Stull Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stull 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	14% 12% 5% 10% 6% 5% 5% 4 5% 7% 28%	101449
	Night	14% 13% 5% 10% 6% 5% 4 3 5% 6% 28%	97563
Transparency	Day	52% 7% 11% 19% 9% 1	105051
	Night	54% 7% 12% 18% 8% 1	102049
Seeing	Day	24% 20% 29% 17% 8% 3	14647
	Night	23% 19% 28% 21% 9% 1	23984

By Month

Percentage Cloud Cover NE (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% 23% 14% 13% 5% 4 3 2 3 3 18%	2517
	11th to 20th	Day	9% 20% 9% 21% 7% 3 4 2 4 4 16%	2694
	21st to end	Day	18% 17% 9% 15% 8% 5% 5% 3 4 4 12%	3124
February	1st to 10th	Day	13% 18% 7% 13% 10% 5% 5% 3 4 4 17%	1710
	11th to 20th	Day	10% 14% 8% 18% 8% 5% 5% 3 4 4 20%	1663
	21st to end	Day	16% 16% 4 12% 7% 4 3 4 5% 6% 23%	1532
March	1st to 10th	Day	17% 15% 7% 11% 5% 3 3 3 4 6% 28%	3366
	11th to 20th	Day	9% 20% 8% 10% 5% 4 4 3 4 5% 29%	3480
	21st to end	Day	14% 14% 4 13% 6% 5% 5% 3 5% 6% 26%	3866
April	1st to 10th	Day	14% 20% 4 9% 4 3 4 3 5% 6% 28%	2280
	11th to 20th	Day	17% 13% 4 6% 5% 5% 5% 4 5% 6% 31%	2254
	21st to end	Day	19% 12% 4 8% 4 4 4 3 5% 5% 31%	2392
May	1st to 10th	Day	22% 10% 3 8% 4 4 4 3 5% 6% 30%	4162
	11th to 20th	Day	19% 11% 4 9% 6% 5% 5% 4 6% 7% 24%	4398
	21st to end	Day	12% 7% 3 8% 5% 4 5% 4 6% 7% 40%	4837
June	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 7% 34%	2660
	11th to 20th	Day	14% 7% 3 8% 6% 5% 5% 4 6% 9% 34%	2660
	21st to end	Day	13% 7% 2 8% 5% 5% 5% 4 6% 9% 35%	2660
July	1st to 10th	Day	10% 6% 3 7% 6% 6% 6% 5% 8% 11% 34%	4440
	11th to 20th	Day	8% 7% 3 7% 6% 5% 6% 5% 9% 10% 34%	4440
	21st to end	Day	9% 7% 3 8% 7% 6% 7% 5% 7% 8% 34%	4575
August	1st to 10th	Day	18% 8% 3 7% 4 5% 5% 5% 7% 8% 30%	4104
	11th to 20th	Day	15% 8% 3 9% 6% 7% 6% 6% 7% 9% 25%	4044
	21st to end	Day	12% 9% 3 7% 6% 5% 6% 4 6% 7% 36%	4183
September	1st to 10th	Day	18% 7% 2 6% 4 5% 4 4 6% 7% 37%	2280
	11th to 20th	Day	15% 7% 2 7% 4 4 4 4 5% 8% 42%	2277
	21st to end	Day	20% 9% 3 8% 5% 5% 4 4 5% 7% 31%	1984
October	1st to 10th	Day	17% 9% 5% 7% 5% 4 4 3 5% 8% 32%	1900
	11th to 20th	Day	18% 13% 3 9% 5% 4 4 4 4 5% 30%	1900
	21st to end	Day	21% 18% 4 7% 4 4 3 3 5% 5% 26%	2090
November	1st to 10th	Day	11% 16% 4 9% 4 4 3 3 5% 6% 33%	1734
	11th to 20th	Day	14% 20% 6% 10% 4 5% 4 3 6% 6% 23%	1525
	21st to end	Day	15% 21% 7% 11% 6% 4 5% 3 4 5% 19%	1520
December	1st to 10th	Day	14% 23% 9% 15% 6% 5% 4 3 4 5% 13%	1520
	11th to 20th	Day	14% 19% 7% 16% 6% 4 3 4 5% 5% 17%	1906
	21st to end	Day	17% 25% 9% 14% 6% 4 3 3 2 4 12%	2772
January	1st to 10th	Night	13% 18% 11% 16% 7% 5% 4 3 4 5% 14%	4429
	11th to 20th	Night	10% 19% 9% 18% 9% 6% 5% 2 4 4 14%	4250
	21st to end	Night	17% 17% 7% 18% 11% 6% 4 3 3 3 12%	4532
February	1st to 10th	Night	12% 16% 6% 14% 11% 6% 5% 3 4 5% 18%	2470
	11th to 20th	Night	10% 14% 7% 17% 9% 6% 6% 4 4 5% 18%	2403
