Spenco Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spenco Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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% 7% 3 7% 4 4 4 4 5% 8% 41%	131674
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 43%	131353
Transparency	Day	40% 6% 11% 24% 16% 2	135275
	Night	40% 5% 10% 23% 18% 3	135814
Seeing	Day	15% 9% 23% 25% 19% 9%	17505
	Night	17% 7% 26% 31% 17% 2	27383

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% 10% 5% 11% 6% 6% 4 4 4 5% 32%	4691
	11th to 20th	Day	10% 8% 3 9% 7% 5% 5% 3 4 7% 40%	4710
	21st to end	Day	19% 12% 4 9% 5% 4 4 3 4 5% 31%	5778
February	1st to 10th	Day	16% 11% 4 7% 4 3 3 3 5% 6% 36%	2100
	11th to 20th	Day	17% 9% 4 8% 4 4 4 3 6% 5% 36%	2048
	21st to end	Day	14% 10% 3 9% 4 5% 5% 4 5% 6% 34%	1730
March	1st to 10th	Day	19% 11% 3 6% 3 3 3 3 3 4 42%	4580
	11th to 20th	Day	12% 8% 3 5% 3 3 4 3 5% 7% 46%	4774
	21st to end	Day	13% 9% 3 7% 5% 5% 4 3 6% 7% 40%	6099
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 9% 42%	2520
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 4 4 7% 42%	2494
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 8% 40%	2531
May	1st to 10th	Day	18% 8% 3 9% 5% 5% 5% 5% 6% 9% 27%	5992
	11th to 20th	Day	16% 7% 3 7% 4 5% 5% 4 6% 8% 35%	5939
	21st to end	Day	12% 7% 2 7% 5% 4 5% 4 6% 8% 39%	6698
June	1st to 10th	Day	11% 6% 2 5% 4 4 5% 5% 6% 8% 46%	2800
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 4 7% 9% 47%	2800
	21st to end	Day	6% 5% 2 4 5% 5% 5% 5% 8% 9% 45%	2800
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 6% 9% 46%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 41%	6360
	21st to end	Day	7% 5% 3 9% 6% 6% 7% 5% 8% 11% 34%	6848
August	1st to 10th	Day	5% 3 2 5% 3 4 4 3 6% 10% 54%	3982
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 5% 7% 10% 44%	3849
	21st to end	Day	5% 3 2 5% 3 4 4 3 5% 9% 57%	4312
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 5% 7% 49%	2400
	11th to 20th	Day	9% 4 2 6% 5% 5% 4 4 5% 8% 48%	2245
	21st to end	Day	11% 5% 2 6% 4 4 3 3 4 6% 52%	2200
October	1st to 10th	Day	9% 4 1 5% 3 3 3 3 4 7% 57%	2200
	11th to 20th	Day	12% 5% 2 5% 3 4 3 3 4 7% 51%	2019
	21st to end	Day	17% 7% 2 5% 3 3 4 3 5% 5% 46%	2033
November	1st to 10th	Day	9% 6% 3 5% 3 3 4 4 6% 6% 51%	1852
	11th to 20th	Day	15% 6% 3 6% 4 4 3 3 5% 6% 45%	1890
	21st to end	Day	20% 9% 3 6% 3 3 3 3 4 6% 41%	1890
December	1st to 10th	Day	13% 10% 3 6% 4 3 3 2 3 5% 48%	2992
	11th to 20th	Day	14% 10% 4 10% 6% 4 4 4 5% 6% 33%	3088
	21st to end	Day	26% 14% 5% 9% 5% 4 4 3 4 4 23%	4070
January	1st to 10th	Night	16% 12% 5% 7% 4 4 4 4 4 6% 36%	8232
	11th to 20th	Night	12% 10% 3 9% 4 4 5% 3 4 5% 41%	8280
	21st to end	Night	18% 10% 4 7% 4 4 3 3 4 6% 38%	8427
February	1st to 10th	Night	18% 11% 4 9% 5% 4 4 3 4 6% 32%	2520
	11th to 20th	Night	18% 9% 3 8% 4 4 3 3 4 5% 39%	2458
