Grasslands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grasslands Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	2 3 1 4 3 4 4 4 5% 7% 64%	152344
	Night	2 3 1 4 3 3 4 3 5% 7% 65%	147160
Transparency	Day	19% 5% 7% 12% 25% 33%	147238
	Night	18% 5% 7% 12% 26% 32%	143273
Seeing	Day	7% 12% 26% 30% 19% 6%	17244
	Night	4 5% 19% 38% 29% 5%	27315

By Month

Percentage Cloud Cover SW (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	2 4 2 5% 3 3 3 4 6% 6% 61%	5318
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 80%	5340
	21st to end	Day	3 5% 2 6% 4 4 4 4 5% 7% 57%	5844
February	1st to 10th	Day	4 4 1 4 3 3 3 3 4 7% 65%	1922
	11th to 20th	Day	4 6% 2 4 3 3 4 3 4 6% 62%	2050
	21st to end	Day	3 4 1 4 3 2 3 2 4 6% 68%	1873
March	1st to 10th	Day	2 3 1 4 3 2 4 3 4 6% 68%	5064
	11th to 20th	Day	1 2 1 3 3 3 4 3 5% 6% 69%	5110
	21st to end	Day	1 3 1 4 3 4 4 4 6% 9% 63%	5607
April	1st to 10th	Day	1 2 1 3 3 3 3 3 4 6% 70%	2310
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 5% 78%	2459
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 79%	2520
May	1st to 10th	Day	1 2 1 3 2 2 2 2 2 3 81%	7504
	11th to 20th	Day	1 1 1 2 2 2 3 3 4 5% 76%	7596
	21st to end	Day	1 1 1 2 2 2 2 1 2 3 84%	8370
June	1st to 10th	Day	2 2 1 2 2 3 3 3 3 6% 73%	2730
	11th to 20th	Day	1 1 1 2 2 2 3 2 3 4 79%	2730
	21st to end	Day	3 3 1 4 5% 4 4 3 6% 7% 60%	2730
July	1st to 10th	Day	1 2 1 5% 5% 6% 6% 4 7% 9% 54%	7649
	11th to 20th	Day	2 4 2 7% 6% 7% 7% 6% 9% 10% 39%	7650
	21st to end	Day	3 4 2 7% 6% 7% 8% 6% 9% 12% 36%	8304
August	1st to 10th	Day	3 3 2 6% 5% 6% 7% 6% 8% 10% 44%	5909
	11th to 20th	Day	2 3 1 5% 5% 6% 8% 6% 9% 12% 42%	5763
	21st to end	Day	2 3 1 6% 5% 6% 8% 5% 8% 9% 47%	5943
September	1st to 10th	Day	6% 5% 2 6% 6% 5% 6% 4 6% 7% 46%	2200
	11th to 20th	Day	4 2 1 4 4 4 4 5% 6% 7% 59%	2200
	21st to end	Day	3 2 1 4 3 4 3 3 5% 6% 67%	2200
October	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 77%	3417
	11th to 20th	Day	2 1 1 2 2 1 2 2 3 4 81%	3250
	21st to end	Day	1 1 1 1 1 2 2 3 5% 83%	3548
November	1st to 10th	Day	1 1 1 2 2 2 3 2 5% 6% 75%	2059
	11th to 20th	Day	2 2 1 4 2 3 4 2 3 5% 71%	2100
	21st to end	Day	3 5% 1 4 3 3 4 3 5% 6% 62%	1927
December	1st to 10th	Day	3 2 1 2 2 2 2 2 3 5% 76%	3040
	11th to 20th	Day	3 4 2 4 3 2 3 2 5% 7% 64%	3499
	21st to end	Day	2 3 1 4 3 3 3 3 5% 6% 68%	4609
January	1st to 10th	Night	2 3 2 5% 4 4 4 4 5% 6% 60%	7605
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 83%	7650
	21st to end	Night	4 5% 2 5% 4 4 5% 4 6% 7% 53%	8370
February	1st to 10th	Night	2 3 1 3 3 4 3 3 5% 6% 66%	2693
	11th to 20th	Night	3 4 2 4 4 4 3 3 5% 6% 62%	2458
	21st to end	Night	2 3 2 4 3 4 3 4 6% 5% 65%	1930
March	1st to 10th	Night	1 2 1 3 3 3 3 3 4 5% 70%	5050
