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 2026-01-17.

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	9% 7% 3 8% 6% 6% 6% 5% 7% 8% 36%	127320
	Night	9% 8% 3 8% 5% 5% 5% 4 6% 8% 38%	124381
Transparency	Day	40% 10% 12% 18% 14% 4	131031
	Night	38% 10% 13% 19% 15% 4	128726
Seeing	Day	19% 16% 25% 21% 14% 6%	16571
	Night	16% 14% 22% 28% 17% 3	26665

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	7% 7% 4 9% 5% 4 4 4 6% 7% 42%	4146
	11th to 20th	Day	6% 6% 3 9% 6% 5% 5% 4 5% 6% 46%	4114
	21st to end	Day	10% 10% 4 9% 5% 4 4 4 4 5% 39%	4466
February	1st to 10th	Day	11% 8% 3 9% 6% 6% 5% 5% 5% 6% 36%	1900
	11th to 20th	Day	10% 11% 5% 9% 6% 5% 4 4 5% 6% 35%	1948
	21st to end	Day	10% 9% 3 9% 6% 4 5% 5% 6% 5% 38%	1650
March	1st to 10th	Day	13% 7% 3 8% 5% 6% 5% 4 6% 6% 37%	4792
	11th to 20th	Day	8% 6% 3 8% 4 4 4 3 5% 7% 49%	4980
	21st to end	Day	11% 9% 3 8% 5% 4 4 3 5% 6% 42%	5465
April	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 5% 6% 8% 41%	2200
	11th to 20th	Day	11% 8% 3 6% 5% 5% 4 4 6% 7% 41%	2409
	21st to end	Day	11% 7% 3 7% 5% 5% 4 5% 6% 6% 41%	2600
May	1st to 10th	Day	9% 8% 3 10% 7% 7% 6% 6% 8% 9% 27%	5862
	11th to 20th	Day	13% 8% 3 9% 7% 7% 8% 5% 7% 8% 25%	5861
	21st to end	Day	10% 8% 3 9% 6% 6% 6% 5% 7% 9% 32%	6433
June	1st to 10th	Day	9% 6% 3 8% 7% 7% 7% 6% 8% 9% 30%	2600
	11th to 20th	Day	6% 7% 3 8% 8% 7% 7% 6% 8% 9% 31%	2600
	21st to end	Day	7% 6% 3 7% 7% 6% 7% 6% 8% 11% 32%	2600
July	1st to 10th	Day	6% 5% 3 9% 7% 7% 8% 6% 8% 9% 33%	5900
	11th to 20th	Day	7% 7% 3 11% 8% 8% 8% 6% 8% 9% 24%	5900
	21st to end	Day	8% 8% 4 12% 9% 9% 8% 6% 8% 10% 18%	6424
August	1st to 10th	Day	10% 7% 3 9% 6% 7% 7% 6% 9% 11% 24%	4234
	11th to 20th	Day	7% 5% 3 8% 7% 7% 7% 6% 10% 11% 29%	4174
	21st to end	Day	6% 5% 2 7% 6% 5% 6% 4 7% 8% 44%	4324
September	1st to 10th	Day	9% 8% 3 8% 6% 7% 6% 5% 6% 7% 36%	2200
	11th to 20th	Day	13% 8% 3 9% 5% 6% 5% 4 6% 9% 32%	2200
	21st to end	Day	14% 6% 3 7% 5% 6% 5% 4 5% 7% 38%	2200
October	1st to 10th	Day	11% 6% 3 7% 4 4 4 3 6% 6% 46%	2178
	11th to 20th	Day	8% 6% 2 6% 4 4 4 3 5% 6% 51%	2000
	21st to end	Day	11% 6% 2 7% 4 3 3 3 5% 6% 50%	2200
November	1st to 10th	Day	8% 7% 2 5% 3 4 5% 4 4 6% 53%	1932
	11th to 20th	Day	8% 7% 2 6% 4 3 4 4 6% 6% 48%	1890
	21st to end	Day	8% 8% 3 6% 4 4 4 4 4 8% 46%	1890
December	1st to 10th	Day	7% 8% 3 7% 5% 5% 5% 5% 6% 6% 44%	3020
	11th to 20th	Day	8% 7% 4 9% 5% 5% 6% 6% 7% 8% 34%	3463
	21st to end	Day	11% 7% 3 9% 5% 6% 5% 4 6% 8% 37%	4565
January	1st to 10th	Night	10% 7% 3 10% 6% 5% 5% 5% 6% 6% 37%	6019
	11th to 20th	Night	7% 7% 3 10% 7% 6% 5% 3 5% 6% 41%	5979
	21st to end	Night	9% 10% 3 8% 6% 4 5% 4 5% 6% 41%	6490
February	1st to 10th	Night	9% 9% 4 8% 5% 5% 5% 4 6% 7% 37%	2495
	11th to 20th	Night	12% 11% 4 10% 6% 5% 5% 3 5% 6% 34%	2338
	21st to end	Night	10% 9% 4 8% 6% 4 5% 4 6% 7% 37%	1980
