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 2024-12-09.

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	12% 9% 3 8% 5% 4 4 4 6% 7% 38%	101807
	Night	12% 9% 3 8% 5% 4 4 4 5% 6% 40%	97953
Transparency	Day	44% 5% 13% 25% 11% 1	105441
	Night	43% 5% 12% 24% 14% 2	102459
Seeing	Day	17% 11% 31% 26% 12% 3	16094
	Night	16% 9% 33% 31% 10% 1	26018

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	9% 6% 2 6% 5% 5% 5% 5% 8% 10% 40%	4440
	11th to 20th	Day	5% 3 1 4 3 3 4 4 5% 9% 59%	4440
	21st to end	Day	5% 3 2 5% 5% 5% 5% 5% 7% 10% 48%	4625
August	1st to 10th	Day	12% 6% 2 6% 4 5% 4 4 5% 8% 45%	4104
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 41%	4041
	21st to end	Day	8% 5% 3 7% 4 4 4 3 6% 6% 49%	4357
September	1st to 10th	Day	12% 5% 2 6% 3 3 3 4 5% 7% 49%	2280
	11th to 20th	Day	11% 6% 1 5% 5% 4 4 3 5% 7% 50%	2204
	21st to end	Day	13% 5% 3 6% 3 3 5% 3 5% 7% 46%	2028
October	1st to 10th	Day	15% 7% 2 6% 4 3 4 3 5% 6% 44%	1900
	11th to 20th	Day	14% 10% 2 6% 4 4 4 2 4 6% 44%	1900
	21st to end	Day	19% 9% 4 8% 3 3 3 2 4 6% 39%	2090
November	1st to 10th	Day	11% 10% 3 8% 4 3 4 3 5% 8% 40%	1656
	11th to 20th	Day	11% 13% 6% 10% 5% 4 5% 4 5% 6% 31%	1600
	21st to end	Day	12% 10% 5% 8% 6% 4 4 4 5% 8% 35%	1600
December	1st to 10th	Day	11% 13% 3 11% 9% 6% 5% 5% 5% 6% 27%	1584
	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	10% 4 2 5% 4 4 5% 5% 8% 9% 45%	2530
	11th to 20th	Night	3 2 1 5% 4 3 4 3 5% 9% 62%	2530
	21st to end	Night	4 3 1 4 4 4 4 3 7% 8% 57%	2934
August	1st to 10th	Night	11% 6% 2 5% 4 3 4 3 5% 7% 48%	2836
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 5% 8% 50%	2798
	21st to end	Night	9% 6% 2 6% 3 4 3 4 5% 8% 51%	3290
September	1st to 10th	Night	9% 5% 2 4 3 2 4 3 5% 7% 58%	1900
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 5% 8% 53%	1971
	21st to end	Night	10% 6% 2 5% 3 4 4 3 5% 7% 52%	2133
October	1st to 10th	Night	14% 7% 2 5% 3 3 4 3 4 5% 51%	2280
	11th to 20th	Night	15% 9% 3 5% 3 3 4 3 5% 6% 44%	2280
	21st to end	Night	17% 10% 3 7% 3 3 3 3 5% 6% 40%	2508
November	1st to 10th	Night	13% 8% 3 6% 3 4 4 3 5% 6% 45%	2637
	11th to 20th	Night	12% 13% 4 8% 5% 4 4 3 4 5% 37%	2800
	21st to end	Night	13% 10% 4 8% 5% 4 3 3 5% 7% 38%	2800
December	1st to 10th	Night	10% 11% 4 11% 7% 5% 4 4 6% 7% 32%	2779
	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	38% 5% 22% 24% 11%	4371
	11th to 20th	Day	24% 4 16% 35% 18% 2	4365
	21st to end	Day	31% 6% 16% 30% 16% 1	4479
August	1st to 10th	Day	39% 4 11% 31% 13% 2	4090
	11th to 20th	Day	40% 4 16% 31% 8%	4080
	21st to end	Day	35% 5% 17% 29% 14% 1	4356
September	1st to 10th	Day	34% 5% 13% 31% 16%	2280
	11th to 20th	Day	34% 6% 16% 31% 12% 1	2204
	21st to end	Day	37% 5% 12% 32% 13% 1	2028
