Atwood Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Atwood Observing Site clear sky chart from 2005-11-06 to 2026-03-03.

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	11% 7% 3 6% 4 4 4 4 5% 7% 45%	130516
	Night	10% 8% 3 6% 4 4 4 3 5% 6% 49%	130883
Transparency	Day	37% 6% 11% 24% 18% 5%	134124
	Night	35% 5% 11% 24% 20% 6%	135351
Seeing	Day	17% 15% 29% 24% 13% 3	17170
	Night	17% 14% 28% 26% 13% 1	27130

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	12% 10% 4 5% 3 4 4 3 4 6% 45%	5289
	11th to 20th	Day	6% 7% 3 7% 4 3 3 3 4 5% 56%	5310
	21st to end	Day	16% 8% 3 6% 3 2 3 3 3 4 48%	5812
February	1st to 10th	Day	14% 10% 3 6% 5% 4 3 3 4 5% 42%	1890
	11th to 20th	Day	12% 10% 3 8% 5% 5% 3 3 5% 5% 41%	1885
	21st to end	Day	13% 11% 4 6% 4 4 4 3 4 6% 41%	1847
March	1st to 10th	Day	13% 9% 3 6% 4 4 4 3 5% 6% 44%	4958
	11th to 20th	Day	8% 6% 2 5% 4 4 5% 4 5% 6% 52%	4980
	21st to end	Day	12% 9% 3 7% 5% 5% 4 4 6% 8% 38%	5462
April	1st to 10th	Day	12% 7% 3 7% 5% 4 5% 4 5% 6% 42%	2350
	11th to 20th	Day	14% 8% 3 6% 4 4 5% 3 5% 6% 41%	2374
	21st to end	Day	17% 9% 3 7% 5% 5% 5% 4 4 4 38%	2400
May	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 4 6% 6% 33%	5681
	11th to 20th	Day	11% 7% 3 8% 6% 5% 5% 4 6% 7% 38%	5862
	21st to end	Day	18% 7% 3 8% 5% 5% 5% 4 5% 6% 34%	6434
June	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 4 6% 9% 42%	2645
	11th to 20th	Day	5% 5% 2 7% 6% 5% 5% 6% 7% 9% 42%	2729
	21st to end	Day	4 4 1 5% 5% 5% 6% 4 7% 9% 49%	2600
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 7% 5% 7% 10% 39%	5900
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 4 6% 9% 50%	5900
	21st to end	Day	6% 4 2 5% 5% 6% 6% 6% 8% 9% 42%	6422
August	1st to 10th	Day	5% 4 1 4 4 4 5% 4 6% 9% 55%	4234
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 7% 8% 44%	4022
	21st to end	Day	5% 4 2 5% 5% 5% 5% 4 6% 8% 52%	4312
September	1st to 10th	Day	11% 6% 2 6% 3 4 4 4 5% 6% 49%	2400
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 43%	2310
	21st to end	Day	8% 4 2 5% 4 4 4 5% 5% 7% 52%	2130
October	1st to 10th	Day	8% 5% 2 5% 4 3 4 3 5% 6% 56%	2000
	11th to 20th	Day	11% 6% 2 5% 3 2 3 2 4 5% 57%	2000
	21st to end	Day	18% 7% 1 5% 3 3 3 3 3 4 50%	2200
November	1st to 10th	Day	10% 7% 2 5% 4 3 2 2 5% 5% 56%	2058
	11th to 20th	Day	9% 8% 2 5% 3 3 4 4 5% 6% 52%	2084
	21st to end	Day	13% 8% 4 6% 4 4 3 3 5% 5% 45%	1700
December	1st to 10th	Day	10% 7% 3 5% 4 3 3 3 4 5% 53%	2690
	11th to 20th	Day	9% 10% 3 7% 4 4 4 3 4 5% 48%	3088
	21st to end	Day	13% 16% 6% 7% 4 4 4 3 5% 5% 34%	4558
January	1st to 10th	Night	10% 8% 4 9% 5% 4 4 3 4 5% 43%	7634
	11th to 20th	Night	7% 6% 2 5% 4 3 3 2 4 6% 58%	7680
	21st to end	Night	15% 10% 4 8% 4 4 3 3 4 5% 42%	8402
February	1st to 10th	Night	13% 10% 3 7% 4 4 3 3 5% 6% 41%	2730
	11th to 20th	Night	13% 8% 3 6% 4 4 3 3 5% 6% 43%	2621
