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 2024-07-16.

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% 9% 3 8% 5% 4 4 4 6% 7% 37%	89841
	Night	12% 9% 3 8% 5% 4 4 3 5% 6% 39%	89580
Transparency	Day	46% 6% 13% 24% 11% 1	89598
	Night	45% 5% 12% 23% 13% 2	89428
Seeing	Day	17% 11% 31% 26% 12% 3	15554
	Night	16% 9% 33% 31% 10% 1	25473

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	10% 18% 7% 16% 7% 5% 4 4 4 6% 18%	2527
	11th to 20th	Day	6% 14% 5% 12% 8% 7% 8% 6% 8% 8% 18%	2665
	21st to end	Day	7% 20% 7% 17% 8% 5% 4 3 4 5% 19%	3135
February	1st to 10th	Day	8% 11% 6% 10% 8% 7% 5% 5% 6% 7% 26%	1710
	11th to 20th	Day	8% 12% 4 10% 7% 6% 6% 4 6% 7% 30%	1664
	21st to end	Day	12% 11% 4 10% 6% 5% 4 3 6% 7% 32%	1447
March	1st to 10th	Day	14% 10% 3 8% 4 4 4 3 5% 6% 39%	3436
	11th to 20th	Day	11% 10% 3 7% 4 4 4 4 5% 8% 41%	3480
	21st to end	Day	25% 13% 5% 11% 7% 3 3 3 4 6% 21%	3814
April	1st to 10th	Day	17% 10% 3 7% 4 4 3 3 6% 7% 34%	2270
	11th to 20th	Day	19% 13% 3 8% 3 4 4 3 5% 6% 32%	2290
	21st to end	Day	22% 11% 3 7% 4 4 4 4 5% 6% 30%	2470
May	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 5% 6% 8% 36%	4146
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 5% 7% 45%	4399
	21st to end	Day	18% 6% 2 7% 5% 4 5% 4 6% 7% 37%	4824
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 6% 8% 46%	2660
	11th to 20th	Day	13% 6% 2 6% 5% 5% 4 4 6% 8% 40%	2660
	21st to end	Day	10% 4 2 6% 5% 5% 5% 4 7% 10% 43%	2660
July	1st to 10th	Day	6% 5% 1 5% 4 4 4 4 7% 10% 50%	2660
	11th to 20th	Day	7% 4 1 4 4 3 4 4 7% 8% 54%	2593
	21st to end	Day	6% 3 2 4 4 4 5% 4 7% 10% 50%	2636
August	1st to 10th	Day	9% 4 2 7% 5% 6% 5% 4 7% 9% 42%	2328
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 7% 46%	2301
	21st to end	Day	8% 4 2 5% 3 4 4 3 6% 7% 52%	2483
September	1st to 10th	Day	13% 5% 3 6% 3 3 4 4 5% 7% 47%	2160
	11th to 20th	Day	11% 6% 1 5% 4 3 4 3 5% 7% 50%	2088
	21st to end	Day	14% 6% 3 6% 3 3 5% 3 5% 7% 44%	1922
October	1st to 10th	Day	16% 7% 3 6% 5% 4 4 3 5% 5% 43%	1800
	11th to 20th	Day	14% 10% 2 6% 4 4 4 2 4 6% 43%	1800
	21st to end	Day	19% 9% 4 8% 3 3 3 2 4 5% 38%	1980
November	1st to 10th	Day	11% 10% 3 8% 5% 3 4 3 5% 8% 41%	1572
	11th to 20th	Day	11% 13% 6% 10% 6% 4 5% 4 5% 6% 31%	1520
	21st to end	Day	13% 10% 4 6% 6% 4 4 4 5% 8% 36%	1520
December	1st to 10th	Day	11% 13% 3 11% 9% 5% 5% 5% 5% 6% 27%	1520
	11th to 20th	Day	10% 15% 5% 10% 7% 5% 4 4 5% 6% 29%	1918
	21st to end	Day	19% 15% 6% 18% 5% 4 4 2 4 5% 18%	2783
January	1st to 10th	Night	9% 15% 7% 20% 6% 4 4 3 4 4 24%	4428
	11th to 20th	Night	6% 11% 5% 15% 8% 6% 8% 6% 9% 6% 20%	4303
	21st to end	Night	10% 19% 8% 14% 5% 4 3 4 4 5% 25%	4532
February	1st to 10th	Night	8% 13% 5% 10% 6% 6% 4 3 5% 7% 34%	2470
	11th to 20th	Night	8% 10% 3 8% 5% 6% 5% 3 5% 7% 39%	2403
