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 2025-01-07.

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	13% 8% 3 7% 4 4 4 4 6% 8% 39%	104899
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 40%	104235
Transparency	Day	42% 6% 12% 24% 15% 2	108503
	Night	43% 5% 10% 23% 17% 2	108772
Seeing	Day	15% 9% 27% 26% 17% 6%	15854
	Night	17% 9% 28% 31% 14% 1	25762

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	13% 12% 7% 12% 5% 5% 4 4 5% 6% 26%	2575
	11th to 20th	Day	14% 10% 6% 12% 7% 9% 7% 6% 6% 4 19%	2530
	21st to end	Day	22% 20% 6% 8% 4 3 4 2 4 5% 20%	3071
February	1st to 10th	Day	16% 11% 5% 11% 6% 4 4 3 4 7% 28%	1781
	11th to 20th	Day	14% 10% 4 8% 7% 5% 4 3 6% 7% 31%	1848
	21st to end	Day	17% 14% 4 9% 5% 3 4 3 5% 6% 32%	1570
March	1st to 10th	Day	18% 17% 4 7% 4 3 3 3 3 5% 33%	3120
	11th to 20th	Day	10% 8% 3 7% 4 3 4 3 5% 7% 46%	3306
	21st to end	Day	16% 11% 4 9% 6% 6% 5% 4 5% 6% 30%	4160
April	1st to 10th	Day	13% 9% 3 6% 5% 5% 5% 4 6% 7% 36%	2280
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 4 6% 40%	2254
	21st to end	Day	20% 7% 3 6% 4 4 4 4 5% 6% 37%	2289
May	1st to 10th	Day	15% 7% 3 7% 6% 5% 5% 4 7% 11% 30%	4082
	11th to 20th	Day	17% 7% 3 6% 4 4 4 4 5% 8% 36%	4194
	21st to end	Day	8% 6% 2 6% 4 5% 5% 4 6% 9% 45%	4824
June	1st to 10th	Day	7% 6% 2 5% 5% 5% 5% 4 7% 9% 45%	2660
	11th to 20th	Day	8% 4 1 6% 4 4 5% 4 7% 8% 49%	2660
	21st to end	Day	8% 5% 2 6% 5% 4 5% 5% 8% 9% 42%	2660
July	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 4 7% 10% 40%	4440
	11th to 20th	Day	5% 4 2 6% 4 4 5% 5% 8% 11% 45%	4440
	21st to end	Day	8% 5% 2 7% 5% 4 5% 5% 9% 11% 38%	4813
August	1st to 10th	Day	8% 3 1 4 3 3 3 3 6% 10% 53%	4048
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 5% 7% 11% 40%	3729
	21st to end	Day	5% 4 2 5% 4 4 6% 4 7% 10% 50%	4180
September	1st to 10th	Day	12% 5% 3 6% 5% 4 3 3 5% 8% 47%	2280
	11th to 20th	Day	9% 4 2 6% 5% 4 5% 3 6% 6% 50%	2176
	21st to end	Day	11% 5% 2 5% 3 4 4 3 5% 8% 51%	2090
October	1st to 10th	Day	10% 5% 2 5% 3 4 3 3 5% 7% 52%	2024
	11th to 20th	Day	15% 7% 2 5% 3 3 4 3 5% 6% 46%	1900
	21st to end	Day	18% 8% 3 7% 4 3 3 3 4 6% 41%	1943
November	1st to 10th	Day	10% 8% 2 5% 3 3 3 2 4 6% 52%	1762
	11th to 20th	Day	16% 9% 3 6% 4 3 3 2 5% 7% 42%	1800
	21st to end	Day	16% 11% 4 8% 4 4 4 3 5% 5% 37%	1800
December	1st to 10th	Day	9% 9% 4 7% 4 4 3 3 4 6% 45%	2620
	11th to 20th	Day	13% 14% 5% 11% 6% 5% 5% 4 5% 6% 27%	3008
	21st to end	Day	27% 17% 6% 11% 4 4 3 2 4 4 16%	3982
January	1st to 10th	Night	15% 14% 6% 10% 4 4 4 3 4 5% 30%	4513
	11th to 20th	Night	14% 17% 5% 11% 5% 5% 8% 4 4 3 24%	4440
	21st to end	Night	25% 15% 7% 9% 4 3 3 2 3 6% 22%	4592
February	1st to 10th	Night	17% 12% 5% 8% 5% 4 4 3 5% 6% 31%	2398
	11th to 20th	Night	15% 8% 4 9% 5% 4 4 3 5% 6% 38%	2218
	21st to end	Night	18% 10% 3 8% 4 4 3 3 5% 5% 37%	1884
