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 2024-11-04.

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	17% 7% 2 6% 4 4 4 4 5% 7% 38%	101249
	Night	18% 8% 2 6% 4 4 4 4 5% 6% 39%	97765
Transparency	Day	24% 48% 7% 12% 8% 1	104800
	Night	24% 48% 7% 12% 9% 1	102303
Seeing	Day	19% 12% 31% 26% 9% 2	15013
	Night	20% 11% 32% 27% 9% 1	23860

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% 8% 2 6% 3 3 4 3 5% 6% 44%	2517
	11th to 20th	Day	17% 6% 3 7% 4 3 4 4 4 6% 41%	2520
	21st to end	Day	19% 11% 2 6% 4 5% 4 3 4 5% 36%	2772
February	1st to 10th	Day	17% 8% 3 6% 4 4 4 5% 7% 6% 36%	1735
	11th to 20th	Day	14% 7% 3 6% 4 3 5% 4 6% 7% 41%	1848
	21st to end	Day	17% 11% 2 8% 4 4 3 3 5% 5% 37%	1570
March	1st to 10th	Day	26% 13% 2 5% 4 4 4 3 4 5% 29%	3116
	11th to 20th	Day	15% 8% 2 5% 4 5% 4 4 6% 7% 40%	3367
	21st to end	Day	23% 9% 2 6% 4 4 4 3 5% 6% 33%	3992
April	1st to 10th	Day	19% 10% 2 7% 5% 4 3 2 4 6% 38%	2280
	11th to 20th	Day	17% 7% 2 5% 4 5% 5% 4 6% 7% 37%	2296
	21st to end	Day	20% 6% 3 7% 5% 5% 4 3 4 6% 37%	2470
May	1st to 10th	Day	22% 12% 3 8% 5% 4 4 3 4 7% 29%	4106
	11th to 20th	Day	23% 7% 3 8% 4 4 4 3 5% 6% 34%	4160
	21st to end	Day	16% 5% 2 5% 4 3 4 4 6% 9% 43%	4837
June	1st to 10th	Day	20% 8% 2 6% 5% 5% 4 4 6% 8% 33%	2660
	11th to 20th	Day	16% 6% 2 5% 4 4 4 4 6% 9% 39%	2660
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 4 6% 8% 33%	2660
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 7% 11% 40%	4440
	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 2 4 5% 49%	1698
	11th to 20th	Day	19% 6% 2 4 3 3 4 3 5% 5% 46%	1710
	21st to end	Day	20% 7% 3 4 3 3 3 4 4 5% 44%	1710
December	1st to 10th	Day	19% 8% 2 7% 5% 3 4 4 5% 6% 38%	1549
	11th to 20th	Day	22% 7% 2 5% 3 3 3 3 4 5% 43%	1906
	21st to end	Day	24% 10% 2 6% 4 4 4 3 5% 5% 33%	2772
January	1st to 10th	Night	18% 8% 2 5% 3 3 3 3 5% 5% 45%	4429
	11th to 20th	Night	14% 8% 3 8% 5% 5% 4 3 5% 6% 39%	4440
	21st to end	Night	22% 12% 3 7% 5% 4 4 4 4 5% 32%	4884
February	1st to 10th	Night	15% 10% 2 6% 5% 4 4 4 6% 5% 37%	2438
	11th to 20th	Night	14% 9% 3 6% 5% 4 5% 3 5% 5% 41%	2218
	21st to end	Night	17% 9% 2 5% 3 3 4 4 5% 6% 42%	1884
March	1st to 10th	Night	28% 10% 2 5% 3 3 4 3 4 5% 33%	3724
	11th to 20th	Night	13% 8% 2 8% 5% 5% 5% 4 5% 6% 36%	3590
	21st to end	Night	25% 7% 3 5% 4 4 4 4 6% 6% 33%	3636
April	1st to 10th	Night	19% 9% 3 6% 5% 4 3 4 5% 6% 37%	1900
	11th to 20th	Night	21% 7% 3 6% 5% 3 4 3 4 5% 41%	1833
	21st to end	Night	20% 7% 3 7% 4 4 4 3 4 6% 37%	1710
May	1st to 10th	Night	23% 12% 4 7% 4 4 4 4 5% 7% 25%	2834
	11th to 20th	Night	19% 8% 3 8% 6% 6% 5% 5% 6% 8% 27%	2762
	21st to end	Night	17% 7% 3 7% 4 3 4 3 5% 7% 40%	2738
June	1st to 10th	Night	19% 8% 3 6% 5% 4 4 4 6% 4 37%	1520