	21st to end	Night	13% 15% 6% 16% 7% 5% 4 3 5% 4 21%	1922
March	1st to 10th	Night	16% 15% 5% 11% 5% 5% 4 3 4 5% 27%	3470
	11th to 20th	Night	13% 18% 8% 12% 6% 4 4 3 4 5% 23%	3479
	21st to end	Night	14% 13% 4 12% 5% 4 3 3 4 5% 33%	3764
April	1st to 10th	Night	16% 15% 4 9% 5% 5% 4 4 4 7% 28%	1900
	11th to 20th	Night	17% 14% 4 7% 5% 4 4 3 4 6% 33%	1875
	21st to end	Night	15% 12% 4 8% 5% 4 3 3 5% 6% 33%	1803
May	1st to 10th	Night	23% 11% 3 7% 5% 5% 4 3 4 6% 30%	2776
	11th to 20th	Night	18% 10% 3 7% 5% 5% 4 4 5% 7% 31%	2495
	21st to end	Night	13% 8% 2 6% 4 4 4 3 6% 8% 42%	2738
June	1st to 10th	Night	16% 7% 2 7% 5% 5% 4 4 7% 9% 36%	1520
	11th to 20th	Night	11% 8% 2 6% 4 4 4 5% 7% 8% 39%	1520
	21st to end	Night	11% 7% 2 6% 5% 4 5% 4 6% 7% 43%	1520
July	1st to 10th	Night	11% 8% 4 8% 5% 4 5% 4 6% 8% 38%	2520
	11th to 20th	Night	9% 6% 2 7% 4 5% 5% 4 7% 8% 43%	2520
	21st to end	Night	11% 7% 3 7% 5% 4 4 3 7% 7% 42%	2960
August	1st to 10th	Night	17% 9% 3 6% 4 4 5% 4 6% 7% 35%	2825
	11th to 20th	Night	16% 9% 3 8% 3 4 4 3 6% 7% 36%	2785
	21st to end	Night	10% 7% 3 6% 5% 5% 5% 4 6% 8% 40%	3440
September	1st to 10th	Night	16% 6% 2 6% 4 3 3 3 4 8% 44%	1900
	11th to 20th	Night	14% 7% 2 5% 3 4 4 3 5% 7% 46%	1900
	21st to end	Night	18% 9% 2 7% 6% 4 4 4 5% 7% 35%	2195
October	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 5% 7% 37%	2280
	11th to 20th	Night	17% 13% 3 7% 5% 4 4 4 4 7% 32%	2280
	21st to end	Night	24% 14% 3 7% 5% 4 4 3 5% 6% 27%	2508
November	1st to 10th	Night	11% 15% 4 8% 5% 4 4 3 5% 6% 36%	2441
	11th to 20th	Night	14% 18% 4 11% 6% 5% 4 4 5% 6% 24%	2651
	21st to end	Night	13% 18% 6% 12% 7% 4 4 4 5% 6% 21%	2660
December	1st to 10th	Night	15% 22% 8% 14% 7% 5% 4 4 5% 4 13%	2660
	11th to 20th	Night	15% 20% 8% 15% 7% 5% 4 3 4 4 16%	3288
	21st to end	Night	19% 19% 9% 13% 7% 5% 5% 4 5% 4 11%	4884

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	71% 4 8% 12% 4 1	2518
	11th to 20th	Day	72% 5% 7% 12% 5%	2694
	21st to end	Day	75% 5% 9% 8% 3 1	3124
February	1st to 10th	Day	70% 6% 10% 10% 4	1710
	11th to 20th	Day	66% 5% 7% 15% 7%	1672
	21st to end	Day	60% 5% 11% 17% 6% 2	1532
March	1st to 10th	Day	57% 5% 10% 21% 6% 1	3369
	11th to 20th	Day	56% 6% 14% 16% 7% 1	3480
	21st to end	Day	56% 6% 7% 21% 8% 1	3866
April	1st to 10th	Day	56% 5% 10% 16% 11% 1	2280
	11th to 20th	Day	52% 7% 10% 17% 13% 1	2254
	21st to end	Day	53% 6% 9% 17% 13% 2	2392
May	1st to 10th	Day	54% 5% 9% 20% 10% 2	4174
	11th to 20th	Day	54% 10% 12% 16% 7% 1	4402
	21st to end	Day	40% 9% 12% 24% 13% 2	4837
June	1st to 10th	Day	47% 8% 12% 19% 14% 1	2660
	11th to 20th	Day	45% 8% 14% 21% 11% 1	2660
	21st to end	Day	45% 7% 13% 23% 12%	2660
July	1st to 10th	Day	41% 8% 14% 25% 11% 1	4370
	11th to 20th	Day	38% 10% 15% 22% 14%	4365
	21st to end	Day	44% 9% 14% 20% 13% 1	4429
August	1st to 10th	Day	48% 9% 13% 20% 9%	4091
