Sullivan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sullivan Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	12% 8% 3 7% 5% 4 5% 4 6% 8% 38%	131646
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	131077
Transparency	Day	43% 5% 11% 24% 15% 2	135205
	Night	43% 4 10% 24% 16% 2	135593
Seeing	Day	15% 9% 27% 25% 17% 6%	17616
	Night	17% 9% 29% 30% 13% 1	27551

By Month

Percentage Cloud Cover SE (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% 10% 6% 13% 6% 5% 5% 3 5% 7% 29%	4691
	11th to 20th	Day	9% 10% 5% 13% 8% 8% 6% 5% 5% 5% 26%	4710
	21st to end	Day	16% 16% 5% 10% 7% 3 4 3 4 5% 26%	5692
February	1st to 10th	Day	16% 11% 5% 10% 6% 4 4 3 5% 8% 28%	1969
	11th to 20th	Day	15% 10% 4 9% 7% 5% 4 3 6% 7% 31%	2048
	21st to end	Day	16% 13% 3 9% 4 3 4 3 5% 6% 34%	1730
March	1st to 10th	Day	16% 15% 3 7% 4 2 3 3 2 4 40%	4580
	11th to 20th	Day	9% 10% 3 6% 4 4 4 3 6% 8% 45%	4826
	21st to end	Day	16% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	6094
April	1st to 10th	Day	15% 9% 3 6% 5% 5% 5% 4 6% 7% 35%	2520
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 5% 6% 40%	2494
	21st to end	Day	19% 7% 3 6% 4 4 4 4 5% 6% 37%	2530
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 10% 30%	5875
	11th to 20th	Day	15% 8% 3 6% 4 4 4 4 5% 8% 38%	6016
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	6935
June	1st to 10th	Day	8% 6% 2 5% 5% 5% 4 4 7% 9% 45%	2800
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 8% 47%	2800
	21st to end	Day	7% 5% 2 6% 5% 4 5% 5% 9% 9% 43%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 6% 10% 45%	6360
	11th to 20th	Day	7% 6% 3 8% 5% 5% 6% 5% 8% 11% 36%	6360
	21st to end	Day	10% 6% 2 7% 4 5% 6% 5% 8% 11% 37%	6924
August	1st to 10th	Day	8% 4 1 5% 3 3 3 3 6% 10% 53%	4176
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 5% 7% 11% 41%	3849
	21st to end	Day	5% 4 2 5% 4 4 5% 4 7% 10% 50%	4312
September	1st to 10th	Day	11% 5% 2 6% 5% 4 3 3 5% 8% 47%	2400
	11th to 20th	Day	8% 4 2 6% 4 4 5% 3 6% 6% 51%	2290
	21st to end	Day	10% 5% 2 5% 4 4 4 3 5% 8% 51%	2200
October	1st to 10th	Day	10% 4 2 5% 4 4 3 3 5% 7% 52%	2130
	11th to 20th	Day	15% 7% 2 5% 3 3 5% 3 5% 6% 46%	2000
	21st to end	Day	18% 8% 3 7% 4 3 3 3 4 5% 41%	2045
November	1st to 10th	Day	10% 8% 3 5% 3 3 3 3 4 6% 52%	1852
	11th to 20th	Day	16% 10% 3 6% 4 3 3 2 5% 7% 41%	1890
	21st to end	Day	16% 11% 4 8% 4 4 4 3 5% 6% 37%	1890
December	1st to 10th	Day	9% 9% 4 7% 4 4 3 3 5% 7% 44%	2700
	11th to 20th	Day	13% 13% 5% 11% 6% 5% 5% 4 5% 6% 27%	3088
	21st to end	Day	27% 17% 6% 11% 4 4 3 2 4 5% 16%	4070
January	1st to 10th	Night	13% 12% 6% 11% 4 4 4 3 4 6% 32%	8232
	11th to 20th	Night	10% 14% 5% 11% 6% 5% 7% 4 5% 5% 29%	8280
	21st to end	Night	17% 11% 5% 9% 5% 5% 5% 3 4 6% 31%	8504
February	1st to 10th	Night	17% 12% 5% 8% 5% 4 4 3 5% 6% 31%	2650