	11th to 20th	Night	19% 6% 2 6% 4 4 4 4 6% 8% 38%	1520
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 6% 7% 36%	1520
July	1st to 10th	Night	12% 7% 2 7% 6% 5% 6% 5% 6% 10% 34%	2520
	11th to 20th	Night	10% 5% 2 7% 5% 5% 5% 5% 7% 9% 39%	2520
	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 5% 3 4 5% 47%	2453
	11th to 20th	Night	17% 7% 2 5% 4 3 4 2 3 6% 46%	2470
	21st to end	Night	16% 7% 2 4 4 4 3 3 4 6% 44%	2470
December	1st to 10th	Night	20% 9% 2 6% 5% 4 4 3 4 6% 38%	2629
	11th to 20th	Night	20% 7% 1 5% 4 3 3 4 5% 6% 41%	3288
	21st to end	Night	22% 9% 3 6% 5% 4 4 2 4 5% 35%	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	24% 39% 8% 22% 7% 1	2518
	11th to 20th	Day	26% 39% 7% 18% 9% 1	2520
	21st to end	Day	35% 42% 6% 12% 4	2772
February	1st to 10th	Day	16% 66% 6% 7% 4 1	1735
	11th to 20th	Day	17% 64% 3 6% 8% 1	1857
	21st to end	Day	15% 65% 4 8% 6% 2	1570
March	1st to 10th	Day	39% 42% 4 6% 7% 1	3119
	11th to 20th	Day	22% 43% 10% 12% 9% 3	3367
	21st to end	Day	34% 41% 3 8% 9% 4	3992
April	1st to 10th	Day	19% 64% 2 7% 5% 2	2280
	11th to 20th	Day	15% 67% 4 7% 6% 1	2296
	21st to end	Day	18% 65% 3 6% 8%	2470
May	1st to 10th	Day	33% 46% 8% 8% 5%	4120
	11th to 20th	Day	33% 43% 11% 8% 4	4164
	21st to end	Day	21% 43% 9% 14% 12%	4837
June	1st to 10th	Day	15% 67% 4 9% 6%	2660
	11th to 20th	Day	13% 66% 5% 9% 8% 1	2660
	21st to end	Day	17% 65% 4 8% 7%	2660
July	1st to 10th	Day	28% 39% 8% 14% 10% 1	4370
	11th to 20th	Day	23% 39% 11% 18% 9%	4365
	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% 59% 6% 10% 6% 1	1698
	11th to 20th	Day	18% 59% 3 7% 11% 1	1710
	21st to end	Day	16% 59% 6% 9% 8% 1	1710
December	1st to 10th	Day	19% 61% 6% 6% 7%	1549
	11th to 20th	Day	24% 48% 6% 9% 8% 5%	1906
	21st to end	Day	34% 39% 6% 12% 7% 1	2772
January	1st to 10th	Night	24% 38% 9% 19% 7% 3	4429
	11th to 20th	Night	26% 39% 8% 16% 9% 3	4440
	21st to end	Night	38% 42% 6% 10% 3 1	4884
February	1st to 10th	Night	17% 65% 5% 6% 5% 2	2438
	11th to 20th	Night	16% 63% 4 7% 8% 2	2220
	21st to end	Night	15% 65% 2 9% 6% 3	1884
March	1st to 10th	Night	37% 41% 6% 8% 5% 2	3734
	11th to 20th	Night	24% 45% 7% 10% 12% 2	3591
	21st to end	Night	33% 41% 5% 7% 10% 5%	3636
April	1st to 10th	Night	19% 65% 3 5% 7% 1	1900
	11th to 20th	Night	16% 66% 3 7% 7% 1	1833
	21st to end	Night	17% 65% 3 7% 7% 1	1710
May	1st to 10th	Night	35% 46% 8% 8% 3	2840
	11th to 20th	Night	33% 45% 11% 7% 3	2763
	21st to end	Night	26% 43% 11% 12% 9% 1	2739
June	1st to 10th	Night	16% 65% 3 9% 6%	1520
	11th to 20th	Night	13% 67% 5% 9% 6%	1520
	21st to end	Night	16% 65% 3 10% 6% 1	1520
July	1st to 10th	Night	28% 39% 8% 14% 11%	2480
	11th to 20th	Night	25% 39% 11% 14% 11%	2472
	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% 59% 4 11% 7% 1	2453