March	1st to 10th	Night	19% 15% 3 6% 4 3 4 3 4 4 35%	3733
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 50%	3683
	21st to end	Night	18% 10% 3 8% 5% 4 4 3 4 6% 35%	3470
April	1st to 10th	Night	13% 6% 3 7% 5% 4 4 3 5% 7% 43%	1900
	11th to 20th	Night	16% 8% 2 6% 4 3 2 3 4 6% 46%	1875
	21st to end	Night	19% 7% 2 7% 5% 5% 5% 3 5% 5% 37%	1891
May	1st to 10th	Night	17% 10% 2 7% 5% 4 4 4 6% 8% 34%	2813
	11th to 20th	Night	17% 6% 2 6% 4 3 4 3 5% 7% 41%	2700
	21st to end	Night	9% 6% 2 6% 5% 5% 4 4 6% 8% 43%	2743
June	1st to 10th	Night	8% 6% 2 4 4 4 5% 4 5% 10% 48%	1520
	11th to 20th	Night	7% 4 2 4 5% 3 4 4 5% 8% 54%	1520
	21st to end	Night	6% 5% 3 6% 5% 4 4 4 7% 9% 47%	1520
July	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 8% 46%	2530
	11th to 20th	Night	4 4 1 4 4 3 4 3 7% 8% 57%	2530
	21st to end	Night	7% 7% 3 6% 5% 4 5% 4 7% 11% 40%	2750
August	1st to 10th	Night	6% 4 2 5% 3 3 4 4 7% 8% 55%	2880
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 7% 8% 49%	3100
	21st to end	Night	6% 3 1 4 4 4 4 4 6% 8% 57%	3476
September	1st to 10th	Night	12% 4 2 5% 3 3 3 3 4 8% 53%	1900
	11th to 20th	Night	7% 4 2 5% 4 3 4 3 6% 8% 54%	1984
	21st to end	Night	10% 6% 2 4 3 3 4 4 5% 7% 52%	2090
October	1st to 10th	Night	9% 4 2 5% 4 3 3 3 5% 6% 56%	2152
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 6% 52%	2280
	21st to end	Night	17% 8% 3 5% 3 4 4 3 5% 6% 42%	2654
November	1st to 10th	Night	10% 6% 2 5% 3 4 5% 3 4 5% 53%	2536
	11th to 20th	Night	13% 8% 2 6% 4 4 4 3 5% 6% 45%	2600
	21st to end	Night	19% 9% 3 7% 4 3 3 3 4 6% 39%	2600
December	1st to 10th	Night	12% 8% 3 7% 5% 4 3 3 4 6% 45%	4565
	11th to 20th	Night	17% 9% 4 9% 5% 5% 4 3 4 6% 35%	5199
	21st to end	Night	28% 16% 6% 8% 5% 4 4 3 4 4 18%	6996

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	58% 4 9% 21% 7%	2576
	11th to 20th	Day	65% 6% 11% 12% 5%	2530
	21st to end	Day	68% 3 9% 14% 5%	3071
February	1st to 10th	Day	57% 4 11% 17% 10% 1	1781
	11th to 20th	Day	52% 5% 11% 22% 10% 1	1858
	21st to end	Day	54% 4 9% 21% 9% 3	1570
March	1st to 10th	Day	55% 4 7% 19% 13% 2	3120
	11th to 20th	Day	37% 5% 14% 25% 18% 1	3306
	21st to end	Day	52% 7% 12% 22% 6% 1	4160
April	1st to 10th	Day	44% 6% 12% 23% 14% 1	2280
	11th to 20th	Day	46% 4 11% 25% 12% 3	2254
	21st to end	Day	48% 5% 12% 22% 11% 3	2289
May	1st to 10th	Day	46% 9% 14% 24% 8%	4120
	11th to 20th	Day	44% 7% 12% 26% 10% 1	4198
	21st to end	Day	34% 8% 11% 21% 23% 2	4837
June	1st to 10th	Day	33% 7% 14% 26% 16% 4	2660
	11th to 20th	Day	31% 6% 11% 26% 22% 3	2660
	21st to end	Day	35% 8% 16% 22% 16% 3	2660
July	1st to 10th	Day	39% 6% 10% 29% 14% 2	4371
	11th to 20th	Day	31% 7% 14% 29% 18% 1	4365
	21st to end	Day	36% 8% 19% 22% 14% 1	4662
August	1st to 10th	Day	27% 6% 15% 36% 15% 1	4033
	11th to 20th	Day	35% 8% 14% 27% 15% 1	3765
