Grassland Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grassland Mountain Observatory clear sky chart from 2005-11-06 to 2024-11-25.

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	10% 8% 3 8% 6% 6% 6% 5% 6% 8% 35%	101293
	Night	9% 8% 3 8% 5% 5% 5% 4 6% 8% 37%	98137
Transparency	Day	42% 10% 12% 19% 14% 4	105048
	Night	39% 11% 13% 19% 14% 4	102508
Seeing	Day	18% 15% 25% 21% 14% 6%	15071
	Night	16% 13% 22% 28% 17% 3	25133

By Month

Percentage Cloud Cover SE (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	9% 8% 3 9% 5% 5% 4 5% 7% 8% 37%	2836
	11th to 20th	Day	9% 8% 4 10% 6% 6% 6% 5% 6% 6% 35%	2840
	21st to end	Day	13% 13% 5% 9% 4 4 4 4 5% 5% 34%	3124
February	1st to 10th	Day	11% 8% 3 9% 6% 6% 5% 5% 5% 6% 35%	1805
	11th to 20th	Day	10% 11% 5% 9% 6% 5% 4 4 5% 6% 35%	1848
	21st to end	Day	10% 10% 4 9% 6% 4 5% 5% 5% 6% 36%	1570
March	1st to 10th	Day	17% 9% 4 7% 4 4 4 4 5% 7% 35%	3334
	11th to 20th	Day	10% 7% 3 7% 4 4 4 3 4 5% 48%	3480
	21st to end	Day	14% 9% 3 7% 5% 4 4 4 5% 6% 38%	3815
April	1st to 10th	Day	8% 7% 3 7% 5% 5% 5% 5% 6% 8% 41%	2090
	11th to 20th	Day	12% 8% 3 6% 5% 5% 4 4 6% 7% 40%	2286
	21st to end	Day	11% 7% 3 7% 5% 5% 4 5% 5% 6% 41%	2470
May	1st to 10th	Day	10% 8% 3 9% 7% 6% 6% 5% 7% 7% 32%	4082
	11th to 20th	Day	13% 9% 3 9% 7% 6% 7% 4 6% 7% 29%	4081
	21st to end	Day	7% 6% 2 8% 6% 6% 6% 6% 8% 10% 35%	4476
June	1st to 10th	Day	9% 7% 3 8% 7% 7% 7% 6% 8% 9% 29%	2470
	11th to 20th	Day	6% 7% 3 8% 7% 7% 7% 6% 8% 9% 31%	2470
	21st to end	Day	8% 7% 3 7% 7% 6% 7% 6% 8% 10% 32%	2470
July	1st to 10th	Day	7% 6% 3 9% 7% 7% 8% 6% 9% 9% 30%	4120
	11th to 20th	Day	8% 8% 3 10% 8% 7% 7% 6% 7% 8% 29%	4120
	21st to end	Day	11% 9% 5% 14% 10% 9% 8% 6% 7% 8% 14%	4466
August	1st to 10th	Day	9% 7% 3 9% 7% 7% 8% 6% 9% 11% 25%	4104
	11th to 20th	Day	7% 5% 3 8% 7% 7% 7% 6% 10% 10% 29%	4044
	21st to end	Day	6% 5% 2 7% 5% 5% 5% 4 7% 8% 44%	4192
September	1st to 10th	Day	9% 9% 3 8% 6% 7% 6% 5% 6% 7% 36%	2090
	11th to 20th	Day	14% 8% 3 9% 5% 6% 5% 4 5% 8% 32%	2090
	21st to end	Day	14% 6% 3 7% 5% 6% 5% 4 5% 7% 38%	2090
October	1st to 10th	Day	12% 6% 3 7% 5% 4 3 3 5% 6% 46%	2069
	11th to 20th	Day	9% 6% 2 6% 4 4 4 3 5% 6% 50%	1900
	21st to end	Day	11% 6% 2 7% 4 3 3 3 5% 7% 51%	2090
November	1st to 10th	Day	8% 7% 2 5% 3 4 5% 4 4 6% 52%	1838
	11th to 20th	Day	9% 8% 2 6% 5% 3 4 4 6% 6% 47%	1800