	21st to end	Night	11% 9% 3 9% 6% 5% 4 4 5% 6% 37%	2006
March	1st to 10th	Night	9% 8% 3 7% 4 4 4 3 5% 5% 48%	3435
	11th to 20th	Night	9% 8% 3 7% 5% 5% 6% 4 5% 7% 42%	3480
	21st to end	Night	26% 12% 3 9% 4 3 3 2 3 5% 29%	3817
April	1st to 10th	Night	14% 11% 2 8% 4 4 4 4 5% 6% 37%	1910
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 3 4 5% 36%	1836
	21st to end	Night	18% 10% 3 7% 4 3 3 4 5% 6% 37%	1710
May	1st to 10th	Night	15% 7% 3 5% 4 4 4 4 5% 6% 43%	2744
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 5% 7% 50%	2506
	21st to end	Night	16% 6% 3 7% 3 3 4 3 4 7% 45%	2743
June	1st to 10th	Night	9% 5% 2 4 5% 3 4 4 6% 7% 52%	1520
	11th to 20th	Night	10% 5% 2 6% 5% 5% 5% 5% 7% 7% 44%	1520
	21st to end	Night	8% 4 1 4 4 3 4 3 5% 7% 56%	1421
July	1st to 10th	Night	5% 3 1 4 3 3 4 4 7% 9% 56%	1520
	11th to 20th	Night	4 2 1 5% 3 4 3 3 6% 8% 60%	1488
	21st to end	Night	4 4 1 4 3 4 3 3 6% 9% 59%	1678
August	1st to 10th	Night	5% 5% 2 5% 4 4 5% 4 7% 9% 50%	1611
	11th to 20th	Night	8% 5% 2 4 3 3 4 3 5% 8% 55%	1600
	21st to end	Night	7% 4 1 5% 3 3 3 3 4 7% 58%	1872
September	1st to 10th	Night	9% 5% 2 4 3 2 4 3 5% 7% 57%	1800
	11th to 20th	Night	11% 4 2 4 4 3 3 3 5% 7% 53%	1867
	21st to end	Night	10% 6% 2 6% 3 4 4 3 5% 7% 51%	2020
October	1st to 10th	Night	15% 7% 2 6% 3 3 4 3 4 5% 49%	2160
	11th to 20th	Night	15% 9% 3 6% 3 3 4 3 5% 7% 43%	2160
	21st to end	Night	18% 10% 3 7% 3 3 3 3 5% 5% 40%	2376
November	1st to 10th	Night	12% 8% 3 6% 3 4 4 3 5% 6% 45%	2501
	11th to 20th	Night	12% 13% 4 8% 5% 4 4 3 4 5% 37%	2660
	21st to end	Night	13% 10% 3 8% 4 4 3 3 5% 7% 39%	2660
December	1st to 10th	Night	10% 11% 4 11% 7% 5% 4 4 5% 7% 32%	2660
	11th to 20th	Night	11% 12% 5% 11% 7% 6% 5% 4 6% 6% 29%	3279
	21st to end	Night	21% 13% 5% 14% 6% 4 4 4 4 4 20%	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	68% 5% 11% 13% 4	2528
	11th to 20th	Day	61% 4 12% 17% 6%	2665
	21st to end	Day	69% 4 11% 12% 3	3135
February	1st to 10th	Day	56% 6% 12% 17% 8% 1	1710
	11th to 20th	Day	52% 7% 12% 21% 7% 1	1673
	21st to end	Day	52% 7% 13% 17% 11%	1447
March	1st to 10th	Day	45% 5% 12% 27% 9% 1	3436
	11th to 20th	Day	41% 6% 10% 29% 11% 2	3480
	21st to end	Day	65% 6% 9% 15% 4	3814
April	1st to 10th	Day	48% 6% 12% 23% 10% 1	2270
	11th to 20th	Day	52% 5% 10% 21% 10% 1	2290
	21st to end	Day	54% 6% 12% 20% 8% 1	2470
May	1st to 10th	Day	43% 7% 13% 25% 12% 1	4184
	11th to 20th	Day	37% 7% 10% 29% 15% 2	4403
	21st to end	Day	46% 6% 12% 26% 10% 1	4837
June	1st to 10th	Day	35% 7% 15% 27% 15% 1	2660
	11th to 20th	Day	42% 6% 13% 22% 16% 2	2660
	21st to end	Day	36% 8% 16% 27% 12% 2	2660
July	1st to 10th	Day	27% 7% 19% 32% 14% 1	2591
	11th to 20th	Day	27% 7% 16% 28% 19% 4	2518
	21st to end	Day	27% 8% 18% 30% 15% 3	2489
August	1st to 10th	Day	37% 7% 15% 28% 10% 3	2314
	11th to 20th	Day	37% 7% 14% 30% 12%	2301
