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 2024-11-04.

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%	104895
	Night	5% 4 1 4 2 3 3 3 4 6% 65%	102921
Transparency	Day	22% 3 7% 20% 26% 22%	104656
	Night	22% 3 7% 19% 25% 24%	102749
Seeing	Day	5% 3 13% 32% 36% 10%	15653
	Night	6% 2 10% 29% 38% 14%	25454

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	5% 3 1 3 2 2 3 2 4 6% 69%	2570
	11th to 20th	Day	8% 5% 2 5% 3 4 3 3 5% 9% 53%	2860
	21st to end	Day	13% 5% 1 4 3 3 3 3 5% 8% 52%	3372
February	1st to 10th	Day	10% 4 2 3 3 2 2 4 3 6% 61%	1900
	11th to 20th	Day	8% 3 1 3 3 3 3 3 5% 8% 61%	1850
	21st to end	Day	8% 4 1 5% 3 4 4 3 4 6% 59%	1570
March	1st to 10th	Day	9% 5% 2 6% 3 4 4 4 5% 6% 52%	3138
	11th to 20th	Day	4 3 1 4 2 3 3 3 5% 9% 62%	3330
	21st to end	Day	7% 4 1 4 4 4 5% 4 7% 9% 50%	4154
April	1st to 10th	Day	5% 5% 2 4 3 3 3 4 5% 8% 58%	2280
	11th to 20th	Day	4 4 1 4 2 2 3 3 5% 7% 65%	2249
	21st to end	Day	3 4 1 3 2 3 3 3 5% 7% 66%	2280
May	1st to 10th	Day	3 3 2 3 2 2 3 3 4 7% 69%	3800
	11th to 20th	Day	2 10% 3 5% 3 3 3 3 5% 9% 54%	4067
	21st to end	Day	2 6% 2 4 3 3 3 3 5% 10% 60%	4477
June	1st to 10th	Day	4 6% 3 5% 3 3 3 3 5% 7% 58%	2570
	11th to 20th	Day	2 5% 2 3 2 2 3 3 5% 9% 64%	2660
	21st to end	Day	1 3 1 3 2 2 3 2 5% 7% 70%	2660
July	1st to 10th	Day	1 2 1 2 1 1 2 2 4 8% 75%	4420
	11th to 20th	Day	1 2 1 3 2 2 3 3 4 7% 72%	4331
	21st to end	Day	1 2 2 1 2 2 2 3 4 82%	4190
August	1st to 10th	Day	2 2 1 2 1 2 1 2 3 6% 79%	3783
	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%	1714
	11th to 20th	Day	5% 3 1 3 2 2 3 3 5% 5% 68%	1710
	21st to end	Day	4 3 2 4 3 3 3 4 5% 7% 63%	1710
December	1st to 10th	Day	7% 4 2 4 4 3 4 4 4 7% 56%	1710
	11th to 20th	Day	15% 7% 2 5% 4 4 3 3 5% 7% 44%	2169
	21st to end	Day	12% 5% 2 6% 5% 5% 4 4 6% 8% 44%	2817
January	1st to 10th	Night	7% 4 2 4 2 3 3 2 3 5% 65%	4374
	11th to 20th	Night	8% 6% 2 5% 4 3 3 3 5% 7% 54%	4110
	21st to end	Night	12% 6% 2 4 3 2 3 3 4 6% 54%	4298
February	1st to 10th	Night	7% 5% 2 5% 2 2 3 3 5% 5% 61%	2280
	11th to 20th	Night	9% 5% 1 4 2 3 3 2 4 6% 61%	2218
	21st to end	Night	10% 4 2 4 4 3 4 2 4 6% 58%	1884
March	1st to 10th	Night	6% 5% 2 6% 3 4 4 4 7% 8% 51%	3736
	11th to 20th	Night	4 5% 1 4 3 4 4 3 6% 7% 61%	3645
	21st to end	Night	9% 5% 2 4 3 3 3 3 4 7% 59%	3498
April	1st to 10th	Night	6% 5% 2 4 3 3 3 3 5% 7% 60%	1900
	11th to 20th	Night	3 5% 2 3 2 3 4 3 4 6% 66%	1879
	21st to end	Night	4 5% 1 3 2 3 3 3 5% 7% 65%	1900
May	1st to 10th	Night	3 4 2 3 3 3 2 2 4 5% 70%	3180
	11th to 20th	Night	2 6% 2 6% 4 4 4 3 5% 7% 57%	2834
	21st to end	Night	1 4 2 3 2 2 2 2 4 6% 71%	3110
June	1st to 10th	Night	1 5% 2 6% 3 4 5% 4 5% 7% 59%	1604
	11th to 20th	Night	2 5% 1 4 2 2 2 3 6% 7% 67%	1520
	21st to end	Night	1 4 1 4 2 2 3 3 6% 6% 67%	1520
July	1st to 10th	Night	1 2 1 2 1 2 3 3 3 6% 77%	2540
	11th to 20th	Night	1 1 1 3 3 3 3 4 5% 7% 69%	2627
	21st to end	Night	1 1 1 2 1 2 2 2 4 5% 80%	3380
August	1st to 10th	Night	2 1 1 2 2 2 2 2 4 7% 76%	3166
	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	3 3 2 2 3 2 3 3 5% 6% 70%	2436
	11th to 20th	Night	4 3 2 4 2 2 2 2 5% 6% 69%	2470
	21st to end	Night	4 4 1 3 3 3 3 4 6% 6% 62%	2470
December	1st to 10th	Night	8% 4 1 5% 4 3 3 3 5% 6% 60%	2470
	11th to 20th	Night	16% 7% 2 5% 3 3 3 2 5% 7% 46%	3041
	21st to end	Night	12% 6% 2 5% 4 4 5% 4 5% 6% 45%	4835

