Sperry Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sperry Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	17% 6% 2 6% 4 4 4 4 5% 7% 40%	126176
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	122532
Transparency	Day	41% 6% 12% 22% 17% 3	129703
	Night	41% 5% 11% 22% 17% 3	127092
Seeing	Day	20% 15% 30% 22% 10% 3	16056
	Night	20% 12% 32% 27% 9% 1	25182

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	13% 6% 2 4 4 4 4 4 6% 5% 50%	3597
	11th to 20th	Day	14% 8% 3 6% 5% 4 5% 4 5% 5% 42%	3570
	21st to end	Day	18% 10% 4 7% 4 3 3 2 4 5% 40%	4356
February	1st to 10th	Day	17% 8% 3 7% 4 4 4 3 5% 6% 40%	1800
	11th to 20th	Day	15% 8% 2 7% 5% 4 4 3 4 6% 43%	1945
	21st to end	Day	19% 8% 2 6% 4 4 4 3 4 7% 38%	1650
March	1st to 10th	Day	22% 6% 2 4 3 3 3 3 5% 5% 43%	4476
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 4 5% 7% 43%	4828
	21st to end	Day	18% 6% 2 7% 3 3 3 3 4 6% 44%	5896
April	1st to 10th	Day	18% 8% 2 5% 4 4 5% 3 5% 6% 38%	2400
	11th to 20th	Day	18% 6% 2 6% 4 4 4 4 5% 8% 40%	2374
	21st to end	Day	19% 6% 2 5% 4 4 4 3 5% 7% 40%	2402
May	1st to 10th	Day	29% 7% 2 5% 4 4 4 4 5% 7% 29%	5886
	11th to 20th	Day	25% 8% 3 6% 4 4 3 3 5% 6% 35%	6309
	21st to end	Day	21% 5% 2 5% 4 4 5% 3 6% 8% 36%	6949
June	1st to 10th	Day	16% 5% 2 5% 4 5% 6% 4 7% 9% 37%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 4 5% 4 6% 8% 37%	2800
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 6% 9% 40%	2800
July	1st to 10th	Day	10% 7% 2 7% 5% 5% 6% 5% 7% 10% 36%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 6% 8% 11% 32%	6360
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	6926
August	1st to 10th	Day	20% 5% 2 6% 5% 4 5% 4 6% 9% 33%	4276
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 4 5% 8% 44%	3850
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 8% 47%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 4 5% 4 4 7% 44%	2316
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 5% 8% 49%	2209
	21st to end	Day	20% 5% 2 7% 4 3 4 4 4 6% 40%	2090
October	1st to 10th	Day	20% 6% 2 5% 3 3 4 4 5% 7% 41%	1967
	11th to 20th	Day	15% 5% 2 5% 4 3 4 4 4 7% 48%	1900
	21st to end	Day	20% 5% 2 4 3 4 4 3 4 6% 44%	1957
November	1st to 10th	Day	14% 6% 2 5% 3 4 3 2 4 5% 53%	1761
	11th to 20th	Day	18% 5% 2 4 3 3 4 3 5% 6% 49%	1800
	21st to end	Day	16% 6% 2 4 3 4 4 4 5% 8% 44%	1708
December	1st to 10th	Day	16% 7% 2 5% 3 3 3 3 5% 7% 46%	2600
	11th to 20th	Day	23% 7% 2 3 3 2 3 3 4 5% 44%	2986
	21st to end	Day	18% 7% 2 6% 4 4 4 4 6% 7% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 50%	6349
	11th to 20th	Night	14% 8% 2 5% 3 4 4 3 4 5% 47%	6315
	21st to end	Night	18% 10% 2 5% 4 4 4 3 4 5% 41%	6582
February	1st to 10th	Night	16% 9% 3 5% 4 4 3 4 5% 5% 42%	2600
	11th to 20th	Night	18% 8% 3 6% 5% 3 3 3 5% 5% 42%	2339
	21st to end	Night	19% 8% 2 5% 3 4 4 3 4 5% 42%	1980