	21st to end	Night	18% 9% 3 7% 5% 4 4 4 4 5% 39%	2076
March	1st to 10th	Night	20% 10% 3 6% 4 4 4 3 4 5% 36%	5493
	11th to 20th	Night	12% 6% 2 6% 3 3 4 3 5% 6% 50%	5436
	21st to end	Night	17% 8% 3 8% 5% 5% 5% 3 5% 6% 36%	5072
April	1st to 10th	Night	13% 7% 3 5% 5% 4 4 4 5% 7% 45%	2100
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 44%	2075
	21st to end	Night	17% 7% 2 6% 5% 3 4 4 5% 7% 39%	2086
May	1st to 10th	Night	16% 9% 3 9% 5% 4 4 4 6% 9% 32%	4123
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 7% 36%	4086
	21st to end	Night	13% 6% 2 7% 5% 5% 4 4 7% 9% 38%	4164
June	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 6% 10% 47%	1600
	11th to 20th	Night	6% 4 1 6% 4 4 4 4 6% 9% 53%	1600
	21st to end	Night	4 4 1 4 3 3 5% 4 8% 10% 54%	1600
July	1st to 10th	Night	10% 5% 2 7% 3 3 4 4 6% 8% 49%	3618
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 4 7% 10% 49%	3617
	21st to end	Night	6% 5% 3 7% 5% 5% 6% 4 8% 12% 39%	4006
August	1st to 10th	Night	5% 2 2 4 3 4 3 3 5% 8% 59%	3162
	11th to 20th	Night	6% 4 2 5% 4 4 5% 3 6% 9% 53%	3200
	21st to end	Night	4 3 2 4 3 3 4 3 5% 10% 59%	3586
September	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 6% 55%	2000
	11th to 20th	Night	8% 4 2 5% 4 4 3 4 6% 7% 53%	2135
	21st to end	Night	12% 5% 2 4 4 3 3 3 5% 6% 52%	2200
October	1st to 10th	Night	7% 4 2 6% 3 3 3 3 4 5% 60%	2200
	11th to 20th	Night	14% 4 1 5% 3 3 3 2 4 6% 55%	2377
	21st to end	Night	18% 8% 2 6% 3 3 3 3 4 6% 45%	2805
November	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 4 5% 54%	2666
	11th to 20th	Night	13% 6% 2 6% 5% 3 3 2 4 7% 50%	2730
	21st to end	Night	19% 6% 3 6% 4 4 3 2 4 5% 44%	2730
December	1st to 10th	Night	17% 10% 3 6% 3 3 3 3 4 5% 43%	4404
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 4 6% 40%	5339
	21st to end	Night	27% 13% 4 8% 5% 4 4 3 4 4 26%	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	54% 4 13% 22% 6% 1	4692
	11th to 20th	Day	45% 3 8% 28% 14% 1	4710
	21st to end	Day	56% 3 9% 18% 9% 5%	5778
February	1st to 10th	Day	48% 4 12% 23% 11% 2	2100
	11th to 20th	Day	50% 4 9% 21% 13% 2	2058
	21st to end	Day	51% 5% 8% 18% 14% 4	1730
March	1st to 10th	Day	48% 3 4 20% 21% 3	4580
	11th to 20th	Day	38% 4 8% 27% 19% 3	4774
	21st to end	Day	43% 5% 12% 26% 14% 1	6101
April	1st to 10th	Day	39% 6% 14% 25% 16% 1	2520
	11th to 20th	Day	41% 5% 12% 23% 15% 3	2494
	21st to end	Day	42% 6% 11% 28% 12% 2	2531
May	1st to 10th	Day	52% 8% 11% 19% 10%	6027
	11th to 20th	Day	46% 8% 10% 23% 12% 1	5943
	21st to end	Day	41% 7% 11% 21% 18% 2	6712
June	1st to 10th	Day	34% 7% 12% 25% 19% 3	2800
	11th to 20th	Day	29% 9% 12% 26% 22% 3	2800
	21st to end	Day	31% 9% 14% 24% 17% 5%	2800
July	1st to 10th	Day	35% 6% 10% 29% 18% 2	6291
	11th to 20th	Day	33% 9% 16% 28% 12% 1	6285
	21st to end	Day	41% 9% 18% 24% 8% 1	6653
August	1st to 10th	Day	25% 7% 15% 35% 19% 1	3969