	21st to end	Day	27% 6% 15% 33% 14% 4	4179
September	1st to 10th	Day	37% 5% 9% 25% 20% 4	2280
	11th to 20th	Day	33% 5% 9% 25% 24% 4	2176
	21st to end	Day	33% 4 9% 27% 24% 3	2090
October	1st to 10th	Day	22% 3 8% 22% 41% 3	3362
	11th to 20th	Day	28% 3 8% 23% 35% 3	3140
	21st to end	Day	39% 2 8% 24% 26% 2	3181
November	1st to 10th	Day	34% 3 13% 27% 19% 4	1762
	11th to 20th	Day	43% 3 10% 24% 16% 4	1800
	21st to end	Day	50% 3 11% 22% 13% 1	1800
December	1st to 10th	Day	41% 3 14% 30% 12%	2620
	11th to 20th	Day	57% 4 9% 22% 6% 1	3005
	21st to end	Day	72% 4 9% 11% 4	3982
January	1st to 10th	Night	56% 4 11% 19% 9%	4513
	11th to 20th	Night	66% 3 8% 15% 8%	4440
	21st to end	Night	65% 3 8% 16% 7%	4592
February	1st to 10th	Night	55% 3 10% 19% 10% 2	2398
	11th to 20th	Night	47% 4 11% 22% 14% 2	2219
	21st to end	Night	49% 4 11% 22% 11% 2	1884
March	1st to 10th	Night	52% 5% 8% 22% 12% 1	3733
	11th to 20th	Night	35% 5% 10% 26% 21% 2	3683
	21st to end	Night	51% 5% 11% 25% 8% 1	3470
April	1st to 10th	Night	42% 5% 10% 25% 16% 2	1900
	11th to 20th	Night	41% 4 9% 24% 18% 4	1875
	21st to end	Night	49% 5% 9% 20% 13% 3	1891
May	1st to 10th	Night	47% 7% 15% 22% 9% 1	2840
	11th to 20th	Night	42% 7% 12% 27% 11% 1	2701
	21st to end	Night	36% 8% 11% 21% 21% 3	2751
June	1st to 10th	Night	31% 7% 11% 27% 20% 4	1520
	11th to 20th	Night	29% 6% 10% 26% 24% 5%	1520
	21st to end	Night	32% 6% 16% 23% 19% 5%	1520
July	1st to 10th	Night	35% 5% 11% 26% 18% 3	2490
	11th to 20th	Night	22% 6% 12% 34% 23% 3	2482
	21st to end	Night	35% 7% 16% 23% 16% 3	2662
August	1st to 10th	Night	25% 7% 16% 35% 17% 1	2871
	11th to 20th	Night	31% 6% 14% 30% 18% 1	3130
	21st to end	Night	24% 6% 14% 33% 18% 4	3476
September	1st to 10th	Night	30% 5% 9% 26% 22% 7%	1900
	11th to 20th	Night	27% 5% 9% 26% 28% 5%	1984
	21st to end	Night	31% 5% 10% 24% 25% 4	2090
October	1st to 10th	Night	20% 4 7% 23% 44% 4	3594
	11th to 20th	Night	24% 2 7% 26% 37% 4	3779
	21st to end	Night	34% 3 8% 25% 24% 5%	4366
November	1st to 10th	Night	34% 5% 10% 25% 21% 6%	2536
	11th to 20th	Night	40% 3 9% 26% 20% 2	2600
	21st to end	Night	48% 4 10% 22% 15% 1	2600
December	1st to 10th	Night	42% 3 12% 28% 14% 1	4565
	11th to 20th	Night	52% 4 11% 22% 11%	5201
	21st to end	Night	70% 4 9% 11% 4 1	6996

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% 20% 7% 1	288
	11th to 20th	Day	27% 16% 34% 20% 3	270
	21st to end	Day	21% 17% 38% 19% 5%	297
February	1st to 10th	Day	26% 16% 35% 20% 4	270
	11th to 20th	Day	23% 18% 34% 20% 4 1	270
	21st to end	Day	29% 12% 40% 16% 3	225
March	1st to 10th	Day	21% 19% 35% 21% 4	270
	11th to 20th	Day	25% 18% 38% 15% 4	270
	21st to end	Day	28% 13% 35% 16% 7% 1	501
April	1st to 10th	Day	18% 15% 35% 25% 6% 2	460
	11th to 20th	Day	21% 17% 28% 19% 14% 1	452
	21st to end	Day	23% 12% 35% 20% 8% 2	460