	11th to 20th	Night	17% 59% 3 9% 10% 1	2470
	21st to end	Night	15% 59% 7% 11% 7% 1	2470
December	1st to 10th	Night	20% 60% 4 9% 6%	2629
	11th to 20th	Night	23% 50% 6% 8% 12% 1	3288
	21st to end	Night	34% 38% 5% 16% 7%	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	25% 22% 28% 22% 4	180
	11th to 20th	Day	22% 27% 29% 18% 3 1	180
	21st to end	Day	17% 21% 38% 19% 5%	198
February	1st to 10th	Day	25% 20% 37% 17% 1	355
	11th to 20th	Day	22% 25% 36% 15% 2	456
	21st to end	Day	22% 19% 36% 19% 4	382
March	1st to 10th	Day	24% 17% 36% 19% 3	460
	11th to 20th	Day	20% 23% 38% 16% 3 1	460
	21st to end	Day	23% 18% 37% 15% 5% 2	506
April	1st to 10th	Day	23% 20% 31% 18% 6% 1	460
	11th to 20th	Day	20% 18% 36% 18% 7%	452
	21st to end	Day	24% 10% 37% 20% 8% 1	460
May	1st to 10th	Day	24% 13% 37% 19% 6% 1	463
	11th to 20th	Day	17% 11% 35% 29% 7% 1	487
	21st to end	Day	19% 7% 31% 32% 10% 1	529
June	1st to 10th	Day	20% 6% 30% 31% 12% 1	490
	11th to 20th	Day	13% 8% 35% 33% 8% 2	490
	21st to end	Day	18% 6% 36% 28% 10% 2	490
July	1st to 10th	Day	15% 5% 23% 36% 17% 3	485
	11th to 20th	Day	12% 7% 23% 40% 16% 2	484
	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	16% 15% 31% 26% 10% 2	374
	11th to 20th	Day	25% 21% 27% 22% 4 1	370
	21st to end	Day	24% 19% 29% 22% 6% 1	370
December	1st to 10th	Day	20% 20% 34% 21% 6%	209
	11th to 20th	Day	23% 19% 39% 17% 1	180
	21st to end	Day	24% 22% 30% 18% 5% 1	198
January	1st to 10th	Night	21% 18% 40% 18% 3	760
	11th to 20th	Night	25% 17% 38% 17% 3	760
	21st to end	Night	19% 20% 38% 18% 4	836
February	1st to 10th	Night	22% 16% 38% 22% 1	760
	11th to 20th	Night	21% 20% 41% 16% 3	743
	21st to end	Night	22% 15% 36% 23% 3	628
March	1st to 10th	Night	25% 17% 37% 19% 3	760
	11th to 20th	Night	21% 16% 38% 19% 6%	760
	21st to end	Night	23% 15% 35% 21% 6%	836
April	1st to 10th	Night	24% 14% 34% 24% 4	760
	11th to 20th	Night	22% 12% 34% 25% 7%	709
	21st to end	Night	23% 10% 33% 29% 4 1	570
May	1st to 10th	Night	25% 7% 33% 26% 8% 1	570
	11th to 20th	Night	21% 7% 33% 30% 8% 1	537
	21st to end	Night	21% 6% 26% 36% 11%	411
June	1st to 10th	Night	22% 5% 26% 34% 12% 2	380
	11th to 20th	Night	20% 6% 24% 38% 12% 1	380
	21st to end	Night	20% 3 24% 40% 13%	380
July	1st to 10th	Night	17% 2 20% 44% 16% 1	370
	11th to 20th	Night	12% 3 17% 47% 20% 1	368
	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% 35% 29% 8% 1	755
	11th to 20th	Night	21% 16% 34% 22% 7% 1	760
	21st to end	Night	21% 18% 34% 23% 4 1	760
December	1st to 10th	Night	24% 14% 39% 18% 4	760
	11th to 20th	Night	21% 19% 34% 22% 3	760
	21st to end	Night	26% 16% 37% 20% 1	836

Page generated at 2024-11-01 22:47:37. Version ac07

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