Olin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Olin Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	18% 7% 2 6% 4 4 4 3 5% 7% 40%	125445
	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	122615
Transparency	Day	27% 40% 8% 13% 10% 2	128991
	Night	28% 39% 7% 13% 11% 2	127156
Seeing	Day	21% 16% 31% 22% 9% 2	16040
	Night	21% 12% 31% 26% 9% 1	24980

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% 8% 2 5% 3 4 4 3 5% 5% 47%	3597
	11th to 20th	Day	15% 9% 2 5% 4 4 3 4 4 5% 47%	3570
	21st to end	Day	19% 8% 4 6% 4 4 3 2 4 4 42%	3978
February	1st to 10th	Day	18% 7% 2 5% 5% 5% 5% 3 5% 6% 38%	1926
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 4 5% 7% 41%	1948
	21st to end	Day	20% 8% 3 7% 4 4 3 4 5% 6% 38%	1650
March	1st to 10th	Day	23% 9% 2 5% 3 3 3 2 4 5% 41%	4476
	11th to 20th	Day	14% 9% 2 5% 4 4 5% 4 6% 7% 41%	4695
	21st to end	Day	22% 8% 2 6% 4 4 4 4 6% 7% 34%	5893
April	1st to 10th	Day	19% 8% 2 5% 4 4 4 3 5% 6% 39%	2400
	11th to 20th	Day	20% 7% 2 5% 3 4 3 3 5% 7% 41%	2374
	21st to end	Day	21% 6% 2 5% 3 4 4 3 5% 7% 40%	2550
May	1st to 10th	Day	29% 10% 3 6% 4 3 4 3 5% 7% 27%	5886
	11th to 20th	Day	21% 9% 3 6% 4 4 4 3 5% 6% 35%	5984
	21st to end	Day	21% 7% 2 6% 4 4 4 4 6% 7% 35%	6949
June	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 5% 6% 8% 36%	2800
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 5% 5% 9% 38%	2800
	21st to end	Day	14% 6% 3 6% 4 4 5% 4 7% 9% 39%	2800
July	1st to 10th	Day	18% 8% 3 6% 4 4 4 4 6% 9% 34%	6360
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 6% 10% 36%	6360
	21st to end	Day	17% 6% 2 5% 4 3 4 4 5% 8% 41%	6926
August	1st to 10th	Day	20% 8% 3 6% 5% 5% 5% 4 5% 8% 32%	4226
	11th to 20th	Day	13% 5% 2 7% 4 4 4 4 6% 9% 43%	3834
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 3 5% 4 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 6% 51%	2114
	21st to end	Day	20% 7% 2 6% 4 4 4 4 4 5% 41%	2090
October	1st to 10th	Day	22% 6% 2 5% 4 3 4 3 5% 6% 41%	2090
	11th to 20th	Day	15% 5% 2 5% 3 3 3 4 6% 7% 48%	1873
	21st to end	Day	18% 6% 2 6% 4 3 4 3 4 6% 44%	1881
November	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 4 5% 50%	1761
	11th to 20th	Day	17% 6% 1 4 3 3 4 3 6% 5% 50%	1800
	21st to end	Day	17% 6% 2 5% 4 3 4 3 4 6% 45%	1800
December	1st to 10th	Day	15% 7% 2 6% 4 3 4 3 4 6% 46%	2648
	11th to 20th	Day	22% 7% 2 5% 3 2 2 2 4 5% 48%	2986
	21st to end	Day	20% 10% 3 7% 4 3 3 2 3 5% 40%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 5% 7% 50%	6349
	11th to 20th	Night	15% 8% 3 7% 4 3 3 3 3 4 46%	6315
	21st to end	Night	17% 9% 2 7% 5% 5% 4 4 5% 6% 37%	6974
February	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 4 6% 40%	2471
	11th to 20th	Night	18% 8% 3 5% 4 4 3 3 4 7% 41%	2338
	21st to end	Night	19% 9% 3 5% 4 3 4 3 5% 4 40%	1980