March	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 5% 44%	5364
	11th to 20th	Night	14% 10% 2 5% 3 3 4 3 4 5% 47%	5127
	21st to end	Night	18% 8% 3 6% 4 3 4 3 4 5% 41%	5021
April	1st to 10th	Night	20% 7% 2 6% 4 5% 5% 3 5% 7% 37%	2000
	11th to 20th	Night	18% 8% 2 6% 3 4 4 3 5% 8% 40%	1975
	21st to end	Night	18% 6% 2 5% 4 3 4 4 4 7% 43%	1995
May	1st to 10th	Night	28% 9% 3 7% 4 4 4 3 5% 7% 28%	4054
	11th to 20th	Night	24% 9% 3 6% 5% 3 3 3 4 5% 35%	3626
	21st to end	Night	22% 7% 2 6% 4 4 4 3 5% 8% 35%	3926
June	1st to 10th	Night	17% 6% 2 5% 4 4 5% 5% 6% 8% 40%	1600
	11th to 20th	Night	15% 6% 2 5% 4 5% 4 4 6% 9% 40%	1600
	21st to end	Night	12% 5% 2 5% 4 5% 5% 4 6% 10% 44%	1600
July	1st to 10th	Night	10% 7% 3 8% 5% 5% 5% 4 6% 9% 39%	3600
	11th to 20th	Night	16% 8% 3 7% 4 5% 5% 4 7% 9% 32%	3600
	21st to end	Night	14% 6% 2 5% 4 4 5% 4 5% 8% 43%	3926
August	1st to 10th	Night	23% 6% 2 6% 5% 4 4 3 6% 8% 32%	2838
	11th to 20th	Night	14% 7% 2 7% 4 5% 5% 4 5% 7% 40%	3197
	21st to end	Night	10% 5% 2 5% 3 4 4 3 5% 7% 52%	3586
September	1st to 10th	Night	15% 6% 2 6% 5% 4 4 4 4 6% 44%	1931
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 4 7% 52%	1970
	21st to end	Night	19% 7% 2 5% 4 4 3 3 4 5% 44%	2090
October	1st to 10th	Night	18% 7% 2 5% 4 4 3 3 5% 6% 42%	2209
	11th to 20th	Night	15% 5% 2 5% 3 3 4 2 4 6% 51%	2280
	21st to end	Night	20% 7% 2 5% 3 2 3 3 4 5% 45%	2641
November	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 4 5% 53%	2537
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 4 5% 48%	2600
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 7% 45%	2690
December	1st to 10th	Night	14% 8% 3 5% 4 3 4 3 4 6% 47%	4580
	11th to 20th	Night	25% 7% 2 5% 3 3 3 3 3 5% 43%	5208
	21st to end	Night	21% 9% 3 6% 3 3 3 3 4 5% 41%	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	34% 5% 9% 30% 20% 3	3598
	11th to 20th	Day	43% 5% 10% 21% 16% 6%	3600
	21st to end	Day	48% 4 10% 21% 16% 1	4356
February	1st to 10th	Day	45% 6% 13% 22% 13% 2	1800
	11th to 20th	Day	44% 4 9% 24% 16% 3	1954
	21st to end	Day	46% 4 10% 21% 14% 3	1650
March	1st to 10th	Day	41% 4 11% 20% 15% 8%	4479
	11th to 20th	Day	38% 6% 13% 26% 13% 4	4828
	21st to end	Day	42% 5% 10% 13% 25% 6%	5896
April	1st to 10th	Day	45% 6% 9% 19% 16% 5%	2400
	11th to 20th	Day	42% 6% 11% 21% 17% 3	2374
	21st to end	Day	42% 6% 9% 20% 18% 4	2402
May	1st to 10th	Day	54% 6% 13% 15% 10% 1	5897
	11th to 20th	Day	50% 6% 10% 19% 14% 2	6313
	21st to end	Day	45% 7% 10% 22% 15% 1	6949
June	1st to 10th	Day	41% 8% 13% 21% 16% 1	2800
	11th to 20th	Day	44% 8% 12% 22% 12% 2	2800
	21st to end	Day	38% 9% 14% 21% 16% 1	2800
July	1st to 10th	Day	41% 9% 12% 21% 15% 1	6290
	11th to 20th	Day	42% 9% 18% 19% 10% 1	6285
	21st to end	Day	40% 6% 11% 19% 21% 2	6728
August	1st to 10th	Day	47% 8% 13% 21% 10% 1	4262
	11th to 20th	Day	37% 7% 13% 29% 13% 1	3886