	11th to 20th	Night	1 2 1 3 3 3 3 3 4 6% 72%	5110
	21st to end	Night	2 3 1 4 3 3 3 3 4 6% 67%	5610
April	1st to 10th	Night	2 3 1 3 2 2 3 3 4 6% 72%	2310
	11th to 20th	Night	1 3 1 2 2 2 3 2 3 4 76%	2106
	21st to end	Night	1 2 1 2 2 1 2 2 2 5% 81%	2100
May	1st to 10th	Night	2 2 1 2 2 1 2 2 2 3 81%	5438
	11th to 20th	Night	1 1 1 3 2 2 3 3 4 4 77%	5296
	21st to end	Night	1 1 1 1 2 2 3 4 85%	5837
June	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 75%	1890
	11th to 20th	Night	2 2 1 2 1 1 1 2 3 5% 80%	1890
	21st to end	Night	2 3 1 4 3 3 4 4 6% 7% 63%	1890
July	1st to 10th	Night	1 2 1 5% 5% 5% 6% 5% 7% 9% 52%	5340
	11th to 20th	Night	2 4 3 8% 6% 8% 9% 8% 11% 12% 31%	5340
	21st to end	Night	2 3 2 6% 6% 6% 8% 6% 12% 15% 36%	5805
August	1st to 10th	Night	2 2 1 6% 5% 6% 7% 6% 10% 13% 42%	4123
	11th to 20th	Night	2 3 1 5% 5% 7% 8% 7% 10% 15% 39%	4175
	21st to end	Night	2 2 1 4 4 5% 5% 5% 8% 11% 50%	5030
September	1st to 10th	Night	6% 5% 2 5% 4 4 5% 4 6% 9% 50%	2200
	11th to 20th	Night	2 3 1 4 3 3 3 3 4 7% 67%	2200
	21st to end	Night	2 2 1 3 2 2 3 3 4 7% 72%	2200
October	1st to 10th	Night	1 1 1 3 2 2 3 3 5% 5% 75%	3774
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 80%	3898
	21st to end	Night	1 1 1 1 2 1 3 4 86%	4252
November	1st to 10th	Night	1 1 2 2 2 2 2 4 6% 77%	2460
	11th to 20th	Night	2 3 1 3 3 3 3 2 4 6% 72%	2520
	21st to end	Night	3 4 2 4 4 3 4 3 5% 7% 63%	2688
December	1st to 10th	Night	2 2 1 3 2 2 2 2 3 4 77%	4370
	11th to 20th	Night	2 3 2 5% 4 4 4 3 4 5% 66%	4941
	21st to end	Night	2 2 1 4 3 4 3 4 5% 6% 66%	6611

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	22% 3 5% 12% 30% 28%	5321
	11th to 20th	Day	11% 2 3 6% 30% 48%	5340
	21st to end	Day	25% 4 5% 11% 25% 30%	5844
February	1st to 10th	Day	21% 3 4 11% 26% 35%	1922
	11th to 20th	Day	24% 4 4 14% 28% 26%	2060
	21st to end	Day	20% 2 3 10% 28% 37%	1873
March	1st to 10th	Day	15% 5% 4 6% 31% 39%	5064
	11th to 20th	Day	14% 4 6% 13% 35% 28%	5110
	21st to end	Day	17% 5% 5% 10% 37% 26%	5607
April	1st to 10th	Day	15% 3 3 9% 37% 33%	2310
	11th to 20th	Day	11% 2 3 7% 29% 48%	2462
	21st to end	Day	10% 2 4 8% 32% 44%	2520
May	1st to 10th	Day	10% 2 3 6% 26% 54%	7504
	11th to 20th	Day	9% 3 4 9% 28% 47%	7599
	21st to end	Day	8% 2 2 3 16% 69%	8371
June	1st to 10th	Day	14% 3 3 8% 27% 46%	2730
	11th to 20th	Day	9% 2 4 6% 20% 59%	2730
	21st to end	Day	22% 6% 7% 10% 21% 34%	2730
July	1st to 10th	Day	22% 8% 8% 18% 21% 24%	7586
	11th to 20th	Day	32% 13% 17% 21% 13% 3	7581
	21st to end	Day	32% 12% 18% 19% 13% 6%	8120
August	1st to 10th	Day	35% 13% 17% 18% 11% 5%	4338
	11th to 20th	Day	30% 12% 15% 21% 19% 3	4186
	21st to end	Day	30% 11% 13% 18% 20% 9%	4282
September	1st to 10th	Day	34% 8% 11% 14% 17% 15%	2200
	11th to 20th	Day	21% 6% 9% 15% 27% 22%	2200