March	1st to 10th	Night	22% 9% 2 5% 3 3 3 3 4 5% 40%	5364
	11th to 20th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 40%	5262
	21st to end	Night	22% 9% 2 6% 4 4 4 4 5% 5% 36%	5036
April	1st to 10th	Night	21% 8% 2 6% 4 5% 3 4 4 6% 38%	2000
	11th to 20th	Night	20% 7% 2 5% 4 4 3 3 4 6% 41%	1975
	21st to end	Night	22% 7% 2 5% 4 4 4 3 4 5% 40%	1848
May	1st to 10th	Night	28% 11% 3 7% 4 4 4 2 5% 6% 27%	4054
	11th to 20th	Night	23% 10% 3 6% 4 4 4 3 5% 7% 30%	3951
	21st to end	Night	23% 8% 3 6% 4 4 4 4 5% 7% 32%	3926
June	1st to 10th	Night	19% 7% 3 5% 5% 4 5% 3 5% 7% 38%	1600
	11th to 20th	Night	20% 7% 2 5% 3 3 4 4 6% 9% 37%	1600
	21st to end	Night	15% 8% 2 6% 4 3 4 4 5% 10% 39%	1600
July	1st to 10th	Night	21% 9% 3 7% 4 4 4 3 5% 8% 31%	3600
	11th to 20th	Night	23% 10% 3 7% 3 3 4 4 6% 8% 31%	3600
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	3926
August	1st to 10th	Night	24% 7% 3 7% 5% 4 5% 5% 6% 7% 28%	2759
	11th to 20th	Night	11% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	2990
	21st to end	Night	8% 4 2 4 3 3 4 4 5% 8% 54%	3476
September	1st to 10th	Night	15% 7% 2 7% 4 4 5% 4 6% 6% 41%	1900
	11th to 20th	Night	14% 7% 2 5% 3 2 3 2 4 6% 52%	2066
	21st to end	Night	19% 7% 3 7% 4 4 5% 4 5% 5% 38%	2090
October	1st to 10th	Night	19% 7% 2 6% 4 4 3 3 4 6% 41%	2090
	11th to 20th	Night	15% 5% 2 5% 4 4 3 4 5% 5% 49%	2303
	21st to end	Night	20% 7% 2 6% 4 3 4 2 5% 6% 42%	2717
November	1st to 10th	Night	13% 6% 2 5% 4 4 4 2 5% 6% 48%	2537
	11th to 20th	Night	19% 7% 2 5% 3 2 4 3 3 5% 48%	2600
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 6% 47%	2600
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 2 4 4 49%	4514
	11th to 20th	Night	26% 7% 2 5% 3 3 3 3 3 4 42%	5208
	21st to end	Night	19% 9% 3 6% 3 3 3 2 3 5% 43%	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	27% 27% 5% 21% 16% 3	3598
	11th to 20th	Day	26% 28% 9% 17% 13% 8%	3600
	21st to end	Day	36% 28% 7% 14% 13% 1	3978
February	1st to 10th	Day	18% 63% 7% 8% 4 1	1926
	11th to 20th	Day	19% 61% 3 8% 9% 1	1957
	21st to end	Day	16% 62% 6% 8% 6% 1	1650
March	1st to 10th	Day	34% 31% 8% 9% 9% 9%	4479
	11th to 20th	Day	25% 33% 11% 21% 8% 1	4695
	21st to end	Day	36% 30% 7% 9% 12% 5%	5893
April	1st to 10th	Day	20% 62% 4 6% 6% 2	2400
	11th to 20th	Day	16% 63% 3 8% 10% 1	2374
	21st to end	Day	18% 62% 3 9% 6% 1	2550
May	1st to 10th	Day	43% 33% 13% 7% 4	5897
	11th to 20th	Day	38% 32% 11% 11% 8%	5988
	21st to end	Day	36% 32% 8% 15% 9%	6949
June	1st to 10th	Day	17% 63% 4 9% 7%	2800
	11th to 20th	Day	16% 62% 5% 9% 6% 1	2800
	21st to end	Day	15% 61% 5% 10% 8% 1	2800
July	1st to 10th	Day	37% 30% 8% 14% 10% 1	6290
	11th to 20th	Day	34% 28% 14% 17% 7%	6285
	21st to end	Day	31% 27% 8% 20% 13% 1	6728
August	1st to 10th	Day	35% 38% 11% 12% 4	4212
	11th to 20th	Day	24% 39% 11% 22% 5%	3873
