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 2024-04-10.

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 5% 5% 3 4 4 5% 7% 41%	82955
	Night	19% 7% 3 5% 5% 3 4 4 4 6% 41%	85811
Transparency	Day	29% 35% 9% 15% 10% 1	82660
	Night	31% 34% 9% 15% 10% 1	85640
Seeing	Day	20% 15% 31% 23% 9% 2	14282
	Night	21% 12% 31% 26% 9% 1	23426

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 5% 4 3 4 4 5% 4 46%	2517
	11th to 20th	Day	17% 9% 2 4 5% 3 4 5% 4 6% 39%	2520
	21st to end	Day	23% 11% 4 5% 4 3 3 2 4 5% 35%	2776
February	1st to 10th	Day	19% 7% 2 5% 5% 3 5% 4 5% 6% 38%	1829
	11th to 20th	Day	14% 7% 2 5% 6% 3 5% 4 5% 7% 42%	1848
	21st to end	Day	21% 8% 3 6% 4 3 3 4 5% 6% 37%	1570
March	1st to 10th	Day	27% 10% 3 5% 4 2 3 2 3 5% 36%	3116
	11th to 20th	Day	11% 5% 2 4 4 3 5% 5% 6% 8% 45%	3279
	21st to end	Day	21% 7% 3 5% 5% 3 3 5% 5% 6% 36%	4117
April	1st to 10th	Day	19% 7% 3 4 4 2 4 4 5% 6% 40%	2270
	11th to 20th	Day	19% 6% 2 5% 3 3 4 4 5% 8% 42%	2134
	21st to end	Day	22% 6% 2 4 4 3 4 4 4 7% 39%	2294
May	1st to 10th	Day	24% 8% 3 5% 5% 2 3 4 4 6% 35%	2340
	11th to 20th	Day	20% 6% 2 4 4 3 5% 4 5% 7% 41%	2340
	21st to end	Day	16% 6% 2 4 4 3 3 5% 5% 8% 44%	2725
June	1st to 10th	Day	19% 5% 2 4 5% 4 5% 6% 6% 9% 35%	2520
	11th to 20th	Day	16% 6% 2 5% 5% 3 4 6% 5% 9% 38%	2520
	21st to end	Day	13% 6% 3 5% 5% 3 5% 5% 6% 10% 39%	2520
July	1st to 10th	Day	14% 5% 2 5% 4 3 6% 5% 6% 9% 42%	2520
	11th to 20th	Day	13% 6% 2 4 5% 3 5% 5% 6% 9% 41%	2520
	21st to end	Day	13% 5% 3 5% 6% 3 5% 6% 7% 9% 39%	2744
August	1st to 10th	Day	12% 5% 3 5% 5% 3 5% 6% 7% 10% 39%	2366
	11th to 20th	Day	14% 5% 2 5% 5% 3 4 6% 5% 8% 42%	2191
	21st to end	Day	11% 6% 2 6% 5% 3 5% 5% 6% 9% 41%	2376
September	1st to 10th	Day	15% 5% 2 5% 5% 3 4 5% 5% 6% 43%	2160
	11th to 20th	Day	13% 5% 2 4 4 2 3 4 6% 6% 51%	2003
	21st to end	Day	20% 6% 2 5% 4 3 3 4 4 6% 43%	1980
October	1st to 10th	Day	23% 6% 2 5% 4 2 4 4 4 6% 41%	1980
	11th to 20th	Day	16% 5% 2 4 3 3 3 5% 6% 7% 46%	1775
	21st to end	Day	19% 6% 2 5% 5% 3 3 5% 4 6% 42%	1782
November	1st to 10th	Day	15% 6% 2 4 5% 3 4 4 4 5% 49%	1671
	11th to 20th	Day	18% 5% 2 3 3 2 4 4 6% 5% 48%	1710
	21st to end	Day	17% 5% 2 5% 4 3 4 4 4 6% 45%	1710
December	1st to 10th	Day	22% 6% 2 5% 5% 2 5% 5% 5% 6% 38%	1554
	11th to 20th	Day	23% 5% 2 4 4 2 2 2 4 5% 46%	1906
	21st to end	Day	20% 10% 3 6% 5% 3 3 3 4 6% 36%	2772
January	1st to 10th	Night	17% 6% 2 4 4 2 3 3 4 7% 47%	4429
	11th to 20th	Night	18% 9% 3 6% 4 2 3 3 3 4 43%	4440