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	19% 2 5% 27% 30% 18%	2570
	11th to 20th	Day	31% 2 15% 19% 18% 15%	2860
	21st to end	Day	31% 3 5% 22% 23% 16%	3372
February	1st to 10th	Day	26% 2 4 17% 26% 24%	1900
	11th to 20th	Day	24% 2 7% 19% 27% 22%	1860
	21st to end	Day	26% 4 7% 24% 23% 17%	1570
March	1st to 10th	Day	29% 3 9% 26% 16% 16%	3138
	11th to 20th	Day	20% 4 15% 31% 16% 15%	3330
	21st to end	Day	30% 4 13% 29% 16% 9%	4154
April	1st to 10th	Day	25% 3 11% 28% 21% 12%	2280
	11th to 20th	Day	20% 2 8% 26% 28% 16%	2252
	21st to end	Day	19% 4 9% 26% 30% 11%	2280
May	1st to 10th	Day	19% 4 9% 20% 36% 12%	3800
	11th to 20th	Day	33% 2 8% 26% 25% 7%	4070
	21st to end	Day	28% 3 8% 33% 24% 4	4477
June	1st to 10th	Day	29% 2 12% 22% 25% 10%	2570
	11th to 20th	Day	20% 3 7% 17% 33% 20%	2660
	21st to end	Day	23% 3 3 11% 32% 29%	2660
July	1st to 10th	Day	18% 3 5% 11% 16% 48%	4351
	11th to 20th	Day	19% 3 5% 14% 27% 31%	4256
	21st to end	Day	12% 2 7% 22% 29% 29%	4058
August	1st to 10th	Day	15% 2 6% 17% 36% 24%	3770
	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	20% 3 5% 20% 23% 30%	1714
	11th to 20th	Day	19% 3 7% 15% 20% 35%	1710
	21st to end	Day	20% 3 7% 15% 26% 29%	1710
December	1st to 10th	Day	27% 3 6% 18% 23% 23%	1710
	11th to 20th	Day	39% 3 7% 16% 21% 15%	2168
	21st to end	Day	39% 3 10% 22% 15% 11%	2817
January	1st to 10th	Night	22% 2 7% 17% 31% 22%	4374
	11th to 20th	Night	32% 2 10% 22% 17% 17%	4110
	21st to end	Night	31% 3 6% 20% 22% 18%	4298
February	1st to 10th	Night	27% 2 5% 15% 24% 27%	2280
	11th to 20th	Night	27% 3 5% 18% 22% 25%	2219
	21st to end	Night	29% 3 6% 19% 23% 21%	1884
March	1st to 10th	Night	26% 4 9% 26% 23% 11%	3736
	11th to 20th	Night	23% 4 14% 27% 18% 15%	3645
	21st to end	Night	26% 4 14% 26% 21% 8%	3498
April	1st to 10th	Night	27% 3 9% 27% 22% 13%	1900
	11th to 20th	Night	21% 4 6% 25% 27% 17%	1879
	21st to end	Night	24% 3 9% 25% 26% 13%	1900
May	1st to 10th	Night	18% 6% 6% 22% 35% 12%	3180
	11th to 20th	Night	26% 5% 9% 28% 25% 7%	2835
	21st to end	Night	16% 7% 7% 36% 30% 4	3110
June	1st to 10th	Night	32% 3 12% 20% 25% 9%	1604
	11th to 20th	Night	21% 3 6% 17% 35% 17%	1520
	21st to end	Night	25% 5% 3 11% 29% 28%	1520
July	1st to 10th	Night	13% 3 4 12% 15% 53%	2500
	11th to 20th	Night	15% 3 4 13% 28% 37%	2579
	21st to end	Night	11% 3 4 24% 27% 29%	3274
August	1st to 10th	Night	14% 3 4 19% 34% 26%	3156
	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% 23% 34%	2436
	11th to 20th	Night	19% 3 6% 15% 21% 37%	2470
	21st to end	Night	23% 3 6% 18% 20% 31%	2470
December	1st to 10th	Night	26% 3 6% 17% 21% 28%	2470
	11th to 20th	Night	39% 2 7% 15% 21% 16%	3041
	21st to end	Night	37% 3 8% 24% 16% 12%	4835

