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 2024-11-02.

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% 65%	100957
	Night	8% 5% 1 3 2 2 3 3 4 5% 64%	98202
Transparency	Day	22% 4 7% 21% 26% 19%	100704
	Night	23% 4 8% 23% 25% 17%	98045
Seeing	Day	10% 12% 32% 33% 11% 2	15273
	Night	9% 3 22% 41% 23% 1	24818

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	10% 10% 3 5% 3 4 4 3 5% 7% 46%	2539
	11th to 20th	Day	12% 7% 3 7% 5% 5% 6% 4 7% 7% 38%	2677
	21st to end	Day	15% 9% 2 7% 4 4 5% 4 6% 8% 36%	3146
February	1st to 10th	Day	14% 10% 2 4 3 2 3 3 6% 4 49%	1867
	11th to 20th	Day	10% 13% 2 5% 3 4 2 3 5% 5% 47%	1850
	21st to end	Day	16% 13% 4 6% 3 4 3 2 4 5% 41%	1570
March	1st to 10th	Day	12% 16% 3 7% 4 4 4 3 4 7% 34%	3138
	11th to 20th	Day	7% 9% 2 5% 4 3 3 3 4 7% 54%	3333
	21st to end	Day	14% 18% 3 6% 5% 3 3 3 4 6% 35%	4147
April	1st to 10th	Day	10% 10% 2 4 4 3 3 2 5% 7% 49%	2280
	11th to 20th	Day	11% 9% 2 5% 4 2 4 3 5% 6% 50%	2249
	21st to end	Day	10% 8% 1 4 3 3 3 3 5% 7% 53%	2280
May	1st to 10th	Day	6% 5% 2 3 2 2 2 2 4 5% 67%	3951
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 6% 72%	4071
	21st to end	Day	5% 3 1 2 2 2 2 2 3 4 74%	4690
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	5% 4 1 3 2 3 3 2 3 5% 69%	2038
	11th to 20th	Day	4 2 1 3 2 3 4 2 3 4 72%	1900
	21st to end	Day	5% 2 1 4 2 2 2 2 4 5% 69%	2042
November	1st to 10th	Day	6% 9% 2 4 2 3 3 3 5% 5% 58%	1683
	11th to 20th	Day	7% 4 2 2 2 2 3 2 5% 6% 64%	1710
	21st to end	Day	7% 6% 2 4 3 4 3 3 5% 6% 57%	1710
December	1st to 10th	Day	12% 4 2 4 3 2 3 3 5% 7% 55%	1677
	11th to 20th	Day	18% 9% 3 7% 4 4 4 3 5% 6% 37%	1928
	21st to end	Day	11% 10% 3 6% 5% 4 4 4 5% 8% 39%	2794
January	1st to 10th	Night	12% 9% 2 5% 3 3 3 3 5% 6% 50%	4406
	11th to 20th	Night	14% 10% 3 6% 4 3 4 3 5% 7% 40%	4282
	21st to end	Night	15% 9% 3 5% 4 4 5% 4 6% 9% 37%	4521
February	1st to 10th	Night	13% 9% 2 4 3 3 3 2 5% 5% 50%	2309
	11th to 20th	Night	12% 8% 2 4 2 2 4 3 4 6% 53%	2218
	21st to end	Night	14% 9% 4 4 3 3 3 2 4 5% 49%	1884
March	1st to 10th	Night	15% 12% 2 5% 3 3 4 4 6% 7% 39%	3736
	11th to 20th	Night	7% 6% 2 4 3 4 3 3 5% 7% 57%	3631
	21st to end	Night	17% 10% 2 5% 4 3 3 4 4 6% 41%	3504
April	1st to 10th	Night	10% 7% 1 3 3 4 3 3 4 6% 57%	1900
	11th to 20th	Night	8% 5% 2 4 3 3 4 3 4 6% 59%	1879
	21st to end	Night	7% 6% 1 3 2 3 3 2 5% 7% 62%	1900
May	1st to 10th	Night	6% 4 1 2 2 2 2 2 3 5% 72%	3020
	11th to 20th	Night	3 2 1 2 1 1 2 2 3 5% 77%	2834
