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 2025-01-22.

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% 7% 3 7% 4 4 4 4 5% 7% 41%	104935
	Night	15% 8% 3 6% 4 4 4 3 5% 6% 43%	104501
Transparency	Day	40% 6% 11% 24% 17% 2	108581
	Night	40% 5% 10% 24% 18% 3	108983
Seeing	Day	15% 8% 23% 25% 20% 9%	15734
	Night	17% 7% 25% 31% 17% 2	25638

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% 5% 10% 5% 5% 4 4 4 6% 33%	2607
	11th to 20th	Day	17% 9% 2 8% 8% 6% 6% 3 4 6% 31%	2610
	21st to end	Day	26% 16% 5% 7% 4 3 3 3 3 4 25%	3111
February	1st to 10th	Day	17% 12% 4 6% 4 3 3 3 5% 6% 36%	1900
	11th to 20th	Day	17% 9% 4 8% 4 4 4 3 6% 5% 36%	1848
	21st to end	Day	15% 10% 3 9% 5% 5% 5% 4 5% 6% 34%	1570
March	1st to 10th	Day	23% 13% 3 7% 3 3 3 3 3 4 35%	3120
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 5% 7% 47%	3269
	21st to end	Day	13% 9% 2 6% 5% 5% 4 3 6% 7% 39%	4165
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 9% 43%	2280
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 4 4 7% 42%	2254
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 7% 39%	2289
May	1st to 10th	Day	16% 6% 3 8% 5% 5% 6% 5% 7% 10% 30%	4082
	11th to 20th	Day	18% 7% 2 7% 4 4 5% 3 5% 8% 37%	4081
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 8% 41%	4657
June	1st to 10th	Day	11% 6% 2 5% 4 4 5% 5% 6% 8% 45%	2660
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 4 7% 9% 49%	2660
	21st to end	Day	6% 5% 2 4 5% 5% 5% 5% 8% 9% 45%	2660
July	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 7% 9% 42%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 10% 49%	4440
	21st to end	Day	9% 6% 3 10% 6% 6% 7% 5% 7% 9% 31%	4765
August	1st to 10th	Day	5% 3 2 5% 3 4 4 3 6% 11% 54%	3860
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 5% 7% 10% 43%	3729
	21st to end	Day	5% 4 2 5% 3 4 4 3 5% 9% 57%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 5% 7% 48%	2280
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 5% 8% 48%	2133
	21st to end	Day	11% 5% 2 6% 4 4 3 3 4 6% 52%	2090
October	1st to 10th	Day	9% 4 1 5% 3 3 3 3 4 7% 57%	2090
	11th to 20th	Day	13% 5% 2 5% 3 4 3 3 4 7% 50%	1918
	21st to end	Day	17% 7% 2 5% 3 3 4 3 5% 5% 46%	1932
November	1st to 10th	Day	9% 6% 2 5% 3 3 4 4 6% 6% 52%	1762
	11th to 20th	Day	15% 6% 3 6% 4 4 3 3 5% 6% 45%	1800
	21st to end	Day	19% 8% 3 6% 3 3 3 3 4 6% 41%	1800
December	1st to 10th	Day	13% 10% 3 5% 4 3 3 2 3 5% 48%	2903
	11th to 20th	Day	14% 10% 4 10% 6% 4 4 4 5% 6% 32%	3008
	21st to end	Day	26% 14% 5% 9% 5% 4 4 3 4 4 23%	3982
January	1st to 10th	Night	17% 12% 4 6% 4 4 3 4 5% 6% 34%	4568
	11th to 20th	Night	14% 13% 4 9% 5% 5% 5% 3 4 5% 32%	4580
	21st to end	Night	24% 13% 6% 6% 4 4 3 3 3 6% 27%	4572
February	1st to 10th	Night	19% 11% 4 8% 5% 4 4 3 4 5% 33%	2280
	11th to 20th	Night	16% 8% 3 8% 4 4 3 4 4 5% 40%	2218
