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 2024-12-06.

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%	102098
	Night	12% 9% 3 8% 5% 4 4 4 5% 7% 40%	97676
Transparency	Day	42% 7% 14% 24% 11% 1	105712
	Night	42% 7% 13% 24% 12% 2	102203
Seeing	Day	15% 9% 31% 28% 14% 3	16127
	Night	15% 9% 33% 32% 11% 1	26053

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	11% 16% 3 15% 10% 6% 6% 3 5% 6% 19%	2218
	11th to 20th	Day	6% 9% 4 15% 8% 7% 9% 6% 9% 6% 22%	2665
	21st to end	Day	6% 18% 9% 12% 7% 6% 5% 4 5% 7% 20%	3135
February	1st to 10th	Day	8% 12% 4 9% 7% 6% 5% 5% 6% 8% 30%	1710
	11th to 20th	Day	8% 11% 4 10% 7% 5% 6% 5% 6% 7% 32%	1664
	21st to end	Day	9% 12% 3 11% 6% 5% 4 5% 6% 7% 31%	1446
March	1st to 10th	Day	11% 12% 4 9% 5% 5% 5% 3 5% 6% 36%	3425
	11th to 20th	Day	9% 9% 3 8% 4 4 5% 4 5% 6% 42%	3480
	21st to end	Day	22% 12% 4 11% 8% 5% 4 3 4 6% 21%	3814
April	1st to 10th	Day	17% 9% 4 7% 4 4 3 4 5% 7% 36%	2242
	11th to 20th	Day	19% 12% 4 7% 5% 3 4 3 4 6% 32%	2357
	21st to end	Day	24% 9% 3 7% 4 3 4 4 4 5% 32%	2470
May	1st to 10th	Day	17% 6% 2 6% 5% 4 5% 4 5% 8% 39%	4130
	11th to 20th	Day	14% 6% 2 6% 4 4 3 4 5% 7% 46%	4399
	21st to end	Day	18% 5% 2 7% 4 5% 5% 4 7% 8% 36%	4824
June	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 8% 46%	2700
	11th to 20th	Day	14% 6% 2 7% 5% 5% 4 4 6% 7% 39%	2850
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 7% 9% 45%	2850
July	1st to 10th	Day	9% 4 2 6% 5% 5% 6% 5% 8% 11% 39%	4750
	11th to 20th	Day	6% 3 1 3 3 3 3 3 6% 9% 60%	4575
	21st to end	Day	5% 4 2 5% 4 4 4 5% 7% 9% 49%	4598
August	1st to 10th	Day	12% 6% 2 6% 4 3 4 4 6% 8% 45%	4104
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	4041
	21st to end	Day	10% 5% 2 5% 4 4 5% 3 6% 8% 49%	4446
September	1st to 10th	Day	11% 4 1 5% 4 4 4 3 6% 7% 51%	2280
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 6% 7% 47%	2157
	21st to end	Day	16% 5% 2 4 4 3 4 2 4 7% 48%	2058
October	1st to 10th	Day	18% 7% 2 6% 3 4 4 3 5% 7% 43%	1900
	11th to 20th	Day	16% 9% 2 6% 3 3 4 3 6% 6% 42%	1900
	21st to end	Day	17% 11% 3 7% 4 4 3 3 4 7% 38%	2078
November	1st to 10th	Day	13% 11% 3 8% 4 4 4 3 5% 7% 37%	1619
	11th to 20th	Day	9% 12% 6% 13% 7% 5% 4 4 4 6% 30%	1600
	21st to end	Day	13% 10% 5% 11% 6% 4 5% 4 6% 7% 32%	1600
December	1st to 10th	Day	8% 13% 4 12% 7% 7% 6% 5% 5% 6% 26%	1560
	11th to 20th	Day	10% 13% 7% 12% 6% 5% 4 3 4 6% 29%	1918
	21st to end	Day	21% 15% 4 16% 4 3 3 3 4 6% 21%	2535
January	1st to 10th	Night	9% 12% 6% 17% 7% 5% 4 3 6% 6% 26%	4737
	11th to 20th	Night	6% 11% 4 13% 10% 9% 9% 6% 8% 6% 20%	4283
	21st to end	Night	8% 18% 9% 12% 5% 5% 4 3 5% 5% 25%	4532
February	1st to 10th	Night	6% 11% 4 10% 6% 5% 5% 4 5% 7% 35%	2470
	11th to 20th	Night	6% 9% 3 8% 5% 5% 5% 5% 7% 6% 40%	2403
	21st to end	Night	11% 9% 3 8% 5% 5% 5% 4 6% 7% 36%	2023
March	1st to 10th	Night	9% 7% 3 7% 3 4 3 4 5% 8% 47%	3429
	11th to 20th	Night	8% 7% 3 8% 5% 4 5% 5% 6% 8% 42%	3480
	21st to end	Night	25% 12% 4 9% 5% 3 3 2 4 5% 28%	3817
April	1st to 10th	Night	14% 9% 3 8% 4 3 5% 3 6% 6% 39%	1938
	11th to 20th	Night	19% 11% 3 7% 3 3 4 4 5% 5% 37%	1769
