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 2024-11-04.

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	18% 7% 2 6% 4 4 4 4 5% 7% 39%	101171
	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	97854
Transparency	Day	24% 48% 7% 12% 9% 1	104735
	Night	24% 47% 6% 12% 9% 1	102379
Seeing	Day	20% 15% 31% 23% 9% 2	15096
	Night	21% 12% 31% 26% 9% 1	24067

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	16% 7% 3 6% 3 3 4 3 5% 4 45%	2517
	11th to 20th	Day	17% 9% 2 5% 4 4 4 4 5% 6% 39%	2520
	21st to end	Day	23% 11% 4 6% 3 3 3 2 5% 5% 35%	2776
February	1st to 10th	Day	19% 7% 2 5% 5% 4 5% 3 5% 6% 38%	1829
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 4 5% 7% 42%	1848
	21st to end	Day	21% 9% 3 7% 4 4 3 4 5% 6% 36%	1570
March	1st to 10th	Day	27% 10% 3 6% 3 3 3 2 3 5% 36%	3116
	11th to 20th	Day	10% 6% 2 4 4 4 5% 4 6% 8% 45%	3279
	21st to end	Day	21% 8% 3 6% 4 4 4 3 6% 6% 36%	4117
April	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 5% 6% 40%	2280
	11th to 20th	Day	20% 7% 2 5% 3 4 4 3 5% 7% 41%	2254
	21st to end	Day	21% 6% 2 5% 3 4 4 4 5% 7% 40%	2422
May	1st to 10th	Day	27% 11% 3 6% 4 3 4 3 5% 6% 30%	4106
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 5% 6% 33%	4162
	21st to end	Day	16% 6% 2 5% 4 3 4 4 5% 8% 45%	4837
June	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 5% 6% 8% 36%	2660
	11th to 20th	Day	15% 7% 2 5% 5% 4 4 5% 5% 9% 39%	2660
	21st to end	Day	13% 6% 3 6% 4 4 4 4 6% 9% 39%	2660
July	1st to 10th	Day	17% 6% 3 7% 5% 5% 5% 4 6% 9% 36%	4440
	11th to 20th	Day	16% 6% 2 5% 4 5% 5% 4 6% 9% 40%	4440
	21st to end	Day	21% 7% 3 6% 5% 4 4 4 6% 7% 33%	4814
August	1st to 10th	Day	20% 8% 3 6% 5% 5% 5% 4 6% 8% 32%	4172
	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% 7% 2 5% 4 4 4 3 4 5% 49%	1698
	11th to 20th	Day	18% 6% 1 4 3 3 4 3 6% 5% 48%	1710
	21st to end	Day	17% 6% 2 5% 4 3 4 3 4 6% 45%	1710
December	1st to 10th	Day	21% 7% 2 6% 4 4 4 4 5% 6% 38%	1554
	11th to 20th	Day	23% 6% 2 5% 3 2 2 2 4 5% 46%	1906
	21st to end	Day	20% 11% 3 7% 4 4 3 2 4 6% 36%	2772
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 7% 47%	4429
	11th to 20th	Night	18% 9% 3 7% 4 3 3 2 3 4 43%	4440
	21st to end	Night	24% 10% 3 7% 5% 4 3 3 4 5% 32%	4876
February	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 4 6% 41%	2348
	11th to 20th	Night	18% 8% 3 5% 4 4 3 3 4 7% 42%	2218
	21st to end	Night	20% 9% 3 4 4 3 4 3 4 4 41%	1884
March	1st to 10th	Night	27% 11% 3 4 4 3 3 2 4 5% 35%	3724
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 4 6% 7% 43%	3678
	21st to end	Night	24% 8% 2 6% 3 3 4 3 5% 6% 37%	3512
April	1st to 10th	Night	21% 8% 2 6% 4 5% 3 4 4 6% 38%	1900
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 6% 41%	1875
	21st to end	Night	22% 7% 2 5% 4 3 4 3 4 5% 40%	1756
May	1st to 10th	Night	25% 11% 3 7% 4 4 4 3 5% 6% 28%	2834
	11th to 20th	Night	24% 9% 4 7% 5% 5% 4 3 5% 7% 26%	2759
	21st to end	Night	19% 7% 2 5% 3 3 4 4 6% 7% 39%	2738
June	1st to 10th	Night	19% 7% 3 5% 5% 3 4 3 4 7% 39%	1520
	11th to 20th	Night	19% 7% 2 5% 3 3 4 4 6% 9% 38%	1520
	21st to end	Night	15% 8% 2 6% 5% 3 4 4 5% 10% 38%	1520
July	1st to 10th	Night	19% 9% 3 7% 5% 5% 5% 4 6% 8% 31%	2520
	11th to 20th	Night	20% 6% 2 5% 3 3 4 4 7% 9% 35%	2520
	21st to end	Night	20% 9% 3 7% 4 4 4 4 6% 6% 33%	2739
August	1st to 10th	Night	24% 7% 3 7% 5% 4 5% 5% 6% 7% 27%	2725
	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	14% 6% 2 5% 4 4 4 3 4 6% 48%	2453
	11th to 20th	Night	19% 7% 2 5% 3 2 4 3 3 5% 47%	2470
	21st to end	Night	16% 6% 1 6% 4 4 4 3 4 6% 47%	2470
December	1st to 10th	Night	21% 7% 3 5% 4 4 4 3 5% 5% 40%	2622
	11th to 20th	Night	21% 6% 2 5% 4 3 4 3 4 6% 42%	3288
	21st to end	Night	22% 10% 3 7% 3 3 3 3 4 5% 37%	4884