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	13% 7% 17% 42% 19% 2	198
	11th to 20th	Day	11% 4 18% 39% 26% 2	370
	21st to end	Day	11% 7% 19% 38% 22% 3	472
February	1st to 10th	Day	12% 4 22% 35% 22% 6%	460
	11th to 20th	Day	8% 7% 22% 38% 21% 5%	455
	21st to end	Day	7% 7% 25% 39% 20% 2	382
March	1st to 10th	Day	8% 10% 22% 38% 20% 2	460
	11th to 20th	Day	9% 6% 21% 36% 23% 4	460
	21st to end	Day	7% 7% 22% 40% 22% 2	506
April	1st to 10th	Day	7% 3 17% 37% 30% 6%	460
	11th to 20th	Day	4 4 25% 37% 25% 6%	452
	21st to end	Day	2 5% 17% 37% 31% 9%	460
May	1st to 10th	Day	3 4 16% 39% 32% 5%	463
	11th to 20th	Day	3 3 13% 36% 38% 6%	488
	21st to end	Day	4 4 17% 35% 33% 8%	529
June	1st to 10th	Day	5% 2 9% 33% 37% 13%	490
	11th to 20th	Day	3 10% 28% 45% 14%	490
	21st to end	Day	3 1 6% 25% 51% 14%	490
July	1st to 10th	Day	2 2 18% 55% 22%	486
	11th to 20th	Day	2 3 13% 52% 29%	484
	21st to end	Day	1 2 14% 56% 27%	531
August	1st to 10th	Day	2 1 2 19% 55% 21%	487
	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 7% 11% 33% 36% 12%	292
	11th to 20th	Day	5% 2 16% 42% 29% 6%	270
	21st to end	Day	3 5% 17% 39% 28% 8%	270
December	1st to 10th	Day	9% 4 15% 43% 26% 3	270
	11th to 20th	Day	5% 4 19% 38% 30% 4	270
	21st to end	Day	13% 4 24% 37% 21% 1	203
January	1st to 10th	Night	10% 3 14% 40% 26% 7%	760
	11th to 20th	Night	8% 4 16% 39% 29% 4	760
	21st to end	Night	11% 4 18% 36% 28% 4	836
February	1st to 10th	Night	9% 5% 13% 36% 30% 7%	760
	11th to 20th	Night	9% 2 16% 36% 29% 7%	744
	21st to end	Night	11% 4 16% 39% 26% 4	628
March	1st to 10th	Night	7% 5% 16% 41% 26% 4	760
	11th to 20th	Night	6% 4 12% 36% 32% 9%	760
	21st to end	Night	8% 3 17% 34% 33% 6%	836
April	1st to 10th	Night	7% 4 11% 36% 36% 7%	760
	11th to 20th	Night	4 4 15% 32% 38% 7%	751
	21st to end	Night	5% 3 10% 30% 41% 11%	760
May	1st to 10th	Night	5% 3 11% 30% 41% 10%	760
	11th to 20th	Night	4 3 9% 28% 42% 14%	561
	21st to end	Night	3 3 9% 29% 43% 13%	617
June	1st to 10th	Night	2 1 7% 28% 43% 19%	464
	11th to 20th	Night	2 1 5% 21% 51% 20%	380
	21st to end	Night	3 1 5% 22% 52% 18%	380
July	1st to 10th	Night	2 1 6% 53% 38%	370
	11th to 20th	Night	2 1 6% 41% 49%	416
	21st to end	Night	2 1 2 7% 48% 39%	781
August	1st to 10th	Night	3 2 13% 52% 30%	756
	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% 33% 38% 12%	754
	11th to 20th	Night	4 1 9% 37% 39% 10%	760
	21st to end	Night	6% 2 15% 39% 31% 7%	760
December	1st to 10th	Night	8% 3 15% 37% 30% 7%	760
	11th to 20th	Night	12% 3 15% 38% 27% 4	760
	21st to end	Night	10% 4 20% 36% 25% 4	836

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