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 2025-03-27.

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 7% 2 6% 4 4 5% 4 6% 7% 51%	113102
	Night	5% 8% 3 6% 4 4 4 3 5% 6% 52%	115096
Transparency	Day	29% 2 8% 23% 24% 13%	112862
	Night	30% 2 9% 25% 23% 11%	114926
Seeing	Day	10% 11% 27% 32% 18% 3	15800
	Night	9% 5% 20% 39% 23% 3	26104

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 8% 5% 10% 6% 5% 5% 3 6% 11% 40%	3960
	11th to 20th	Day	3 9% 3 7% 5% 5% 6% 4 6% 8% 45%	4100
	21st to end	Day	3 9% 3 8% 4 3 3 3 4 6% 54%	4510
February	1st to 10th	Day	4 10% 4 8% 6% 5% 5% 4 6% 6% 41%	1800
	11th to 20th	Day	6% 12% 4 9% 5% 5% 5% 4 6% 6% 39%	1755
	21st to end	Day	5% 11% 5% 10% 6% 5% 4 3 4 5% 43%	1665
March	1st to 10th	Day	4 13% 4 9% 6% 5% 5% 4 5% 5% 40%	3554
	11th to 20th	Day	3 12% 4 10% 6% 5% 4 4 6% 6% 40%	3600
	21st to end	Day	5% 15% 4 11% 7% 5% 4 4 5% 5% 34%	3892
April	1st to 10th	Day	7% 9% 3 8% 4 4 5% 3 5% 7% 46%	2266
	11th to 20th	Day	6% 9% 3 8% 6% 5% 6% 4 5% 6% 42%	2251
	21st to end	Day	4 8% 3 8% 5% 5% 6% 4 6% 7% 43%	2299
May	1st to 10th	Day	5% 9% 4 10% 6% 5% 6% 4 6% 6% 40%	4110
	11th to 20th	Day	4 5% 2 6% 5% 5% 5% 5% 7% 8% 49%	4120
	21st to end	Day	6% 6% 2 6% 3 4 4 4 6% 7% 52%	4825
June	1st to 10th	Day	4 4 2 5% 4 3 4 4 6% 8% 57%	2660
	11th to 20th	Day	4 4 2 4 4 3 3 3 5% 6% 63%	2660
	21st to end	Day	3 3 1 4 3 4 4 4 5% 7% 63%	2660
July	1st to 10th	Day	2 3 1 3 3 3 3 3 4 6% 69%	4412
	11th to 20th	Day	2 3 1 3 4 5% 6% 6% 11% 13% 47%	4110
	21st to end	Day	3 3 1 3 4 5% 4 5% 7% 8% 57%	4454
August	1st to 10th	Day	4 3 2 5% 5% 6% 8% 7% 11% 11% 39%	4092
	11th to 20th	Day	4 4 1 5% 5% 6% 6% 4 6% 7% 53%	3980
	21st to end	Day	5% 3 1 5% 5% 5% 5% 5% 7% 9% 51%	4180
September	1st to 10th	Day	6% 3 1 4 3 4 4 3 6% 5% 60%	2280
	11th to 20th	Day	4 4 1 4 3 4 5% 4 5% 6% 59%	2230
	21st to end	Day	6% 3 1 4 3 3 3 3 4 5% 66%	2026
October	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 5% 73%	3170
	11th to 20th	Day	5% 5% 2 6% 4 5% 5% 5% 7% 8% 48%	3150
	21st to end	Day	6% 5% 2 5% 3 3 3 3 5% 5% 61%	3438
November	1st to 10th	Day	4 5% 3 5% 4 4 3 3 4 6% 59%	1939
	11th to 20th	Day	6% 7% 2 5% 3 3 4 3 4 6% 59%	1704
	21st to end	Day	5% 9% 2 5% 4 3 4 3 5% 6% 57%	1600
December	1st to 10th	Day	4 6% 2 4 2 2 3 3 4 6% 62%	2620
	11th to 20th	Day	4 8% 3 7% 4 4 4 4 6% 8% 49%	3026
	21st to end	Day	4 12% 4 7% 5% 5% 5% 4 6% 7% 43%	4004
January	1st to 10th	Night	3 8% 4 11% 5% 4 5% 4 6% 8% 43%	6033
	11th to 20th	Night	4 12% 3 7% 4 4 4 3 5% 6% 47%	5880
	21st to end	Night	3 11% 4 6% 3 3 3 3 4 5% 54%	6468
February	1st to 10th	Night	4 11% 3 7% 5% 4 4 4 6% 6% 46%	2600
	11th to 20th	Night	6% 11% 3 8% 5% 5% 5% 3 5% 6% 43%	2532
	21st to end	Night	4 10% 4 9% 5% 5% 4 3 5% 6% 46%	1961
March	1st to 10th	Night	4 10% 4 9% 5% 4 4 3 5% 6% 46%	3540