	21st to end	Night	20% 9% 3 6% 4 3 3 3 5% 6% 37%	1710
May	1st to 10th	Night	16% 7% 2 5% 3 4 4 3 5% 6% 46%	2773
	11th to 20th	Night	14% 6% 1 5% 4 4 4 3 5% 7% 47%	2506
	21st to end	Night	18% 6% 2 5% 3 3 3 3 6% 6% 43%	2743
June	1st to 10th	Night	10% 4 1 5% 3 4 4 4 7% 9% 48%	1475
	11th to 20th	Night	8% 5% 2 5% 4 4 3 4 7% 8% 48%	1330
	21st to end	Night	9% 3 1 4 4 3 4 3 6% 6% 57%	1330
July	1st to 10th	Night	11% 4 2 5% 4 4 5% 4 7% 10% 45%	2220
	11th to 20th	Night	3 2 1 4 4 3 3 3 5% 8% 63%	2392
	21st to end	Night	5% 3 1 4 3 3 4 4 7% 9% 56%	2961
August	1st to 10th	Night	11% 6% 2 6% 3 3 4 4 4 7% 49%	2836
	11th to 20th	Night	13% 4 2 4 3 4 4 3 5% 7% 50%	2798
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 8% 53%	3218
September	1st to 10th	Night	8% 3 2 5% 3 3 3 4 5% 7% 58%	1900
	11th to 20th	Night	9% 5% 2 4 3 4 4 3 5% 7% 54%	2019
	21st to end	Night	10% 6% 2 5% 3 3 3 2 4 6% 56%	2102
October	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 4 6% 50%	2280
	11th to 20th	Night	15% 9% 2 5% 4 3 4 3 5% 6% 45%	2280
	21st to end	Night	17% 11% 3 7% 4 3 3 2 5% 7% 39%	2517
November	1st to 10th	Night	12% 8% 3 7% 5% 4 4 4 5% 7% 41%	2679
	11th to 20th	Night	12% 12% 5% 10% 5% 4 4 3 4 5% 37%	2800
	21st to end	Night	12% 9% 4 9% 6% 4 4 4 5% 6% 38%	2800
December	1st to 10th	Night	9% 10% 5% 12% 7% 5% 5% 4 5% 7% 31%	2737
	11th to 20th	Night	12% 11% 4 10% 8% 5% 6% 4 6% 6% 28%	3279
	21st to end	Night	20% 14% 5% 13% 6% 4 4 3 4 5% 21%	5110

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	65% 7% 11% 14% 4	2219
	11th to 20th	Day	57% 8% 14% 16% 5%	2665
	21st to end	Day	61% 9% 13% 12% 4	3135
February	1st to 10th	Day	50% 7% 12% 22% 8% 1	1710
	11th to 20th	Day	51% 6% 16% 20% 7% 1	1673
	21st to end	Day	50% 7% 13% 19% 10%	1446
March	1st to 10th	Day	47% 8% 12% 23% 8% 1	3425
	11th to 20th	Day	40% 7% 12% 29% 10% 2	3480
	21st to end	Day	64% 7% 8% 16% 4	3814
April	1st to 10th	Day	47% 7% 12% 22% 11% 1	2242
	11th to 20th	Day	52% 6% 11% 18% 11% 1	2357
	21st to end	Day	53% 7% 11% 20% 8% 1	2470
May	1st to 10th	Day	40% 9% 14% 23% 14% 1	4168
	11th to 20th	Day	36% 7% 12% 28% 14% 2	4403
	21st to end	Day	40% 11% 15% 25% 8% 1	4837
June	1st to 10th	Day	33% 9% 16% 28% 12% 1	2700
	11th to 20th	Day	42% 9% 13% 20% 15% 1	2850
	21st to end	Day	32% 10% 18% 27% 11% 1	2850
July	1st to 10th	Day	35% 10% 21% 21% 12%	4676
	11th to 20th	Day	21% 6% 18% 36% 17% 2	4495
	21st to end	Day	28% 9% 18% 30% 14% 1	4453
August	1st to 10th	Day	36% 7% 13% 30% 12% 2	4090
	11th to 20th	Day	39% 8% 16% 30% 8%	4080
	21st to end	Day	32% 7% 19% 28% 13% 1	4445
September	1st to 10th	Day	30% 7% 14% 36% 12%	2280
	11th to 20th	Day	34% 7% 18% 29% 11%	2157
	21st to end	Day	36% 7% 12% 32% 12% 1	2058
October	1st to 10th	Day	32% 4 8% 33% 23% 1	3160
	11th to 20th	Day	36% 6% 13% 22% 22% 2	3140
	21st to end	Day	45% 4 16% 26% 9%	3405
November	1st to 10th	Day	46% 7% 12% 20% 12% 2	1619
	11th to 20th	Day	55% 6% 12% 17% 9% 1	1600
	21st to end	Day	50% 6% 15% 20% 7% 2	1600
December	1st to 10th	Day	55% 9% 14% 14% 7%	1560
	11th to 20th	Day	56% 6% 16% 18% 4	1915
	21st to end	Day	65% 6% 11% 13% 5%	2535
