The Grainger Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Grainger Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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% 38%	119754
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	119346
Transparency	Day	27% 41% 7% 13% 9% 2	123332
	Night	28% 40% 7% 13% 10% 2	123929
Seeing	Day	19% 13% 31% 25% 9% 2	15891
	Night	21% 12% 32% 26% 9% 1	24736

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	14% 6% 2 7% 5% 5% 6% 4 7% 7% 39%	3597
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 4 6% 46%	3570
	21st to end	Day	16% 9% 2 5% 4 4 4 3 4 5% 46%	3960
February	1st to 10th	Day	17% 8% 3 6% 4 4 4 5% 7% 6% 36%	1827
	11th to 20th	Day	14% 7% 3 7% 4 4 5% 4 6% 7% 41%	1948
	21st to end	Day	16% 11% 2 7% 4 5% 3 3 6% 5% 37%	1650
March	1st to 10th	Day	23% 10% 2 6% 4 4 4 3 4 5% 33%	4476
	11th to 20th	Day	17% 9% 3 6% 4 4 4 3 5% 6% 38%	4811
	21st to end	Day	27% 9% 2 5% 4 3 4 3 5% 6% 31%	5726
April	1st to 10th	Day	19% 10% 2 7% 5% 4 3 2 4 6% 37%	2400
	11th to 20th	Day	18% 7% 2 5% 4 5% 5% 4 6% 7% 37%	2418
	21st to end	Day	20% 6% 3 7% 5% 5% 4 3 4 6% 38%	2600
May	1st to 10th	Day	31% 10% 2 7% 4 4 4 3 4 6% 25%	5886
	11th to 20th	Day	21% 8% 3 8% 5% 4 4 4 5% 7% 32%	5968
	21st to end	Day	20% 6% 2 6% 4 4 4 4 5% 8% 35%	6949
June	1st to 10th	Day	20% 8% 2 6% 5% 4 5% 4 6% 9% 33%	2800
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 6% 9% 39%	2800
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 4 6% 8% 34%	2800
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 9% 35%	4580
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 11% 40%	4580
	21st to end	Day	17% 7% 3 6% 5% 4 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 7% 8% 35%	4147
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 7% 8% 36%	4045
	21st to end	Day	9% 4 2 5% 4 4 5% 5% 8% 9% 46%	4195
September	1st to 10th	Day	13% 5% 1 5% 4 4 4 4 5% 7% 47%	2262
	11th to 20th	Day	15% 5% 2 5% 3 3 3 2 4 5% 52%	2090
	21st to end	Day	17% 5% 2 6% 4 3 4 3 4 6% 46%	2090
October	1st to 10th	Day	22% 6% 2 5% 3 4 4 4 5% 6% 40%	2090
	11th to 20th	Day	14% 5% 2 6% 4 5% 3 3 4 7% 46%	1844
	21st to end	Day	22% 6% 2 6% 3 3 4 4 4 6% 40%	1881
November	1st to 10th	Day	16% 6% 3 5% 4 3 3 3 4 5% 50%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 5% 5% 48%	1800
	21st to end	Day	21% 6% 3 4 4 3 3 4 4 5% 43%	1800
December	1st to 10th	Day	14% 9% 2 5% 3 3 4 3 5% 7% 46%	2643
	11th to 20th	Day	19% 6% 2 5% 3 3 3 3 5% 5% 46%	2986
	21st to end	Day	22% 10% 2 5% 4 3 4 3 5% 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 6% 3 4 4 4 6% 6% 45%	6349
	11th to 20th	Night	13% 9% 3 8% 5% 5% 4 3 5% 6% 38%	6315
	21st to end	Night	16% 9% 3 6% 5% 4 4 3 5% 5% 40%	6996
February	1st to 10th	Night	16% 10% 2 6% 5% 4 4 4 6% 5% 37%	2565
	11th to 20th	Night	14% 9% 3 6% 5% 4 5% 3 5% 5% 41%	2338
