Atwater
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Atwater clear sky chart from 2005-11-06 to 2026-03-18.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 38%	131786
	Night	11% 8% 3 9% 5% 4 5% 4 5% 7% 39%	129966
Transparency	Day	42% 7% 14% 25% 11% 1	135361
	Night	43% 6% 13% 24% 12% 1	134469
Seeing	Day	15% 9% 31% 28% 14% 3	17828
	Night	15% 9% 33% 32% 11% 1	27893

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	8% 14% 4 16% 12% 7% 7% 4 5% 6% 17%	4124
	11th to 20th	Day	5% 8% 5% 14% 8% 7% 9% 7% 10% 7% 21%	4985
	21st to end	Day	3 11% 6% 10% 6% 6% 6% 5% 6% 7% 33%	5812
February	1st to 10th	Day	8% 12% 4 9% 7% 6% 6% 5% 6% 8% 29%	1890
	11th to 20th	Day	8% 11% 4 9% 7% 5% 6% 5% 6% 7% 32%	1844
	21st to end	Day	9% 11% 3 10% 6% 5% 5% 5% 6% 8% 32%	1594
March	1st to 10th	Day	11% 9% 3 7% 4 4 4 3 5% 7% 42%	5034
	11th to 20th	Day	9% 9% 3 8% 4 4 5% 4 5% 8% 42%	5057
	21st to end	Day	21% 12% 4 10% 7% 5% 4 3 4 6% 25%	5462
April	1st to 10th	Day	17% 9% 4 7% 4 4 3 4 5% 7% 37%	2360
	11th to 20th	Day	18% 12% 4 7% 5% 3 4 3 4 6% 32%	2483
	21st to end	Day	24% 9% 3 7% 4 3 4 4 4 5% 33%	2600
May	1st to 10th	Day	13% 6% 2 5% 4 3 4 3 4 6% 50%	5924
	11th to 20th	Day	18% 8% 2 5% 3 3 3 3 4 6% 45%	6319
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 7% 7% 37%	6935
June	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 8% 45%	2842
	11th to 20th	Day	15% 7% 3 7% 5% 5% 4 4 6% 7% 38%	3000
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 7% 9% 44%	3000
July	1st to 10th	Day	8% 4 2 5% 4 4 5% 5% 7% 11% 45%	6800
	11th to 20th	Day	8% 4 1 4 3 3 3 3 5% 9% 56%	6547
	21st to end	Day	7% 5% 2 6% 4 4 4 5% 7% 9% 46%	6611
August	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 45%	4234
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	4171
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 8% 49%	4586
September	1st to 10th	Day	11% 4 2 5% 3 4 4 3 6% 8% 50%	2400
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 7% 7% 46%	2270
	21st to end	Day	16% 5% 2 4 4 3 4 2 4 7% 49%	2166
October	1st to 10th	Day	17% 6% 2 6% 3 4 4 3 5% 7% 44%	2000
	11th to 20th	Day	16% 9% 2 6% 4 3 4 3 6% 6% 41%	2000
	21st to end	Day	17% 12% 3 6% 4 4 3 3 4 6% 37%	2187
November	1st to 10th	Day	13% 11% 3 8% 4 4 4 3 5% 7% 37%	1701
	11th to 20th	Day	9% 12% 6% 13% 7% 5% 4 4 4 6% 31%	1680
	21st to end	Day	13% 10% 5% 11% 6% 4 5% 4 5% 7% 31%	1680
December	1st to 10th	Day	6% 10% 4 13% 10% 8% 7% 5% 6% 7% 24%	2690
	11th to 20th	Day	9% 15% 7% 11% 6% 5% 5% 4 4 6% 28%	3088
	21st to end	Day	17% 17% 5% 21% 5% 3 3 3 3 5% 17%	3710
January	1st to 10th	Night	8% 9% 5% 15% 9% 7% 5% 4 6% 7% 26%	8799
	11th to 20th	Night	5% 10% 4 11% 9% 7% 10% 8% 7% 5% 23%	7983
	21st to end	Night	4 11% 6% 9% 5% 5% 5% 4 6% 7% 37%	8402
February	1st to 10th	Night	7% 11% 4 10% 6% 6% 5% 4 5% 7% 34%	2730
	11th to 20th	Night	6% 8% 4 8% 6% 5% 5% 5% 7% 7% 40%	2663
