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 2024-01-16.

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% 8% 3 7% 6% 3 5% 5% 5% 7% 38%	74728
	Night	12% 8% 4 7% 6% 3 4 4 5% 7% 40%	75016
Transparency	Day	45% 6% 14% 23% 11% 1	74434
	Night	44% 5% 13% 23% 12% 2	74830
Seeing	Day	17% 10% 31% 26% 13% 3	14814
	Night	16% 9% 33% 31% 10% 1	24841

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	8% 12% 6% 9% 9% 5% 7% 6% 5% 8% 25%	1517
	11th to 20th	Day	10% 15% 6% 12% 10% 4 7% 4 6% 5% 21%	1564
	21st to end	Day	9% 14% 6% 12% 10% 4 7% 5% 6% 7% 21%	1782
February	1st to 10th	Day	8% 10% 6% 9% 10% 6% 6% 5% 6% 7% 27%	1620
	11th to 20th	Day	9% 11% 5% 9% 9% 4 6% 6% 6% 7% 28%	1574
	21st to end	Day	13% 11% 5% 8% 8% 4 5% 4 6% 7% 30%	1364
March	1st to 10th	Day	15% 12% 5% 8% 6% 3 4 4 5% 7% 31%	1980
	11th to 20th	Day	16% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	1980
	21st to end	Day	19% 11% 4 8% 6% 3 4 4 5% 7% 30%	2178
April	1st to 10th	Day	17% 9% 4 7% 4 4 3 4 6% 8% 33%	2150
	11th to 20th	Day	20% 12% 3 6% 5% 3 4 3 4 6% 33%	2168
	21st to end	Day	23% 10% 3 6% 5% 3 5% 5% 5% 6% 30%	2340
May	1st to 10th	Day	18% 8% 3 6% 6% 4 6% 5% 6% 6% 33%	2376
	11th to 20th	Day	18% 7% 3 6% 4 2 4 4 6% 8% 38%	2479
	21st to end	Day	16% 6% 3 6% 6% 3 4 5% 6% 8% 37%	2725
June	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 4 6% 8% 46%	2520
	11th to 20th	Day	13% 6% 2 5% 6% 3 6% 5% 6% 8% 41%	2520
	21st to end	Day	9% 4 2 5% 5% 4 5% 5% 6% 10% 44%	2520
July	1st to 10th	Day	6% 4 2 4 4 2 4 6% 7% 10% 51%	2520
	11th to 20th	Day	7% 4 1 3 4 2 4 5% 6% 9% 54%	2520
	21st to end	Day	6% 3 2 4 5% 3 5% 5% 7% 10% 51%	2636
August	1st to 10th	Day	9% 4 2 6% 7% 4 5% 5% 7% 9% 42%	2328
	11th to 20th	Day	11% 6% 3 5% 5% 3 4 5% 5% 8% 46%	2301
	21st to end	Day	8% 4 2 5% 4 3 4 4 6% 7% 52%	2483
September	1st to 10th	Day	13% 5% 3 5% 4 3 3 5% 5% 7% 47%	2160
	11th to 20th	Day	12% 6% 1 4 5% 2 4 4 5% 7% 50%	2088
	21st to end	Day	14% 5% 3 6% 3 2 5% 4 5% 7% 45%	1922
October	1st to 10th	Day	16% 7% 3 6% 5% 3 4 4 5% 6% 43%	1800
	11th to 20th	Day	15% 10% 2 5% 5% 3 4 3 4 6% 43%	1800
	21st to end	Day	19% 9% 4 7% 4 2 3 3 4 5% 39%	1980
November	1st to 10th	Day	11% 9% 4 7% 5% 3 4 4 4 8% 41%	1572
	11th to 20th	Day	11% 12% 6% 9% 6% 3 5% 5% 5% 7% 31%	1520
	21st to end	Day	13% 9% 4 6% 6% 2 5% 5% 5% 8% 36%	1520
December	1st to 10th	Day	11% 12% 4 10% 10% 4 5% 6% 5% 6% 28%	1520
	11th to 20th	Day	11% 13% 6% 8% 7% 4 4 5% 5% 7% 29%	1918
	21st to end	Day	20% 14% 6% 16% 7% 3 4 3 4 5% 19%	2783
January	1st to 10th	Night	7% 9% 6% 10% 9% 4 7% 5% 5% 5% 32%	2648
	11th to 20th	Night	10% 11% 6% 10% 9% 3 6% 6% 6% 7% 26%	2522
	21st to end	Night	12% 13% 5% 11% 8% 3 5% 6% 6% 6% 25%	2574
February	1st to 10th	Night	8% 12% 6% 9% 8% 4 5% 4 4 7% 34%	2340