	11th to 20th	Day	49% 10% 15% 20% 5%	4083
	21st to end	Day	44% 8% 11% 29% 8% 1	4183
September	1st to 10th	Day	43% 9% 10% 24% 11% 2	2280
	11th to 20th	Day	40% 7% 11% 26% 15% 1	2277
	21st to end	Day	52% 6% 10% 20% 11% 1	1984
October	1st to 10th	Day	43% 8% 13% 20% 16% 1	3160
	11th to 20th	Day	48% 5% 13% 16% 14% 4	3140
	21st to end	Day	46% 5% 15% 24% 9% 1	3428
November	1st to 10th	Day	50% 7% 12% 17% 11% 2	1734
	11th to 20th	Day	59% 7% 11% 16% 5% 2	1525
	21st to end	Day	64% 8% 10% 13% 5% 1	1520
December	1st to 10th	Day	73% 6% 9% 9% 3	1520
	11th to 20th	Day	67% 6% 13% 9% 4 1	1906
	21st to end	Day	76% 6% 7% 9% 1	2772
January	1st to 10th	Night	70% 6% 10% 10% 4	4429
	11th to 20th	Night	70% 7% 10% 11% 2	4250
	21st to end	Night	78% 4 6% 9% 3	4532
February	1st to 10th	Night	66% 6% 11% 11% 5%	2470
	11th to 20th	Night	65% 7% 11% 12% 5% 1	2405
	21st to end	Night	63% 6% 10% 13% 7% 1	1922
March	1st to 10th	Night	58% 7% 12% 16% 6% 1	3480
	11th to 20th	Night	61% 7% 12% 15% 6%	3480
	21st to end	Night	50% 7% 9% 20% 12% 2	3764
April	1st to 10th	Night	56% 6% 11% 17% 10%	1900
	11th to 20th	Night	52% 6% 11% 18% 12% 1	1875
	21st to end	Night	49% 6% 12% 18% 13% 2	1803
May	1st to 10th	Night	54% 7% 9% 20% 8% 1	2783
	11th to 20th	Night	50% 9% 17% 17% 7% 1	2496
	21st to end	Night	38% 10% 14% 20% 16% 3	2739
June	1st to 10th	Night	44% 7% 12% 24% 13% 1	1520
	11th to 20th	Night	39% 9% 15% 23% 12% 2	1520
	21st to end	Night	38% 8% 14% 25% 13% 1	1520
July	1st to 10th	Night	43% 7% 14% 20% 14% 1	2480
	11th to 20th	Night	34% 10% 16% 28% 12% 1	2472
	21st to end	Night	40% 7% 12% 25% 16% 1	2865
August	1st to 10th	Night	46% 8% 16% 22% 7% 1	2814
	11th to 20th	Night	43% 8% 16% 26% 7%	2812
	21st to end	Night	38% 9% 13% 31% 8% 1	3440
September	1st to 10th	Night	38% 7% 15% 25% 12% 2	1900
	11th to 20th	Night	37% 7% 12% 25% 17% 1	1900
	21st to end	Night	47% 6% 14% 20% 12% 1	2195
October	1st to 10th	Night	46% 7% 11% 23% 14%	3800
	11th to 20th	Night	45% 6% 12% 18% 13% 6%	3779
	21st to end	Night	44% 6% 16% 24% 9% 1	4120
November	1st to 10th	Night	48% 7% 12% 19% 11% 2	2441
	11th to 20th	Night	58% 6% 12% 15% 7% 2	2651
	21st to end	Night	61% 7% 13% 14% 4	2660
December	1st to 10th	Night	71% 8% 10% 9% 3	2660
	11th to 20th	Night	69% 6% 10% 12% 3	3288
	21st to end	Night	74% 7% 9% 9% 1	4884

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	31% 31% 23% 11% 4	180
	11th to 20th	Day	31% 35% 27% 5% 1	231
	21st to end	Day	26% 35% 30% 7% 2	297
February	1st to 10th	Day	37% 33% 24% 6%	270
	11th to 20th	Day	34% 36% 28% 1 1	270
	21st to end	Day	39% 29% 26% 6% 1	344
March	1st to 10th	Day	32% 27% 27% 11% 3	460
	11th to 20th	Day	29% 30% 28% 9% 3	460
	21st to end	Day	36% 24% 26% 10% 4 1	506
April	1st to 10th	Day	34% 22% 27% 11% 4 1	460
	11th to 20th	Day	29% 27% 25% 10% 6% 2	452
	21st to end	Day	29% 22% 31% 12% 4 2	460