	21st to end	Day	18% 5% 8% 16% 29% 24%	2200
October	1st to 10th	Day	9% 4 5% 10% 19% 54%	3417
	11th to 20th	Day	8% 3 3 7% 34% 44%	3250
	21st to end	Day	7% 2 3 7% 26% 56%	3548
November	1st to 10th	Day	10% 4 4 12% 36% 34%	2059
	11th to 20th	Day	17% 3 4 11% 33% 32%	2100
	21st to end	Day	22% 4 5% 15% 25% 29%	1927
December	1st to 10th	Day	13% 2 4 10% 36% 36%	3040
	11th to 20th	Day	20% 2 5% 9% 26% 38%	3498
	21st to end	Day	17% 3 5% 13% 38% 24%	4609
January	1st to 10th	Night	22% 4 6% 14% 33% 21%	7605
	11th to 20th	Night	7% 2 2 11% 32% 46%	7650
	21st to end	Night	28% 5% 6% 12% 22% 27%	8370
February	1st to 10th	Night	19% 3 5% 13% 22% 38%	2693
	11th to 20th	Night	23% 3 7% 13% 28% 25%	2459
	21st to end	Night	20% 4 4 12% 26% 35%	1930
March	1st to 10th	Night	15% 3 5% 8% 31% 38%	5050
	11th to 20th	Night	12% 4 6% 15% 37% 26%	5110
	21st to end	Night	18% 4 5% 12% 36% 24%	5610
April	1st to 10th	Night	15% 2 3 11% 38% 30%	2310
	11th to 20th	Night	13% 3 3 9% 29% 44%	2106
	21st to end	Night	10% 2 3 9% 34% 43%	2100
May	1st to 10th	Night	10% 2 3 7% 33% 45%	5436
	11th to 20th	Night	10% 2 4 8% 22% 54%	5297
	21st to end	Night	6% 1 2 5% 22% 64%	5838
June	1st to 10th	Night	12% 3 2 7% 29% 47%	1890
	11th to 20th	Night	9% 2 3 5% 23% 59%	1890
	21st to end	Night	17% 7% 8% 10% 20% 39%	1890
July	1st to 10th	Night	24% 7% 11% 17% 18% 24%	5295
	11th to 20th	Night	37% 14% 19% 16% 11% 4	5286
	21st to end	Night	29% 13% 24% 17% 11% 6%	5677
August	1st to 10th	Night	32% 14% 17% 19% 12% 5%	3018
	11th to 20th	Night	27% 11% 18% 23% 18% 3	3028
	21st to end	Night	25% 9% 17% 20% 20% 8%	3621
September	1st to 10th	Night	29% 8% 13% 18% 19% 14%	2200
	11th to 20th	Night	17% 5% 8% 17% 30% 23%	2200
	21st to end	Night	13% 5% 8% 17% 34% 24%	2200
October	1st to 10th	Night	10% 4 6% 11% 20% 50%	3774
	11th to 20th	Night	9% 3 4 13% 28% 43%	3898
	21st to end	Night	5% 1 3 10% 28% 53%	4252
November	1st to 10th	Night	10% 2 4 15% 35% 33%	2460
	11th to 20th	Night	15% 3 3 16% 31% 32%	2520
	21st to end	Night	23% 3 5% 13% 30% 26%	2688
December	1st to 10th	Night	12% 3 4 10% 34% 36%	4370
	11th to 20th	Night	21% 4 4 9% 25% 37%	4941
	21st to end	Night	18% 5% 6% 13% 31% 27%	6611

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	13% 16% 23% 36% 12%	330
	11th to 20th	Day	7% 8% 31% 35% 17% 2	330
	21st to end	Day	5% 17% 32% 34% 9% 2	363
February	1st to 10th	Day	9% 13% 30% 29% 16% 4	362
	11th to 20th	Day	13% 27% 30% 22% 7% 1	535
	21st to end	Day	14% 26% 31% 21% 7% 1	446
March	1st to 10th	Day	11% 25% 36% 19% 7% 2	540
	11th to 20th	Day	11% 22% 27% 22% 12% 5%	540
	21st to end	Day	12% 25% 31% 20% 8% 4	594
April	1st to 10th	Day	5% 19% 37% 22% 11% 5%	540
	11th to 20th	Day	7% 26% 35% 17% 12% 3	532
	21st to end	Day	6% 21% 35% 23% 11% 4	540
May	1st to 10th	Day	7% 17% 36% 23% 13% 3	543