	11th to 20th	Night	8% 10% 4 7% 6% 5% 6% 4 5% 7% 39%	2273
	21st to end	Night	12% 9% 4 8% 7% 4 5% 5% 5% 7% 35%	1891
March	1st to 10th	Night	13% 10% 4 7% 5% 3 5% 4 4 7% 37%	1996
	11th to 20th	Night	13% 10% 4 6% 6% 3 6% 4 3 6% 38%	1980
	21st to end	Night	18% 11% 4 5% 4 3 4 4 5% 7% 34%	2178
April	1st to 10th	Night	14% 10% 4 6% 5% 2 5% 5% 5% 6% 38%	1810
	11th to 20th	Night	20% 9% 4 7% 6% 3 4 4 4 4 35%	1739
	21st to end	Night	18% 10% 3 6% 5% 3 3 5% 5% 6% 37%	1620
May	1st to 10th	Night	15% 8% 3 4 6% 3 4 5% 6% 7% 41%	1583
	11th to 20th	Night	15% 5% 2 5% 4 2 4 4 5% 8% 46%	1416
	21st to end	Night	15% 5% 3 6% 5% 3 5% 5% 4 7% 43%	1551
June	1st to 10th	Night	9% 4 2 3 5% 2 5% 5% 6% 8% 52%	1440
	11th to 20th	Night	10% 5% 2 6% 5% 3 5% 5% 7% 8% 45%	1440
	21st to end	Night	8% 3 2 4 5% 2 5% 4 4 8% 57%	1347
July	1st to 10th	Night	5% 2 1 3 4 2 4 5% 7% 9% 57%	1440
	11th to 20th	Night	4 2 1 4 4 2 4 4 6% 9% 60%	1440
	21st to end	Night	4 3 2 3 4 3 3 4 6% 9% 60%	1678
August	1st to 10th	Night	5% 4 2 4 5% 3 5% 6% 7% 9% 50%	1611
	11th to 20th	Night	8% 5% 2 3 4 2 4 4 5% 8% 55%	1600
	21st to end	Night	7% 4 1 4 4 2 4 4 4 8% 59%	1872
September	1st to 10th	Night	9% 5% 2 3 4 2 3 4 4 7% 57%	1800
	11th to 20th	Night	11% 4 2 4 4 2 4 4 5% 7% 53%	1867
	21st to end	Night	10% 6% 2 5% 4 3 4 4 4 7% 51%	2020
October	1st to 10th	Night	15% 7% 2 5% 4 2 3 4 4 5% 49%	2160
	11th to 20th	Night	16% 9% 3 4 4 2 4 4 5% 7% 43%	2160
	21st to end	Night	18% 10% 3 6% 4 2 3 3 5% 6% 40%	2376
November	1st to 10th	Night	12% 7% 3 5% 4 3 4 4 5% 7% 46%	2501
	11th to 20th	Night	12% 12% 5% 7% 6% 3 4 4 4 5% 37%	2660
	21st to end	Night	14% 9% 4 7% 5% 2 4 4 4 7% 40%	2660
December	1st to 10th	Night	10% 11% 4 9% 8% 3 5% 4 5% 7% 32%	2660
	11th to 20th	Night	11% 12% 5% 8% 9% 4 5% 5% 5% 6% 29%	3279
	21st to end	Night	22% 13% 6% 12% 8% 3 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	57% 6% 14% 17% 6%	1518
	11th to 20th	Day	64% 4 10% 14% 7%	1564
	21st to end	Day	63% 5% 14% 11% 6% 1	1782
February	1st to 10th	Day	56% 6% 12% 17% 8% 1	1620
	11th to 20th	Day	54% 7% 12% 19% 7% 1	1583
	21st to end	Day	53% 7% 12% 17% 11%	1364
March	1st to 10th	Day	53% 7% 13% 19% 6% 1	1980
	11th to 20th	Day	49% 7% 12% 23% 8%	1980
	21st to end	Day	55% 7% 12% 20% 5% 1	2178
April	1st to 10th	Day	49% 7% 12% 22% 9% 1	2150
	11th to 20th	Day	53% 5% 10% 21% 10% 1	2168
	21st to end	Day	55% 6% 12% 20% 7% 1	2340
May	1st to 10th	Day	50% 7% 13% 22% 8% 1	2376
	11th to 20th	Day	44% 8% 12% 25% 11% 1	2483
	21st to end	Day	45% 6% 12% 23% 12% 1	2725
June	1st to 10th	Day	36% 7% 15% 27% 14% 1	2520
	11th to 20th	Day	41% 7% 13% 22% 16% 1	2520
	21st to end	Day	34% 8% 17% 27% 12% 2	2520
July	1st to 10th	Day	26% 8% 18% 33% 14% 1	2451
	11th to 20th	Day	27% 7% 16% 27% 19% 4	2445
	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	52% 5% 14% 21% 8%	2648