	21st to end	Night	10% 8% 3 7% 4 4 3 3 4 5% 47%	1976
March	1st to 10th	Night	16% 11% 3 6% 4 4 3 2 4 4 43%	4941
	11th to 20th	Night	9% 10% 2 5% 3 3 3 3 5% 7% 50%	4980
	21st to end	Night	11% 9% 3 7% 5% 4 4 3 5% 9% 42%	5467
April	1st to 10th	Night	10% 8% 2 5% 4 4 4 3 5% 7% 48%	2049
	11th to 20th	Night	12% 9% 2 5% 4 3 3 3 3 6% 49%	1975
	21st to end	Night	17% 7% 3 6% 4 4 3 3 5% 7% 42%	2000
May	1st to 10th	Night	15% 10% 3 6% 4 4 3 3 5% 7% 41%	4211
	11th to 20th	Night	12% 8% 3 8% 4 4 5% 4 7% 7% 38%	4053
	21st to end	Night	19% 10% 2 6% 4 3 4 3 5% 7% 37%	4443
June	1st to 10th	Night	7% 4 2 4 3 4 3 4 5% 10% 53%	1734
	11th to 20th	Night	5% 4 1 3 3 3 3 3 7% 12% 56%	1648
	21st to end	Night	3 3 1 4 3 3 4 3 6% 8% 62%	1800
July	1st to 10th	Night	7% 4 2 6% 5% 5% 5% 4 7% 10% 44%	4078
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 8% 64%	4077
	21st to end	Night	5% 5% 2 5% 4 3 4 4 6% 9% 51%	4449
August	1st to 10th	Night	3 2 1 3 3 3 4 4 7% 9% 61%	2926
	11th to 20th	Night	5% 4 2 5% 4 4 4 3 7% 9% 53%	3029
	21st to end	Night	4 3 1 3 4 3 3 3 6% 8% 61%	3586
September	1st to 10th	Night	8% 4 2 4 2 4 4 3 5% 8% 57%	2000
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 5% 6% 49%	2085
	21st to end	Night	8% 6% 2 4 3 3 3 3 5% 6% 56%	2249
October	1st to 10th	Night	5% 4 2 5% 2 3 3 3 5% 8% 61%	2400
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 4 5% 59%	2400
	21st to end	Night	15% 9% 3 4 2 3 3 2 3 4 51%	2640
November	1st to 10th	Night	11% 7% 2 4 3 3 2 2 4 4 57%	2460
	11th to 20th	Night	10% 6% 2 4 3 2 4 3 5% 5% 56%	2531
	21st to end	Night	13% 9% 3 5% 3 3 3 3 5% 6% 48%	2917
December	1st to 10th	Night	9% 9% 3 5% 3 3 3 3 4 6% 52%	4720
	11th to 20th	Night	10% 11% 4 7% 4 4 4 2 4 3 47%	5339
	21st to end	Night	16% 14% 6% 7% 4 4 4 3 4 5% 35%	6653

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	41% 3 8% 23% 22% 3	5290
	11th to 20th	Day	31% 4 7% 23% 29% 6%	5310
	21st to end	Day	40% 3 5% 20% 23% 9%	5812
February	1st to 10th	Day	45% 6% 9% 22% 15% 4	1890
	11th to 20th	Day	45% 4 9% 20% 18% 4	1894
	21st to end	Day	45% 4 7% 17% 18% 9%	1847
March	1st to 10th	Day	41% 5% 7% 18% 15% 15%	4958
	11th to 20th	Day	31% 5% 8% 19% 24% 12%	4980
	21st to end	Day	43% 5% 10% 27% 14% 1	5464
April	1st to 10th	Day	40% 7% 11% 20% 19% 3	2350
	11th to 20th	Day	43% 7% 9% 20% 17% 4	2374
	21st to end	Day	48% 6% 8% 22% 14% 2	2400
May	1st to 10th	Day	49% 6% 9% 21% 14% 1	5716
	11th to 20th	Day	42% 8% 11% 24% 13% 1	5866
	21st to end	Day	49% 7% 11% 17% 16% 1	6447
June	1st to 10th	Day	37% 9% 14% 26% 13% 1	2645
	11th to 20th	Day	35% 9% 14% 26% 15% 2	2729
	21st to end	Day	29% 8% 16% 27% 16% 4	2600
July	1st to 10th	Day	36% 7% 16% 26% 12% 2	5836
	11th to 20th	Day	29% 7% 16% 33% 14% 1	5831
	21st to end	Day	33% 8% 18% 29% 11% 1	6240
August	1st to 10th	Day	25% 7% 16% 30% 18% 5%	4220
	11th to 20th	Day	37% 6% 16% 29% 12% 1	4061
