night	29% 3 7% 20% 26% 14%	6117
	21st to end	Night	35% 5% 9% 16% 21% 14%	6468
February	1st to 10th	Night	36% 3 6% 25% 20% 10%	2430
	11th to 20th	Night	32% 3 10% 23% 25% 7%	2339
	21st to end	Night	35% 3 11% 27% 18% 6%	1980
March	1st to 10th	Night	40% 7% 7% 18% 20% 8%	5376
	11th to 20th	Night	29% 8% 12% 30% 14% 6%	5206
	21st to end	Night	38% 7% 17% 24% 11% 2	5035
April	1st to 10th	Night	27% 6% 14% 30% 17% 6%	2000
	11th to 20th	Night	26% 6% 13% 28% 21% 5%	1979
	21st to end	Night	21% 5% 15% 39% 18% 1	2000
May	1st to 10th	Night	17% 5% 8% 41% 27% 3	3115
	11th to 20th	Night	11% 4 8% 42% 32% 3	2925
	21st to end	Night	11% 4 8% 40% 35% 2	2918
June	1st to 10th	Night	15% 3 12% 28% 34% 7%	1520
	11th to 20th	Night	8% 3 7% 23% 40% 19%	1520
	21st to end	Night	6% 2 3 15% 42% 31%	1520
July	1st to 10th	Night	3 1 1 7% 19% 68%	2500
	11th to 20th	Night	9% 2 5% 10% 27% 48%	2492
	21st to end	Night	7% 2 4 19% 27% 41%	2850
August	1st to 10th	Night	9% 2 3 15% 38% 33%	3152
	11th to 20th	Night	3 1 2 10% 28% 57%	3156
	21st to end	Night	6% 1 1 9% 40% 43%	3498
September	1st to 10th	Night	5% 2 2 11% 36% 44%	1900
	11th to 20th	Night	8% 2 4 20% 41% 24%	1984
	21st to end	Night	9% 3 3 20% 40% 26%	2090
October	1st to 10th	Night	9% 2 4 12% 31% 42%	3573
	11th to 20th	Night	11% 3 3 14% 37% 31%	3777
	21st to end	Night	14% 2 4 17% 42% 21%	4201
November	1st to 10th	Night	22% 3 8% 23% 27% 16%	2535
	11th to 20th	Night	21% 3 9% 21% 32% 15%	2600
	21st to end	Night	28% 4 9% 17% 29% 13%	2600
December	1st to 10th	Night	24% 4 7% 22% 26% 17%	4294
	11th to 20th	Night	35% 7% 10% 22% 21% 6%	5176
	21st to end	Night	38% 6% 11% 22% 18% 5%	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	26% 11% 34% 20% 9% 2	200
	11th to 20th	Day	16% 15% 31% 25% 12% 1	290
	21st to end	Day	18% 10% 34% 26% 10% 1	440
February	1st to 10th	Day	21% 15% 31% 24% 8% 1	484
	11th to 20th	Day	19% 19% 36% 21% 6%	495
	21st to end	Day	19% 19% 40% 19% 4	414
March	1st to 10th	Day	25% 17% 35% 19% 4	500
	11th to 20th	Day	18% 22% 31% 21% 8%	500
	21st to end	Day	22% 23% 30% 21% 4	550
April	1st to 10th	Day	16% 20% 32% 25% 7%	500
	11th to 20th	Day	19% 20% 32% 25% 4	492
	21st to end	Day	12% 16% 34% 26% 11% 2	500
May	1st to 10th	Day	13% 20% 34% 24% 8% 1	503
	11th to 20th	Day	12% 14% 34% 27% 12% 1	528
	21st to end	Day	12% 11% 32% 31% 11% 2	550
June	1st to 10th	Day	4 14% 29% 38% 13% 2	490
	11th to 20th	Day	4 14% 33% 36% 12% 2	490
	21st to end	Day	1 9% 34% 38% 15% 2	490
July	1st to 10th	Day	1 3 32% 44% 17% 3	486