	21st to end	Night	19% 9% 3 7% 5% 4 4 4 5% 5% 37%	1884
March	1st to 10th	Night	24% 10% 2 6% 4 3 4 3 4 5% 35%	3733
	11th to 20th	Night	13% 6% 2 6% 3 4 4 3 4 6% 50%	3720
	21st to end	Night	17% 8% 2 7% 5% 4 5% 3 5% 6% 38%	3466
April	1st to 10th	Night	12% 7% 3 5% 5% 4 4 3 5% 7% 45%	1900
	11th to 20th	Night	16% 6% 2 5% 4 4 4 3 5% 6% 44%	1875
	21st to end	Night	18% 7% 2 6% 5% 3 4 4 5% 7% 38%	1888
May	1st to 10th	Night	16% 8% 2 7% 5% 4 5% 4 6% 8% 38%	2813
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 4 6% 43%	2812
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	2900
June	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 6% 10% 47%	1520
	11th to 20th	Night	5% 3 1 5% 4 4 4 4 6% 9% 54%	1520
	21st to end	Night	4 4 1 4 3 3 5% 4 8% 9% 53%	1520
July	1st to 10th	Night	10% 5% 2 6% 3 3 3 4 6% 8% 49%	2530
	11th to 20th	Night	5% 4 2 4 4 4 4 3 5% 9% 57%	2530
	21st to end	Night	8% 7% 3 9% 6% 5% 6% 4 8% 8% 35%	2796
August	1st to 10th	Night	5% 2 2 4 3 4 3 3 6% 8% 60%	3065
	11th to 20th	Night	6% 4 2 5% 4 4 5% 3 6% 9% 53%	3100
	21st to end	Night	4 3 2 4 3 3 4 3 5% 10% 59%	3476
September	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 6% 55%	1900
	11th to 20th	Night	9% 4 2 5% 4 4 3 4 6% 8% 52%	2028
	21st to end	Night	12% 5% 2 4 4 3 3 3 5% 6% 52%	2090
October	1st to 10th	Night	7% 4 2 6% 3 3 3 3 4 5% 59%	2090
	11th to 20th	Night	14% 5% 1 5% 3 3 4 2 4 6% 54%	2258
	21st to end	Night	17% 8% 2 5% 3 3 3 3 4 6% 46%	2665
November	1st to 10th	Night	12% 6% 2 4 3 3 4 3 4 5% 54%	2536
	11th to 20th	Night	14% 6% 2 6% 5% 3 3 2 4 7% 49%	2600
	21st to end	Night	18% 6% 3 5% 4 4 3 2 4 5% 45%	2600
December	1st to 10th	Night	18% 10% 3 5% 3 3 3 3 4 5% 44%	4273
	11th to 20th	Night	18% 8% 3 6% 4 4 3 3 4 6% 40%	5199
	21st to end	Night	27% 13% 4 8% 5% 4 4 3 4 4 25%	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	51% 5% 13% 22% 8% 1	2608
	11th to 20th	Day	55% 4 11% 21% 9%	2610
	21st to end	Day	63% 4 7% 18% 6% 1	3111
February	1st to 10th	Day	49% 4 10% 23% 12% 2	1900
	11th to 20th	Day	49% 4 10% 21% 13% 3	1858
	21st to end	Day	51% 5% 9% 17% 14% 4	1570
March	1st to 10th	Day	55% 4 5% 18% 14% 4	3120
	11th to 20th	Day	37% 4 10% 29% 16% 4	3269
	21st to end	Day	43% 5% 13% 22% 16% 1	4165
April	1st to 10th	Day	38% 6% 13% 25% 16% 1	2280
	11th to 20th	Day	42% 5% 11% 23% 16% 3	2254
	21st to end	Day	43% 6% 11% 27% 11% 2	2289
May	1st to 10th	Day	46% 9% 14% 21% 9% 1	4120
	11th to 20th	Day	44% 9% 10% 24% 13% 1	4085
	21st to end	Day	39% 8% 11% 19% 22% 1	4670
June	1st to 10th	Day	34% 8% 13% 24% 19% 3	2660
	11th to 20th	Day	28% 9% 12% 27% 22% 3	2660
	21st to end	Day	31% 9% 14% 24% 17% 5%	2660
July	1st to 10th	Day	37% 7% 12% 26% 16% 3	4371
	11th to 20th	Day	28% 7% 13% 34% 16% 2	4365
	21st to end	Day	47% 9% 14% 20% 8% 1	4615
August	1st to 10th	Day	25% 6% 15% 34% 19% 1	3847
	11th to 20th	Day	34% 7% 13% 27% 18% 2	3765