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	26% 38% 5% 19% 9% 2	2518
	11th to 20th	Day	27% 39% 8% 18% 7% 1	2520
	21st to end	Day	38% 40% 5% 11% 6% 1	2776
February	1st to 10th	Day	17% 66% 6% 7% 3 1	1829
	11th to 20th	Day	17% 64% 2 8% 9% 1	1857
	21st to end	Day	16% 66% 5% 7% 4 1	1570
March	1st to 10th	Day	35% 43% 7% 8% 6% 1	3119
	11th to 20th	Day	13% 45% 11% 22% 9% 1	3279
	21st to end	Day	30% 41% 5% 10% 11% 3	4117
April	1st to 10th	Day	18% 65% 4 6% 5% 1	2280
	11th to 20th	Day	14% 66% 3 7% 9%	2254
	21st to end	Day	17% 66% 3 8% 5% 1	2422
May	1st to 10th	Day	36% 44% 10% 5% 4 1	4120
	11th to 20th	Day	33% 44% 11% 9% 4	4166
	21st to end	Day	21% 43% 7% 15% 13%	4837
June	1st to 10th	Day	15% 66% 3 9% 6%	2660
	11th to 20th	Day	14% 65% 5% 9% 6% 1	2660
	21st to end	Day	14% 64% 5% 9% 8%	2660
July	1st to 10th	Day	32% 39% 7% 12% 9% 1	4370
	11th to 20th	Day	28% 38% 12% 14% 8% 1	4365
	21st to end	Day	35% 37% 8% 11% 8% 1	4661
August	1st to 10th	Day	35% 38% 11% 12% 4	4158
	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% 59% 6% 8% 9% 1	1698
	11th to 20th	Day	14% 60% 4 9% 11% 2	1710
	21st to end	Day	15% 59% 5% 13% 6% 2	1710
December	1st to 10th	Day	20% 60% 5% 5% 9%	1554
	11th to 20th	Day	22% 48% 6% 9% 13% 2	1906
	21st to end	Day	33% 39% 5% 16% 6%	2772
January	1st to 10th	Night	24% 37% 7% 22% 7% 4	4429
	11th to 20th	Night	27% 38% 8% 15% 10% 2	4440
	21st to end	Night	40% 41% 5% 9% 4 1	4876
February	1st to 10th	Night	16% 65% 5% 9% 3 1	2348
	11th to 20th	Night	15% 63% 3 7% 9% 2	2220
	21st to end	Night	16% 65% 3 7% 7% 2	1884
March	1st to 10th	Night	38% 41% 4 10% 5% 1	3734
	11th to 20th	Night	18% 44% 11% 15% 11% 1	3679
	21st to end	Night	30% 42% 4 5% 15% 3	3512
April	1st to 10th	Night	18% 65% 3 7% 5% 1	1900
	11th to 20th	Night	15% 67% 4 7% 7% 1	1875
	21st to end	Night	17% 65% 3 8% 6% 1	1756
May	1st to 10th	Night	37% 42% 9% 9% 3	2840
	11th to 20th	Night	38% 45% 9% 6% 2	2760
	21st to end	Night	24% 44% 9% 15% 7% 1	2739
June	1st to 10th	Night	15% 66% 2 9% 8% 1	1520
	11th to 20th	Night	13% 66% 4 11% 5% 1	1520
	21st to end	Night	13% 64% 5% 11% 6% 1	1520
July	1st to 10th	Night	36% 39% 6% 11% 8%	2480
	11th to 20th	Night	30% 38% 11% 15% 6%	2472
	21st to end	Night	36% 38% 8% 7% 10% 1	2651
August	1st to 10th	Night	39% 40% 7% 8% 4 1	2715
	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% 59% 5% 9% 9% 1	2453
	11th to 20th	Night	16% 59% 4 8% 10% 3	2470
	21st to end	Night	15% 60% 5% 13% 5% 2	2470
December	1st to 10th	Night	21% 60% 4 8% 6%	2622
	11th to 20th	Night	21% 49% 6% 10% 12% 2	3288
	21st to end	Night	32% 40% 6% 13% 8%	4884