	11th to 20th	Night	4 11% 4 7% 5% 5% 5% 4 5% 6% 45%	3600
	21st to end	Night	5% 13% 5% 13% 7% 6% 5% 4 5% 5% 32%	3899
April	1st to 10th	Night	7% 11% 3 8% 5% 4 3 3 4 6% 45%	1914
	11th to 20th	Night	7% 10% 3 7% 6% 4 4 3 5% 5% 46%	1877
	21st to end	Night	8% 8% 2 7% 5% 4 5% 4 6% 5% 46%	1876
May	1st to 10th	Night	8% 10% 3 9% 5% 4 5% 4 4 6% 41%	2860
	11th to 20th	Night	6% 5% 2 6% 4 4 6% 6% 7% 8% 47%	2781
	21st to end	Night	8% 6% 2 6% 4 3 4 3 4 5% 55%	2762
June	1st to 10th	Night	3 4 2 4 3 4 4 4 5% 7% 60%	1520
	11th to 20th	Night	6% 5% 2 4 3 4 3 3 5% 5% 61%	1520
	21st to end	Night	3 3 1 3 4 4 3 4 5% 8% 62%	1520
July	1st to 10th	Night	2 2 1 3 3 4 4 3 4 4 68%	2540
	11th to 20th	Night	2 2 1 5% 3 5% 5% 5% 7% 11% 54%	2859
	21st to end	Night	2 4 2 5% 4 4 4 4 6% 7% 58%	3109
August	1st to 10th	Night	3 4 2 6% 6% 6% 8% 6% 10% 9% 40%	2847
	11th to 20th	Night	4 4 2 6% 5% 5% 6% 4 6% 8% 50%	2859
	21st to end	Night	5% 4 2 5% 3 5% 6% 5% 6% 8% 51%	3487
September	1st to 10th	Night	5% 4 2 4 3 4 3 4 5% 6% 61%	1900
	11th to 20th	Night	5% 4 1 4 3 3 3 2 5% 6% 62%	1947
	21st to end	Night	6% 4 1 4 3 2 3 2 3 4 68%	2152
October	1st to 10th	Night	5% 3 1 2 2 2 2 2 3 5% 74%	3800
	11th to 20th	Night	6% 5% 1 4 4 4 4 4 5% 8% 55%	3778
	21st to end	Night	8% 8% 2 4 2 2 2 2 2 6% 61%	4120
November	1st to 10th	Night	4 7% 3 4 3 3 3 2 4 5% 61%	2340
	11th to 20th	Night	5% 7% 2 4 3 3 3 2 4 5% 62%	2690
	21st to end	Night	7% 8% 3 5% 4 4 3 3 5% 5% 55%	2800
December	1st to 10th	Night	4 6% 2 4 2 2 3 2 4 6% 66%	4570
	11th to 20th	Night	5% 10% 3 6% 4 4 4 3 4 6% 50%	5181
	21st to end	Night	5% 11% 4 7% 4 4 4 3 5% 7% 45%	6974

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% 2 17% 31% 13% 1	3961
	11th to 20th	Day	33% 1 16% 22% 21% 6%	4100
	21st to end	Day	29% 1 14% 26% 22% 7%	4510
February	1st to 10th	Day	38% 3 15% 25% 15% 4	1800
	11th to 20th	Day	39% 2 10% 28% 18% 3	1764
	21st to end	Day	38% 2 6% 25% 22% 6%	1665
March	1st to 10th	Day	39% 1 5% 31% 21% 3	3554
	11th to 20th	Day	40% 1 9% 35% 13% 3	3600
	21st to end	Day	46% 2 5% 27% 17% 3	3892
April	1st to 10th	Day	33% 2 7% 26% 23% 10%	2266
	11th to 20th	Day	35% 3 7% 22% 25% 7%	2254
	21st to end	Day	32% 3 6% 25% 27% 6%	2299
May	1st to 10th	Day	37% 1 4 17% 32% 9%	4110
	11th to 20th	Day	28% 2 5% 26% 28% 11%	4123
	21st to end	Day	28% 1 4 17% 40% 10%	4825
June	1st to 10th	Day	22% 2 4 21% 31% 21%	2660
	11th to 20th	Day	21% 2 4 11% 25% 37%	2660
	21st to end	Day	18% 2 2 14% 27% 37%	2660
July	1st to 10th	Day	16% 3 4 9% 15% 53%	4343
	11th to 20th	Day	22% 3 8% 21% 26% 20%	4041
	21st to end	Day	21% 3 9% 18% 20% 29%	4314
August	1st to 10th	Day	30% 7% 17% 16% 15% 15%	4078
	11th to 20th	Day	29% 5% 8% 16% 24% 18%	4019
	21st to end	Day	26% 6% 9% 18% 23% 19%	4179
September	1st to 10th	Day	24% 3 5% 15% 26% 27%	2280
	11th to 20th	Day	23% 3 8% 18% 27% 21%	2230