	21st to end	Day	29% 7% 14% 33% 13% 3	4311
September	1st to 10th	Day	35% 6% 11% 23% 17% 8%	2400
	11th to 20th	Day	38% 7% 10% 21% 22% 3	2310
	21st to end	Day	31% 5% 11% 23% 26% 4	2130
October	1st to 10th	Day	18% 3 6% 15% 38% 20%	3260
	11th to 20th	Day	20% 3 4 21% 35% 17%	3240
	21st to end	Day	30% 4 9% 30% 22% 5%	3538
November	1st to 10th	Day	32% 3 8% 26% 22% 9%	2058
	11th to 20th	Day	33% 5% 8% 22% 22% 10%	2084
	21st to end	Day	40% 4 8% 18% 24% 5%	1700
December	1st to 10th	Day	34% 4 7% 28% 24% 3	2690
	11th to 20th	Day	39% 4 6% 23% 25% 2	3085
	21st to end	Day	51% 5% 8% 20% 14% 3	4558
January	1st to 10th	Night	42% 3 9% 25% 17% 3	7635
	11th to 20th	Night	27% 5% 8% 19% 33% 8%	7680
	21st to end	Night	45% 3 7% 18% 21% 6%	8402
February	1st to 10th	Night	44% 5% 9% 22% 16% 4	2730
	11th to 20th	Night	42% 5% 9% 21% 18% 5%	2622
	21st to end	Night	39% 5% 8% 17% 21% 10%	1976
March	1st to 10th	Night	44% 5% 6% 15% 18% 12%	4941
	11th to 20th	Night	34% 5% 7% 24% 19% 12%	4980
	21st to end	Night	40% 5% 12% 27% 14% 2	5467
April	1st to 10th	Night	36% 6% 10% 24% 19% 5%	2049
	11th to 20th	Night	37% 5% 8% 25% 20% 5%	1975
	21st to end	Night	42% 7% 11% 24% 15% 2	2000
May	1st to 10th	Night	43% 5% 14% 24% 12% 2	4240
	11th to 20th	Night	42% 8% 14% 22% 11% 3	4054
	21st to end	Night	46% 7% 13% 18% 13% 3	4452
June	1st to 10th	Night	26% 7% 15% 32% 17% 2	1734
	11th to 20th	Night	20% 9% 15% 35% 19% 2	1648
	21st to end	Night	20% 7% 17% 29% 22% 5%	1800
July	1st to 10th	Night	33% 8% 19% 24% 14% 2	4035
	11th to 20th	Night	20% 5% 18% 39% 17% 1	4026
	21st to end	Night	28% 7% 17% 34% 14%	4320
August	1st to 10th	Night	19% 6% 18% 34% 22% 1	2916
	11th to 20th	Night	27% 6% 22% 28% 16% 1	3056
	21st to end	Night	22% 5% 15% 36% 19% 3	3586
September	1st to 10th	Night	27% 5% 12% 29% 18% 8%	2000
	11th to 20th	Night	34% 6% 12% 22% 22% 4	2085
	21st to end	Night	28% 5% 10% 21% 29% 5%	2249
October	1st to 10th	Night	14% 4 7% 14% 35% 26%	3920
	11th to 20th	Night	19% 2 6% 18% 33% 21%	3899
	21st to end	Night	29% 3 6% 30% 23% 8%	4252
November	1st to 10th	Night	31% 4 8% 23% 26% 9%	2460
	11th to 20th	Night	29% 5% 8% 24% 26% 9%	2531
	21st to end	Night	37% 5% 9% 21% 23% 5%	2917
December	1st to 10th	Night	34% 4 9% 32% 18% 3	4720
	11th to 20th	Night	42% 3 5% 21% 21% 7%	5341
	21st to end	Night	52% 4 9% 21% 11% 4	6653

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	27% 25% 32% 14% 2	330
	11th to 20th	Day	25% 32% 34% 7% 1	330
	21st to end	Day	25% 31% 31% 10% 2	363
February	1st to 10th	Day	30% 26% 32% 9% 2	330
	11th to 20th	Day	35% 29% 28% 7% 1	330
	21st to end	Day	36% 29% 28% 5% 2	273
March	1st to 10th	Day	28% 23% 36% 13% 1	306
	11th to 20th	Day	34% 24% 31% 10% 1	300
	21st to end	Day	30% 32% 28% 8% 2	330
April	1st to 10th	Day	26% 26% 30% 14% 4 1	450
	11th to 20th	Day	27% 25% 30% 14% 4	492