	21st to end	Day	9% 8% 3 6% 4 4 4 4 4 7% 47%	1746
December	1st to 10th	Day	12% 10% 5% 8% 5% 5% 4 4 6% 6% 34%	1710
	11th to 20th	Day	9% 7% 4 6% 4 4 6% 6% 8% 7% 39%	2153
	21st to end	Day	14% 8% 3 8% 4 6% 6% 4 6% 7% 33%	3124
January	1st to 10th	Night	12% 8% 3 11% 7% 6% 5% 6% 6% 6% 32%	4109
	11th to 20th	Night	8% 9% 4 10% 7% 6% 5% 3 5% 7% 36%	4120
	21st to end	Night	13% 13% 4 9% 5% 4 5% 4 5% 5% 34%	4532
February	1st to 10th	Night	9% 8% 4 8% 5% 5% 5% 5% 6% 8% 38%	2371
	11th to 20th	Night	12% 11% 4 10% 6% 4 5% 3 5% 6% 34%	2218
	21st to end	Night	10% 9% 4 9% 6% 4 5% 4 6% 7% 35%	1884
March	1st to 10th	Night	16% 13% 5% 10% 6% 5% 5% 4 5% 5% 26%	3517
	11th to 20th	Night	7% 8% 3 7% 4 4 4 4 5% 8% 44%	3480
	21st to end	Night	12% 7% 3 6% 5% 5% 5% 4 5% 6% 41%	3817
April	1st to 10th	Night	9% 7% 3 7% 5% 4 4 4 6% 8% 43%	2090
	11th to 20th	Night	9% 7% 3 7% 6% 4 4 3 5% 7% 45%	1856
	21st to end	Night	9% 6% 2 8% 5% 4 4 4 6% 8% 44%	1710
May	1st to 10th	Night	9% 8% 3 8% 5% 6% 6% 5% 7% 10% 33%	2813
	11th to 20th	Night	11% 8% 4 9% 6% 5% 6% 5% 7% 9% 31%	2812
	21st to end	Night	6% 6% 3 8% 5% 6% 6% 5% 8% 11% 37%	3079
June	1st to 10th	Night	7% 5% 2 7% 6% 5% 7% 6% 8% 13% 34%	1710
	11th to 20th	Night	4 5% 3 6% 5% 4 7% 5% 8% 13% 40%	1710
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 4 7% 12% 40%	1710
July	1st to 10th	Night	6% 6% 3 8% 6% 7% 6% 6% 10% 10% 32%	2840
	11th to 20th	Night	5% 5% 3 8% 6% 5% 6% 6% 9% 11% 37%	2840
	21st to end	Night	8% 9% 4 12% 9% 7% 8% 6% 8% 8% 21%	3087
August	1st to 10th	Night	7% 6% 3 8% 6% 6% 7% 6% 8% 12% 32%	2825
	11th to 20th	Night	6% 6% 4 10% 7% 6% 6% 6% 7% 10% 33%	2785
	21st to end	Night	5% 4 2 5% 4 5% 5% 4 8% 9% 48%	3427
September	1st to 10th	Night	8% 6% 3 8% 5% 6% 5% 5% 7% 9% 39%	2090
	11th to 20th	Night	12% 9% 4 7% 6% 4 4 4 5% 8% 36%	2090
	21st to end	Night	12% 7% 3 7% 4 4 4 3 5% 7% 46%	2090
October	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 4 6% 48%	2109
	11th to 20th	Night	9% 8% 3 6% 5% 4 5% 4 5% 6% 46%	2280
	21st to end	Night	9% 8% 2 5% 3 3 4 3 5% 6% 52%	2508
November	1st to 10th	Night	9% 7% 3 6% 5% 4 5% 4 5% 5% 46%	2455
	11th to 20th	Night	10% 8% 3 7% 4 4 4 4 5% 6% 47%	2600
	21st to end	Night	9% 8% 3 8% 4 3 4 4 6% 6% 46%	2529
December	1st to 10th	Night	11% 11% 4 10% 6% 5% 5% 5% 6% 6% 30%	2470
	11th to 20th	Night	10% 9% 4 11% 5% 5% 5% 4 4 6% 37%	3042
	21st to end	Night	16% 8% 3 9% 6% 5% 6% 5% 7% 8% 28%	4532