	11th to 20th	Day	34% 7% 13% 27% 17% 2	3885
	21st to end	Day	25% 5% 11% 35% 20% 3	4311
September	1st to 10th	Day	35% 7% 9% 25% 20% 5%	2400
	11th to 20th	Day	33% 6% 9% 25% 24% 3	2245
	21st to end	Day	34% 5% 7% 23% 27% 4	2200
October	1st to 10th	Day	18% 2 6% 24% 43% 6%	3590
	11th to 20th	Day	23% 3 8% 25% 33% 7%	3259
	21st to end	Day	32% 3 6% 26% 28% 5%	3258
November	1st to 10th	Day	33% 5% 11% 27% 19% 5%	1852
	11th to 20th	Day	40% 4 7% 25% 19% 5%	1890
	21st to end	Day	46% 4 8% 24% 16% 2	1890
December	1st to 10th	Day	41% 3 10% 29% 16% 1	2992
	11th to 20th	Day	51% 3 11% 23% 11% 1	3085
	21st to end	Day	63% 4 12% 14% 7%	4070
January	1st to 10th	Night	50% 3 13% 25% 8% 1	8233
	11th to 20th	Night	47% 4 5% 22% 19% 3	8280
	21st to end	Night	49% 3 9% 21% 17% 1	8427
February	1st to 10th	Night	53% 4 10% 21% 11% 1	2520
	11th to 20th	Night	49% 4 7% 24% 14% 2	2459
	21st to end	Night	49% 4 9% 20% 15% 3	2076
March	1st to 10th	Night	51% 4 6% 16% 20% 3	5493
	11th to 20th	Night	36% 4 10% 23% 20% 7%	5436
	21st to end	Night	48% 5% 11% 22% 12% 2	5072
April	1st to 10th	Night	37% 5% 12% 27% 17% 3	2100
	11th to 20th	Night	40% 5% 11% 21% 19% 5%	2075
	21st to end	Night	44% 6% 9% 21% 17% 2	2086
May	1st to 10th	Night	48% 8% 13% 22% 9% 1	4150
	11th to 20th	Night	46% 8% 11% 22% 13% 1	4087
	21st to end	Night	39% 8% 13% 23% 14% 2	4172
June	1st to 10th	Night	34% 7% 11% 24% 21% 3	1600
	11th to 20th	Night	27% 8% 13% 22% 26% 5%	1600
	21st to end	Night	23% 8% 16% 27% 19% 7%	1600
July	1st to 10th	Night	33% 6% 10% 26% 22% 3	3580
	11th to 20th	Night	30% 9% 17% 29% 13% 2	3572
	21st to end	Night	35% 9% 20% 25% 11% 1	3890
August	1st to 10th	Night	22% 6% 14% 38% 18% 1	3151
	11th to 20th	Night	27% 7% 14% 30% 21% 1	3230
	21st to end	Night	21% 5% 11% 37% 24% 3	3586
September	1st to 10th	Night	30% 6% 8% 27% 23% 6%	2000
	11th to 20th	Night	30% 5% 9% 26% 26% 4	2135
	21st to end	Night	33% 5% 8% 24% 23% 7%	2200
October	1st to 10th	Night	19% 3 5% 14% 50% 9%	3590
	11th to 20th	Night	22% 2 5% 24% 38% 7%	3863
	21st to end	Night	32% 3 7% 23% 27% 9%	4530
November	1st to 10th	Night	33% 3 10% 25% 25% 5%	2666
	11th to 20th	Night	36% 3 8% 24% 22% 7%	2730
	21st to end	Night	43% 4 8% 24% 19% 2	2730
December	1st to 10th	Night	45% 4 9% 24% 17% 1	4404
	11th to 20th	Night	46% 4 11% 22% 17% 1	5341
	21st to end	Night	61% 4 10% 17% 7% 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	23% 16% 32% 21% 8%	330
	11th to 20th	Day	21% 18% 37% 20% 3	330
	21st to end	Day	21% 19% 39% 17% 4	363
February	1st to 10th	Day	22% 14% 38% 21% 4 1	330
	11th to 20th	Day	27% 21% 36% 13% 2 1	330
	21st to end	Day	25% 17% 33% 21% 3	273
March	1st to 10th	Day	22% 18% 37% 16% 6% 1	330
	11th to 20th	Day	25% 17% 37% 17% 5% 1	367
	21st to end	Day	25% 15% 33% 19% 8% 1	594
April	1st to 10th	Day	17% 14% 33% 26% 9% 1	540
	11th to 20th	Day	19% 16% 27% 25% 13% 1	532
