Grasswood Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grasswood Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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 4 1 3 2 3 3 3 5% 7% 65%	129338
	Night	4 4 1 4 2 3 3 3 4 6% 65%	127737
Transparency	Day	22% 3 8% 19% 26% 22%	129060
	Night	21% 3 7% 19% 26% 23%	127529
Seeing	Day	5% 4 14% 33% 35% 10%	16649
	Night	6% 2 11% 30% 38% 14%	26431

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	4 3 1 2 2 2 3 2 4 5% 73%	3684
	11th to 20th	Day	6% 3 1 3 2 3 2 2 4 7% 66%	4100
	21st to end	Day	10% 5% 1 4 3 3 3 3 6% 9% 52%	4848
February	1st to 10th	Day	10% 5% 2 3 3 2 2 4 4 6% 60%	2000
	11th to 20th	Day	8% 3 1 3 3 3 3 3 5% 8% 60%	1950
	21st to end	Day	7% 4 1 5% 3 4 4 3 4 6% 59%	1650
March	1st to 10th	Day	10% 5% 2 5% 3 3 4 3 5% 6% 55%	4518
	11th to 20th	Day	9% 4 2 4 2 3 3 3 5% 7% 58%	4775
	21st to end	Day	7% 5% 2 4 4 4 5% 4 7% 9% 50%	5958
April	1st to 10th	Day	5% 5% 2 4 3 3 3 4 5% 8% 60%	2400
	11th to 20th	Day	4 4 2 4 3 2 3 3 5% 7% 63%	2369
	21st to end	Day	3 4 1 3 2 3 3 3 5% 8% 65%	2400
May	1st to 10th	Day	3 5% 2 4 2 3 3 3 4 8% 63%	5440
	11th to 20th	Day	1 8% 3 5% 3 3 3 3 5% 8% 57%	5837
	21st to end	Day	1 5% 2 5% 3 3 3 3 5% 10% 60%	6423
June	1st to 10th	Day	3 6% 3 5% 3 3 3 3 5% 8% 57%	2706
	11th to 20th	Day	2 5% 2 4 2 2 3 3 5% 9% 64%	2800
	21st to end	Day	2 3 1 3 2 2 3 2 5% 7% 70%	2800
July	1st to 10th	Day	1 1 1 2 1 1 2 2 3 8% 78%	6319
	11th to 20th	Day	1 2 1 3 2 2 3 3 4 7% 72%	6192
	21st to end	Day	1 2 1 2 1 2 2 2 4 6% 77%	6020
August	1st to 10th	Day	1 2 1 2 1 2 1 2 3 6% 79%	3832
	11th to 20th	Day	1 1 1 1 1 1 1 1 3 5% 84%	3724
	21st to end	Day	1 2 1 2 2 2 3 7% 79%	4180
September	1st to 10th	Day	2 2 3 1 2 2 2 4 8% 73%	2280
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 7% 74%	2176
	21st to end	Day	1 2 1 3 2 2 3 2 4 7% 72%	2090
October	1st to 10th	Day	2 1 2 1 2 2 2 3 8% 77%	3490
	11th to 20th	Day	2 4 2 4 2 2 3 2 4 7% 67%	3166
	21st to end	Day	3 3 1 3 2 3 3 3 5% 6% 67%	3448
November	1st to 10th	Day	3 3 1 3 2 3 3 2 5% 7% 69%	1777
	11th to 20th	Day	5% 3 1 3 2 2 3 3 5% 5% 69%	1800
	21st to end	Day	5% 3 2 4 3 4 3 4 5% 7% 61%	1800
December	1st to 10th	Day	4 2 1 3 3 3 4 5% 5% 8% 61%	2950
	11th to 20th	Day	10% 5% 2 4 3 3 5% 4 5% 7% 52%	3409
	21st to end	Day	9% 5% 2 6% 4 4 4 4 7% 11% 45%	4027
January	1st to 10th	Night	5% 3 1 3 2 2 3 2 3 5% 70%	6248
	11th to 20th	Night	6% 4 2 4 3 2 2 2 4 6% 66%	5880
	21st to end	Night	9% 5% 2 4 3 3 3 3 5% 6% 57%	6141
February	1st to 10th	Night	8% 5% 2 5% 2 2 2 3 5% 6% 60%	2400
	11th to 20th	Night	9% 5% 1 4 2 3 3 2 5% 7% 60%	2338
	21st to end	Night	10% 4 2 4 4 3 4 2 3 5% 59%	1980
March	1st to 10th	Night	6% 6% 2 5% 3 3 4 4 6% 7% 55%	5376
	11th to 20th	Night	6% 6% 2 5% 3 4 4 3 5% 5% 58%	5220