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	38% 11% 10% 18% 19% 3	2837
	11th to 20th	Day	44% 11% 9% 11% 22% 3	2840
	21st to end	Day	48% 10% 7% 13% 20% 2	3124
February	1st to 10th	Day	48% 9% 8% 15% 17% 3	1805
	11th to 20th	Day	47% 9% 9% 15% 15% 4	1858
	21st to end	Day	47% 8% 9% 14% 17% 6%	1570
March	1st to 10th	Day	45% 9% 8% 17% 14% 7%	3334
	11th to 20th	Day	36% 8% 8% 20% 20% 9%	3480
	21st to end	Day	44% 8% 8% 25% 10% 4	3815
April	1st to 10th	Day	38% 9% 11% 21% 17% 3	2090
	11th to 20th	Day	40% 9% 10% 17% 20% 5%	2286
	21st to end	Day	40% 10% 10% 25% 13% 2	2470
May	1st to 10th	Day	46% 12% 13% 19% 9% 1	4120
	11th to 20th	Day	50% 11% 12% 20% 7% 1	4085
	21st to end	Day	37% 14% 14% 19% 15% 1	4489
June	1st to 10th	Day	46% 13% 16% 16% 8% 1	2470
	11th to 20th	Day	43% 14% 16% 15% 11% 1	2470
	21st to end	Day	42% 13% 15% 16% 11% 2	2470
July	1st to 10th	Day	42% 16% 17% 18% 7%	4055
	11th to 20th	Day	47% 12% 17% 17% 5% 1	4051
	21st to end	Day	63% 13% 11% 9% 4	4324
August	1st to 10th	Day	45% 13% 18% 20% 3	4091
	11th to 20th	Day	38% 13% 24% 14% 9% 1	4083
	21st to end	Day	34% 10% 13% 32% 10% 1	4192
September	1st to 10th	Day	43% 10% 11% 17% 14% 4	2090
	11th to 20th	Day	48% 9% 9% 18% 14% 2	2090
	21st to end	Day	44% 7% 13% 18% 15% 4	2090
October	1st to 10th	Day	28% 7% 8% 18% 27% 13%	3433
	11th to 20th	Day	24% 5% 7% 21% 20% 23%	3140
	21st to end	Day	23% 6% 8% 28% 20% 15%	3428
November	1st to 10th	Day	31% 8% 9% 19% 26% 8%	1838
	11th to 20th	Day	35% 7% 7% 21% 25% 6%	1800
	21st to end	Day	36% 8% 9% 22% 21% 5%	1746
December	1st to 10th	Day	48% 9% 9% 17% 14% 3	1710
	11th to 20th	Day	37% 12% 11% 15% 21% 4	2150
	21st to end	Day	44% 10% 11% 20% 13% 2	3124
January	1st to 10th	Night	46% 13% 9% 13% 16% 3	4109
	11th to 20th	Night	41% 14% 6% 11% 21% 6%	4120
	21st to end	Night	49% 9% 8% 13% 20% 2	4532
February	1st to 10th	Night	42% 10% 11% 17% 17% 3	2371
	11th to 20th	Night	50% 7% 8% 17% 14% 4	2219
	21st to end	Night	46% 9% 10% 16% 14% 4	1884
March	1st to 10th	Night	53% 12% 9% 11% 12% 3	3517
	11th to 20th	Night	35% 9% 10% 22% 16% 8%	3480
	21st to end	Night	41% 9% 10% 23% 13% 4	3817
April	1st to 10th	Night	37% 9% 13% 22% 16% 2	2090
	11th to 20th	Night	38% 8% 10% 21% 20% 4	1856
	21st to end	Night	36% 9% 13% 27% 13% 1	1710
May	1st to 10th	Night	43% 12% 15% 22% 7% 1	2840
	11th to 20th	Night	46% 13% 15% 21% 5%	2813
	21st to end	Night	34% 15% 17% 24% 9% 1	3087
June	1st to 10th	Night	37% 15% 20% 20% 8%	1710
	11th to 20th	Night	32% 14% 24% 21% 9% 1	1710
	21st to end	Night	33% 13% 20% 21% 11% 3	1710
July	1st to 10th	Night	38% 18% 19% 19% 5%	2795
	11th to 20th	Night	34% 16% 25% 20% 4 1	2786
	21st to end	Night	53% 16% 15% 12% 4	2988
August	1st to 10th	Night	37% 15% 26% 18% 4	2814
	11th to 20th	Night	36% 16% 25% 15% 7%	2812
	21st to end	Night	28% 11% 20% 33% 8% 1	3427
September	1st to 10th	Night	40% 11% 14% 19% 13% 3	2090
	11th to 20th	Night	45% 9% 13% 18% 15% 1	2090
	21st to end	Night	38% 8% 11% 22% 17% 3	2090
October	1st to 10th	Night	28% 7% 11% 20% 26% 9%	3512
	11th to 20th	Night	24% 5% 8% 22% 21% 20%	3779
	21st to end	Night	24% 7% 7% 28% 19% 16%	4120
November	1st to 10th	Night	37% 8% 9% 19% 21% 6%	2455
	11th to 20th	Night	36% 7% 7% 20% 23% 7%	2600
	21st to end	Night	37% 7% 8% 24% 20% 5%	2529
December	1st to 10th	Night	49% 11% 10% 15% 11% 3	2470
	11th to 20th	Night	43% 12% 7% 13% 20% 4	3044
	21st to end	Night	46% 11% 11% 18% 12% 1	4532