	21st to end	Night	3 2 1 2 1 2 1 2 2 4 81%	2871
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	3 3 1 2 2 2 2 2 3 5% 74%	2137
	11th to 20th	Night	3 2 1 3 3 2 3 2 3 4 74%	2280
	21st to end	Night	4 2 1 3 1 1 2 2 3 5% 76%	2555
November	1st to 10th	Night	7% 6% 1 2 3 3 2 3 4 5% 64%	2418
	11th to 20th	Night	7% 4 1 3 2 2 3 3 5% 7% 63%	2470
	21st to end	Night	7% 6% 2 4 3 2 3 3 4 6% 58%	2470
December	1st to 10th	Night	12% 5% 1 5% 3 2 3 3 5% 8% 54%	2498
	11th to 20th	Night	20% 7% 2 5% 3 3 4 4 6% 6% 40%	3276
	21st to end	Night	10% 7% 2 7% 5% 4 5% 4 6% 8% 41%	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	36% 4 12% 30% 15% 2	2539
	11th to 20th	Day	40% 6% 14% 22% 11% 7%	2677
	21st to end	Day	42% 5% 15% 22% 13% 4	3146
February	1st to 10th	Day	35% 5% 8% 24% 20% 7%	1867
	11th to 20th	Day	38% 4 9% 20% 22% 7%	1860
	21st to end	Day	45% 4 8% 25% 15% 2	1570
March	1st to 10th	Day	45% 7% 11% 17% 15% 4	3138
	11th to 20th	Day	26% 8% 15% 31% 11% 8%	3333
	21st to end	Day	45% 9% 12% 21% 12% 2	4147
April	1st to 10th	Day	35% 6% 12% 25% 19% 3	2280
	11th to 20th	Day	34% 5% 12% 25% 20% 3	2252
	21st to end	Day	29% 6% 12% 35% 17% 1	2280
May	1st to 10th	Day	18% 5% 10% 32% 30% 5%	3951
	11th to 20th	Day	14% 5% 7% 36% 34% 4	4074
	21st to end	Day	15% 5% 7% 37% 34% 2	4690
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	19% 3 4 19% 28% 26%	2038
	11th to 20th	Day	16% 3 4 22% 31% 23%	1900
	21st to end	Day	17% 4 5% 21% 31% 22%	2042
November	1st to 10th	Day	26% 3 8% 21% 27% 15%	1683
	11th to 20th	Day	21% 3 9% 23% 31% 13%	1710
	21st to end	Day	27% 4 9% 17% 28% 15%	1710
December	1st to 10th	Day	29% 5% 8% 20% 24% 13%	1677
	11th to 20th	Day	46% 6% 9% 16% 16% 7%	1928
	21st to end	Day	40% 5% 10% 23% 16% 5%	2794
January	1st to 10th	Night	34% 3 17% 26% 18% 3	4406
	11th to 20th	Night	41% 4 10% 26% 13% 6%	4282
	21st to end	Night	40% 7% 12% 21% 15% 5%	4521
February	1st to 10th	Night	36% 3 7% 26% 20% 9%	2309
	11th to 20th	Night	31% 3 10% 23% 26% 7%	2219
	21st to end	Night	36% 3 11% 27% 18% 5%	1884
March	1st to 10th	Night	40% 7% 9% 21% 18% 5%	3736
	11th to 20th	Night	24% 7% 14% 32% 16% 8%	3631
	21st to end	Night	42% 7% 13% 27% 10% 2	3504
April	1st to 10th	Night	27% 6% 13% 30% 16% 6%	1900
	11th to 20th	Night	26% 6% 13% 29% 20% 6%	1879
	21st to end	Night	20% 5% 15% 40% 18% 1	1900
May	1st to 10th	Night	16% 5% 8% 41% 27% 3	3020
	11th to 20th	Night	11% 3 8% 42% 32% 3	2835
	21st to end	Night	11% 4 8% 40% 35% 2	2871