	21st to end	Day	21% 2 3 18% 39% 17%	2026
October	1st to 10th	Day	16% 1 4 15% 38% 26%	3170
	11th to 20th	Day	28% 4 5% 27% 29% 7%	3150
	21st to end	Day	24% 1 9% 31% 31% 4	3438
November	1st to 10th	Day	23% 2 7% 28% 34% 6%	1939
	11th to 20th	Day	24% 2 9% 35% 27% 4	1704
	21st to end	Day	26% 1 12% 33% 24% 4	1600
December	1st to 10th	Day	21% 2 12% 41% 22% 2	2620
	11th to 20th	Day	31% 2 15% 32% 19%	3024
	21st to end	Day	37% 3 15% 21% 22% 2	4004
January	1st to 10th	Night	38% 1 14% 30% 16% 2	6033
	11th to 20th	Night	35% 1 14% 22% 23% 5%	5880
	21st to end	Night	33% 1 15% 24% 22% 6%	6468
February	1st to 10th	Night	33% 4 15% 29% 16% 4	2600
	11th to 20th	Night	38% 2 10% 28% 20% 2	2533
	21st to end	Night	35% 2 6% 28% 24% 5%	1961
March	1st to 10th	Night	36% 1 5% 32% 24% 2	3540
	11th to 20th	Night	39% 1 8% 31% 18% 3	3600
	21st to end	Night	48% 1 7% 26% 16% 2	3899
April	1st to 10th	Night	37% 1 6% 22% 22% 10%	1914
	11th to 20th	Night	38% 2 6% 21% 22% 12%	1877
	21st to end	Night	34% 2 6% 22% 28% 7%	1876
May	1st to 10th	Night	39% 2 5% 17% 28% 9%	2860
	11th to 20th	Night	30% 2 7% 27% 27% 8%	2782
	21st to end	Night	30% 2 4 15% 36% 14%	2762
June	1st to 10th	Night	21% 2 5% 19% 30% 24%	1520
	11th to 20th	Night	23% 2 5% 12% 21% 37%	1520
	21st to end	Night	18% 2 3 15% 25% 37%	1520
July	1st to 10th	Night	18% 3 5% 8% 14% 52%	2500
	11th to 20th	Night	19% 3 8% 26% 23% 21%	2806
	21st to end	Night	22% 4 11% 14% 22% 27%	3011
August	1st to 10th	Night	33% 7% 16% 14% 15% 16%	2838
	11th to 20th	Night	30% 5% 11% 14% 20% 19%	2886
	21st to end	Night	27% 6% 8% 14% 24% 21%	3487
September	1st to 10th	Night	23% 2 6% 15% 22% 32%	1900
	11th to 20th	Night	21% 2 9% 19% 24% 26%	1947
	21st to end	Night	21% 2 3 16% 37% 21%	2152
October	1st to 10th	Night	15% 2 3 13% 38% 29%	3800
	11th to 20th	Night	25% 4 7% 27% 28% 9%	3778
	21st to end	Night	26% 1 4 32% 32% 4	4120
November	1st to 10th	Night	25% 2 7% 27% 32% 7%	2340
	11th to 20th	Night	22% 2 8% 35% 30% 4	2690
	21st to end	Night	30% 2 7% 29% 28% 4	2800
December	1st to 10th	Night	20% 1 10% 42% 25% 2	4570
	11th to 20th	Night	33% 2 9% 40% 15% 1	5182
	21st to end	Night	37% 3 14% 27% 18% 1	6974

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	15% 8% 31% 27% 13% 6%	300
	11th to 20th	Day	13% 10% 28% 33% 14% 1	300
	21st to end	Day	8% 7% 32% 37% 12% 4	330
February	1st to 10th	Day	8% 22% 34% 28% 8% 1	300
	11th to 20th	Day	15% 14% 36% 25% 9% 1	300
	21st to end	Day	12% 17% 32% 30% 7% 1	269
March	1st to 10th	Day	15% 20% 29% 24% 10% 2	500
	11th to 20th	Day	14% 18% 31% 26% 10% 2	500
	21st to end	Day	18% 24% 33% 18% 7% 1	531
April	1st to 10th	Day	16% 17% 33% 23% 9% 1	460
	11th to 20th	Day	17% 25% 32% 17% 7% 1	452
	21st to end	Day	15% 18% 35% 23% 9%	460
May	1st to 10th	Day	16% 19% 32% 24% 9% 1	463