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	29% 28% 32% 9% 2	270
	11th to 20th	Day	33% 26% 25% 13% 2	270
	21st to end	Day	28% 34% 27% 8% 2	297
February	1st to 10th	Day	27% 30% 31% 10% 2	270
	11th to 20th	Day	37% 33% 24% 5% 1	270
	21st to end	Day	41% 21% 30% 6% 1	225
March	1st to 10th	Day	27% 23% 29% 14% 7%	393
	11th to 20th	Day	28% 28% 25% 15% 4 1	460
	21st to end	Day	26% 26% 27% 15% 5%	506
April	1st to 10th	Day	23% 23% 27% 19% 8% 1	460
	11th to 20th	Day	23% 20% 25% 22% 8% 3	452
	21st to end	Day	19% 20% 26% 19% 13% 3	460
May	1st to 10th	Day	19% 19% 27% 18% 12% 4	463
	11th to 20th	Day	17% 13% 29% 22% 14% 6%	487
	21st to end	Day	15% 9% 23% 26% 17% 10%	529
June	1st to 10th	Day	13% 9% 27% 26% 17% 9%	490
	11th to 20th	Day	7% 9% 27% 23% 22% 12%	490
	21st to end	Day	11% 8% 30% 24% 17% 11%	490
July	1st to 10th	Day	8% 4 19% 29% 23% 18%	485
	11th to 20th	Day	9% 5% 17% 26% 24% 18%	484
	21st to end	Day	8% 4 19% 25% 28% 17%	532
August	1st to 10th	Day	13% 7% 23% 21% 20% 16%	486
	11th to 20th	Day	10% 6% 21% 25% 25% 14%	479
	21st to end	Day	11% 4 16% 27% 26% 16%	539
September	1st to 10th	Day	10% 4 18% 30% 26% 12%	490
	11th to 20th	Day	16% 6% 22% 20% 27% 9%	490
	21st to end	Day	18% 7% 19% 31% 20% 4	490
October	1st to 10th	Day	17% 9% 23% 26% 18% 7%	490
	11th to 20th	Day	15% 17% 24% 25% 18% 2	490
	21st to end	Day	17% 14% 27% 27% 12% 2	539
November	1st to 10th	Day	16% 19% 29% 25% 10% 2	376
	11th to 20th	Day	23% 24% 31% 17% 5%	300
	21st to end	Day	21% 23% 32% 17% 6% 1	282
December	1st to 10th	Day	29% 26% 29% 13% 3	270
	11th to 20th	Day	19% 29% 32% 15% 5%	270
	21st to end	Day	24% 25% 35% 12% 4	297
January	1st to 10th	Night	22% 24% 26% 23% 4	760
	11th to 20th	Night	20% 22% 30% 23% 4	760
	21st to end	Night	19% 25% 29% 23% 3	836
February	1st to 10th	Night	18% 19% 32% 27% 4	760
	11th to 20th	Night	24% 29% 26% 20% 1	743
	21st to end	Night	22% 29% 29% 18% 2	628
March	1st to 10th	Night	18% 23% 31% 24% 4	760
	11th to 20th	Night	22% 19% 30% 21% 7% 1	760
	21st to end	Night	19% 20% 31% 23% 6%	836
April	1st to 10th	Night	19% 20% 26% 26% 8%	760
	11th to 20th	Night	19% 14% 28% 26% 12% 1	629
	21st to end	Night	14% 13% 24% 35% 13% 1	570
May	1st to 10th	Night	14% 12% 27% 29% 16% 1	570
	11th to 20th	Night	13% 8% 22% 32% 22% 3	561
	21st to end	Night	11% 3 16% 35% 30% 5%	617
June	1st to 10th	Night	8% 3 15% 39% 29% 5%	570
	11th to 20th	Night	6% 4 15% 34% 31% 10%	570
	21st to end	Night	8% 2 14% 34% 35% 6%	570
July	1st to 10th	Night	9% 2 12% 28% 40% 8%	555
	11th to 20th	Night	8% 3 10% 26% 42% 11%	552
	21st to end	Night	6% 2 8% 30% 40% 13%	624
August	1st to 10th	Night	9% 2 9% 32% 40% 8%	567
	11th to 20th	Night	9% 3 12% 32% 37% 6%	564
	21st to end	Night	10% 3 8% 30% 40% 8%	660
September	1st to 10th	Night	11% 3 13% 31% 34% 7%	760
	11th to 20th	Night	18% 4 15% 28% 30% 6%	760
	21st to end	Night	16% 5% 17% 30% 28% 4	760
October	1st to 10th	Night	16% 7% 19% 28% 24% 6%	760
	11th to 20th	Night	13% 14% 22% 31% 16% 3	760
	21st to end	Night	16% 14% 22% 33% 13% 2	836
November	1st to 10th	Night	15% 12% 22% 32% 15% 3	780
	11th to 20th	Night	19% 15% 24% 31% 11% 1	800
	21st to end	Night	18% 15% 25% 29% 12% 1	779
December	1st to 10th	Night	23% 19% 25% 27% 6%	760
	11th to 20th	Night	18% 18% 28% 29% 7% 1	760
	21st to end	Night	20% 20% 28% 28% 4	836

Page generated at 2024-11-22 08:02:25. Version ac07

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