Grand Mesa Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Mesa Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	4 6% 2 6% 4 4 5% 4 6% 8% 51%	147122
	Night	5% 8% 3 6% 4 4 4 3 5% 6% 52%	143449
Transparency	Day	28% 3 9% 22% 25% 13%	141985
	Night	30% 2 9% 24% 24% 11%	139595
Seeing	Day	10% 12% 28% 31% 16% 3	17714
	Night	9% 6% 22% 38% 22% 3	27831

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	3 10% 4 9% 5% 5% 5% 4 6% 10% 38%	5140
	11th to 20th	Day	2 7% 2 6% 4 4 5% 3 5% 7% 56%	5340
	21st to end	Day	2 8% 3 8% 4 3 3 3 5% 6% 54%	5844
February	1st to 10th	Day	4 10% 4 8% 6% 5% 5% 4 6% 6% 42%	1890
	11th to 20th	Day	6% 12% 4 9% 5% 5% 5% 4 6% 6% 38%	1845
	21st to end	Day	4 10% 5% 10% 5% 4 4 3 4 6% 44%	1746
March	1st to 10th	Day	3 12% 3 8% 5% 4 6% 3 5% 5% 44%	5064
	11th to 20th	Day	3 14% 5% 10% 6% 5% 4 4 5% 6% 37%	5110
	21st to end	Day	5% 14% 4 11% 6% 4 4 3 5% 6% 37%	5607
April	1st to 10th	Day	6% 8% 3 8% 4 4 5% 3 5% 6% 47%	2504
	11th to 20th	Day	6% 9% 3 8% 5% 5% 6% 4 5% 7% 42%	2491
	21st to end	Day	4 7% 3 7% 5% 6% 5% 4 6% 8% 44%	2541
May	1st to 10th	Day	4 6% 3 7% 5% 5% 6% 5% 7% 8% 45%	7626
	11th to 20th	Day	3 6% 2 6% 4 5% 6% 4 7% 7% 51%	7673
	21st to end	Day	4 5% 2 5% 3 4 4 4 6% 10% 54%	9026
June	1st to 10th	Day	4 4 2 4 4 3 4 4 6% 8% 57%	2940
	11th to 20th	Day	4 4 1 4 4 3 3 3 5% 6% 64%	2940
	21st to end	Day	3 2 1 4 3 3 4 4 5% 7% 65%	2940
July	1st to 10th	Day	2 3 1 3 3 3 3 3 4 6% 69%	6461
	11th to 20th	Day	2 3 1 3 4 4 6% 6% 10% 12% 49%	6010
	21st to end	Day	3 2 1 3 3 5% 6% 6% 8% 10% 53%	6532
August	1st to 10th	Day	3 3 1 4 4 5% 7% 6% 10% 11% 45%	5779
	11th to 20th	Day	3 3 1 4 4 5% 5% 4 6% 7% 58%	5737
	21st to end	Day	6% 4 2 5% 5% 5% 6% 5% 7% 10% 45%	5984
September	1st to 10th	Day	6% 3 1 4 3 4 4 3 6% 5% 60%	2400
	11th to 20th	Day	4 4 1 5% 3 4 5% 4 5% 6% 59%	2347
	21st to end	Day	6% 3 1 4 3 3 3 3 4 5% 65%	2132
October	1st to 10th	Day	5% 3 1 3 2 2 3 2 3 4 73%	3270
	11th to 20th	Day	5% 5% 2 6% 4 5% 5% 4 7% 8% 49%	3250
	21st to end	Day	6% 5% 2 5% 3 3 3 3 5% 5% 61%	3548
November	1st to 10th	Day	4 5% 3 5% 4 3 3 3 5% 6% 59%	2038
	11th to 20th	Day	6% 7% 2 5% 3 3 4 3 4 6% 58%	1789
	21st to end	Day	4 9% 2 5% 4 3 4 2 5% 6% 56%	1680
December	1st to 10th	Day	4 6% 2 4 2 2 3 3 4 6% 62%	2700
	11th to 20th	Day	4 8% 3 7% 4 4 4 4 6% 8% 50%	3106
	21st to end	Day	4 11% 4 7% 5% 4 5% 4 6% 7% 43%	4092
January	1st to 10th	Night	4 11% 5% 11% 5% 4 5% 3 6% 7% 40%	7823
	11th to 20th	Night	3 9% 2 6% 3 3 3 3 4 5% 58%	7650
	21st to end	Night	3 11% 4 6% 3 3 3 3 4 5% 56%	8370
February	1st to 10th	Night	4 10% 3 7% 5% 4 4 4 6% 6% 47%	2730
	11th to 20th	Night	6% 12% 3 9% 5% 5% 5% 3 6% 6% 42%	2662
	21st to end	Night	3 10% 4 9% 5% 4 4 3 5% 7% 46%	2056
March	1st to 10th	Night	4 10% 5% 9% 5% 4 4 3 4 6% 47%	5050
	11th to 20th	Night	3 11% 3 8% 5% 5% 5% 4 5% 6% 45%	5110