	21st to end	Night	10% 9% 3 8% 5% 5% 5% 5% 6% 7% 37%	2229
March	1st to 10th	Night	10% 6% 2 6% 4 4 3 3 5% 8% 48%	5040
	11th to 20th	Night	9% 7% 2 7% 5% 4 5% 4 7% 10% 41%	5057
	21st to end	Night	23% 10% 4 9% 5% 4 4 3 4 5% 29%	5467
April	1st to 10th	Night	14% 9% 3 8% 5% 3 5% 3 6% 6% 40%	2040
	11th to 20th	Night	18% 10% 3 7% 3 3 4 4 5% 6% 37%	1863
	21st to end	Night	20% 9% 3 7% 4 3 3 3 5% 6% 36%	1800
May	1st to 10th	Night	12% 5% 2 4 3 3 3 2 5% 5% 55%	3989
	11th to 20th	Night	15% 7% 2 4 4 4 4 3 4 6% 48%	3596
	21st to end	Night	18% 8% 3 6% 4 4 4 3 6% 6% 39%	3942
June	1st to 10th	Night	10% 5% 1 5% 3 4 4 4 7% 9% 48%	1552
	11th to 20th	Night	9% 5% 2 5% 4 5% 3 5% 7% 8% 48%	1400
	21st to end	Night	9% 4 2 4 4 3 4 3 6% 6% 55%	1400
July	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 10% 50%	3178
	11th to 20th	Night	5% 3 2 5% 4 3 4 3 6% 9% 56%	3414
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 8% 49%	4256
August	1st to 10th	Night	12% 6% 2 6% 3 3 4 4 4 7% 48%	2926
	11th to 20th	Night	14% 4 2 5% 3 4 4 3 5% 7% 50%	2888
	21st to end	Night	8% 4 2 5% 4 4 3 3 5% 8% 54%	3320
September	1st to 10th	Night	8% 3 2 5% 3 3 3 4 6% 7% 57%	2000
	11th to 20th	Night	9% 5% 2 4 3 4 4 3 5% 7% 53%	2126
	21st to end	Night	9% 6% 2 5% 3 3 3 2 4 6% 56%	2213
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 6% 51%	2400
	11th to 20th	Night	15% 9% 2 5% 4 3 4 3 5% 6% 44%	2400
	21st to end	Night	17% 11% 3 7% 4 3 3 2 4 6% 39%	2650
November	1st to 10th	Night	11% 8% 3 7% 5% 4 4 4 5% 7% 42%	2816
	11th to 20th	Night	11% 12% 5% 10% 5% 4 4 3 4 5% 37%	2940
	21st to end	Night	12% 10% 4 9% 5% 4 4 4 5% 6% 37%	2940
December	1st to 10th	Night	6% 8% 5% 13% 8% 7% 6% 4 6% 8% 31%	4720
	11th to 20th	Night	10% 12% 6% 13% 8% 5% 5% 4 5% 5% 26%	5339
	21st to end	Night	18% 17% 6% 17% 6% 4 4 3 4 5% 18%	7488

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	66% 6% 11% 11% 7%	4125
	11th to 20th	Day	56% 7% 12% 18% 8%	4985
	21st to end	Day	47% 8% 14% 21% 10%	5812
February	1st to 10th	Day	51% 7% 12% 21% 8% 1	1890
	11th to 20th	Day	50% 6% 15% 22% 7% 1	1853
	21st to end	Day	49% 7% 15% 19% 10%	1594
March	1st to 10th	Day	40% 6% 16% 25% 11% 2	5034
	11th to 20th	Day	40% 6% 14% 28% 10% 2	5057
	21st to end	Day	60% 7% 10% 19% 5%	5464
April	1st to 10th	Day	47% 7% 12% 23% 11% 1	2360
	11th to 20th	Day	52% 6% 12% 17% 11% 1	2483
	21st to end	Day	52% 7% 11% 20% 9% 1	2600
May	1st to 10th	Day	33% 7% 14% 29% 17% 1	5962
	11th to 20th	Day	40% 5% 10% 22% 15% 8%	6323
	21st to end	Day	41% 9% 12% 29% 8% 1	6949
June	1st to 10th	Day	33% 9% 16% 29% 12% 1	2842
	11th to 20th	Day	43% 9% 13% 19% 14% 1	3000
	21st to end	Day	33% 10% 18% 27% 12% 1	3000
July	1st to 10th	Day	31% 9% 21% 30% 10%	6726
	11th to 20th	Day	25% 6% 17% 36% 14% 1	6467
	21st to end	Day	31% 8% 18% 30% 13% 1	6424
August	1st to 10th	Day	37% 7% 14% 29% 12% 2	4220
	11th to 20th	Day	39% 8% 16% 30% 8%	4210