	21st to end	Day	18% 40% 13% 19% 10% 1	4180
September	1st to 10th	Day	15% 61% 5% 11% 8%	2280
	11th to 20th	Day	15% 60% 4 9% 10% 1	2114
	21st to end	Day	21% 60% 4 8% 8%	2090
October	1st to 10th	Day	22% 38% 6% 14% 15% 5%	3480
	11th to 20th	Day	14% 37% 4 14% 27% 3	3089
	21st to end	Day	17% 37% 10% 19% 15% 2	3081
November	1st to 10th	Day	17% 57% 6% 8% 10% 1	1761
	11th to 20th	Day	14% 57% 4 10% 13% 3	1800
	21st to end	Day	17% 57% 5% 14% 6% 2	1800
December	1st to 10th	Day	24% 36% 9% 20% 10%	2648
	11th to 20th	Day	28% 31% 9% 12% 16% 4	2986
	21st to end	Day	36% 28% 4 15% 13% 4	3960
January	1st to 10th	Night	24% 27% 7% 23% 16% 3	6349
	11th to 20th	Night	30% 27% 9% 16% 13% 5%	6360
	21st to end	Night	38% 30% 7% 12% 10% 3	6974
February	1st to 10th	Night	18% 62% 5% 9% 4 1	2471
	11th to 20th	Night	16% 60% 3 8% 10% 2	2340
	21st to end	Night	18% 62% 4 7% 7% 2	1980
March	1st to 10th	Night	36% 30% 7% 11% 9% 7%	5374
	11th to 20th	Night	31% 31% 12% 16% 8% 2	5263
	21st to end	Night	37% 30% 4 7% 18% 4	5036
April	1st to 10th	Night	20% 62% 4 7% 5% 2	2000
	11th to 20th	Night	16% 64% 5% 7% 8% 1	1975
	21st to end	Night	18% 62% 3 8% 7% 1	1848
May	1st to 10th	Night	44% 31% 11% 8% 5%	4060
	11th to 20th	Night	41% 33% 10% 10% 4 2	3952
	21st to end	Night	38% 32% 10% 14% 6%	3927
June	1st to 10th	Night	17% 63% 2 9% 9% 1	1600
	11th to 20th	Night	16% 63% 4 11% 6% 1	1600
	21st to end	Night	14% 61% 6% 11% 7% 1	1600
July	1st to 10th	Night	41% 29% 7% 14% 8% 1	3560
	11th to 20th	Night	41% 28% 10% 14% 6% 1	3552
	21st to end	Night	31% 28% 9% 18% 13% 1	3809
August	1st to 10th	Night	39% 40% 7% 8% 5% 1	2749
	11th to 20th	Night	26% 40% 11% 19% 4	3017
	21st to end	Night	13% 39% 13% 22% 14% 1	3476
September	1st to 10th	Night	16% 62% 3 12% 7% 1	1900
	11th to 20th	Night	15% 60% 4 8% 11% 2	2066
	21st to end	Night	22% 61% 3 7% 8%	2090
October	1st to 10th	Night	24% 38% 8% 16% 12% 2	3480
	11th to 20th	Night	11% 37% 5% 16% 27% 4	3826
	21st to end	Night	16% 37% 8% 20% 17% 1	4467
November	1st to 10th	Night	17% 57% 5% 9% 10% 1	2537
	11th to 20th	Night	16% 57% 4 8% 12% 4	2600
	21st to end	Night	16% 57% 5% 14% 6% 2	2600
December	1st to 10th	Night	26% 35% 7% 20% 11% 1	4514
	11th to 20th	Night	33% 32% 7% 11% 16% 2	5208
	21st to end	Night	33% 29% 6% 13% 15% 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	27% 26% 25% 17% 6%	200
	11th to 20th	Day	25% 33% 28% 12% 3	200
	21st to end	Day	23% 25% 33% 14% 5%	223
February	1st to 10th	Day	27% 24% 32% 14% 2	426
	11th to 20th	Day	24% 29% 34% 11% 1	496
	21st to end	Day	24% 25% 35% 13% 3	414
March	1st to 10th	Day	24% 23% 33% 16% 3	500
	11th to 20th	Day	19% 25% 37% 14% 4 1	500
	21st to end	Day	27% 21% 34% 14% 4	550
April	1st to 10th	Day	25% 25% 30% 14% 6% 1	500
	11th to 20th	Day	26% 20% 32% 17% 4	492
	21st to end	Day	28% 16% 34% 16% 4 2	500
May	1st to 10th	Day	30% 17% 30% 17% 5% 1	503