	21st to end	Night	24% 10% 3 6% 5% 3 4 4 4 5% 32%	4876
February	1st to 10th	Night	16% 9% 3 6% 5% 3 3 4 4 6% 41%	2348
	11th to 20th	Night	18% 7% 3 4 5% 3 4 4 4 7% 42%	2218
	21st to end	Night	21% 8% 3 4 4 3 4 4 4 5% 41%	1884
March	1st to 10th	Night	27% 11% 3 3 5% 2 3 3 4 5% 35%	3724
	11th to 20th	Night	11% 7% 3 6% 5% 3 5% 5% 5% 7% 43%	3678
	21st to end	Night	24% 7% 2 5% 4 2 4 5% 5% 5% 37%	3512
April	1st to 10th	Night	21% 8% 2 5% 4 4 4 4 4 6% 38%	1900
	11th to 20th	Night	20% 6% 3 5% 4 3 3 5% 4 6% 42%	1775
	21st to end	Night	23% 7% 2 4 5% 3 4 4 4 6% 39%	1664
May	1st to 10th	Night	23% 8% 3 5% 5% 4 5% 4 5% 6% 32%	1620
	11th to 20th	Night	20% 7% 4 5% 5% 3 4 3 5% 8% 35%	1567
	21st to end	Night	19% 6% 3 6% 5% 3 4 5% 5% 7% 37%	1550
June	1st to 10th	Night	20% 6% 3 4 6% 2 4 4 4 8% 38%	1440
	11th to 20th	Night	20% 6% 2 5% 4 2 4 5% 6% 8% 37%	1440
	21st to end	Night	15% 7% 3 5% 5% 2 4 5% 5% 11% 40%	1440
July	1st to 10th	Night	14% 6% 2 6% 6% 4 5% 5% 5% 8% 37%	1440
	11th to 20th	Night	16% 6% 3 4 5% 2 5% 5% 7% 10% 36%	1440
	21st to end	Night	12% 7% 3 6% 6% 3 4 5% 6% 8% 40%	1574
August	1st to 10th	Night	9% 6% 2 6% 5% 3 5% 6% 8% 8% 40%	1553
	11th to 20th	Night	11% 6% 3 5% 7% 4 6% 6% 7% 9% 37%	1706
	21st to end	Night	13% 5% 3 4 5% 2 5% 5% 6% 8% 45%	1980
September	1st to 10th	Night	15% 7% 2 6% 5% 3 5% 5% 6% 6% 40%	1800
	11th to 20th	Night	15% 6% 2 5% 3 2 3 3 4 6% 52%	1957
	21st to end	Night	19% 6% 3 6% 5% 3 5% 5% 4 5% 40%	1980
October	1st to 10th	Night	20% 7% 3 6% 4 3 3 3 4 5% 41%	1980
	11th to 20th	Night	16% 5% 2 5% 5% 3 3 5% 4 6% 47%	2181
	21st to end	Night	21% 7% 2 5% 5% 2 4 3 4 5% 41%	2574
November	1st to 10th	Night	14% 6% 3 5% 4 3 4 3 4 6% 48%	2407
	11th to 20th	Night	19% 7% 2 4 3 2 3 4 3 5% 47%	2470
	21st to end	Night	16% 5% 2 5% 4 3 4 3 4 6% 47%	2470
December	1st to 10th	Night	21% 7% 3 4 4 3 4 4 5% 6% 40%	2622
	11th to 20th	Night	21% 6% 2 4 4 2 4 4 4 6% 42%	3288
	21st to end	Night	22% 10% 3 6% 3 2 3 3 3 5% 38%	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	31% 27% 8% 24% 9%	2518
	11th to 20th	Day	35% 28% 10% 19% 8% 1	2520
	21st to end	Day	43% 23% 7% 19% 7% 1	2776
February	1st to 10th	Day	29% 35% 9% 15% 10% 1	1829
	11th to 20th	Day	26% 37% 10% 17% 8% 2	1857
	21st to end	Day	32% 36% 8% 13% 9% 2	1570
March	1st to 10th	Day	40% 24% 11% 14% 10% 1	3119
	11th to 20th	Day	23% 28% 13% 25% 11% 1	3279
	21st to end	Day	37% 23% 7% 14% 15% 4	4117
April	1st to 10th	Day	27% 37% 9% 14% 11% 2	2270
	11th to 20th	Day	28% 37% 9% 13% 11% 2	2134
	21st to end	Day	28% 38% 8% 13% 10% 3	2294