	21st to end	Night	6% 13% 4 11% 6% 5% 5% 4 5% 6% 36%	5610
April	1st to 10th	Night	7% 11% 3 8% 5% 4 3 3 4 6% 45%	2115
	11th to 20th	Night	7% 9% 4 8% 5% 4 4 4 5% 5% 45%	2077
	21st to end	Night	8% 9% 2 7% 4 4 5% 4 6% 5% 46%	2074
May	1st to 10th	Night	6% 8% 2 8% 5% 4 5% 4 5% 6% 46%	5321
	11th to 20th	Night	7% 8% 2 6% 4 3 4 4 6% 7% 49%	5201
	21st to end	Night	6% 5% 2 6% 4 3 4 3 5% 7% 55%	5181
June	1st to 10th	Night	3 3 2 5% 3 4 4 4 5% 7% 60%	1680
	11th to 20th	Night	6% 5% 2 4 3 3 3 3 5% 5% 62%	1680
	21st to end	Night	3 2 1 3 3 4 3 4 5% 7% 64%	1680
July	1st to 10th	Night	3 3 2 4 4 5% 4 3 5% 5% 64%	3720
	11th to 20th	Night	2 2 1 5% 4 5% 5% 5% 7% 11% 52%	4189
	21st to end	Night	2 4 2 5% 4 4 5% 4 6% 8% 57%	4562
August	1st to 10th	Night	3 4 2 6% 5% 5% 7% 5% 8% 10% 44%	4033
	11th to 20th	Night	3 3 2 5% 4 4 5% 4 6% 8% 57%	4112
	21st to end	Night	5% 4 2 6% 4 6% 7% 5% 7% 8% 46%	4993
September	1st to 10th	Night	5% 4 2 4 3 4 3 3 5% 6% 62%	2000
	11th to 20th	Night	5% 4 1 4 3 3 3 2 5% 7% 62%	2050
	21st to end	Night	6% 4 2 4 3 3 3 2 4 5% 66%	2266
October	1st to 10th	Night	5% 3 1 2 2 2 2 2 3 4 74%	3920
	11th to 20th	Night	7% 5% 1 4 3 4 4 4 5% 7% 55%	3898
	21st to end	Night	8% 8% 2 4 2 2 2 2 2 6% 62%	4252
November	1st to 10th	Night	4 7% 3 4 3 3 3 2 4 5% 62%	2460
	11th to 20th	Night	5% 7% 2 4 3 4 3 2 4 5% 61%	2825
	21st to end	Night	7% 8% 3 5% 4 4 3 2 5% 5% 54%	2940
December	1st to 10th	Night	4 6% 2 4 2 2 3 3 4 6% 65%	4710
	11th to 20th	Night	5% 10% 3 6% 4 4 4 3 4 6% 51%	5321
	21st to end	Night	5% 11% 4 7% 4 4 4 3 5% 7% 45%	7128

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	37% 3 20% 27% 13% 1	5143
	11th to 20th	Day	26% 1 16% 27% 24% 6%	5340
	21st to end	Day	30% 1 14% 25% 23% 7%	5844
February	1st to 10th	Day	37% 3 15% 26% 16% 3	1890
	11th to 20th	Day	41% 2 10% 27% 18% 3	1854
	21st to end	Day	37% 2 7% 27% 22% 6%	1746
March	1st to 10th	Day	35% 1 7% 27% 24% 7%	5064
	11th to 20th	Day	42% 1 10% 28% 15% 3	5110
	21st to end	Day	45% 2 6% 29% 16% 2	5607
April	1st to 10th	Day	32% 2 7% 25% 24% 9%	2504
	11th to 20th	Day	35% 3 7% 22% 26% 7%	2494
	21st to end	Day	32% 3 6% 24% 29% 6%	2541
May	1st to 10th	Day	29% 4 7% 22% 31% 6%	7626
	11th to 20th	Day	28% 2 6% 23% 33% 9%	7676
	21st to end	Day	23% 2 5% 21% 41% 7%	9027
June	1st to 10th	Day	22% 2 4 20% 32% 20%	2940
	11th to 20th	Day	20% 2 4 11% 25% 39%	2940
	21st to end	Day	17% 2 2 13% 27% 38%	2940
July	1st to 10th	Day	17% 3 5% 8% 15% 53%	6393
	11th to 20th	Day	22% 3 8% 18% 28% 20%	5941
	21st to end	Day	21% 4 11% 17% 23% 24%	6348
August	1st to 10th	Day	29% 7% 17% 16% 15% 15%	4208
	11th to 20th	Day	28% 5% 8% 16% 25% 18%	4148
	21st to end	Day	26% 6% 9% 18% 22% 18%	4311
September	1st to 10th	Day	23% 3 6% 15% 26% 27%	2400
	11th to 20th	Day	24% 3 8% 19% 27% 20%	2347