	11th to 20th	Day	20% 16% 38% 20% 5% 1	527
	21st to end	Day	21% 8% 28% 29% 11% 2	573
June	1st to 10th	Day	18% 12% 31% 25% 13% 2	530
	11th to 20th	Day	18% 11% 33% 28% 8% 1	530
	21st to end	Day	17% 11% 31% 27% 12% 2	530
July	1st to 10th	Day	19% 6% 30% 28% 15% 2	525
	11th to 20th	Day	14% 8% 26% 34% 14% 4	524
	21st to end	Day	14% 8% 34% 32% 9% 2	576
August	1st to 10th	Day	12% 8% 30% 33% 12% 5%	502
	11th to 20th	Day	16% 5% 27% 31% 16% 5%	479
	21st to end	Day	10% 7% 30% 31% 16% 6%	539
September	1st to 10th	Day	19% 9% 26% 26% 15% 5%	490
	11th to 20th	Day	14% 6% 32% 27% 13% 7%	490
	21st to end	Day	22% 9% 32% 24% 10% 2	490
October	1st to 10th	Day	26% 10% 26% 22% 12% 4	490
	11th to 20th	Day	17% 15% 31% 24% 12% 1	462
	21st to end	Day	24% 14% 32% 21% 7% 2	429
November	1st to 10th	Day	16% 16% 31% 25% 10% 2	395
	11th to 20th	Day	22% 23% 31% 18% 7%	400
	21st to end	Day	21% 22% 29% 22% 7% 1	400
December	1st to 10th	Day	21% 22% 36% 14% 6%	235
	11th to 20th	Day	28% 26% 30% 13% 4	200
	21st to end	Day	25% 23% 33% 12% 5% 2	220
January	1st to 10th	Night	21% 22% 35% 20% 3	800
	11th to 20th	Night	27% 21% 32% 18% 2	800
	21st to end	Night	19% 23% 36% 20% 2	880
February	1st to 10th	Night	23% 19% 35% 22% 1	800
	11th to 20th	Night	26% 18% 40% 15% 1	783
	21st to end	Night	25% 19% 36% 17% 3	660
March	1st to 10th	Night	24% 17% 35% 20% 3	800
	11th to 20th	Night	21% 15% 37% 23% 4 1	800
	21st to end	Night	23% 17% 35% 20% 4 1	880
April	1st to 10th	Night	27% 13% 32% 23% 4	800
	11th to 20th	Night	24% 11% 36% 24% 5%	791
	21st to end	Night	24% 10% 36% 23% 7% 1	648
May	1st to 10th	Night	25% 9% 31% 26% 9% 1	600
	11th to 20th	Night	22% 10% 36% 26% 6% 1	562
	21st to end	Night	23% 3 25% 35% 12% 3	433
June	1st to 10th	Night	21% 5% 27% 33% 14% 1	400
	11th to 20th	Night	24% 4 22% 39% 11% 1	400
	21st to end	Night	17% 5% 24% 40% 15% 1	400
July	1st to 10th	Night	18% 4 19% 36% 22% 1	390
	11th to 20th	Night	15% 4 18% 42% 19% 2	388
	21st to end	Night	15% 4 20% 44% 16% 1	438
August	1st to 10th	Night	12% 3 21% 42% 20% 1	516
	11th to 20th	Night	15% 3 21% 37% 22% 3	679
	21st to end	Night	15% 3 17% 39% 22% 4	836
September	1st to 10th	Night	18% 3 25% 35% 17% 3	760
	11th to 20th	Night	16% 5% 24% 30% 19% 5%	760
	21st to end	Night	22% 6% 27% 27% 16% 2	760
October	1st to 10th	Night	23% 9% 27% 24% 14% 4	760
	11th to 20th	Night	19% 11% 33% 26% 11% 1	760
	21st to end	Night	24% 11% 37% 19% 8% 1	836
November	1st to 10th	Night	16% 13% 33% 25% 12% 1	780
	11th to 20th	Night	23% 18% 33% 19% 6% 1	800
	21st to end	Night	18% 20% 34% 21% 7% 1	800
December	1st to 10th	Night	26% 18% 31% 20% 5% 1	800
	11th to 20th	Night	25% 20% 32% 19% 4	800
	21st to end	Night	25% 16% 35% 21% 3	880

Page generated at 2025-08-02 01:32:29. Version ac07

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