	21st to end	Night	16% 9% 2 5% 4 4 5% 4 5% 6% 41%	1980
March	1st to 10th	Night	25% 9% 2 5% 3 4 4 3 5% 6% 35%	5364
	11th to 20th	Night	16% 10% 3 7% 5% 4 5% 4 5% 6% 36%	5146
	21st to end	Night	26% 8% 2 5% 4 4 4 4 6% 6% 32%	5202
April	1st to 10th	Night	20% 9% 3 6% 5% 4 4 4 5% 6% 37%	2000
	11th to 20th	Night	20% 7% 3 6% 5% 3 4 3 4 5% 40%	1930
	21st to end	Night	20% 7% 3 7% 4 4 4 3 5% 6% 38%	1800
May	1st to 10th	Night	32% 11% 3 6% 3 3 3 3 5% 6% 23%	4054
	11th to 20th	Night	16% 9% 4 8% 6% 6% 6% 4 6% 8% 28%	3954
	21st to end	Night	20% 8% 3 9% 5% 4 5% 3 4 6% 32%	3926
June	1st to 10th	Night	20% 8% 3 7% 5% 4 4 4 5% 4 37%	1600
	11th to 20th	Night	19% 6% 2 6% 4 4 5% 4 6% 8% 38%	1600
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 6% 7% 37%	1600
July	1st to 10th	Night	12% 7% 2 7% 6% 5% 6% 5% 6% 10% 34%	2600
	11th to 20th	Night	10% 5% 3 7% 5% 5% 5% 5% 7% 9% 39%	2600
	21st to end	Night	15% 7% 3 6% 4 5% 5% 4 5% 7% 38%	2739
August	1st to 10th	Night	20% 6% 2 7% 5% 5% 5% 4 5% 7% 33%	2752
	11th to 20th	Night	13% 8% 3 7% 5% 5% 5% 4 7% 8% 34%	2782
	21st to end	Night	9% 5% 2 5% 3 3 4 4 6% 8% 51%	3444
September	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 5% 7% 49%	1917
	11th to 20th	Night	14% 6% 2 3 4 2 3 2 4 7% 52%	2090
	21st to end	Night	16% 5% 2 5% 4 4 3 3 5% 7% 45%	2090
October	1st to 10th	Night	21% 6% 2 7% 4 3 3 3 5% 6% 39%	2090
	11th to 20th	Night	16% 5% 2 5% 4 5% 3 4 5% 6% 45%	2330
	21st to end	Night	22% 7% 2 5% 4 4 3 3 4 6% 39%	2717
November	1st to 10th	Night	15% 5% 2 5% 4 4 4 3 4 5% 48%	2537
	11th to 20th	Night	16% 7% 2 5% 4 3 4 2 4 6% 47%	2600
	21st to end	Night	16% 7% 2 4 4 4 3 3 4 6% 44%	2600
December	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 6% 41%	4535
	11th to 20th	Night	22% 7% 2 6% 3 3 3 3 4 5% 42%	5208
	21st to end	Night	18% 10% 3 6% 4 4 4 3 4 5% 39%	6996

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	30% 29% 8% 20% 13% 1	3598
	11th to 20th	Day	26% 28% 8% 19% 15% 3	3600
	21st to end	Day	31% 30% 5% 14% 17% 3	3960
February	1st to 10th	Day	18% 63% 6% 8% 5% 1	1827
	11th to 20th	Day	18% 61% 4 7% 9% 2	1957
	21st to end	Day	15% 62% 4 9% 7% 2	1650
March	1st to 10th	Day	39% 30% 5% 8% 9% 9%	4479
	11th to 20th	Day	32% 31% 11% 16% 7% 2	4811
	21st to end	Day	41% 30% 4 10% 11% 5%	5726
April	1st to 10th	Day	21% 61% 2 7% 6% 2	2400
	11th to 20th	Day	16% 64% 4 7% 7% 2	2418
	21st to end	Day	19% 62% 3 7% 8% 1	2600
May	1st to 10th	Day	45% 34% 10% 8% 3	5897
	11th to 20th	Day	39% 32% 11% 12% 5% 1	5972
	21st to end	Day	35% 32% 10% 15% 8%	6949
June	1st to 10th	Day	16% 64% 4 10% 6%	2800
	11th to 20th	Day	15% 62% 5% 9% 8% 1	2800
	21st to end	Day	18% 61% 4 9% 7% 1	2800
July	1st to 10th	Day	29% 38% 8% 14% 10% 1	4510
	11th to 20th	Day	23% 38% 11% 18% 9%	4505
	21st to end	Day	31% 38% 8% 14% 8% 1	4661
August	1st to 10th	Day	30% 40% 10% 15% 5%	4133