	21st to end	Day	21% 2 3 18% 39% 17%	2132
October	1st to 10th	Day	16% 1 4 15% 38% 26%	3270
	11th to 20th	Day	28% 4 5% 27% 30% 7%	3250
	21st to end	Day	24% 1 9% 31% 31% 4	3548
November	1st to 10th	Day	23% 2 8% 29% 33% 5%	2038
	11th to 20th	Day	24% 2 10% 35% 26% 3	1789
	21st to end	Day	27% 2 13% 32% 23% 4	1680
December	1st to 10th	Day	21% 2 13% 41% 21% 2	2700
	11th to 20th	Day	31% 2 15% 32% 19%	3104
	21st to end	Day	37% 3 15% 21% 22% 2	4092
January	1st to 10th	Night	41% 2 15% 28% 13% 1	7824
	11th to 20th	Night	27% 1 13% 29% 23% 6%	7650
	21st to end	Night	31% 1 14% 24% 22% 8%	8370
February	1st to 10th	Night	32% 3 14% 31% 16% 3	2730
	11th to 20th	Night	40% 2 10% 27% 19% 2	2663
	21st to end	Night	34% 2 7% 29% 23% 5%	2056
March	1st to 10th	Night	36% 1 5% 25% 28% 5%	5050
	11th to 20th	Night	38% 1 8% 28% 22% 3	5110
	21st to end	Night	44% 2 8% 29% 15% 2	5610
April	1st to 10th	Night	37% 2 6% 22% 22% 10%	2115
	11th to 20th	Night	38% 2 6% 21% 22% 11%	2077
	21st to end	Night	34% 2 6% 21% 29% 7%	2074
May	1st to 10th	Night	34% 3 8% 20% 27% 8%	5319
	11th to 20th	Night	32% 2 6% 25% 26% 10%	5202
	21st to end	Night	27% 2 6% 19% 37% 9%	5182
June	1st to 10th	Night	21% 2 5% 18% 31% 23%	1680
	11th to 20th	Night	22% 2 5% 12% 22% 38%	1680
	21st to end	Night	17% 2 3 14% 25% 39%	1680
July	1st to 10th	Night	21% 3 5% 8% 12% 51%	3680
	11th to 20th	Night	21% 3 8% 23% 24% 20%	4136
	21st to end	Night	23% 4 10% 15% 25% 23%	4434
August	1st to 10th	Night	33% 6% 16% 14% 15% 16%	2928
	11th to 20th	Night	30% 5% 11% 14% 21% 20%	2977
	21st to end	Night	28% 6% 8% 14% 23% 21%	3597
September	1st to 10th	Night	22% 2 6% 16% 22% 31%	2000
	11th to 20th	Night	20% 2 9% 19% 25% 25%	2050
	21st to end	Night	21% 2 4 17% 36% 20%	2266
October	1st to 10th	Night	15% 2 3 13% 38% 29%	3920
	11th to 20th	Night	25% 4 7% 27% 28% 8%	3898
	21st to end	Night	26% 1 4 32% 32% 4	4252
November	1st to 10th	Night	24% 2 7% 28% 33% 7%	2460
	11th to 20th	Night	23% 2 8% 35% 29% 4	2825
	21st to end	Night	30% 2 8% 30% 27% 4	2940
December	1st to 10th	Night	21% 1 10% 42% 24% 2	4710
	11th to 20th	Night	33% 2 9% 40% 15% 1	5322
	21st to end	Night	37% 3 15% 26% 19% 1	7128

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	17% 9% 32% 25% 12% 5%	330
	11th to 20th	Day	13% 11% 30% 31% 14% 1	330
	21st to end	Day	9% 8% 36% 34% 11% 3	363
February	1st to 10th	Day	7% 21% 35% 27% 8% 1	330
	11th to 20th	Day	17% 15% 36% 23% 8% 1	330
	21st to end	Day	12% 17% 35% 28% 7% 1	294
March	1st to 10th	Day	15% 21% 30% 24% 9% 1	540
	11th to 20th	Day	16% 18% 30% 25% 9% 1	540
	21st to end	Day	18% 24% 34% 18% 6% 1	594
April	1st to 10th	Day	17% 18% 33% 22% 9% 1	540
	11th to 20th	Day	20% 26% 31% 16% 6% 1	532
	21st to end	Day	17% 20% 35% 21% 8%	540
May	1st to 10th	Day	15% 19% 34% 24% 8% 1	543