	21st to end	Night	9% 5% 2 4 2 3 3 3 5% 7% 58%	5016
April	1st to 10th	Night	5% 5% 2 3 3 3 3 3 5% 7% 61%	2000
	11th to 20th	Night	3 5% 2 3 2 3 4 3 5% 6% 66%	1979
	21st to end	Night	4 5% 1 3 2 3 3 3 5% 7% 64%	2000
May	1st to 10th	Night	2 5% 2 4 4 4 3 3 4 5% 64%	4560
	11th to 20th	Night	1 5% 2 4 3 3 4 3 4 6% 64%	4074
	21st to end	Night	1 4 2 3 2 2 3 2 4 6% 71%	4474
June	1st to 10th	Night	1 5% 2 6% 3 3 5% 4 5% 7% 59%	1688
	11th to 20th	Night	2 4 2 4 2 3 2 3 6% 7% 67%	1600
	21st to end	Night	1 4 1 4 2 2 3 3 6% 6% 68%	1600
July	1st to 10th	Night	1 1 1 2 1 2 3 3 3 6% 78%	3640
	11th to 20th	Night	1 2 1 5% 3 3 4 4 4 6% 68%	3769
	21st to end	Night	1 2 1 2 1 2 2 2 3 5% 79%	4855
August	1st to 10th	Night	2 1 1 2 2 2 2 2 4 6% 77%	3206
	11th to 20th	Night	1 2 1 1 2 1 1 2 3 5% 81%	3126
	21st to end	Night	1 1 1 2 2 2 3 2 3 6% 78%	3498
September	1st to 10th	Night	2 3 1 3 2 3 2 2 4 6% 72%	1900
	11th to 20th	Night	3 2 1 2 1 2 2 2 4 5% 76%	2004
	21st to end	Night	1 1 1 2 2 2 2 2 4 6% 77%	2090
October	1st to 10th	Night	2 1 1 2 2 2 2 3 3 5% 77%	3490
	11th to 20th	Night	3 5% 3 3 2 2 3 2 4 6% 67%	3767
	21st to end	Night	3 3 1 2 2 2 2 2 3 5% 77%	4121
November	1st to 10th	Night	2 3 1 2 3 2 3 3 4 6% 71%	2521
	11th to 20th	Night	3 3 2 3 2 2 2 2 5% 6% 70%	2600
	21st to end	Night	4 4 1 3 3 3 3 4 6% 6% 61%	2600
December	1st to 10th	Night	5% 3 1 4 3 3 3 3 4 6% 66%	4240
	11th to 20th	Night	11% 5% 2 4 3 3 4 4 6% 8% 52%	4811
	21st to end	Night	9% 5% 2 5% 4 4 6% 5% 7% 9% 45%	6925

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	16% 2 4 23% 30% 26%	3684
	11th to 20th	Day	22% 2 11% 17% 18% 30%	4100
	21st to end	Day	27% 3 5% 20% 24% 20%	4848
February	1st to 10th	Day	26% 3 5% 17% 26% 23%	2000
	11th to 20th	Day	24% 2 7% 21% 26% 21%	1960
	21st to end	Day	25% 3 6% 23% 23% 20%	1650
March	1st to 10th	Day	28% 3 10% 20% 22% 17%	4518
	11th to 20th	Day	25% 4 11% 26% 23% 11%	4775
	21st to end	Day	29% 4 12% 31% 17% 7%	5958
April	1st to 10th	Day	24% 3 11% 27% 23% 12%	2400
	11th to 20th	Day	20% 2 8% 26% 28% 15%	2372
	21st to end	Day	20% 4 10% 27% 29% 11%	2400
May	1st to 10th	Day	20% 5% 12% 21% 30% 12%	5440
	11th to 20th	Day	29% 3 8% 25% 29% 7%	5840
	21st to end	Day	27% 4 8% 31% 24% 7%	6424
June	1st to 10th	Day	29% 2 11% 22% 25% 10%	2706
	11th to 20th	Day	20% 3 7% 17% 32% 21%	2800
	21st to end	Day	23% 3 3 10% 32% 29%	2800
July	1st to 10th	Day	16% 2 6% 12% 20% 44%	6251
	11th to 20th	Day	21% 3 6% 11% 33% 26%	6117
	21st to end	Day	15% 2 9% 18% 33% 23%	5847
August	1st to 10th	Day	15% 2 6% 17% 36% 24%	3819
	11th to 20th	Day	11% 2 3 13% 33% 38%	3760
	21st to end	Day	14% 3 4 11% 32% 36%	4179
September	1st to 10th	Day	17% 3 2 12% 32% 33%	2280
	11th to 20th	Day	18% 2 3 17% 32% 28%	2176