	11th to 20th	Day	31% 39% 10% 16% 4	4084
	21st to end	Day	17% 40% 12% 19% 12% 1	4195
September	1st to 10th	Day	13% 61% 5% 11% 8% 1	2262
	11th to 20th	Day	17% 60% 3 9% 10% 1	2090
	21st to end	Day	16% 60% 4 11% 8% 1	2090
October	1st to 10th	Day	27% 39% 6% 16% 11% 1	3480
	11th to 20th	Day	17% 37% 4 21% 19% 2	3047
	21st to end	Day	19% 38% 4 23% 15% 2	3081
November	1st to 10th	Day	17% 57% 7% 11% 7% 1	1761
	11th to 20th	Day	17% 56% 3 7% 14% 2	1800
	21st to end	Day	17% 57% 6% 10% 9% 1	1800
December	1st to 10th	Day	24% 36% 9% 24% 7%	2643
	11th to 20th	Day	27% 31% 11% 12% 14% 6%	2986
	21st to end	Day	36% 28% 5% 15% 12% 5%	3960
January	1st to 10th	Night	26% 27% 8% 26% 11% 2	6349
	11th to 20th	Night	33% 28% 9% 17% 10% 3	6360
	21st to end	Night	35% 30% 6% 12% 12% 5%	6996
February	1st to 10th	Night	18% 62% 5% 7% 5% 2	2565
	11th to 20th	Night	17% 60% 4 8% 8% 2	2340
	21st to end	Night	18% 62% 3 9% 6% 3	1980
March	1st to 10th	Night	39% 30% 6% 8% 10% 7%	5374
	11th to 20th	Night	33% 32% 11% 12% 10% 1	5147
	21st to end	Night	40% 29% 4 8% 15% 5%	5202
April	1st to 10th	Night	21% 62% 3 5% 7% 2	2000
	11th to 20th	Night	18% 63% 3 7% 7% 1	1930
	21st to end	Night	18% 62% 3 7% 9% 1	1800
May	1st to 10th	Night	46% 33% 9% 7% 5% 1	4060
	11th to 20th	Night	39% 33% 12% 11% 6%	3955
	21st to end	Night	40% 31% 9% 12% 6% 1	3927
June	1st to 10th	Night	18% 62% 3 10% 7%	1600
	11th to 20th	Night	15% 64% 5% 10% 6%	1600
	21st to end	Night	17% 61% 3 11% 6% 1	1600
July	1st to 10th	Night	28% 38% 8% 14% 11%	2560
	11th to 20th	Night	26% 38% 11% 14% 11%	2552
	21st to end	Night	31% 37% 7% 13% 11% 1	2651
August	1st to 10th	Night	36% 39% 8% 10% 7% 1	2742
	11th to 20th	Night	30% 42% 10% 12% 6%	2809
	21st to end	Night	16% 39% 11% 17% 16%	3444
September	1st to 10th	Night	14% 60% 6% 11% 8% 1	1917
	11th to 20th	Night	16% 60% 3 10% 10% 1	2090
	21st to end	Night	17% 60% 4 12% 7%	2090
October	1st to 10th	Night	26% 39% 11% 15% 9% 1	3480
	11th to 20th	Night	12% 37% 7% 19% 22% 3	3866
	21st to end	Night	19% 37% 7% 22% 13% 2	4467
November	1st to 10th	Night	17% 57% 4 12% 8% 2	2537
	11th to 20th	Night	17% 56% 3 9% 12% 2	2600
	21st to end	Night	16% 57% 7% 12% 7% 1	2600
December	1st to 10th	Night	29% 36% 6% 21% 8%	4535
	11th to 20th	Night	33% 32% 8% 9% 17% 1	5208
	21st to end	Night	35% 28% 6% 16% 13% 2	6996

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	26% 23% 29% 20% 4	200
	11th to 20th	Day	23% 25% 32% 18% 3 1	200
	21st to end	Day	18% 23% 37% 17% 4	220
February	1st to 10th	Day	26% 20% 37% 16% 1	387
	11th to 20th	Day	24% 25% 35% 14% 2	496
	21st to end	Day	21% 21% 36% 19% 4	414
March	1st to 10th	Day	26% 18% 35% 18% 3	500
	11th to 20th	Day	20% 23% 37% 16% 3 1	500
	21st to end	Day	24% 19% 37% 14% 5% 2	550
April	1st to 10th	Day	25% 22% 30% 17% 5% 1	500
	11th to 20th	Day	22% 18% 36% 17% 7%	492
	21st to end	Day	24% 11% 38% 19% 8% 1	500