	21st to end	Day	30% 7% 14% 31% 18% 2	2483
September	1st to 10th	Day	36% 5% 13% 30% 16%	2160
	11th to 20th	Day	34% 7% 16% 31% 12% 1	2088
	21st to end	Day	39% 5% 13% 32% 10% 1	1922
October	1st to 10th	Day	43% 4 10% 28% 14% 1	1800
	11th to 20th	Day	42% 4 12% 27% 13% 1	1800
	21st to end	Day	47% 4 13% 24% 11% 1	1980
November	1st to 10th	Day	42% 6% 14% 21% 15% 3	1572
	11th to 20th	Day	52% 6% 13% 19% 10%	1520
	21st to end	Day	46% 5% 19% 22% 7% 1	1520
December	1st to 10th	Day	57% 7% 16% 13% 7% 1	1520
	11th to 20th	Day	55% 6% 19% 15% 5%	1915
	21st to end	Day	69% 4 10% 12% 4 1	2783
January	1st to 10th	Night	63% 5% 11% 16% 5%	4428
	11th to 20th	Night	60% 5% 12% 17% 5%	4303
	21st to end	Night	63% 4 11% 16% 6%	4532
February	1st to 10th	Night	52% 4 12% 23% 9% 1	2470
	11th to 20th	Night	45% 6% 14% 24% 10% 2	2404
	21st to end	Night	47% 5% 11% 23% 12% 2	2006
March	1st to 10th	Night	37% 5% 12% 30% 15% 1	3435
	11th to 20th	Night	39% 5% 10% 29% 16% 1	3480
	21st to end	Night	60% 4 9% 19% 6% 1	3817
April	1st to 10th	Night	46% 6% 12% 23% 12% 1	1910
	11th to 20th	Night	50% 4 8% 18% 16% 3	1836
	21st to end	Night	47% 5% 10% 22% 15% 2	1710
May	1st to 10th	Night	41% 6% 11% 19% 21% 1	2771
	11th to 20th	Night	32% 7% 11% 25% 19% 5%	2507
	21st to end	Night	40% 4 12% 26% 16% 2	2751
June	1st to 10th	Night	31% 6% 13% 30% 18% 2	1520
	11th to 20th	Night	37% 7% 13% 22% 19% 3	1520
	21st to end	Night	27% 6% 15% 30% 19% 3	1421
July	1st to 10th	Night	23% 6% 16% 33% 19% 3	1480
	11th to 20th	Night	21% 6% 17% 31% 21% 4	1440
	21st to end	Night	20% 5% 17% 35% 19% 4	1585
August	1st to 10th	Night	28% 8% 15% 29% 16% 4	1601
	11th to 20th	Night	27% 6% 17% 30% 18% 1	1600
	21st to end	Night	25% 6% 15% 30% 20% 4	1872
September	1st to 10th	Night	28% 5% 12% 32% 21% 1	1800
	11th to 20th	Night	30% 7% 14% 32% 16% 1	1867
	21st to end	Night	33% 5% 13% 31% 17% 1	2020
October	1st to 10th	Night	37% 4 10% 29% 17% 3	2160
	11th to 20th	Night	41% 4 13% 24% 16% 2	2160
	21st to end	Night	45% 5% 12% 24% 13% 1	2376
November	1st to 10th	Night	39% 5% 14% 25% 15% 2	2501
	11th to 20th	Night	49% 6% 12% 20% 12% 1	2660
	21st to end	Night	45% 5% 16% 24% 9% 1	2660
December	1st to 10th	Night	52% 6% 16% 19% 7%	2660
	11th to 20th	Night	54% 6% 16% 18% 5% 1	3281
	21st to end	Night	67% 5% 9% 15% 4	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	14% 16% 36% 23% 10% 2	270
	11th to 20th	Day	20% 14% 38% 24% 4 1	270
	21st to end	Day	14% 12% 37% 27% 9% 1	297
February	1st to 10th	Day	13% 15% 47% 20% 4	270
	11th to 20th	Day	14% 15% 43% 23% 5%	270
	21st to end	Day	20% 16% 38% 25% 1	225
March	1st to 10th	Day	20% 15% 41% 21% 3	270
	11th to 20th	Day	25% 12% 33% 24% 5% 1	270
	21st to end	Day	30% 13% 27% 20% 8% 1	297
April	1st to 10th	Day	27% 14% 33% 21% 4	450
	11th to 20th	Day	29% 14% 25% 22% 8% 1	488
	21st to end	Day	28% 14% 27% 20% 8% 3	650