	21st to end	Day	33% 6% 11% 27% 21% 2	4312
September	1st to 10th	Day	39% 6% 11% 21% 19% 3	2316
	11th to 20th	Day	34% 4 11% 23% 24% 3	2209
	21st to end	Day	45% 6% 9% 18% 19% 3	2090
October	1st to 10th	Day	38% 6% 9% 18% 24% 6%	3266
	11th to 20th	Day	25% 4 11% 24% 30% 5%	3140
	21st to end	Day	27% 4 14% 25% 26% 4	3208
November	1st to 10th	Day	35% 4 12% 24% 22% 3	1761
	11th to 20th	Day	37% 4 10% 24% 22% 4	1800
	21st to end	Day	38% 5% 13% 29% 11% 2	1708
December	1st to 10th	Day	38% 4 15% 27% 13% 2	2600
	11th to 20th	Day	42% 4 14% 24% 14% 2	2986
	21st to end	Day	43% 7% 10% 23% 15% 2	3960
January	1st to 10th	Night	36% 4 9% 26% 22% 4	6349
	11th to 20th	Night	39% 4 14% 21% 16% 6%	6360
	21st to end	Night	45% 4 9% 20% 17% 5%	6582
February	1st to 10th	Night	44% 5% 12% 24% 12% 2	2600
	11th to 20th	Night	45% 4 9% 20% 18% 4	2341
	21st to end	Night	43% 5% 7% 25% 17% 4	1980
March	1st to 10th	Night	43% 4 10% 22% 13% 9%	5374
	11th to 20th	Night	39% 5% 15% 23% 13% 4	5128
	21st to end	Night	45% 5% 7% 15% 22% 6%	5021
April	1st to 10th	Night	47% 6% 9% 18% 16% 4	2000
	11th to 20th	Night	44% 5% 10% 18% 20% 2	1975
	21st to end	Night	41% 6% 9% 21% 19% 4	1995
May	1st to 10th	Night	56% 6% 13% 14% 10% 1	4060
	11th to 20th	Night	51% 6% 12% 14% 14% 2	3627
	21st to end	Night	47% 6% 13% 22% 10% 2	3927
June	1st to 10th	Night	41% 7% 11% 20% 20% 2	1600
	11th to 20th	Night	40% 8% 14% 25% 12% 1	1600
	21st to end	Night	35% 8% 16% 23% 16% 2	1600
July	1st to 10th	Night	42% 9% 14% 20% 14% 2	3560
	11th to 20th	Night	47% 8% 15% 20% 11% 1	3552
	21st to end	Night	37% 7% 15% 21% 19% 1	3809
August	1st to 10th	Night	49% 7% 16% 17% 10% 1	2828
	11th to 20th	Night	42% 8% 11% 28% 11% 1	3227
	21st to end	Night	31% 6% 10% 30% 21% 1	3586
September	1st to 10th	Night	40% 7% 11% 23% 18% 2	1931
	11th to 20th	Night	33% 6% 12% 22% 25% 3	1970
	21st to end	Night	43% 4 8% 22% 21% 2	2090
October	1st to 10th	Night	38% 6% 11% 23% 20% 2	3690
	11th to 20th	Night	24% 4 10% 27% 30% 4	3779
	21st to end	Night	30% 4 11% 30% 23% 2	4340
November	1st to 10th	Night	33% 5% 11% 24% 22% 4	2537
	11th to 20th	Night	39% 5% 8% 24% 20% 4	2600
	21st to end	Night	38% 5% 14% 27% 15% 1	2690
December	1st to 10th	Night	39% 4 13% 30% 12% 2	4580
	11th to 20th	Night	45% 4 10% 23% 15% 3	5208
	21st to end	Night	46% 5% 10% 18% 17% 4	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	21% 31% 30% 15% 4 1	200
	11th to 20th	Day	25% 32% 33% 9% 2	200
	21st to end	Day	23% 23% 37% 11% 4 1	307
February	1st to 10th	Day	27% 24% 36% 11% 3	300
	11th to 20th	Day	25% 22% 38% 13% 2	493
	21st to end	Day	25% 22% 35% 15% 4	414
March	1st to 10th	Day	23% 25% 32% 17% 4	500
	11th to 20th	Day	21% 23% 36% 14% 4 1	500
	21st to end	Day	27% 22% 33% 13% 4 1	550
April	1st to 10th	Day	28% 21% 32% 13% 5% 1	500
	11th to 20th	Day	24% 22% 30% 14% 9% 1	492