	21st to end	Day	32% 7% 19% 28% 12% 1	4585
September	1st to 10th	Day	31% 7% 15% 35% 12%	2400
	11th to 20th	Day	35% 7% 19% 28% 11%	2270
	21st to end	Day	35% 6% 11% 33% 13% 1	2166
October	1st to 10th	Day	31% 4 8% 33% 23% 1	3260
	11th to 20th	Day	37% 5% 13% 21% 22% 2	3240
	21st to end	Day	45% 4 16% 26% 9%	3514
November	1st to 10th	Day	46% 7% 13% 20% 12% 2	1701
	11th to 20th	Day	54% 6% 12% 18% 9% 1	1680
	21st to end	Day	51% 6% 15% 20% 7% 2	1680
December	1st to 10th	Day	57% 8% 14% 15% 5%	2690
	11th to 20th	Day	58% 4 14% 17% 6%	3085
	21st to end	Day	71% 4 10% 10% 5%	3710
January	1st to 10th	Night	56% 6% 14% 16% 7%	8800
	11th to 20th	Night	58% 7% 9% 16% 9% 1	7983
	21st to end	Night	44% 6% 13% 27% 10%	8402
February	1st to 10th	Night	48% 6% 14% 19% 11% 1	2730
	11th to 20th	Night	42% 7% 15% 24% 11% 2	2664
	21st to end	Night	44% 7% 15% 20% 12% 2	2229
March	1st to 10th	Night	34% 6% 13% 30% 15% 2	5040
	11th to 20th	Night	35% 7% 16% 30% 13%	5057
	21st to end	Night	58% 5% 10% 18% 9% 1	5467
April	1st to 10th	Night	44% 5% 13% 26% 11% 1	2040
	11th to 20th	Night	46% 6% 8% 22% 15% 2	1863
	21st to end	Night	48% 6% 9% 20% 14% 2	1800
May	1st to 10th	Night	29% 7% 13% 29% 19% 3	4016
	11th to 20th	Night	36% 7% 10% 19% 21% 8%	3597
	21st to end	Night	42% 7% 12% 25% 12% 1	3950
June	1st to 10th	Night	29% 9% 16% 30% 14% 2	1552
	11th to 20th	Night	32% 8% 18% 24% 16% 3	1400
	21st to end	Night	27% 8% 15% 29% 17% 3	1400
July	1st to 10th	Night	28% 8% 20% 26% 17% 1	3145
	11th to 20th	Night	23% 7% 17% 36% 15% 2	3374
	21st to end	Night	32% 7% 16% 31% 11% 2	4132
August	1st to 10th	Night	35% 7% 13% 26% 15% 4	2916
	11th to 20th	Night	34% 7% 16% 32% 12% 1	2915
	21st to end	Night	29% 6% 19% 30% 14% 2	3320
September	1st to 10th	Night	26% 8% 16% 33% 17% 1	2000
	11th to 20th	Night	29% 7% 17% 30% 17%	2126
	21st to end	Night	30% 5% 12% 34% 17% 1	2213
October	1st to 10th	Night	23% 4 7% 37% 26% 4	3920
	11th to 20th	Night	31% 5% 12% 27% 22% 3	3899
	21st to end	Night	42% 5% 13% 27% 13% 1	4274
November	1st to 10th	Night	39% 7% 15% 24% 13% 2	2816
	11th to 20th	Night	49% 6% 13% 19% 12% 1	2940
	21st to end	Night	47% 6% 17% 22% 7% 1	2940
December	1st to 10th	Night	50% 6% 19% 18% 6%	4720
	11th to 20th	Night	58% 7% 11% 18% 5%	5341
	21st to end	Night	68% 6% 9% 13% 4	7488

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	15% 10% 32% 27% 12% 3	330
	11th to 20th	Day	13% 11% 37% 34% 5%	330
	21st to end	Day	13% 10% 40% 26% 11% 1	363
February	1st to 10th	Day	12% 17% 42% 26% 3 1	330
	11th to 20th	Day	15% 12% 36% 27% 8% 2	330
	21st to end	Day	16% 10% 43% 26% 5%	273
March	1st to 10th	Day	18% 14% 38% 23% 7%	330
	11th to 20th	Day	18% 12% 37% 24% 9% 1	321
	21st to end	Day	25% 10% 27% 24% 11% 3	330
April	1st to 10th	Day	24% 14% 30% 25% 7%	460
	11th to 20th	Day	26% 14% 27% 23% 8% 1	601
	21st to end	Day	24% 10% 34% 20% 10% 2	700