	11th to 20th	Night	16% 9% 4 9% 4 4 3 3 5% 6% 37%	2458
	21st to end	Night	17% 10% 3 8% 4 3 3 3 5% 5% 39%	2076
March	1st to 10th	Night	18% 14% 4 6% 4 3 3 3 3 4 37%	5493
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 49%	5384
	21st to end	Night	17% 10% 3 8% 5% 5% 5% 3 5% 6% 33%	5076
April	1st to 10th	Night	13% 7% 3 7% 6% 4 4 3 4 7% 42%	2100
	11th to 20th	Night	16% 8% 2 6% 4 3 2 3 4 6% 46%	2075
	21st to end	Night	18% 6% 2 7% 5% 5% 5% 3 5% 6% 38%	2089
May	1st to 10th	Night	17% 10% 3 8% 4 4 4 3 5% 7% 35%	4042
	11th to 20th	Night	15% 6% 3 6% 4 4 5% 3 6% 9% 39%	3868
	21st to end	Night	13% 7% 3 7% 4 5% 4 4 6% 8% 39%	3942
June	1st to 10th	Night	8% 6% 2 4 4 4 5% 4 6% 10% 48%	1600
	11th to 20th	Night	8% 5% 2 5% 5% 3 4 4 5% 8% 53%	1600
	21st to end	Night	6% 5% 3 6% 5% 4 4 4 7% 10% 47%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	3618
	11th to 20th	Night	5% 5% 2 6% 5% 4 5% 4 7% 10% 47%	3617
	21st to end	Night	8% 7% 3 7% 5% 4 5% 4 7% 11% 39%	3947
August	1st to 10th	Night	6% 4 2 5% 3 3 4 4 7% 8% 55%	2972
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 7% 8% 49%	3200
	21st to end	Night	5% 3 1 4 4 4 4 4 6% 8% 57%	3586
September	1st to 10th	Night	11% 4 2 5% 3 3 3 3 4 8% 54%	2000
	11th to 20th	Night	7% 4 2 5% 4 3 4 3 6% 8% 55%	2089
	21st to end	Night	10% 6% 2 4 3 3 4 4 4 7% 52%	2200
October	1st to 10th	Night	9% 4 2 5% 4 3 3 3 5% 5% 57%	2266
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 6% 52%	2400
	21st to end	Night	18% 9% 3 5% 3 4 4 3 5% 5% 42%	2794
November	1st to 10th	Night	10% 6% 2 5% 3 4 5% 3 4 6% 52%	2666
	11th to 20th	Night	13% 8% 2 6% 4 4 4 3 5% 6% 45%	2730
	21st to end	Night	19% 9% 3 7% 4 3 4 3 4 6% 38%	2730
December	1st to 10th	Night	12% 8% 3 7% 5% 4 3 3 4 6% 44%	4704
	11th to 20th	Night	16% 9% 4 9% 5% 5% 4 3 4 6% 35%	5339
	21st to end	Night	28% 16% 6% 8% 5% 4 4 3 4 4 19%	7150

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	56% 3 11% 22% 8% 1	4692
	11th to 20th	Day	60% 4 9% 14% 11% 2	4710
	21st to end	Day	62% 3 8% 19% 7% 2	5692
February	1st to 10th	Day	55% 4 12% 19% 9% 1	1969
	11th to 20th	Day	53% 4 10% 21% 11% 1	2058
	21st to end	Day	52% 3 10% 22% 10% 3	1730
March	1st to 10th	Day	50% 3 5% 21% 18% 3	4580
	11th to 20th	Day	37% 4 11% 30% 17% 1	4826
	21st to end	Day	52% 6% 11% 23% 7% 1	6096
April	1st to 10th	Day	46% 6% 12% 22% 13% 1	2520
	11th to 20th	Day	46% 4 11% 25% 11% 3	2494
	21st to end	Day	47% 5% 12% 23% 11% 2	2530
May	1st to 10th	Day	48% 7% 12% 21% 11% 1	5910
	11th to 20th	Day	43% 7% 11% 25% 13% 1	6020
	21st to end	Day	41% 7% 11% 22% 17% 2	6949
June	1st to 10th	Day	34% 7% 14% 26% 16% 4	2800
	11th to 20th	Day	33% 6% 11% 26% 21% 3	2800
	21st to end	Day	34% 8% 16% 22% 17% 3	2800
July	1st to 10th	Day	35% 5% 10% 30% 18% 2	6291
	11th to 20th	Day	40% 8% 14% 25% 13% 1	6285
	21st to end	Day	38% 8% 19% 24% 10% 1	6728
August	1st to 10th	Day	27% 6% 15% 37% 15% 1	4161