	21st to end	Day	20% 3 2 19% 30% 26%	2090
October	1st to 10th	Day	17% 3 11% 16% 21% 31%	3490
	11th to 20th	Day	18% 3 7% 16% 31% 25%	3166
	21st to end	Day	19% 2 4 11% 33% 31%	3448
November	1st to 10th	Day	19% 3 5% 19% 23% 32%	1777
	11th to 20th	Day	19% 3 7% 15% 22% 35%	1800
	21st to end	Day	22% 4 8% 14% 25% 28%	1800
December	1st to 10th	Day	21% 4 9% 16% 20% 30%	2950
	11th to 20th	Day	30% 3 7% 18% 27% 15%	3408
	21st to end	Day	35% 5% 14% 22% 16% 8%	4027
January	1st to 10th	Night	18% 1 6% 16% 32% 27%	6248
	11th to 20th	Night	23% 2 8% 19% 20% 29%	5880
	21st to end	Night	27% 3 6% 17% 26% 22%	6141
February	1st to 10th	Night	28% 2 5% 16% 24% 25%	2400
	11th to 20th	Night	27% 3 5% 19% 22% 24%	2339
	21st to end	Night	28% 3 6% 18% 23% 24%	1980
March	1st to 10th	Night	24% 4 9% 21% 28% 14%	5376
	11th to 20th	Night	28% 4 11% 26% 18% 13%	5220
	21st to end	Night	26% 3 15% 27% 21% 6%	5016
April	1st to 10th	Night	25% 3 9% 27% 23% 13%	2000
	11th to 20th	Night	21% 4 6% 25% 27% 17%	1979
	21st to end	Night	24% 3 9% 27% 25% 12%	2000
May	1st to 10th	Night	22% 7% 9% 21% 31% 10%	4557
	11th to 20th	Night	21% 4 9% 29% 29% 9%	4075
	21st to end	Night	14% 6% 7% 35% 31% 7%	4474
June	1st to 10th	Night	31% 3 11% 20% 25% 8%	1688
	11th to 20th	Night	21% 3 6% 17% 35% 18%	1600
	21st to end	Night	24% 5% 3 11% 29% 29%	1600
July	1st to 10th	Night	12% 2 4 16% 15% 51%	3600
	11th to 20th	Night	17% 3 6% 12% 32% 30%	3721
	21st to end	Night	12% 4 5% 21% 38% 22%	4716
August	1st to 10th	Night	14% 3 4 19% 34% 26%	3196
	11th to 20th	Night	12% 2 4 14% 32% 36%	3156
	21st to end	Night	15% 3 4 8% 35% 34%	3498
September	1st to 10th	Night	19% 4 3 7% 32% 35%	1900
	11th to 20th	Night	15% 3 2 14% 35% 31%	2004
	21st to end	Night	15% 3 3 16% 32% 32%	2090
October	1st to 10th	Night	15% 2 8% 17% 20% 39%	3490
	11th to 20th	Night	22% 2 6% 15% 28% 25%	3767
	21st to end	Night	15% 2 4 10% 31% 40%	4121
November	1st to 10th	Night	17% 3 5% 17% 24% 35%	2521
	11th to 20th	Night	18% 2 6% 15% 23% 37%	2600
	21st to end	Night	23% 3 6% 18% 20% 30%	2600
December	1st to 10th	Night	21% 3 6% 15% 24% 31%	4240
	11th to 20th	Night	31% 2 9% 15% 22% 22%	4811
	21st to end	Night	32% 5% 12% 28% 15% 8%	6925

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	12% 8% 18% 41% 19% 2	219
	11th to 20th	Day	10% 5% 20% 38% 25% 3	400
	21st to end	Day	10% 7% 20% 38% 21% 3	513
February	1st to 10th	Day	13% 4 24% 33% 20% 5%	500
	11th to 20th	Day	9% 7% 22% 39% 20% 4	495
	21st to end	Day	7% 6% 27% 39% 19% 2	414
March	1st to 10th	Day	10% 11% 22% 37% 19% 2	500
	11th to 20th	Day	10% 7% 21% 36% 21% 4	500
	21st to end	Day	7% 7% 22% 40% 20% 3	550
April	1st to 10th	Day	7% 4 18% 35% 30% 6%	500
	11th to 20th	Day	4 4 25% 37% 24% 5%	492