March	1st to 10th	Night	13% 10% 5% 9% 5% 5% 5% 4 6% 5% 33%	5059
	11th to 20th	Night	7% 7% 3 6% 3 4 5% 4 6% 8% 48%	4980
	21st to end	Night	9% 7% 3 7% 5% 5% 6% 4 6% 6% 42%	5467
April	1st to 10th	Night	9% 7% 3 7% 5% 4 5% 4 6% 8% 43%	2200
	11th to 20th	Night	9% 7% 2 7% 6% 4 4 3 5% 7% 46%	1954
	21st to end	Night	9% 7% 2 8% 5% 4 5% 4 6% 8% 43%	1800
May	1st to 10th	Night	9% 8% 3 7% 5% 6% 6% 5% 8% 11% 30%	4033
	11th to 20th	Night	15% 8% 4 9% 6% 5% 6% 5% 7% 10% 25%	4032
	21st to end	Night	11% 9% 3 8% 5% 5% 6% 5% 7% 9% 32%	4421
June	1st to 10th	Night	7% 5% 2 7% 6% 5% 7% 6% 8% 13% 34%	1800
	11th to 20th	Night	4 5% 3 6% 5% 5% 7% 5% 9% 13% 39%	1800
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 4 7% 12% 41%	1800
July	1st to 10th	Night	5% 6% 3 8% 6% 7% 6% 6% 9% 11% 35%	4058
	11th to 20th	Night	4 5% 3 8% 6% 6% 7% 6% 10% 12% 32%	4057
	21st to end	Night	7% 8% 4 11% 8% 7% 8% 6% 8% 8% 24%	4427
August	1st to 10th	Night	7% 6% 3 8% 6% 7% 7% 6% 8% 12% 32%	2915
	11th to 20th	Night	6% 6% 4 10% 7% 6% 6% 6% 7% 10% 33%	2875
	21st to end	Night	5% 4 2 5% 5% 5% 5% 4 8% 9% 48%	3536
September	1st to 10th	Night	8% 6% 3 8% 5% 6% 5% 5% 6% 9% 39%	2200
	11th to 20th	Night	12% 9% 4 8% 6% 4 5% 4 5% 8% 36%	2200
	21st to end	Night	12% 7% 3 7% 4 4 4 3 5% 7% 44%	2200
October	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 4 6% 48%	2220
	11th to 20th	Night	8% 7% 3 6% 5% 4 5% 4 5% 7% 47%	2400
	21st to end	Night	10% 8% 3 5% 3 3 4 3 5% 6% 51%	2640
November	1st to 10th	Night	9% 7% 3 6% 5% 4 5% 4 5% 5% 47%	2581
	11th to 20th	Night	9% 8% 3 7% 4 3 4 4 5% 6% 48%	2730
	21st to end	Night	8% 8% 3 8% 4 4 4 4 6% 6% 45%	2730
December	1st to 10th	Night	8% 8% 5% 10% 6% 4 4 4 5% 6% 41%	4380
	11th to 20th	Night	10% 9% 4 12% 6% 6% 6% 4 5% 6% 32%	4952
	21st to end	Night	13% 7% 3 9% 5% 5% 5% 4 6% 7% 35%	6633

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	36% 9% 10% 17% 26% 2	4147
	11th to 20th	Day	36% 8% 9% 11% 32% 5%	4114
	21st to end	Day	42% 10% 8% 15% 20% 6%	4466
February	1st to 10th	Day	46% 9% 9% 16% 16% 3	1900
	11th to 20th	Day	47% 9% 9% 16% 15% 4	1958
	21st to end	Day	45% 8% 9% 14% 19% 6%	1650
March	1st to 10th	Day	41% 7% 9% 14% 18% 11%	4792
	11th to 20th	Day	34% 6% 7% 16% 20% 16%	4980
	21st to end	Day	41% 7% 8% 28% 13% 4	5465
April	1st to 10th	Day	38% 9% 12% 21% 17% 3	2200
	11th to 20th	Day	39% 9% 9% 18% 20% 5%	2409
	21st to end	Day	40% 10% 10% 25% 13% 2	2600
May	1st to 10th	Day	47% 12% 14% 18% 8% 1	5897
	11th to 20th	Day	50% 12% 14% 17% 6% 1	5865
	21st to end	Day	42% 13% 12% 18% 13% 2	6447
June	1st to 10th	Day	45% 13% 17% 17% 8% 1	2600
	11th to 20th	Day	43% 14% 16% 15% 11% 1	2600
	21st to end	Day	41% 13% 16% 17% 10% 2	2600
July	1st to 10th	Day	39% 15% 16% 20% 9%	5835
	11th to 20th	Day	47% 16% 20% 13% 4 1	5831
	21st to end	Day	53% 15% 16% 11% 5%	6240
August	1st to 10th	Day	46% 13% 18% 20% 3	4221
	11th to 20th	Day	38% 13% 24% 14% 9% 1	4213