May	1st to 10th	Day	31% 38% 6% 14% 9% 1	2340
	11th to 20th	Day	27% 39% 8% 13% 10% 2	2344
	21st to end	Day	25% 39% 12% 14% 9% 2	2725
June	1st to 10th	Day	29% 39% 10% 14% 6%	2520
	11th to 20th	Day	26% 41% 12% 14% 7% 1	2520
	21st to end	Day	26% 41% 11% 15% 6% 1	2520
July	1st to 10th	Day	25% 42% 10% 15% 7% 1	2450
	11th to 20th	Day	24% 39% 11% 18% 7% 1	2445
	21st to end	Day	24% 41% 10% 13% 10% 1	2591
August	1st to 10th	Day	25% 41% 13% 14% 8%	2352
	11th to 20th	Day	27% 39% 9% 16% 9% 1	2191
	21st to end	Day	27% 39% 8% 12% 12% 1	2376
September	1st to 10th	Day	25% 39% 9% 13% 13% 1	2160
	11th to 20th	Day	21% 37% 10% 16% 15% 2	2003
	21st to end	Day	28% 37% 6% 14% 14% 1	1980
October	1st to 10th	Day	26% 38% 7% 15% 12% 1	1980
	11th to 20th	Day	25% 38% 10% 18% 8% 1	1775
	21st to end	Day	27% 39% 8% 16% 9% 2	1782
November	1st to 10th	Day	24% 41% 9% 12% 12% 2	1671
	11th to 20th	Day	23% 41% 10% 14% 10% 2	1710
	21st to end	Day	27% 41% 8% 16% 7% 1	1710
December	1st to 10th	Day	28% 41% 9% 13% 7% 1	1554
	11th to 20th	Day	30% 32% 12% 11% 13% 2	1906
	21st to end	Day	38% 28% 9% 17% 7%	2772
January	1st to 10th	Night	29% 26% 10% 26% 7% 2	4429
	11th to 20th	Night	36% 26% 8% 18% 10% 2	4440
	21st to end	Night	45% 25% 7% 16% 6% 1	4876
February	1st to 10th	Night	29% 35% 9% 16% 9% 1	2348
	11th to 20th	Night	30% 36% 7% 16% 9% 2	2220
	21st to end	Night	31% 35% 6% 17% 9% 2	1884
March	1st to 10th	Night	43% 23% 7% 17% 8% 1	3734
	11th to 20th	Night	29% 27% 14% 18% 10% 1	3679
	21st to end	Night	39% 23% 6% 10% 18% 4	3512
April	1st to 10th	Night	30% 36% 8% 13% 11% 2	1900
	11th to 20th	Night	31% 36% 5% 14% 12% 1	1775
	21st to end	Night	31% 37% 6% 13% 10% 3	1664
May	1st to 10th	Night	34% 37% 8% 10% 10% 2	1620
	11th to 20th	Night	30% 39% 7% 12% 9% 3	1568
	21st to end	Night	29% 41% 9% 10% 10%	1551
June	1st to 10th	Night	32% 38% 10% 11% 8% 1	1440
	11th to 20th	Night	28% 39% 10% 15% 7%	1440
	21st to end	Night	28% 40% 10% 14% 7%	1440
July	1st to 10th	Night	30% 42% 8% 13% 6% 1	1400
	11th to 20th	Night	26% 41% 11% 16% 5% 1	1392
	21st to end	Night	26% 41% 10% 13% 8% 2	1486
August	1st to 10th	Night	26% 41% 12% 14% 7%	1543
	11th to 20th	Night	28% 39% 9% 15% 8%	1706
	21st to end	Night	27% 40% 8% 14% 9% 1	1980
September	1st to 10th	Night	29% 38% 6% 14% 12% 2	1800
	11th to 20th	Night	23% 37% 8% 16% 15% 1	1957
	21st to end	Night	29% 38% 5% 11% 13% 3	1980
October	1st to 10th	Night	29% 37% 8% 14% 11% 2	1980
	11th to 20th	Night	28% 38% 9% 18% 8%	2181
	21st to end	Night	30% 38% 9% 13% 9% 1	2574