May	1st to 10th	Day	26% 12% 36% 18% 6% 1	503
	11th to 20th	Day	17% 12% 35% 28% 6% 2	527
	21st to end	Day	20% 8% 31% 31% 9% 1	573
June	1st to 10th	Day	20% 7% 31% 29% 12% 1	530
	11th to 20th	Day	14% 9% 36% 32% 8% 2	530
	21st to end	Day	18% 6% 35% 29% 10% 2	530
July	1st to 10th	Day	15% 5% 24% 36% 16% 3	525
	11th to 20th	Day	11% 7% 23% 40% 16% 2	524
	21st to end	Day	11% 3 27% 41% 15% 2	532
August	1st to 10th	Day	12% 4 25% 38% 18% 3	486
	11th to 20th	Day	15% 5% 24% 34% 18% 5%	479
	21st to end	Day	14% 3 24% 39% 16% 5%	539
September	1st to 10th	Day	14% 6% 26% 31% 17% 7%	490
	11th to 20th	Day	17% 7% 27% 27% 14% 8%	490
	21st to end	Day	16% 9% 30% 29% 11% 4	490
October	1st to 10th	Day	24% 10% 26% 25% 12% 3	490
	11th to 20th	Day	17% 14% 28% 28% 12% 2	420
	21st to end	Day	26% 10% 32% 22% 8%	429
November	1st to 10th	Day	15% 17% 32% 25% 10% 2	395
	11th to 20th	Day	24% 22% 29% 22% 4 1	400
	21st to end	Day	26% 20% 27% 21% 6% 1	400
December	1st to 10th	Day	20% 20% 35% 20% 6%	230
	11th to 20th	Day	23% 21% 39% 16% 2	200
	21st to end	Day	22% 21% 31% 20% 5% 1	220
January	1st to 10th	Night	21% 20% 40% 17% 3	800
	11th to 20th	Night	25% 17% 38% 18% 3	800
	21st to end	Night	19% 22% 38% 18% 4	880
February	1st to 10th	Night	23% 17% 38% 21% 1	800
	11th to 20th	Night	23% 20% 40% 15% 3	783
	21st to end	Night	21% 16% 37% 23% 3	660
March	1st to 10th	Night	25% 17% 37% 18% 3	800
	11th to 20th	Night	21% 16% 37% 20% 6%	800
	21st to end	Night	24% 15% 35% 21% 5%	880
April	1st to 10th	Night	25% 15% 33% 23% 4	800
	11th to 20th	Night	22% 13% 34% 24% 6%	746
	21st to end	Night	23% 11% 33% 29% 4 1	600
May	1st to 10th	Night	27% 7% 32% 25% 8% 1	600
	11th to 20th	Night	20% 8% 34% 30% 8% 1	565
	21st to end	Night	21% 6% 26% 36% 10%	433
June	1st to 10th	Night	22% 5% 25% 35% 12% 2	400
	11th to 20th	Night	20% 6% 25% 38% 12% 1	400
	21st to end	Night	20% 3 24% 40% 13%	400
July	1st to 10th	Night	17% 3 20% 44% 15% 1	390
	11th to 20th	Night	12% 4 17% 47% 19% 1	388
	21st to end	Night	13% 3 16% 48% 17% 2	416
August	1st to 10th	Night	15% 3 17% 41% 23% 1	520
	11th to 20th	Night	14% 4 20% 41% 18% 2	562
	21st to end	Night	14% 4 22% 34% 23% 3	817
September	1st to 10th	Night	14% 4 27% 33% 21% 1	760
	11th to 20th	Night	16% 5% 26% 29% 20% 4	760
	21st to end	Night	18% 4 27% 31% 17% 3	760
October	1st to 10th	Night	22% 7% 26% 28% 16% 1	760
	11th to 20th	Night	18% 10% 31% 29% 11% 1	760
	21st to end	Night	25% 11% 31% 25% 7%	836
November	1st to 10th	Night	16% 11% 36% 29% 8% 1	780
	11th to 20th	Night	20% 17% 34% 21% 6% 1	800
	21st to end	Night	22% 18% 33% 22% 4 1	800
December	1st to 10th	Night	24% 14% 40% 18% 4	800
	11th to 20th	Night	22% 19% 34% 21% 3	800
	21st to end	Night	25% 16% 36% 21% 2	880

Page generated at 2025-07-18 01:10:02. Version ac07

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