	21st to end	Day	25% 5% 12% 35% 19% 3	4179
September	1st to 10th	Day	35% 7% 9% 24% 19% 5%	2280
	11th to 20th	Day	34% 6% 8% 25% 23% 3	2133
	21st to end	Day	33% 5% 7% 24% 26% 4	2090
October	1st to 10th	Day	18% 2 6% 24% 44% 6%	3480
	11th to 20th	Day	23% 4 8% 25% 33% 7%	3158
	21st to end	Day	31% 3 6% 27% 28% 5%	3157
November	1st to 10th	Day	32% 5% 11% 27% 19% 6%	1762
	11th to 20th	Day	40% 4 7% 24% 20% 5%	1800
	21st to end	Day	45% 4 8% 24% 16% 3	1800
December	1st to 10th	Day	40% 3 10% 30% 16% 1	2903
	11th to 20th	Day	52% 3 11% 22% 10% 1	3005
	21st to end	Day	64% 4 12% 14% 7%	3982
January	1st to 10th	Night	50% 4 11% 25% 10% 1	4568
	11th to 20th	Night	54% 5% 8% 21% 11% 2	4580
	21st to end	Night	59% 4 8% 18% 10% 1	4572
February	1st to 10th	Night	53% 4 9% 21% 12% 1	2280
	11th to 20th	Night	47% 4 8% 24% 15% 2	2219
	21st to end	Night	50% 4 9% 18% 15% 4	1884
March	1st to 10th	Night	52% 5% 8% 15% 19% 3	3733
	11th to 20th	Night	35% 5% 13% 22% 16% 7%	3720
	21st to end	Night	46% 5% 11% 23% 13% 1	3466
April	1st to 10th	Night	37% 5% 11% 27% 18% 3	1900
	11th to 20th	Night	40% 4 9% 22% 19% 6%	1875
	21st to end	Night	45% 6% 9% 21% 17% 3	1888
May	1st to 10th	Night	43% 9% 15% 24% 8% 1	2840
	11th to 20th	Night	41% 7% 11% 25% 14% 1	2813
	21st to end	Night	33% 9% 15% 23% 17% 3	2908
June	1st to 10th	Night	33% 7% 11% 24% 22% 3	1520
	11th to 20th	Night	25% 8% 13% 23% 26% 5%	1520
	21st to end	Night	23% 8% 17% 26% 19% 7%	1520
July	1st to 10th	Night	32% 6% 12% 27% 18% 4	2490
	11th to 20th	Night	26% 6% 11% 36% 18% 3	2482
	21st to end	Night	44% 9% 13% 22% 11% 1	2707
August	1st to 10th	Night	22% 6% 14% 38% 18% 1	3054
	11th to 20th	Night	27% 7% 14% 30% 21% 1	3130
	21st to end	Night	21% 5% 11% 37% 23% 3	3476
September	1st to 10th	Night	30% 6% 8% 27% 23% 6%	1900
	11th to 20th	Night	31% 5% 8% 26% 25% 5%	2028
	21st to end	Night	32% 5% 9% 25% 22% 7%	2090
October	1st to 10th	Night	19% 4 5% 14% 50% 9%	3480
	11th to 20th	Night	22% 2 5% 24% 39% 7%	3744
	21st to end	Night	31% 4 7% 23% 27% 9%	4390
November	1st to 10th	Night	32% 3 10% 24% 25% 5%	2536
	11th to 20th	Night	36% 3 8% 23% 22% 7%	2600
	21st to end	Night	42% 4 8% 24% 19% 2	2600
December	1st to 10th	Night	44% 4 9% 24% 18% 1	4273
	11th to 20th	Night	46% 4 11% 22% 17% 1	5201
	21st to end	Night	61% 4 10% 17% 7% 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	25% 15% 31% 21% 7%	300
	11th to 20th	Day	23% 16% 37% 20% 4	300
	21st to end	Day	22% 19% 36% 17% 5%	300
February	1st to 10th	Day	23% 14% 37% 20% 4 1	270
	11th to 20th	Day	27% 21% 34% 14% 2 1	270
	21st to end	Day	28% 16% 32% 20% 4	225
March	1st to 10th	Day	23% 17% 37% 18% 5%	270
	11th to 20th	Day	24% 17% 36% 17% 5% 1	303
	21st to end	Day	27% 14% 31% 20% 8% 1	506
April	1st to 10th	Day	16% 16% 32% 25% 10% 1	460
	11th to 20th	Day	19% 15% 26% 25% 13% 1	452
	21st to end	Day	21% 9% 35% 22% 9% 3	460