	21st to end	Day	26% 22% 32% 14% 5% 1	500
May	1st to 10th	Day	20% 22% 38% 16% 4	623
	11th to 20th	Day	17% 16% 35% 22% 8% 2	725
	21st to end	Day	15% 14% 38% 20% 11% 3	789
June	1st to 10th	Day	10% 9% 26% 32% 20% 3	730
	11th to 20th	Day	6% 5% 32% 35% 19% 3	730
	21st to end	Day	4 5% 36% 35% 17% 3	730
July	1st to 10th	Day	9% 3 22% 33% 27% 6%	721
	11th to 20th	Day	5% 3 23% 37% 26% 6%	718
	21st to end	Day	7% 2 16% 36% 30% 8%	794
August	1st to 10th	Day	9% 3 20% 40% 24% 4	726
	11th to 20th	Day	12% 3 15% 36% 28% 6%	631
	21st to end	Day	8% 3 17% 31% 29% 11%	583
September	1st to 10th	Day	10% 4 17% 35% 26% 7%	530
	11th to 20th	Day	10% 5% 20% 37% 21% 7%	440
	21st to end	Day	12% 9% 29% 30% 16% 3	330
October	1st to 10th	Day	10% 11% 34% 31% 12% 3	330
	11th to 20th	Day	15% 18% 42% 18% 7% 1	330
	21st to end	Day	33% 13% 35% 16% 3 1	363
November	1st to 10th	Day	18% 22% 36% 16% 6% 2	330
	11th to 20th	Day	19% 24% 36% 16% 4 1	330
	21st to end	Day	28% 21% 31% 15% 4 1	330
December	1st to 10th	Day	23% 29% 29% 13% 4 1	330
	11th to 20th	Day	25% 27% 33% 10% 5%	330
	21st to end	Day	28% 26% 35% 10% 2	363
January	1st to 10th	Night	21% 22% 34% 19% 4	1050
	11th to 20th	Night	22% 27% 33% 16% 3	1050
	21st to end	Night	19% 25% 33% 20% 2	1155
February	1st to 10th	Night	23% 21% 35% 18% 2	1050
	11th to 20th	Night	26% 23% 35% 14% 2	987
	21st to end	Night	24% 25% 33% 16% 1	592
March	1st to 10th	Night	23% 19% 32% 22% 4	606
	11th to 20th	Night	26% 17% 32% 23% 3	600
	21st to end	Night	21% 21% 34% 19% 5%	660
April	1st to 10th	Night	17% 23% 32% 24% 4	600
	11th to 20th	Night	19% 21% 30% 24% 6%	593
	21st to end	Night	18% 22% 33% 21% 7% 1	600
May	1st to 10th	Night	16% 12% 34% 31% 8%	600
	11th to 20th	Night	17% 12% 27% 29% 13% 2	591
	21st to end	Night	13% 10% 29% 29% 15% 4	650
June	1st to 10th	Night	9% 6% 19% 38% 28% 2	600
	11th to 20th	Night	6% 2 27% 38% 23% 3	600
	21st to end	Night	3 3 25% 40% 27% 2	600
July	1st to 10th	Night	6% 2 16% 33% 37% 7%	585
	11th to 20th	Night	5% 2 16% 36% 35% 6%	582
	21st to end	Night	6% 2 10% 39% 35% 8%	657
August	1st to 10th	Night	6% 2 14% 39% 34% 5%	597
	11th to 20th	Night	7% 2 16% 37% 35% 4	594
	21st to end	Night	7% 1 14% 35% 38% 5%	660
September	1st to 10th	Night	10% 2 14% 36% 36% 3	600
	11th to 20th	Night	14% 1 13% 38% 29% 5%	600
	21st to end	Night	12% 5% 21% 31% 28% 3	649
October	1st to 10th	Night	8% 5% 26% 34% 23% 3	800
	11th to 20th	Night	14% 15% 32% 28% 11%	800
	21st to end	Night	22% 16% 29% 24% 9% 1	880
November	1st to 10th	Night	16% 16% 32% 26% 10%	820
	11th to 20th	Night	18% 15% 30% 29% 7%	840
	21st to end	Night	22% 18% 31% 21% 7%	1027
December	1st to 10th	Night	20% 17% 38% 20% 4	1050
	11th to 20th	Night	22% 19% 32% 24% 4	1050
	21st to end	Night	23% 22% 31% 20% 3	1155

Page generated at 2026-02-28 00:40:19. Version ac07

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