	11th to 20th	Night	56% 6% 15% 16% 6%	2522
	21st to end	Night	59% 4 14% 15% 7% 1	2574
February	1st to 10th	Night	52% 3 12% 22% 9% 1	2340
	11th to 20th	Night	45% 6% 14% 23% 11% 2	2274
	21st to end	Night	49% 5% 10% 22% 12% 2	1891
March	1st to 10th	Night	46% 6% 17% 24% 6% 1	1996
	11th to 20th	Night	49% 4 11% 22% 13% 1	1980
	21st to end	Night	51% 6% 13% 22% 7% 1	2178
April	1st to 10th	Night	46% 6% 13% 23% 12% 1	1810
	11th to 20th	Night	52% 4 8% 17% 16% 3	1739
	21st to end	Night	47% 5% 10% 22% 14% 2	1620
May	1st to 10th	Night	42% 6% 15% 22% 15% 1	1583
	11th to 20th	Night	37% 8% 14% 23% 15% 3	1417
	21st to end	Night	41% 5% 15% 21% 15% 3	1552
June	1st to 10th	Night	31% 7% 14% 30% 17% 2	1440
	11th to 20th	Night	36% 7% 13% 23% 19% 3	1440
	21st to end	Night	26% 6% 16% 31% 19% 3	1347
July	1st to 10th	Night	22% 7% 16% 34% 20% 3	1400
	11th to 20th	Night	21% 7% 17% 30% 21% 4	1392
	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	21% 13% 37% 24% 4 1	257
	21st to end	Day	15% 13% 39% 27% 6% 1	264
February	1st to 10th	Day	14% 15% 48% 19% 3	240
	11th to 20th	Day	15% 15% 43% 22% 5%	240
	21st to end	Day	22% 14% 36% 26% 2	198
March	1st to 10th	Day	20% 15% 39% 21% 4	240
	11th to 20th	Day	26% 10% 33% 25% 5% 1	240
	21st to end	Day	29% 14% 27% 20% 9% 2	264
April	1st to 10th	Day	27% 14% 32% 22% 5%	410
	11th to 20th	Day	29% 12% 25% 24% 9% 1	446
	21st to end	Day	28% 14% 27% 21% 8% 3	600
May	1st to 10th	Day	21% 13% 34% 24% 7% 2	603
	11th to 20th	Day	20% 14% 28% 24% 10% 4	625
	21st to end	Day	19% 9% 28% 29% 14% 2	679
June	1st to 10th	Day	11% 8% 28% 31% 18% 4	630
	11th to 20th	Day	14% 8% 35% 28% 12% 2	630
	21st to end	Day	10% 8% 34% 30% 14% 4	630
July	1st to 10th	Day	7% 5% 23% 31% 26% 7%	621
	11th to 20th	Day	8% 7% 28% 33% 19% 6%	618
	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	930
	21st to end	Night	20% 11% 35% 28% 6%	990
February	1st to 10th	Night	15% 13% 38% 29% 5%	900
	11th to 20th	Night	14% 14% 40% 26% 6% 1	879
	21st to end	Night	16% 12% 34% 30% 7% 1	707
March	1st to 10th	Night	18% 11% 38% 24% 9% 1	556
	11th to 20th	Night	19% 10% 36% 28% 6% 1	540
	21st to end	Night	22% 11% 31% 25% 10% 1	594
April	1st to 10th	Night	17% 14% 38% 26% 5%	540
	11th to 20th	Night	25% 12% 29% 26% 7% 1	533
	21st to end	Night	21% 12% 34% 25% 7% 1	540
May	1st to 10th	Night	18% 11% 33% 30% 8% 1	540
	11th to 20th	Night	18% 8% 34% 29% 11% 1	531
	21st to end	Night	17% 7% 29% 33% 13% 1	584
June	1st to 10th	Night	11% 5% 29% 41% 15% 1	540
	11th to 20th	Night	12% 5% 33% 41% 9%	540
	21st to end	Night	8% 5% 28% 43% 16% 1	540
July	1st to 10th	Night	5% 3 21% 40% 28% 2	525
	11th to 20th	Night	3 4 25% 46% 20% 1	522
	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 2024-01-13 18:28:31. Version ac07

Copyright 2024 Attilla Danko