May	1st to 10th	Day	19% 10% 28% 29% 11% 3	463
	11th to 20th	Day	18% 6% 24% 26% 19% 6%	487
	21st to end	Day	11% 4 24% 29% 22% 10%	632
June	1st to 10th	Day	13% 4 17% 26% 27% 12%	680
	11th to 20th	Day	6% 4 17% 33% 29% 12%	680
	21st to end	Day	8% 3 20% 34% 25% 11%	680
July	1st to 10th	Day	10% 3 13% 24% 27% 23%	671
	11th to 20th	Day	6% 3 12% 28% 34% 16%	668
	21st to end	Day	7% 3 11% 29% 34% 16%	715
August	1st to 10th	Day	8% 3 8% 29% 34% 18%	486
	11th to 20th	Day	15% 3 11% 27% 30% 14%	479
	21st to end	Day	8% 2 10% 21% 36% 23%	539
September	1st to 10th	Day	10% 2 12% 25% 34% 16%	490
	11th to 20th	Day	8% 2 10% 26% 35% 19%	490
	21st to end	Day	14% 6% 20% 22% 25% 12%	490
October	1st to 10th	Day	11% 3 19% 32% 25% 11%	490
	11th to 20th	Day	13% 10% 35% 24% 12% 7%	318
	21st to end	Day	23% 10% 29% 25% 12% 1	330
November	1st to 10th	Day	12% 8% 33% 29% 15% 4	300
	11th to 20th	Day	16% 16% 29% 25% 12% 2	300
	21st to end	Day	22% 11% 31% 24% 10% 1	300
December	1st to 10th	Day	21% 16% 32% 21% 7% 3	300
	11th to 20th	Day	22% 14% 28% 27% 9%	300
	21st to end	Day	30% 16% 31% 17% 6%	330
January	1st to 10th	Night	20% 14% 35% 27% 5%	1000
	11th to 20th	Night	24% 14% 36% 22% 5%	1000
	21st to end	Night	22% 10% 37% 28% 3	1044
February	1st to 10th	Night	24% 12% 35% 26% 3	760
	11th to 20th	Night	21% 14% 38% 26% 2	743
	21st to end	Night	23% 13% 39% 23% 2	628
March	1st to 10th	Night	22% 11% 35% 28% 4	760
	11th to 20th	Night	22% 8% 34% 29% 7%	752
	21st to end	Night	24% 8% 29% 30% 7% 1	627
April	1st to 10th	Night	15% 9% 34% 33% 9%	570
	11th to 20th	Night	20% 7% 30% 30% 12% 1	563
	21st to end	Night	20% 6% 31% 33% 9% 1	570
May	1st to 10th	Night	17% 5% 26% 36% 14% 1	570
	11th to 20th	Night	15% 4 22% 35% 21% 3	561
	21st to end	Night	8% 1 19% 39% 29% 3	617
June	1st to 10th	Night	11% 4 15% 39% 28% 2	570
	11th to 20th	Night	6% 1 12% 47% 32% 2	570
	21st to end	Night	4 1 16% 42% 33% 4	570
July	1st to 10th	Night	9% 1 9% 32% 41% 8%	555
	11th to 20th	Night	5% 3 10% 31% 45% 5%	552
	21st to end	Night	8% 1 9% 36% 39% 6%	624
August	1st to 10th	Night	8% 1 7% 35% 43% 6%	567
	11th to 20th	Night	7% 1 8% 39% 39% 6%	564
	21st to end	Night	7% 1 7% 32% 44% 8%	627
September	1st to 10th	Night	10% 2 10% 32% 39% 7%	570
	11th to 20th	Night	10% 2 10% 31% 41% 6%	698
	21st to end	Night	13% 3 20% 30% 28% 6%	760
October	1st to 10th	Night	9% 3 19% 37% 28% 5%	760
	11th to 20th	Night	18% 6% 25% 33% 17% 2	760
	21st to end	Night	21% 7% 31% 30% 12% 1	836
November	1st to 10th	Night	14% 6% 29% 35% 14% 1	780
	11th to 20th	Night	16% 8% 33% 31% 12%	800
	21st to end	Night	22% 9% 28% 31% 10%	800
December	1st to 10th	Night	23% 8% 29% 33% 6%	810
	11th to 20th	Night	23% 7% 32% 30% 7%	1000
	21st to end	Night	28% 11% 32% 25% 4	1100

Page generated at 2025-01-19 02:06:44. Version ac07

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