	11th to 20th	Day	10% 13% 29% 31% 16% 1	511
	21st to end	Day	11% 11% 26% 30% 19% 3	734
June	1st to 10th	Day	5% 9% 23% 36% 24% 3	680
	11th to 20th	Day	9% 13% 24% 34% 17% 4	680
	21st to end	Day	5% 9% 24% 40% 20% 3	680
July	1st to 10th	Day	3 5% 17% 38% 32% 5%	666
	11th to 20th	Day	5% 3 15% 44% 29% 5%	484
	21st to end	Day	6% 5% 13% 43% 29% 4	531
August	1st to 10th	Day	6% 3 19% 41% 26% 3	487
	11th to 20th	Day	4 4 20% 41% 26% 5%	477
	21st to end	Day	6% 3 20% 41% 25% 4	539
September	1st to 10th	Day	7% 4 21% 38% 24% 6%	490
	11th to 20th	Day	7% 9% 29% 33% 18% 4	490
	21st to end	Day	9% 9% 24% 32% 22% 4	426
October	1st to 10th	Day	10% 8% 22% 27% 24% 8%	300
	11th to 20th	Day	5% 11% 26% 33% 19% 5%	300
	21st to end	Day	11% 12% 28% 29% 18% 2	330
November	1st to 10th	Day	9% 13% 30% 30% 17% 1	300
	11th to 20th	Day	6% 12% 35% 29% 13% 5%	300
	21st to end	Day	8% 15% 27% 28% 18% 3	300
December	1st to 10th	Day	9% 12% 30% 31% 14% 4	300
	11th to 20th	Day	7% 10% 36% 29% 14% 4	300
	21st to end	Day	13% 9% 34% 30% 13% 2	330
January	1st to 10th	Night	10% 5% 21% 40% 21% 4	875
	11th to 20th	Night	9% 5% 22% 39% 21% 4	800
	21st to end	Night	12% 5% 22% 40% 18% 3	880
February	1st to 10th	Night	10% 6% 25% 43% 13% 2	800
	11th to 20th	Night	10% 7% 29% 39% 14% 2	784
	21st to end	Night	7% 9% 32% 43% 8% 1	660
March	1st to 10th	Night	10% 7% 30% 38% 13% 2	800
	11th to 20th	Night	7% 8% 21% 43% 19% 1	800
	21st to end	Night	11% 8% 27% 39% 14% 2	862
April	1st to 10th	Night	12% 7% 29% 40% 11% 1	584
	11th to 20th	Night	12% 12% 23% 38% 13% 2	563
	21st to end	Night	11% 8% 26% 37% 14% 4	570
May	1st to 10th	Night	13% 9% 22% 39% 15% 2	570
	11th to 20th	Night	11% 4 16% 40% 25% 5%	561
	21st to end	Night	10% 7% 20% 35% 24% 5%	617
June	1st to 10th	Night	5% 4 15% 42% 29% 5%	570
	11th to 20th	Night	7% 5% 19% 43% 21% 4	570
	21st to end	Night	4 3 16% 39% 33% 4	570
July	1st to 10th	Night	4 2 11% 32% 45% 6%	555
	11th to 20th	Night	2 1 6% 39% 45% 7%	552
	21st to end	Night	4 2 8% 31% 47% 7%	624
August	1st to 10th	Night	5% 2 8% 36% 44% 5%	567
	11th to 20th	Night	5% 2 11% 36% 40% 6%	564
	21st to end	Night	5% 3 10% 37% 37% 8%	627
September	1st to 10th	Night	7% 1 10% 40% 37% 5%	570
	11th to 20th	Night	6% 4 15% 35% 36% 4	617
	21st to end	Night	8% 3 17% 32% 33% 7%	760
October	1st to 10th	Night	8% 3 19% 39% 28% 3	760
	11th to 20th	Night	7% 3 20% 39% 28% 4	760
	21st to end	Night	10% 5% 20% 38% 24% 4	836
November	1st to 10th	Night	8% 6% 21% 39% 25% 3	780
	11th to 20th	Night	8% 6% 24% 41% 18% 3	996
	21st to end	Night	10% 7% 22% 37% 20% 4	1000
December	1st to 10th	Night	9% 5% 21% 45% 18% 2	1000
	11th to 20th	Night	12% 7% 23% 38% 18% 2	1000
	21st to end	Night	12% 7% 24% 44% 11% 2	1100

Page generated at 2025-03-24 07:04:26. Version ac07

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