	11th to 20th	Day	35% 8% 14% 27% 15% 1	3885
	21st to end	Day	27% 6% 15% 33% 15% 4	4311
September	1st to 10th	Day	36% 5% 9% 25% 21% 4	2400
	11th to 20th	Day	32% 5% 9% 24% 26% 3	2290
	21st to end	Day	33% 4 9% 26% 26% 3	2200
October	1st to 10th	Day	22% 3 8% 22% 41% 3	3468
	11th to 20th	Day	28% 3 8% 22% 35% 3	3240
	21st to end	Day	39% 2 8% 23% 26% 1	3283
November	1st to 10th	Day	34% 3 13% 27% 19% 4	1852
	11th to 20th	Day	44% 3 10% 24% 16% 4	1890
	21st to end	Day	50% 3 11% 22% 13% 1	1890
December	1st to 10th	Day	42% 3 14% 29% 12%	2700
	11th to 20th	Day	57% 4 9% 23% 6% 1	3085
	21st to end	Day	72% 4 10% 11% 4	4070
January	1st to 10th	Night	54% 4 12% 23% 7%	8233
	11th to 20th	Night	57% 3 8% 16% 14% 3	8280
	21st to end	Night	56% 2 10% 22% 9% 2	8504
February	1st to 10th	Night	54% 3 10% 20% 10% 2	2650
	11th to 20th	Night	48% 4 10% 22% 14% 2	2459
	21st to end	Night	48% 4 10% 23% 13% 2	2076
March	1st to 10th	Night	51% 4 6% 21% 15% 3	5493
	11th to 20th	Night	36% 4 9% 28% 19% 3	5384
	21st to end	Night	52% 4 10% 24% 9% 1	5076
April	1st to 10th	Night	43% 5% 10% 24% 16% 2	2100
	11th to 20th	Night	41% 4 10% 24% 18% 4	2075
	21st to end	Night	48% 5% 10% 22% 12% 3	2089
May	1st to 10th	Night	49% 6% 12% 22% 10% 2	4069
	11th to 20th	Night	41% 7% 11% 25% 14% 2	3869
	21st to end	Night	41% 7% 12% 22% 16% 2	3950
June	1st to 10th	Night	32% 7% 11% 27% 19% 4	1600
	11th to 20th	Night	31% 6% 10% 26% 23% 5%	1600
	21st to end	Night	31% 6% 16% 23% 19% 5%	1600
July	1st to 10th	Night	34% 5% 11% 27% 20% 2	3580
	11th to 20th	Night	31% 7% 15% 29% 16% 2	3572
	21st to end	Night	37% 7% 16% 24% 13% 2	3832
August	1st to 10th	Night	25% 6% 16% 35% 17% 1	2963
	11th to 20th	Night	31% 6% 15% 30% 18% 1	3230
	21st to end	Night	24% 6% 14% 33% 19% 4	3586
September	1st to 10th	Night	30% 5% 9% 27% 22% 7%	2000
	11th to 20th	Night	27% 5% 9% 26% 30% 4	2089
	21st to end	Night	32% 5% 9% 23% 26% 5%	2200
October	1st to 10th	Night	20% 3 7% 23% 44% 4	3708
	11th to 20th	Night	24% 2 8% 25% 37% 4	3899
	21st to end	Night	35% 3 8% 24% 24% 5%	4506
November	1st to 10th	Night	33% 5% 10% 25% 21% 6%	2666
	11th to 20th	Night	40% 3 8% 26% 20% 2	2730
	21st to end	Night	48% 4 10% 22% 15% 1	2730
December	1st to 10th	Night	43% 3 12% 27% 14% 1	4704
	11th to 20th	Night	52% 4 11% 22% 11%	5341
	21st to end	Night	70% 4 9% 12% 5% 1	7150

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	22% 16% 34% 21% 7% 1	330
	11th to 20th	Day	25% 18% 36% 19% 2	330
	21st to end	Day	19% 17% 42% 18% 4	363
February	1st to 10th	Day	23% 15% 38% 20% 4	330
	11th to 20th	Day	24% 20% 36% 17% 3 1	330
	21st to end	Day	27% 11% 42% 16% 2	273
March	1st to 10th	Day	22% 20% 36% 19% 3	330
	11th to 20th	Day	25% 18% 37% 15% 4	330
	21st to end	Day	27% 15% 36% 15% 6% 1	589
April	1st to 10th	Day	20% 16% 34% 23% 6% 1	540
	11th to 20th	Day	21% 18% 28% 18% 13% 1	532