	21st to end	Day	33% 10% 14% 32% 10% 1	4324
September	1st to 10th	Day	43% 11% 11% 17% 14% 4	2200
	11th to 20th	Day	47% 9% 10% 19% 14% 2	2200
	21st to end	Day	44% 8% 13% 17% 14% 4	2200
October	1st to 10th	Day	28% 7% 8% 18% 27% 13%	3542
	11th to 20th	Day	23% 5% 7% 21% 21% 23%	3240
	21st to end	Day	24% 6% 8% 28% 20% 15%	3538
November	1st to 10th	Day	31% 7% 10% 18% 26% 8%	1932
	11th to 20th	Day	33% 7% 8% 22% 24% 6%	1890
	21st to end	Day	36% 7% 9% 23% 20% 5%	1890
December	1st to 10th	Day	35% 8% 9% 18% 25% 5%	3020
	11th to 20th	Day	41% 11% 12% 15% 18% 3	3460
	21st to end	Day	40% 9% 15% 22% 11% 3	4565
January	1st to 10th	Night	42% 9% 10% 16% 20% 3	6019
	11th to 20th	Night	39% 10% 8% 13% 22% 8%	5979
	21st to end	Night	39% 9% 8% 16% 22% 7%	6490
February	1st to 10th	Night	42% 10% 11% 18% 16% 3	2495
	11th to 20th	Night	50% 7% 9% 17% 14% 4	2339
	21st to end	Night	44% 9% 10% 16% 16% 5%	1980
March	1st to 10th	Night	45% 9% 9% 12% 20% 6%	5059
	11th to 20th	Night	32% 7% 9% 19% 20% 14%	4980
	21st to end	Night	38% 8% 13% 25% 12% 4	5467
April	1st to 10th	Night	37% 9% 14% 22% 16% 2	2200
	11th to 20th	Night	37% 8% 10% 22% 20% 4	1954
	21st to end	Night	37% 10% 13% 26% 13% 1	1800
May	1st to 10th	Night	42% 13% 16% 22% 5% 1	4060
	11th to 20th	Night	50% 12% 17% 17% 4	4033
	21st to end	Night	41% 13% 16% 20% 9% 1	4429
June	1st to 10th	Night	37% 15% 20% 20% 8%	1800
	11th to 20th	Night	32% 15% 23% 21% 9% 1	1800
	21st to end	Night	32% 13% 20% 21% 11% 3	1800
July	1st to 10th	Night	37% 16% 19% 23% 6%	4015
	11th to 20th	Night	37% 19% 27% 15% 3 1	4006
	21st to end	Night	48% 17% 17% 12% 5%	4297
August	1st to 10th	Night	38% 15% 25% 17% 4	2904
	11th to 20th	Night	37% 16% 25% 15% 7%	2902
	21st to end	Night	28% 11% 19% 32% 9% 1	3536
September	1st to 10th	Night	40% 11% 15% 19% 13% 3	2200
	11th to 20th	Night	44% 9% 14% 18% 14% 1	2200
	21st to end	Night	39% 9% 12% 21% 17% 3	2200
October	1st to 10th	Night	28% 7% 11% 21% 25% 9%	3623
	11th to 20th	Night	24% 5% 8% 22% 21% 19%	3899
	21st to end	Night	24% 6% 7% 27% 19% 16%	4252
November	1st to 10th	Night	36% 8% 9% 20% 21% 6%	2581
	11th to 20th	Night	35% 7% 8% 21% 23% 7%	2730
	21st to end	Night	38% 7% 7% 24% 19% 5%	2730
December	1st to 10th	Night	41% 8% 10% 17% 20% 4	4380
	11th to 20th	Night	46% 12% 9% 15% 15% 3	4954
	21st to end	Night	41% 10% 15% 17% 15% 1	6633

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	32% 28% 30% 8% 2	330
	11th to 20th	Day	36% 26% 23% 13% 2	318
	21st to end	Day	29% 33% 27% 8% 2	330
February	1st to 10th	Day	25% 32% 31% 10% 2	300
	11th to 20th	Day	41% 32% 22% 5% 1	300
	21st to end	Day	39% 23% 30% 7% 1	249
March	1st to 10th	Day	29% 23% 29% 13% 6%	430
	11th to 20th	Day	29% 27% 25% 15% 4 1	500
	21st to end	Day	27% 26% 28% 14% 5%	550
April	1st to 10th	Day	23% 24% 27% 17% 8% 1	500
	11th to 20th	Day	22% 22% 24% 22% 8% 2	492