	11th to 20th	Day	2 2 23% 50% 17% 6%	484
	21st to end	Day	2 2 26% 49% 16% 6%	531
August	1st to 10th	Day	2 6% 39% 37% 13% 3	487
	11th to 20th	Day	2 7% 29% 44% 13% 6%	477
	21st to end	Day	2 4 29% 47% 15% 4	539
September	1st to 10th	Day	2 4 36% 42% 15% 1	490
	11th to 20th	Day	4 11% 37% 38% 10%	490
	21st to end	Day	4 13% 33% 38% 12%	490
October	1st to 10th	Day	4 15% 31% 39% 11%	490
	11th to 20th	Day	4 12% 35% 38% 11%	490
	21st to end	Day	5% 12% 33% 37% 12%	491
November	1st to 10th	Day	9% 12% 32% 33% 13% 1	300
	11th to 20th	Day	5% 14% 34% 33% 12% 1	300
	21st to end	Day	12% 14% 28% 30% 14% 1	300
December	1st to 10th	Day	14% 8% 36% 29% 12% 1	283
	11th to 20th	Day	16% 12% 33% 23% 17% 1	200
	21st to end	Day	21% 14% 32% 22% 10% 1	220
January	1st to 10th	Night	18% 4 30% 31% 16% 2	800
	11th to 20th	Night	11% 8% 29% 37% 14% 1	800
	21st to end	Night	17% 5% 31% 33% 13% 1	880
February	1st to 10th	Night	18% 5% 30% 34% 13% 1	800
	11th to 20th	Night	15% 6% 29% 34% 16% 1	784
	21st to end	Night	18% 7% 31% 35% 8% 1	660
March	1st to 10th	Night	18% 8% 27% 38% 10% 1	800
	11th to 20th	Night	14% 7% 27% 34% 18% 1	800
	21st to end	Night	16% 6% 28% 38% 11%	880
April	1st to 10th	Night	13% 4 29% 42% 12%	800
	11th to 20th	Night	10% 6% 27% 41% 15%	791
	21st to end	Night	7% 5% 25% 43% 20% 1	800
May	1st to 10th	Night	7% 3 27% 43% 19% 1	700
	11th to 20th	Night	7% 3 21% 45% 23% 1	591
	21st to end	Night	6% 4 24% 40% 25% 1	497
June	1st to 10th	Night	4 1 17% 48% 28% 3	380
	11th to 20th	Night	1 1 19% 48% 29% 2	380
	21st to end	Night	1 2 12% 50% 36% 1	380
July	1st to 10th	Night	7% 45% 46% 1	370
	11th to 20th	Night	1 4 45% 47% 2	368
	21st to end	Night	2 1 8% 39% 47% 4	522
August	1st to 10th	Night	2 10% 47% 40% 1	751
	11th to 20th	Night	1 9% 46% 43% 1	752
	21st to end	Night	2 8% 39% 49% 2	836
September	1st to 10th	Night	1 1 9% 45% 42% 1	760
	11th to 20th	Night	1 2 13% 51% 31% 2	760
	21st to end	Night	2 1 17% 47% 31% 2	760
October	1st to 10th	Night	3 1 19% 53% 22% 1	760
	11th to 20th	Night	4 2 22% 45% 26% 1	760
	21st to end	Night	5% 2 22% 38% 28% 4	836
November	1st to 10th	Night	9% 3 19% 42% 24% 3	780
	11th to 20th	Night	8% 4 24% 44% 19% 2	800
	21st to end	Night	11% 4 28% 39% 16% 2	800
December	1st to 10th	Night	13% 4 29% 39% 14% 2	800
	11th to 20th	Night	18% 4 29% 36% 11% 1	800
	21st to end	Night	17% 6% 34% 31% 10% 1	880

Page generated at 2025-05-23 06:20:59. Version ac07

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