May	1st to 10th	Day	17% 13% 31% 27% 10% 2	463
	11th to 20th	Day	18% 8% 25% 28% 18% 4	548
	21st to end	Day	11% 5% 31% 29% 16% 8%	734
June	1st to 10th	Day	8% 5% 24% 32% 23% 8%	680
	11th to 20th	Day	9% 4 25% 33% 23% 6%	680
	21st to end	Day	9% 5% 28% 31% 20% 7%	680
July	1st to 10th	Day	12% 3 13% 29% 25% 18%	671
	11th to 20th	Day	7% 3 17% 32% 29% 12%	668
	21st to end	Day	9% 1 15% 32% 33% 10%	739
August	1st to 10th	Day	9% 2 15% 32% 31% 12%	586
	11th to 20th	Day	10% 3 19% 25% 28% 15%	479
	21st to end	Day	5% 2 13% 31% 36% 13%	539
September	1st to 10th	Day	11% 2 17% 29% 30% 10%	490
	11th to 20th	Day	8% 1 11% 31% 34% 14%	490
	21st to end	Day	13% 5% 26% 23% 23% 11%	490
October	1st to 10th	Day	12% 4 28% 29% 20% 7%	424
	11th to 20th	Day	16% 15% 33% 23% 11% 2	300
	21st to end	Day	26% 15% 30% 24% 5%	330
November	1st to 10th	Day	15% 11% 32% 23% 14% 4	300
	11th to 20th	Day	20% 15% 35% 21% 7% 2	300
	21st to end	Day	24% 13% 31% 23% 9%	300
December	1st to 10th	Day	15% 17% 40% 20% 5% 3	300
	11th to 20th	Day	18% 15% 38% 21% 6% 1	300
	21st to end	Day	27% 19% 32% 15% 6%	330
January	1st to 10th	Night	19% 15% 36% 25% 4	981
	11th to 20th	Night	21% 15% 37% 22% 5%	950
	21st to end	Night	21% 13% 37% 25% 3	1045
February	1st to 10th	Night	23% 14% 35% 26% 2	878
	11th to 20th	Night	19% 15% 38% 25% 3	743
	21st to end	Night	24% 14% 36% 23% 2	628
March	1st to 10th	Night	22% 13% 36% 26% 4	760
	11th to 20th	Night	20% 12% 36% 26% 6%	695
	21st to end	Night	25% 9% 33% 25% 8%	627
April	1st to 10th	Night	16% 15% 34% 30% 5%	570
	11th to 20th	Night	19% 10% 32% 28% 11%	563
	21st to end	Night	23% 8% 34% 26% 7% 1	570
May	1st to 10th	Night	19% 8% 30% 34% 8% 1	570
	11th to 20th	Night	16% 6% 25% 36% 15% 2	561
	21st to end	Night	11% 4 26% 37% 20% 3	617
June	1st to 10th	Night	9% 4 21% 42% 24% 1	570
	11th to 20th	Night	8% 2 23% 44% 21% 1	570
	21st to end	Night	7% 3 25% 38% 26% 1	570
July	1st to 10th	Night	11% 1 11% 33% 36% 7%	555
	11th to 20th	Night	5% 3 14% 41% 33% 4	552
	21st to end	Night	6% 1 13% 45% 31% 3	624
August	1st to 10th	Night	7% 2 12% 41% 33% 5%	567
	11th to 20th	Night	5% 1 15% 42% 32% 5%	564
	21st to end	Night	6% 1 12% 37% 38% 5%	627
September	1st to 10th	Night	13% 2 14% 36% 31% 4	570
	11th to 20th	Night	7% 4 13% 39% 33% 4	654
	21st to end	Night	13% 5% 22% 31% 26% 5%	760
October	1st to 10th	Night	12% 4 22% 37% 21% 4	760
	11th to 20th	Night	17% 12% 27% 31% 13% 1	760
	21st to end	Night	22% 13% 32% 27% 6%	836
November	1st to 10th	Night	11% 9% 38% 31% 11%	780
	11th to 20th	Night	18% 12% 34% 27% 8% 1	800
	21st to end	Night	23% 14% 30% 27% 6%	800
December	1st to 10th	Night	22% 11% 37% 25% 5% 1	985
	11th to 20th	Night	24% 8% 33% 28% 6% 1	1000
	21st to end	Night	31% 15% 30% 22% 3	1100

Page generated at 2025-01-04 02:24:01. Version ac07

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