	21st to end	Day	28% 18% 30% 18% 4 2	500
May	1st to 10th	Day	30% 19% 28% 15% 6% 2	503
	11th to 20th	Day	24% 13% 33% 20% 8% 1	527
	21st to end	Day	21% 8% 28% 26% 14% 3	573
June	1st to 10th	Day	18% 11% 31% 25% 14% 2	530
	11th to 20th	Day	17% 12% 32% 26% 12% 2	530
	21st to end	Day	16% 8% 32% 28% 14% 2	530
July	1st to 10th	Day	13% 6% 24% 34% 17% 6%	525
	11th to 20th	Day	10% 3 27% 33% 23% 5%	524
	21st to end	Day	14% 6% 28% 33% 15% 3	576
August	1st to 10th	Day	11% 7% 25% 33% 19% 6%	526
	11th to 20th	Day	14% 5% 22% 35% 18% 6%	519
	21st to end	Day	13% 7% 27% 30% 16% 6%	583
September	1st to 10th	Day	17% 8% 25% 26% 17% 7%	502
	11th to 20th	Day	15% 7% 26% 26% 16% 10%	490
	21st to end	Day	23% 11% 28% 23% 11% 4	490
October	1st to 10th	Day	24% 8% 28% 22% 13% 5%	490
	11th to 20th	Day	16% 16% 29% 26% 11% 2	490
	21st to end	Day	24% 12% 33% 20% 9% 2	469
November	1st to 10th	Day	15% 15% 33% 24% 11% 3	395
	11th to 20th	Day	22% 19% 32% 22% 6%	400
	21st to end	Day	20% 19% 34% 20% 6% 1	308
December	1st to 10th	Day	23% 29% 28% 15% 6% 1	200
	11th to 20th	Day	30% 25% 28% 14% 4	200
	21st to end	Day	24% 17% 36% 16% 6%	220
January	1st to 10th	Night	20% 20% 38% 19% 3	800
	11th to 20th	Night	24% 18% 33% 23% 3	800
	21st to end	Night	18% 18% 43% 19% 2	880
February	1st to 10th	Night	22% 15% 40% 22% 1	800
	11th to 20th	Night	23% 20% 38% 17% 1	783
	21st to end	Night	22% 19% 37% 19% 3	660
March	1st to 10th	Night	22% 18% 36% 20% 4	800
	11th to 20th	Night	22% 16% 37% 21% 3 1	800
	21st to end	Night	21% 15% 41% 19% 5%	880
April	1st to 10th	Night	24% 18% 35% 19% 4	800
	11th to 20th	Night	22% 13% 37% 22% 5%	791
	21st to end	Night	21% 13% 36% 25% 5% 1	795
May	1st to 10th	Night	26% 12% 34% 21% 7%	600
	11th to 20th	Night	23% 11% 34% 25% 8%	419
	21st to end	Night	20% 5% 24% 38% 13% 1	433
June	1st to 10th	Night	19% 6% 29% 35% 12%	400
	11th to 20th	Night	15% 6% 28% 37% 14%	400
	21st to end	Night	11% 4 28% 47% 11%	400
July	1st to 10th	Night	12% 6% 23% 39% 19% 2	390
	11th to 20th	Night	14% 4 19% 44% 18% 1	388
	21st to end	Night	12% 3 24% 45% 15% 1	438
August	1st to 10th	Night	13% 3 18% 44% 21% 2	546
	11th to 20th	Night	15% 4 18% 38% 24% 1	790
	21st to end	Night	15% 3 21% 38% 20% 2	880
September	1st to 10th	Night	15% 4 27% 34% 18% 2	773
	11th to 20th	Night	17% 4 22% 33% 20% 4	760
	21st to end	Night	21% 6% 26% 32% 14% 1	760
October	1st to 10th	Night	22% 7% 29% 25% 15% 2	760
	11th to 20th	Night	18% 10% 30% 31% 11%	760
	21st to end	Night	23% 9% 34% 24% 9%	836
November	1st to 10th	Night	15% 11% 34% 28% 11% 1	780
	11th to 20th	Night	22% 15% 33% 24% 7%	800
	21st to end	Night	17% 17% 35% 24% 6% 1	800
December	1st to 10th	Night	24% 16% 33% 22% 5% 1	800
	11th to 20th	Night	23% 15% 37% 22% 3	800
	21st to end	Night	23% 13% 36% 25% 2	880

Page generated at 2025-09-01 01:51:04. Version ac07

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