May	1st to 10th	Day	20% 13% 36% 23% 7% 2	653
	11th to 20th	Day	19% 14% 29% 24% 10% 4	675
	21st to end	Day	19% 9% 29% 28% 13% 2	734
June	1st to 10th	Day	11% 9% 28% 31% 17% 4	680
	11th to 20th	Day	15% 8% 36% 28% 11% 2	680
	21st to end	Day	12% 8% 33% 29% 13% 4	680
July	1st to 10th	Day	8% 5% 23% 31% 26% 7%	671
	11th to 20th	Day	8% 7% 27% 33% 19% 5%	645
	21st to end	Day	6% 5% 25% 39% 20% 5%	684
August	1st to 10th	Day	11% 2 22% 32% 27% 5%	626
	11th to 20th	Day	13% 4 22% 30% 25% 6%	615
	21st to end	Day	10% 5% 28% 34% 18% 5%	602
September	1st to 10th	Day	16% 8% 28% 29% 16% 3	450
	11th to 20th	Day	13% 7% 30% 30% 15% 5%	378
	21st to end	Day	18% 15% 30% 26% 8% 4	270
October	1st to 10th	Day	20% 10% 27% 25% 14% 4	270
	11th to 20th	Day	20% 16% 36% 20% 8% 1	270
	21st to end	Day	25% 18% 37% 12% 7%	297
November	1st to 10th	Day	18% 17% 37% 19% 9% 1	270
	11th to 20th	Day	18% 21% 40% 16% 4	270
	21st to end	Day	20% 19% 37% 20% 4	270
December	1st to 10th	Day	16% 13% 42% 23% 7%	270
	11th to 20th	Day	19% 15% 40% 20% 6%	270
	21st to end	Day	26% 16% 36% 19% 3	297
January	1st to 10th	Night	12% 12% 39% 29% 8% 1	950
	11th to 20th	Night	16% 12% 34% 31% 6% 1	950
	21st to end	Night	19% 11% 34% 29% 6%	1045
February	1st to 10th	Night	15% 13% 38% 29% 5%	950
	11th to 20th	Night	13% 13% 40% 27% 6% 1	929
	21st to end	Night	16% 11% 34% 31% 7% 1	750
March	1st to 10th	Night	18% 10% 40% 23% 8% 1	586
	11th to 20th	Night	19% 11% 37% 27% 6% 1	570
	21st to end	Night	23% 10% 31% 25% 9% 1	627
April	1st to 10th	Night	17% 15% 38% 26% 5%	570
	11th to 20th	Night	24% 13% 29% 26% 7% 1	563
	21st to end	Night	21% 12% 34% 25% 7% 1	570
May	1st to 10th	Night	17% 11% 34% 30% 8% 1	570
	11th to 20th	Night	17% 9% 34% 29% 11% 1	561
	21st to end	Night	17% 7% 29% 33% 13%	617
June	1st to 10th	Night	11% 5% 29% 40% 14% 1	570
	11th to 20th	Night	12% 5% 34% 41% 9%	570
	21st to end	Night	9% 5% 28% 42% 15% 1	570
July	1st to 10th	Night	6% 3 21% 40% 27% 2	555
	11th to 20th	Night	3 4 25% 46% 20% 1	540
	21st to end	Night	5% 2 23% 50% 19% 2	591
August	1st to 10th	Night	7% 1 20% 47% 24% 2	537
	11th to 20th	Night	10% 2 21% 40% 25% 3	534
	21st to end	Night	9% 2 25% 40% 22% 2	594
September	1st to 10th	Night	12% 4 24% 41% 17% 2	540
	11th to 20th	Night	11% 6% 32% 32% 16% 3	540
	21st to end	Night	12% 10% 34% 31% 12% 1	580
October	1st to 10th	Night	17% 8% 33% 28% 13% 1	720
	11th to 20th	Night	21% 11% 34% 27% 7% 1	720
	21st to end	Night	25% 12% 29% 27% 6%	792
November	1st to 10th	Night	16% 13% 36% 26% 8% 1	867
	11th to 20th	Night	17% 14% 38% 24% 7%	950
	21st to end	Night	18% 14% 38% 24% 7%	950
December	1st to 10th	Night	17% 11% 37% 28% 7% 1	950
	11th to 20th	Night	20% 8% 39% 27% 7%	950
	21st to end	Night	24% 11% 35% 24% 5%	1045

Page generated at 2024-07-13 09:19:50. Version ac07

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