May	1st to 10th	Day	19% 10% 34% 24% 11% 2	703
	11th to 20th	Day	19% 10% 29% 24% 15% 3	725
	21st to end	Day	18% 7% 29% 31% 13% 3	789
June	1st to 10th	Day	9% 9% 25% 33% 19% 4	730
	11th to 20th	Day	17% 8% 32% 31% 11% 2	730
	21st to end	Day	13% 6% 33% 30% 14% 3	730
July	1st to 10th	Day	7% 4 23% 34% 25% 7%	721
	11th to 20th	Day	9% 5% 27% 31% 22% 6%	718
	21st to end	Day	7% 4 24% 38% 22% 5%	794
August	1st to 10th	Day	9% 4 24% 34% 24% 4	726
	11th to 20th	Day	14% 3 23% 33% 22% 6%	715
	21st to end	Day	9% 5% 26% 33% 23% 4	753
September	1st to 10th	Day	11% 7% 29% 33% 16% 4	530
	11th to 20th	Day	10% 4 33% 32% 16% 5%	400
	21st to end	Day	18% 12% 28% 22% 15% 6%	330
October	1st to 10th	Day	18% 8% 31% 26% 14% 4	330
	11th to 20th	Day	18% 17% 37% 20% 8% 1	330
	21st to end	Day	26% 14% 36% 17% 6%	363
November	1st to 10th	Day	22% 14% 31% 25% 8% 1	330
	11th to 20th	Day	15% 16% 47% 18% 4	330
	21st to end	Day	21% 15% 38% 20% 5% 1	330
December	1st to 10th	Day	13% 11% 42% 26% 8% 1	330
	11th to 20th	Day	17% 16% 38% 25% 4	330
	21st to end	Day	27% 10% 37% 22% 3	363
January	1st to 10th	Night	10% 11% 39% 30% 9% 2	1050
	11th to 20th	Night	14% 12% 37% 30% 7% 1	1050
	21st to end	Night	15% 11% 39% 28% 7%	1155
February	1st to 10th	Night	12% 11% 40% 29% 8%	1050
	11th to 20th	Night	11% 11% 41% 29% 7% 1	1029
	21st to end	Night	14% 11% 36% 30% 8% 1	845
March	1st to 10th	Night	16% 9% 38% 27% 9% 1	641
	11th to 20th	Night	17% 10% 39% 28% 5% 1	621
	21st to end	Night	21% 10% 30% 26% 12% 1	660
April	1st to 10th	Night	16% 11% 38% 29% 6%	600
	11th to 20th	Night	22% 12% 29% 27% 8% 1	593
	21st to end	Night	22% 9% 33% 27% 9% 1	600
May	1st to 10th	Night	17% 9% 34% 31% 9% 1	600
	11th to 20th	Night	17% 7% 33% 31% 11% 1	591
	21st to end	Night	17% 6% 26% 37% 12% 1	650
June	1st to 10th	Night	11% 7% 28% 38% 15% 1	600
	11th to 20th	Night	11% 3 33% 42% 11%	600
	21st to end	Night	11% 5% 27% 42% 15% 1	600
July	1st to 10th	Night	6% 1 22% 45% 24% 2	585
	11th to 20th	Night	7% 2 26% 44% 20% 1	582
	21st to end	Night	5% 3 23% 47% 21% 2	657
August	1st to 10th	Night	9% 3 21% 46% 21% 2	597
	11th to 20th	Night	12% 2 19% 38% 25% 4	594
	21st to end	Night	8% 4 25% 39% 22% 2	660
September	1st to 10th	Night	9% 6% 26% 38% 18% 3	600
	11th to 20th	Night	11% 6% 29% 35% 19% 1	600
	21st to end	Night	11% 7% 33% 33% 15% 1	613
October	1st to 10th	Night	16% 8% 34% 29% 12% 1	800
	11th to 20th	Night	20% 12% 34% 27% 7% 1	800
	21st to end	Night	23% 12% 31% 28% 6%	890
November	1st to 10th	Night	15% 12% 35% 30% 8%	1025
	11th to 20th	Night	17% 13% 37% 26% 7% 1	1050
	21st to end	Night	16% 14% 38% 25% 6%	1050
December	1st to 10th	Night	13% 10% 40% 28% 8% 1	1050
	11th to 20th	Night	19% 11% 36% 29% 5% 1	1050
	21st to end	Night	24% 10% 35% 26% 5% 1	1155

Page generated at 2026-03-15 01:23:52. Version ac07

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