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	14% 4 6% 20% 31% 26%	2137
	11th to 20th	Night	14% 4 5% 19% 34% 25%	2280
	21st to end	Night	13% 3 4 23% 36% 21%	2555
November	1st to 10th	Night	23% 3 8% 23% 27% 16%	2418
	11th to 20th	Night	21% 3 9% 21% 31% 15%	2470
	21st to end	Night	26% 4 9% 17% 30% 13%	2470
December	1st to 10th	Night	30% 4 10% 21% 23% 13%	2498
	11th to 20th	Night	42% 6% 9% 19% 17% 6%	3276
	21st to end	Night	37% 5% 10% 26% 16% 5%	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	28% 9% 33% 20% 8% 2	180
	11th to 20th	Day	17% 13% 30% 25% 13% 2	265
	21st to end	Day	19% 10% 33% 27% 10% 1	407
February	1st to 10th	Day	20% 13% 31% 25% 9% 2	445
	11th to 20th	Day	18% 18% 35% 22% 6%	455
	21st to end	Day	20% 19% 38% 18% 4	382
March	1st to 10th	Day	24% 16% 36% 19% 4	460
	11th to 20th	Day	17% 22% 31% 22% 9%	460
	21st to end	Day	22% 23% 31% 21% 3	506
April	1st to 10th	Day	17% 19% 32% 25% 7%	460
	11th to 20th	Day	20% 19% 31% 26% 4	452
	21st to end	Day	11% 17% 34% 26% 11% 2	460
May	1st to 10th	Day	13% 20% 35% 24% 8% 1	463
	11th to 20th	Day	12% 12% 34% 28% 12% 1	488
	21st to end	Day	13% 12% 31% 31% 12% 2	529
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	10% 10% 33% 34% 13% 1	273
	11th to 20th	Day	6% 10% 33% 36% 14% 1	270
	21st to end	Day	10% 13% 29% 32% 16% 1	270
December	1st to 10th	Day	15% 8% 38% 27% 11% 1	255
	11th to 20th	Day	15% 12% 35% 22% 16% 1	180
	21st to end	Day	22% 12% 31% 22% 11% 2	198
January	1st to 10th	Night	19% 3 28% 32% 16% 2	760
	11th to 20th	Night	11% 6% 28% 39% 15% 1	760
	21st to end	Night	18% 4 30% 33% 14% 1	836
February	1st to 10th	Night	17% 4 29% 35% 14% 1	760
	11th to 20th	Night	15% 5% 28% 34% 16% 1	744
	21st to end	Night	19% 7% 30% 35% 9% 1	628
March	1st to 10th	Night	17% 7% 26% 39% 10% 1	760
	11th to 20th	Night	14% 6% 26% 34% 19% 1	760
	21st to end	Night	17% 6% 28% 38% 11%	836
April	1st to 10th	Night	13% 3 28% 43% 13%	760
	11th to 20th	Night	9% 6% 26% 42% 16%	751
	21st to end	Night	7% 4 25% 42% 21% 1	760
May	1st to 10th	Night	7% 3 27% 43% 19% 1	665
	11th to 20th	Night	7% 2 20% 45% 24% 1	561
	21st to end	Night	6% 4 23% 41% 25% 1	482
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 18% 42% 25% 3	744
	11th to 20th	Night	8% 3 22% 45% 20% 2	760
	21st to end	Night	10% 3 28% 40% 17% 2	760
December	1st to 10th	Night	13% 4 28% 39% 14% 2	760
	11th to 20th	Night	19% 4 28% 36% 11% 1	760
	21st to end	Night	18% 5% 33% 33% 11% 1	836

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