	21st to end	Day	19% 10% 34% 24% 9% 3	540
May	1st to 10th	Day	20% 11% 28% 29% 10% 2	543
	11th to 20th	Day	17% 9% 24% 26% 18% 6%	551
	21st to end	Day	12% 5% 25% 28% 21% 9%	682
June	1st to 10th	Day	13% 4 17% 27% 27% 12%	730
	11th to 20th	Day	7% 4 17% 32% 29% 11%	730
	21st to end	Day	8% 2 19% 34% 25% 12%	730
July	1st to 10th	Day	10% 3 13% 24% 28% 23%	721
	11th to 20th	Day	7% 3 11% 30% 34% 15%	718
	21st to end	Day	7% 3 10% 28% 35% 17%	768
August	1st to 10th	Day	8% 2 8% 28% 34% 19%	526
	11th to 20th	Day	14% 3 11% 26% 32% 15%	519
	21st to end	Day	8% 2 10% 23% 35% 22%	583
September	1st to 10th	Day	10% 3 12% 26% 35% 15%	530
	11th to 20th	Day	7% 2 10% 25% 35% 21%	530
	21st to end	Day	14% 6% 20% 22% 26% 12%	530
October	1st to 10th	Day	11% 3 18% 32% 26% 10%	530
	11th to 20th	Day	13% 9% 33% 25% 13% 7%	349
	21st to end	Day	25% 10% 29% 23% 11% 1	363
November	1st to 10th	Day	13% 8% 32% 29% 14% 4	330
	11th to 20th	Day	17% 15% 30% 24% 12% 2	330
	21st to end	Day	23% 12% 31% 23% 10% 1	330
December	1st to 10th	Day	20% 16% 33% 21% 7% 3	330
	11th to 20th	Day	22% 15% 30% 26% 8%	330
	21st to end	Day	29% 16% 33% 17% 5%	363
January	1st to 10th	Night	20% 13% 34% 28% 4	1050
	11th to 20th	Night	24% 14% 37% 21% 5%	1050
	21st to end	Night	21% 11% 37% 28% 2	1146
February	1st to 10th	Night	24% 11% 36% 26% 3	840
	11th to 20th	Night	22% 14% 36% 26% 2	823
	21st to end	Night	23% 13% 38% 24% 2	692
March	1st to 10th	Night	22% 12% 34% 27% 4	840
	11th to 20th	Night	21% 9% 34% 28% 6%	831
	21st to end	Night	23% 9% 30% 30% 6% 1	693
April	1st to 10th	Night	16% 9% 34% 32% 9%	630
	11th to 20th	Night	19% 9% 31% 29% 12% 1	623
	21st to end	Night	20% 6% 31% 34% 9% 1	630
May	1st to 10th	Night	18% 6% 27% 36% 13% 1	630
	11th to 20th	Night	15% 4 23% 35% 20% 3	609
	21st to end	Night	9% 2 20% 39% 28% 3	650
June	1st to 10th	Night	11% 4 16% 40% 27% 2	600
	11th to 20th	Night	6% 1 12% 46% 32% 2	600
	21st to end	Night	4 1 16% 41% 34% 4	600
July	1st to 10th	Night	9% 2 9% 30% 42% 8%	585
	11th to 20th	Night	5% 3 10% 32% 45% 5%	582
	21st to end	Night	8% 1 9% 35% 41% 6%	657
August	1st to 10th	Night	8% 1 7% 34% 43% 6%	597
	11th to 20th	Night	7% 1 8% 37% 41% 6%	594
	21st to end	Night	7% 1 7% 34% 43% 8%	660
September	1st to 10th	Night	10% 2 10% 33% 39% 7%	600
	11th to 20th	Night	10% 2 10% 32% 40% 7%	735
	21st to end	Night	13% 4 19% 30% 28% 6%	800
October	1st to 10th	Night	9% 3 18% 37% 28% 5%	800
	11th to 20th	Night	17% 6% 24% 34% 18% 2	800
	21st to end	Night	21% 7% 31% 29% 11% 1	880
November	1st to 10th	Night	14% 6% 30% 35% 14% 1	820
	11th to 20th	Night	16% 9% 33% 31% 12%	840
	21st to end	Night	22% 9% 28% 31% 10%	840
December	1st to 10th	Night	23% 8% 29% 33% 6%	851
	11th to 20th	Night	23% 8% 32% 30% 6%	1050
	21st to end	Night	28% 11% 32% 25% 4	1155

Page generated at 2026-05-14 00:59:00. Version ac07

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