	21st to end	Day	23% 13% 34% 21% 8% 2	540
May	1st to 10th	Day	18% 13% 30% 27% 10% 2	507
	11th to 20th	Day	18% 9% 25% 28% 17% 4	591
	21st to end	Day	11% 6% 31% 29% 15% 8%	789
June	1st to 10th	Day	9% 6% 23% 32% 22% 8%	730
	11th to 20th	Day	10% 4 25% 33% 23% 6%	730
	21st to end	Day	8% 5% 26% 32% 21% 7%	730
July	1st to 10th	Day	12% 3 13% 29% 25% 18%	721
	11th to 20th	Day	7% 4 18% 31% 29% 11%	718
	21st to end	Day	10% 1 15% 31% 33% 10%	794
August	1st to 10th	Day	8% 3 15% 32% 31% 12%	631
	11th to 20th	Day	9% 3 18% 25% 30% 15%	519
	21st to end	Day	5% 2 14% 32% 35% 12%	583
September	1st to 10th	Day	11% 2 18% 29% 30% 10%	530
	11th to 20th	Day	8% 1 11% 31% 34% 15%	530
	21st to end	Day	12% 5% 24% 23% 23% 12%	530
October	1st to 10th	Day	11% 4 27% 30% 22% 7%	460
	11th to 20th	Day	16% 15% 31% 25% 12% 2	330
	21st to end	Day	26% 15% 30% 23% 5%	363
November	1st to 10th	Day	14% 13% 32% 23% 13% 4	330
	11th to 20th	Day	20% 15% 35% 21% 6% 2	330
	21st to end	Day	24% 13% 31% 23% 9%	330
December	1st to 10th	Day	15% 16% 40% 20% 5% 3	330
	11th to 20th	Day	19% 14% 39% 21% 6% 1	330
	21st to end	Day	27% 18% 34% 15% 6%	363
January	1st to 10th	Night	19% 15% 37% 25% 4	1050
	11th to 20th	Night	20% 16% 38% 21% 4	1050
	21st to end	Night	20% 14% 38% 25% 3	1155
February	1st to 10th	Night	23% 14% 36% 25% 2	970
	11th to 20th	Night	21% 16% 36% 24% 3	823
	21st to end	Night	23% 14% 38% 23% 2	692
March	1st to 10th	Night	23% 14% 35% 25% 4	840
	11th to 20th	Night	20% 13% 37% 25% 5%	768
	21st to end	Night	24% 10% 35% 24% 7%	693
April	1st to 10th	Night	16% 16% 35% 29% 4	630
	11th to 20th	Night	19% 11% 32% 28% 10%	623
	21st to end	Night	22% 9% 34% 27% 7% 1	630
May	1st to 10th	Night	19% 9% 30% 34% 8% 1	603
	11th to 20th	Night	16% 6% 26% 36% 14% 2	591
	21st to end	Night	11% 4 26% 36% 20% 3	650
June	1st to 10th	Night	10% 4 21% 42% 23% 1	600
	11th to 20th	Night	9% 2 23% 45% 21% 1	600
	21st to end	Night	7% 3 24% 39% 27% 1	600
July	1st to 10th	Night	11% 2 11% 33% 37% 7%	585
	11th to 20th	Night	5% 3 14% 41% 34% 3	582
	21st to end	Night	7% 1 13% 44% 32% 3	657
August	1st to 10th	Night	7% 2 13% 41% 33% 5%	597
	11th to 20th	Night	6% 1 14% 43% 32% 5%	594
	21st to end	Night	6% 1 12% 38% 38% 5%	660
September	1st to 10th	Night	13% 2 14% 37% 31% 4	600
	11th to 20th	Night	7% 4 13% 40% 33% 4	689
	21st to end	Night	13% 5% 21% 30% 26% 5%	800
October	1st to 10th	Night	12% 4 22% 38% 21% 4	800
	11th to 20th	Night	17% 12% 27% 32% 13% 1	800
	21st to end	Night	22% 13% 32% 26% 6%	880
November	1st to 10th	Night	11% 9% 39% 30% 11%	820
	11th to 20th	Night	18% 13% 35% 27% 7% 1	840
	21st to end	Night	24% 14% 30% 27% 6%	840
December	1st to 10th	Night	22% 11% 37% 25% 5% 1	1034
	11th to 20th	Night	24% 9% 34% 27% 6%	1050
	21st to end	Night	31% 14% 31% 22% 3	1155

Page generated at 2026-04-29 01:21:44. Version ac07

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