November	1st to 10th	Night	21% 42% 9% 14% 11% 3	2407
	11th to 20th	Night	26% 40% 7% 16% 10% 1	2470
	21st to end	Night	25% 40% 9% 17% 8% 1	2470
December	1st to 10th	Night	28% 41% 10% 13% 8% 1	2622
	11th to 20th	Night	29% 34% 10% 13% 13% 1	3288
	21st to end	Night	39% 28% 8% 17% 9%	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% 24% 31% 15% 6% 1	457
	11th to 20th	Day	23% 18% 33% 20% 5%	412
	21st to end	Day	30% 16% 33% 15% 4 2	420
May	1st to 10th	Day	30% 18% 29% 16% 6% 1	423
	11th to 20th	Day	19% 15% 39% 21% 5% 1	447
	21st to end	Day	20% 7% 28% 30% 12% 2	485
June	1st to 10th	Day	18% 10% 31% 24% 15% 2	450
	11th to 20th	Day	18% 9% 33% 30% 9% 1	450
	21st to end	Day	16% 10% 29% 28% 13% 3	450
July	1st to 10th	Day	18% 5% 28% 30% 18% 2	445
	11th to 20th	Day	13% 7% 24% 36% 16% 5%	444
	21st to end	Day	12% 7% 33% 34% 11% 2	488
August	1st to 10th	Day	12% 5% 30% 36% 13% 5%	446
	11th to 20th	Day	16% 4 27% 31% 16% 5%	439
	21st to end	Day	11% 7% 29% 30% 17% 6%	495
September	1st to 10th	Day	20% 8% 25% 26% 16% 6%	450
	11th to 20th	Day	15% 6% 33% 26% 13% 7%	450
	21st to end	Day	20% 8% 32% 25% 11% 2	450
October	1st to 10th	Day	27% 10% 26% 21% 13% 4	450
	11th to 20th	Day	17% 15% 30% 25% 12% 1	425
	21st to end	Day	25% 15% 31% 20% 7% 2	396
November	1st to 10th	Day	16% 15% 30% 26% 10% 2	365
	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% 25% 4	760
	11th to 20th	Night	23% 9% 36% 25% 6%	711
	21st to end	Night	25% 9% 34% 23% 7% 1	584
May	1st to 10th	Night	25% 9% 31% 25% 9% 1	540
	11th to 20th	Night	22% 9% 36% 26% 6% 1	506
	21st to end	Night	23% 3 23% 36% 12% 3	389
June	1st to 10th	Night	22% 4 26% 33% 15% 1	360
	11th to 20th	Night	24% 4 21% 39% 11% 1	360
	21st to end	Night	17% 4 23% 41% 15% 1	360
July	1st to 10th	Night	16% 3 20% 36% 23% 1	350
	11th to 20th	Night	14% 5% 16% 43% 20% 2	348
	21st to end	Night	15% 3 21% 44% 17% 1	394
August	1st to 10th	Night	10% 3 21% 43% 21% 1	479
	11th to 20th	Night	14% 3 20% 38% 22% 3	642
	21st to end	Night	16% 2 16% 39% 22% 4	792
September	1st to 10th	Night	18% 3 25% 35% 17% 3	720
	11th to 20th	Night	16% 5% 24% 31% 18% 5%	720
	21st to end	Night	21% 6% 26% 28% 17% 2	720
October	1st to 10th	Night	24% 9% 28% 23% 13% 4	720
	11th to 20th	Night	19% 11% 32% 26% 10% 1	720
	21st to end	Night	25% 11% 38% 18% 8% 1	792
November	1st to 10th	Night	16% 12% 33% 25% 13% 1	740
	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 2024-04-07 09:28:10. Version ac07

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