	11th to 20th	Day	5% 13% 37% 29% 13% 3	568
	21st to end	Day	4 12% 38% 29% 13% 5%	617
June	1st to 10th	Day	4 8% 34% 35% 15% 5%	570
	11th to 20th	Day	2 5% 34% 35% 19% 5%	570
	21st to end	Day	4 3 26% 40% 22% 6%	570
July	1st to 10th	Day	3 1 14% 43% 30% 10%	566
	11th to 20th	Day	5% 2 8% 42% 32% 12%	564
	21st to end	Day	8% 1 8% 33% 36% 14%	619
August	1st to 10th	Day	6% 1 7% 32% 34% 21%	567
	11th to 20th	Day	6% 2 8% 32% 36% 16%	534
	21st to end	Day	5% 1 8% 33% 36% 17%	583
September	1st to 10th	Day	7% 3 17% 39% 25% 8%	530
	11th to 20th	Day	4 3 19% 38% 27% 9%	530
	21st to end	Day	5% 7% 22% 35% 24% 7%	530
October	1st to 10th	Day	3 9% 24% 32% 23% 9%	425
	11th to 20th	Day	4 6% 27% 33% 22% 9%	330
	21st to end	Day	3 6% 25% 31% 27% 7%	363
November	1st to 10th	Day	5% 12% 23% 33% 20% 6%	330
	11th to 20th	Day	5% 9% 28% 34% 22% 2	330
	21st to end	Day	8% 19% 26% 32% 14% 1	330
December	1st to 10th	Day	9% 11% 25% 35% 19% 2	330
	11th to 20th	Day	7% 12% 32% 27% 18% 4	330
	21st to end	Day	9% 18% 27% 30% 12% 4	363
January	1st to 10th	Night	5% 5% 22% 41% 23% 4	840
	11th to 20th	Night	5% 5% 21% 42% 24% 3	840
	21st to end	Night	7% 8% 25% 39% 19% 2	924
February	1st to 10th	Night	5% 8% 24% 38% 23% 4	840
	11th to 20th	Night	6% 11% 28% 37% 16% 2	824
	21st to end	Night	6% 11% 28% 39% 14% 2	692
March	1st to 10th	Night	4 13% 30% 37% 15% 1	840
	11th to 20th	Night	5% 11% 24% 32% 25% 4	840
	21st to end	Night	5% 9% 24% 39% 21% 2	924
April	1st to 10th	Night	3 8% 25% 42% 20% 2	840
	11th to 20th	Night	3 12% 31% 37% 16% 1	652
	21st to end	Night	2 8% 29% 43% 15% 3	630
May	1st to 10th	Night	3 7% 28% 41% 19% 2	630
	11th to 20th	Night	1 5% 26% 43% 22% 3	621
	21st to end	Night	1 4 27% 43% 25%	683
June	1st to 10th	Night	3 2 15% 50% 27% 2	630
	11th to 20th	Night	2 3 16% 51% 27% 1	630
	21st to end	Night	2 1 13% 39% 39% 5%	630
July	1st to 10th	Night	3 1 7% 34% 49% 6%	615
	11th to 20th	Night	3 1 6% 27% 53% 10%	612
	21st to end	Night	4 2 5% 26% 50% 13%	690
August	1st to 10th	Night	3 1 4 22% 55% 15%	627
	11th to 20th	Night	4 1 5% 23% 54% 14%	606
	21st to end	Night	6% 1 4 21% 54% 14%	671
September	1st to 10th	Night	6% 2 9% 27% 45% 11%	800
	11th to 20th	Night	3 1 8% 37% 43% 8%	800
	21st to end	Night	2 1 13% 37% 39% 8%	800
October	1st to 10th	Night	2 3 14% 40% 37% 4	800
	11th to 20th	Night	3 3 18% 40% 32% 5%	800
	21st to end	Night	2 4 14% 39% 33% 8%	880
November	1st to 10th	Night	3 5% 16% 39% 31% 6%	820
	11th to 20th	Night	4 5% 15% 41% 32% 4	840
	21st to end	Night	6% 10% 19% 42% 19% 4	840
December	1st to 10th	Night	5% 5% 20% 45% 21% 4	840
	11th to 20th	Night	6% 8% 20% 41% 23% 3	840
	21st to end	Night	6% 7% 23% 40% 20% 5%	924

Page generated at 2026-08-12 00:37:15. Version ac07

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