May	1st to 10th	Day	29% 21% 28% 13% 6% 3	463
	11th to 20th	Day	23% 20% 31% 17% 6% 3	487
	21st to end	Day	17% 16% 31% 23% 9% 4	529
June	1st to 10th	Day	20% 18% 31% 22% 8% 2	490
	11th to 20th	Day	17% 14% 32% 23% 12% 2	490
	21st to end	Day	14% 17% 38% 22% 6% 3	490
July	1st to 10th	Day	11% 11% 32% 29% 13% 4	485
	11th to 20th	Day	11% 11% 31% 29% 13% 5%	484
	21st to end	Day	12% 10% 38% 28% 9% 3	532
August	1st to 10th	Day	14% 10% 28% 27% 15% 6%	486
	11th to 20th	Day	15% 10% 31% 26% 13% 6%	479
	21st to end	Day	14% 12% 31% 22% 13% 8%	539
September	1st to 10th	Day	19% 13% 26% 23% 14% 7%	490
	11th to 20th	Day	13% 10% 25% 22% 21% 9%	490
	21st to end	Day	25% 16% 28% 16% 12% 4	490
October	1st to 10th	Day	21% 16% 28% 19% 14% 2	490
	11th to 20th	Day	26% 24% 28% 14% 7% 1	490
	21st to end	Day	32% 24% 24% 17% 2	539
November	1st to 10th	Day	26% 19% 30% 15% 7% 2	391
	11th to 20th	Day	35% 25% 25% 12% 2	185
	21st to end	Day	33% 23% 37% 6% 1	180
December	1st to 10th	Day	38% 26% 27% 4 4 1	180
	11th to 20th	Day	42% 23% 27% 8% 1	180
	21st to end	Day	34% 33% 28% 4 1	198
January	1st to 10th	Night	26% 28% 31% 12% 2	760
	11th to 20th	Night	27% 28% 29% 13% 2	760
	21st to end	Night	22% 32% 30% 14% 1	836
February	1st to 10th	Night	27% 29% 30% 13% 1	760
	11th to 20th	Night	30% 28% 31% 10% 1	743
	21st to end	Night	31% 27% 28% 12% 2	628
March	1st to 10th	Night	26% 28% 31% 14% 2	760
	11th to 20th	Night	29% 25% 26% 15% 4 1	760
	21st to end	Night	25% 25% 31% 14% 5% 1	836
April	1st to 10th	Night	26% 21% 31% 19% 2	760
	11th to 20th	Night	27% 19% 30% 17% 7% 1	751
	21st to end	Night	21% 15% 33% 26% 6%	663
May	1st to 10th	Night	23% 14% 25% 29% 9% 1	539
	11th to 20th	Night	18% 12% 30% 30% 9%	374
	21st to end	Night	14% 11% 27% 32% 16% 1	411
June	1st to 10th	Night	16% 10% 32% 28% 13% 1	380
	11th to 20th	Night	14% 10% 25% 38% 13%	380
	21st to end	Night	11% 10% 32% 37% 11%	380
July	1st to 10th	Night	8% 7% 24% 42% 17% 2	370
	11th to 20th	Night	10% 7% 26% 35% 21% 1	368
	21st to end	Night	13% 5% 25% 35% 20% 2	549
August	1st to 10th	Night	11% 6% 23% 36% 22% 2	567
	11th to 20th	Night	12% 7% 22% 30% 26% 2	564
	21st to end	Night	14% 6% 24% 32% 23% 2	813
September	1st to 10th	Night	18% 8% 25% 29% 16% 3	760
	11th to 20th	Night	16% 7% 20% 25% 27% 4	760
	21st to end	Night	22% 13% 24% 23% 15% 3	760
October	1st to 10th	Night	21% 13% 27% 22% 16% 2	760
	11th to 20th	Night	25% 19% 28% 21% 6% 1	760
	21st to end	Night	32% 19% 30% 16% 3	836
November	1st to 10th	Night	20% 21% 32% 20% 7%	760
	11th to 20th	Night	28% 23% 29% 14% 4 2	760
	21st to end	Night	25% 27% 29% 16% 4	760
December	1st to 10th	Night	31% 28% 27% 10% 4	760
	11th to 20th	Night	28% 29% 29% 12% 2	760
	21st to end	Night	29% 29% 30% 11% 2	836

Page generated at 2024-11-03 02:11:19. Version ac07

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