October	1st to 10th	Day	31% 3 6% 35% 25% 1	3160
	11th to 20th	Day	38% 3 12% 26% 21% 1	3140
	21st to end	Day	43% 3 14% 30% 11%	3428
November	1st to 10th	Day	43% 5% 14% 20% 15% 3	1656
	11th to 20th	Day	52% 5% 13% 19% 10% 1	1600
	21st to end	Day	47% 5% 19% 22% 7% 1	1600
December	1st to 10th	Day	57% 7% 16% 13% 7% 1	1584
	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	35% 4 21% 25% 13% 2	2490
	11th to 20th	Night	21% 4 15% 36% 22% 2	2482
	21st to end	Night	23% 4 18% 32% 20% 2	2841
August	1st to 10th	Night	36% 5% 12% 29% 15% 4	2826
	11th to 20th	Night	33% 4 16% 31% 15% 1	2825
	21st to end	Night	33% 4 18% 27% 16% 2	3290
September	1st to 10th	Night	27% 5% 12% 33% 21% 1	1900
	11th to 20th	Night	30% 6% 15% 32% 16% 1	1971
	21st to end	Night	32% 4 12% 30% 19% 2	2133
October	1st to 10th	Night	23% 3 6% 34% 30% 4	3800
	11th to 20th	Night	30% 3 13% 27% 23% 4	3779
	21st to end	Night	42% 3 11% 30% 13% 1	4120
November	1st to 10th	Night	40% 4 14% 24% 14% 2	2637
	11th to 20th	Night	49% 6% 12% 20% 12% 1	2800
	21st to end	Night	47% 5% 16% 23% 8% 1	2800
December	1st to 10th	Night	51% 6% 17% 19% 7%	2779
	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% 6% 27% 32% 19% 6%	668
	21st to end	Day	6% 4 24% 40% 20% 5%	739
August	1st to 10th	Day	11% 3 23% 32% 26% 5%	676
	11th to 20th	Day	13% 4 22% 31% 24% 6%	665
	21st to end	Day	10% 4 29% 34% 18% 5%	651
September	1st to 10th	Day	15% 8% 28% 29% 17% 3	490
	11th to 20th	Day	12% 6% 30% 31% 15% 5%	414
	21st to end	Day	16% 14% 29% 25% 12% 5%	300
October	1st to 10th	Day	19% 11% 27% 26% 14% 3	300
	11th to 20th	Day	20% 14% 39% 19% 7% 1	300
	21st to end	Day	25% 17% 39% 13% 6%	330
November	1st to 10th	Day	20% 15% 37% 19% 8% 1	300
	11th to 20th	Day	20% 20% 40% 16% 4	300
	21st to end	Day	20% 19% 38% 19% 4	300
December	1st to 10th	Day	16% 12% 43% 22% 6%	294
	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% 45% 21% 1	552
	21st to end	Night	4 2 23% 50% 19% 2	624
August	1st to 10th	Night	8% 1 20% 46% 23% 2	567
	11th to 20th	Night	10% 2 20% 41% 24% 3	564
	21st to end	Night	9% 2 24% 41% 22% 2	627
September	1st to 10th	Night	12% 4 24% 42% 17% 2	570
	11th to 20th	Night	10% 6% 33% 33% 16% 3	570
	21st to end	Night	11% 10% 33% 32% 13% 1	613
October	1st to 10th	Night	17% 8% 33% 29% 13% 1	760
	11th to 20th	Night	20% 11% 35% 27% 6% 1	760
	21st to end	Night	25% 12% 30% 27% 6%	836
November	1st to 10th	Night	18% 12% 36% 26% 8% 1	914
	11th to 20th	Night	17% 15% 39% 23% 6%	1000
	21st to end	Night	18% 14% 38% 24% 7%	1000
December	1st to 10th	Night	17% 11% 37% 28% 7% 1	993
	11th to 20th	Night	20% 8% 39% 27% 7%	950
	21st to end	Night	24% 11% 35% 24% 5%	1045

Page generated at 2024-12-06 02:54:51. Version ac07

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