	21st to end	Day	3 5% 17% 38% 29% 8%	500
May	1st to 10th	Day	4 5% 17% 39% 31% 5%	503
	11th to 20th	Day	3 4 14% 36% 37% 5%	528
	21st to end	Day	4 4 17% 36% 32% 8%	573
June	1st to 10th	Day	5% 3 11% 34% 35% 12%	530
	11th to 20th	Day	3 12% 29% 44% 13%	530
	21st to end	Day	3 1 8% 25% 49% 13%	530
July	1st to 10th	Day	2 3 21% 53% 21%	526
	11th to 20th	Day	2 3 14% 52% 27%	524
	21st to end	Day	2 3 16% 54% 25%	575
August	1st to 10th	Day	2 1 3 20% 54% 21%	504
	11th to 20th	Day	1 1 5% 15% 57% 21%	477
	21st to end	Day	2 4 24% 51% 19%	539
September	1st to 10th	Day	2 1 4 29% 46% 17%	490
	11th to 20th	Day	3 2 9% 36% 41% 10%	490
	21st to end	Day	1 1 10% 33% 44% 11%	490
October	1st to 10th	Day	1 3 10% 36% 43% 7%	490
	11th to 20th	Day	2 3 11% 39% 37% 8%	490
	21st to end	Day	3 3 14% 35% 36% 10%	539
November	1st to 10th	Day	2 8% 12% 31% 36% 12%	313
	11th to 20th	Day	5% 2 18% 42% 27% 6%	300
	21st to end	Day	4 5% 19% 40% 25% 7%	300
December	1st to 10th	Day	8% 3 14% 45% 26% 4	300
	11th to 20th	Day	4 5% 19% 39% 29% 4	300
	21st to end	Day	13% 4 25% 38% 19% 1	225
January	1st to 10th	Night	10% 3 15% 41% 25% 7%	800
	11th to 20th	Night	7% 4 17% 39% 29% 4	800
	21st to end	Night	10% 4 18% 37% 27% 4	880
February	1st to 10th	Night	10% 5% 14% 36% 29% 7%	800
	11th to 20th	Night	10% 3 17% 36% 28% 7%	784
	21st to end	Night	10% 4 16% 41% 26% 3	660
March	1st to 10th	Night	8% 5% 17% 41% 25% 4	800
	11th to 20th	Night	6% 5% 13% 37% 31% 9%	800
	21st to end	Night	8% 3 16% 35% 32% 6%	880
April	1st to 10th	Night	7% 4 11% 35% 35% 7%	800
	11th to 20th	Night	4 4 16% 33% 37% 6%	791
	21st to end	Night	5% 3 10% 31% 41% 10%	800
May	1st to 10th	Night	6% 3 10% 31% 40% 11%	800
	11th to 20th	Night	4 4 10% 29% 40% 13%	591
	21st to end	Night	3 3 10% 29% 43% 13%	650
June	1st to 10th	Night	2 1 7% 30% 42% 18%	488
	11th to 20th	Night	2 1 5% 23% 50% 19%	400
	21st to end	Night	3 1 5% 23% 51% 18%	400
July	1st to 10th	Night	2 1 9% 53% 36%	390
	11th to 20th	Night	2 1 7% 42% 48%	439
	21st to end	Night	2 1 2 9% 48% 37%	822
August	1st to 10th	Night	3 2 13% 52% 30%	772
	11th to 20th	Night	3 1 3 8% 49% 37%	752
	21st to end	Night	2 3 13% 49% 34%	836
September	1st to 10th	Night	3 1 4 15% 49% 28%	760
	11th to 20th	Night	3 1 4 25% 48% 19%	760
	21st to end	Night	2 4 25% 51% 18%	760
October	1st to 10th	Night	4 1 6% 32% 47% 11%	760
	11th to 20th	Night	5% 2 8% 27% 46% 13%	760
	21st to end	Night	4 2 8% 33% 38% 14%	836
November	1st to 10th	Night	3 3 10% 34% 38% 12%	780
	11th to 20th	Night	4 1 10% 37% 38% 10%	800
	21st to end	Night	6% 2 16% 40% 30% 7%	800
December	1st to 10th	Night	8% 3 15% 37% 30% 7%	800
	11th to 20th	Night	12% 3 15% 40% 27% 4	800
	21st to end	Night	10% 4 20% 37% 25% 4	880

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