	11th to 20th	Day	11% 15% 30% 29% 14% 1	593
	21st to end	Day	10% 13% 28% 29% 17% 3	844
June	1st to 10th	Day	6% 9% 26% 35% 21% 3	780
	11th to 20th	Day	9% 13% 29% 32% 15% 3	780
	21st to end	Day	7% 11% 24% 37% 18% 3	780
July	1st to 10th	Day	3 5% 22% 37% 28% 4	766
	11th to 20th	Day	4 4 19% 43% 26% 4	564
	21st to end	Day	6% 6% 15% 42% 27% 3	616
August	1st to 10th	Day	7% 5% 22% 39% 24% 3	527
	11th to 20th	Day	4 4 21% 41% 26% 4	517
	21st to end	Day	7% 4 22% 40% 24% 4	583
September	1st to 10th	Day	6% 4 24% 38% 22% 5%	530
	11th to 20th	Day	7% 11% 29% 32% 17% 4	530
	21st to end	Day	10% 9% 25% 31% 21% 4	462
October	1st to 10th	Day	12% 9% 24% 26% 22% 7%	330
	11th to 20th	Day	8% 14% 25% 30% 18% 5%	330
	21st to end	Day	10% 13% 29% 29% 17% 2	363
November	1st to 10th	Day	8% 13% 32% 30% 16% 1	330
	11th to 20th	Day	7% 14% 35% 28% 12% 4	330
	21st to end	Day	9% 15% 29% 27% 17% 3	330
December	1st to 10th	Day	11% 12% 32% 28% 13% 3	330
	11th to 20th	Day	7% 9% 37% 29% 14% 3	330
	21st to end	Day	13% 12% 34% 28% 12% 1	363
January	1st to 10th	Night	10% 6% 23% 38% 20% 4	919
	11th to 20th	Night	9% 6% 23% 38% 20% 4	840
	21st to end	Night	11% 6% 25% 39% 17% 2	924
February	1st to 10th	Night	10% 7% 27% 42% 12% 2	840
	11th to 20th	Night	11% 9% 29% 37% 13% 1	824
	21st to end	Night	7% 10% 32% 42% 8% 1	692
March	1st to 10th	Night	10% 8% 31% 37% 13% 2	840
	11th to 20th	Night	8% 8% 23% 41% 18% 1	840
	21st to end	Night	11% 9% 28% 38% 13% 1	924
April	1st to 10th	Night	13% 8% 30% 38% 10% 1	645
	11th to 20th	Night	12% 13% 25% 35% 12% 2	623
	21st to end	Night	11% 9% 28% 35% 13% 3	630
May	1st to 10th	Night	13% 9% 24% 38% 14% 2	630
	11th to 20th	Night	11% 6% 18% 38% 23% 4	621
	21st to end	Night	10% 8% 21% 35% 22% 4	683
June	1st to 10th	Night	4 5% 18% 41% 27% 4	630
	11th to 20th	Night	7% 5% 23% 42% 20% 3	630
	21st to end	Night	5% 5% 19% 37% 30% 3	630
July	1st to 10th	Night	5% 3 15% 32% 41% 5%	615
	11th to 20th	Night	3 1 9% 40% 42% 6%	612
	21st to end	Night	5% 3 11% 31% 44% 6%	687
August	1st to 10th	Night	6% 3 10% 35% 41% 5%	597
	11th to 20th	Night	5% 2 11% 37% 39% 6%	594
	21st to end	Night	5% 3 11% 38% 35% 8%	660
September	1st to 10th	Night	7% 1 13% 39% 35% 5%	600
	11th to 20th	Night	6% 6% 16% 34% 34% 4	650
	21st to end	Night	7% 4 18% 33% 32% 7%	800
October	1st to 10th	Night	9% 3 21% 37% 27% 3	800
	11th to 20th	Night	8% 4 21% 37% 27% 3	800
	21st to end	Night	10% 5% 23% 36% 23% 3	880
November	1st to 10th	Night	7% 6% 23% 38% 23% 2	820
	11th to 20th	Night	8% 6% 25% 40% 17% 3	1046
	21st to end	Night	10% 8% 23% 36% 19% 4	1050
December	1st to 10th	Night	10% 6% 23% 43% 17% 2	1050
	11th to 20th	Night	12% 7% 24% 37% 17% 2	1050
	21st to end	Night	12% 8% 25% 42% 10% 2	1155

Page generated at 2026-07-28 01:25:20. Version ac07

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