	21st to end	Day	18% 20% 26% 20% 13% 3	500
May	1st to 10th	Day	19% 20% 28% 18% 11% 4	503
	11th to 20th	Day	18% 14% 29% 21% 13% 6%	527
	21st to end	Day	17% 10% 23% 25% 16% 9%	573
June	1st to 10th	Day	12% 9% 29% 25% 17% 8%	530
	11th to 20th	Day	7% 8% 28% 24% 22% 11%	530
	21st to end	Day	10% 7% 28% 26% 17% 11%	530
July	1st to 10th	Day	8% 4 19% 29% 23% 17%	525
	11th to 20th	Day	9% 5% 17% 27% 25% 18%	524
	21st to end	Day	7% 3 18% 26% 29% 17%	576
August	1st to 10th	Day	14% 8% 23% 21% 19% 14%	526
	11th to 20th	Day	10% 5% 20% 26% 25% 13%	519
	21st to end	Day	11% 5% 18% 27% 25% 15%	583
September	1st to 10th	Day	10% 5% 20% 31% 24% 11%	530
	11th to 20th	Day	15% 6% 23% 21% 26% 9%	530
	21st to end	Day	18% 7% 20% 31% 20% 4	530
October	1st to 10th	Day	15% 10% 22% 28% 18% 7%	530
	11th to 20th	Day	15% 16% 25% 25% 17% 2	530
	21st to end	Day	20% 14% 27% 26% 11% 2	583
November	1st to 10th	Day	17% 19% 30% 24% 9% 1	410
	11th to 20th	Day	22% 25% 32% 16% 5%	330
	21st to end	Day	25% 24% 30% 15% 5% 1	330
December	1st to 10th	Day	30% 28% 28% 11% 3	330
	11th to 20th	Day	22% 27% 33% 15% 4	330
	21st to end	Day	23% 27% 35% 11% 4	363
January	1st to 10th	Night	23% 24% 27% 22% 4	840
	11th to 20th	Night	21% 23% 31% 22% 4	825
	21st to end	Night	19% 26% 30% 22% 3	880
February	1st to 10th	Night	18% 20% 32% 26% 4	800
	11th to 20th	Night	25% 28% 26% 19% 1	783
	21st to end	Night	22% 28% 30% 18% 2	660
March	1st to 10th	Night	19% 23% 31% 23% 4	800
	11th to 20th	Night	22% 19% 31% 21% 7% 1	800
	21st to end	Night	19% 20% 32% 22% 6%	880
April	1st to 10th	Night	20% 21% 26% 26% 7%	800
	11th to 20th	Night	18% 15% 28% 26% 11% 1	662
	21st to end	Night	15% 13% 24% 36% 12% 1	600
May	1st to 10th	Night	13% 12% 28% 30% 15% 1	600
	11th to 20th	Night	14% 8% 23% 31% 21% 3	591
	21st to end	Night	13% 4 16% 34% 29% 5%	650
June	1st to 10th	Night	9% 4 16% 39% 28% 5%	600
	11th to 20th	Night	6% 4 16% 34% 31% 9%	600
	21st to end	Night	8% 2 14% 34% 36% 6%	600
July	1st to 10th	Night	9% 2 12% 29% 41% 8%	585
	11th to 20th	Night	8% 3 10% 26% 43% 11%	582
	21st to end	Night	6% 2 8% 30% 41% 12%	657
August	1st to 10th	Night	11% 3 10% 31% 39% 8%	597
	11th to 20th	Night	10% 4 12% 30% 38% 6%	594
	21st to end	Night	10% 3 9% 31% 39% 8%	695
September	1st to 10th	Night	11% 3 13% 33% 33% 7%	800
	11th to 20th	Night	17% 4 15% 29% 31% 6%	800
	21st to end	Night	16% 5% 17% 32% 27% 4	800
October	1st to 10th	Night	16% 8% 18% 29% 24% 6%	800
	11th to 20th	Night	13% 14% 23% 32% 16% 3	800
	21st to end	Night	17% 14% 22% 32% 13% 2	880
November	1st to 10th	Night	15% 13% 23% 32% 14% 3	820
	11th to 20th	Night	18% 16% 24% 30% 10% 1	840
	21st to end	Night	19% 16% 25% 28% 11% 1	840
December	1st to 10th	Night	22% 20% 27% 25% 5%	840
	11th to 20th	Night	19% 18% 29% 28% 6% 1	840
	21st to end	Night	20% 20% 29% 27% 4	924

Page generated at 2026-01-14 01:14:30. Version ac07

Copyright © 2026 Attilla Danko.