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	24% 25% 26% 18% 7%	180
	11th to 20th	Day	23% 34% 27% 13% 3	180
	21st to end	Day	21% 25% 34% 15% 6%	200
February	1st to 10th	Day	26% 23% 32% 16% 3	389
	11th to 20th	Day	21% 29% 36% 12% 1	456
	21st to end	Day	24% 23% 35% 13% 3 1	382
March	1st to 10th	Day	23% 22% 35% 18% 3	460
	11th to 20th	Day	18% 26% 37% 14% 4 1	460
	21st to end	Day	26% 20% 35% 15% 5%	506
April	1st to 10th	Day	23% 25% 31% 15% 6% 1	460
	11th to 20th	Day	24% 20% 32% 19% 5%	452
	21st to end	Day	28% 15% 32% 17% 5% 2	460
May	1st to 10th	Day	30% 17% 29% 17% 5% 1	463
	11th to 20th	Day	20% 14% 38% 21% 5% 1	487
	21st to end	Day	19% 8% 28% 31% 12% 2	529
June	1st to 10th	Day	18% 11% 30% 25% 14% 2	490
	11th to 20th	Day	17% 10% 34% 30% 9% 1	490
	21st to end	Day	16% 11% 31% 26% 13% 3	490
July	1st to 10th	Day	18% 5% 29% 28% 16% 2	485
	11th to 20th	Day	14% 7% 26% 34% 15% 4	484
	21st to end	Day	15% 7% 33% 33% 10% 2	532
August	1st to 10th	Day	12% 7% 30% 34% 12% 5%	486
	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% 15% 31% 25% 10% 2	374
	11th to 20th	Day	22% 22% 31% 19% 7%	370
	21st to end	Day	19% 21% 29% 24% 7% 1	370
December	1st to 10th	Day	21% 23% 34% 14% 7%	214
	11th to 20th	Day	25% 27% 31% 13% 3	180
	21st to end	Day	24% 24% 34% 11% 5% 3	198
January	1st to 10th	Night	20% 21% 35% 21% 3	760
	11th to 20th	Night	27% 21% 32% 17% 3	760
	21st to end	Night	19% 22% 35% 21% 2	836
February	1st to 10th	Night	22% 18% 35% 23% 1	760
	11th to 20th	Night	24% 18% 41% 16% 1	743
	21st to end	Night	26% 18% 35% 18% 3	628
March	1st to 10th	Night	23% 17% 36% 21% 3	760
	11th to 20th	Night	20% 16% 36% 23% 4 1	760
	21st to end	Night	23% 17% 34% 21% 5% 1	836
April	1st to 10th	Night	26% 13% 32% 24% 4	760
	11th to 20th	Night	23% 10% 36% 25% 6%	751
	21st to end	Night	24% 9% 35% 24% 7% 1	616
May	1st to 10th	Night	25% 9% 31% 26% 9% 1	570
	11th to 20th	Night	22% 9% 36% 27% 6% 1	534
	21st to end	Night	22% 2 24% 36% 12% 3	411
June	1st to 10th	Night	21% 4 26% 34% 14% 1	380
	11th to 20th	Night	23% 4 21% 40% 11% 1	380
	21st to end	Night	17% 5% 23% 39% 14% 1	380
July	1st to 10th	Night	17% 3 20% 36% 23% 1	370
	11th to 20th	Night	14% 4 18% 43% 18% 2	368
	21st to end	Night	16% 4 20% 43% 16% 1	416
August	1st to 10th	Night	12% 3 21% 42% 20% 1	506
	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% 12% 34% 25% 12% 1	755
	11th to 20th	Night	23% 18% 33% 19% 6% 1	760
	21st to end	Night	18% 20% 34% 21% 7% 1	760
December	1st to 10th	Night	26% 17% 32% 20% 4 1	760
	11th to 20th	Night	24% 20% 32% 19% 4	760
	21st to end	Night	25% 16% 36% 20% 3	836

Page generated at 2024-11-01 22:47:31. Version ac07

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