January	1st to 10th	Night	58% 7% 13% 17% 5%	4737
	11th to 20th	Night	59% 9% 11% 15% 5%	4283
	21st to end	Night	59% 6% 11% 17% 6% 1	4532
February	1st to 10th	Night	47% 6% 14% 20% 11% 1	2470
	11th to 20th	Night	41% 7% 17% 22% 11% 2	2404
	21st to end	Night	45% 7% 14% 20% 12% 2	2023
March	1st to 10th	Night	35% 7% 13% 28% 15% 2	3429
	11th to 20th	Night	35% 8% 13% 30% 13%	3480
	21st to end	Night	59% 5% 9% 19% 6% 1	3817
April	1st to 10th	Night	44% 5% 13% 26% 11% 1	1938
	11th to 20th	Night	47% 6% 7% 22% 16% 2	1769
	21st to end	Night	47% 6% 10% 21% 14% 2	1710
May	1st to 10th	Night	38% 8% 13% 23% 15% 3	2800
	11th to 20th	Night	33% 9% 12% 22% 20% 3	2507
	21st to end	Night	38% 8% 15% 25% 13% 1	2751
June	1st to 10th	Night	29% 9% 16% 30% 14% 2	1475
	11th to 20th	Night	31% 8% 17% 24% 17% 3	1330
	21st to end	Night	26% 8% 16% 30% 17% 3	1330
July	1st to 10th	Night	32% 9% 21% 22% 15% 1	2185
	11th to 20th	Night	19% 6% 16% 38% 20% 2	2349
	21st to end	Night	23% 7% 17% 35% 15% 2	2867
August	1st to 10th	Night	34% 7% 13% 27% 15% 4	2826
	11th to 20th	Night	33% 7% 16% 32% 12% 1	2825
	21st to end	Night	29% 7% 19% 30% 14% 2	3218
September	1st to 10th	Night	25% 7% 16% 33% 18% 1	1900
	11th to 20th	Night	29% 7% 16% 30% 18%	2019
	21st to end	Night	31% 5% 12% 34% 17% 1	2102
October	1st to 10th	Night	23% 4 7% 37% 26% 4	3800
	11th to 20th	Night	31% 5% 13% 27% 22% 2	3779
	21st to end	Night	41% 5% 13% 27% 12% 1	4141
November	1st to 10th	Night	39% 7% 15% 23% 13% 2	2679
	11th to 20th	Night	50% 6% 12% 18% 12% 1	2800
	21st to end	Night	46% 6% 17% 22% 7% 2	2800
December	1st to 10th	Night	52% 7% 15% 19% 7%	2737
	11th to 20th	Night	54% 9% 14% 17% 6% 1	3281
	21st to end	Night	63% 8% 10% 14% 4	5110

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	16% 9% 34% 25% 13% 3	270
	11th to 20th	Day	14% 11% 33% 37% 5%	270
	21st to end	Day	14% 8% 38% 26% 12% 1	297
February	1st to 10th	Day	10% 16% 43% 28% 2	270
	11th to 20th	Day	15% 13% 38% 27% 5% 1	270
	21st to end	Day	17% 9% 42% 27% 5%	225
March	1st to 10th	Day	18% 16% 36% 22% 7%	270
	11th to 20th	Day	16% 10% 39% 24% 11% 1	270
	21st to end	Day	26% 11% 25% 25% 11% 3	297
April	1st to 10th	Day	25% 14% 31% 23% 6%	422
	11th to 20th	Day	27% 14% 25% 24% 9% 1	555
	21st to end	Day	24% 10% 34% 20% 11% 2	650
May	1st to 10th	Day	20% 11% 36% 24% 9% 1	653
	11th to 20th	Day	17% 11% 31% 23% 15% 3	675
	21st to end	Day	18% 7% 30% 30% 12% 3	734
June	1st to 10th	Day	9% 9% 25% 32% 20% 4	680
	11th to 20th	Day	16% 8% 32% 31% 11% 2	680
	21st to end	Day	12% 6% 32% 31% 15% 4	680
July	1st to 10th	Day	7% 4 23% 34% 25% 7%	671
	11th to 20th	Day	9% 6% 26% 31% 22% 6%	668
	21st to end	Day	6% 4 23% 38% 23% 6%	739
August	1st to 10th	Day	9% 4 23% 35% 25% 4	676
	11th to 20th	Day	13% 4 22% 33% 22% 6%	665
	21st to end	Day	9% 5% 27% 33% 22% 3	701
September	1st to 10th	Day	11% 7% 28% 34% 17% 3	490
	11th to 20th	Day	10% 4 33% 31% 17% 5%	367
	21st to end	Day	19% 13% 30% 20% 13% 5%	300
October	1st to 10th	Day	19% 7% 31% 24% 14% 4	300
	11th to 20th	Day	16% 17% 36% 22% 8% 1	300
	21st to end	Day	24% 14% 37% 17% 7%	330
November	1st to 10th	Day	22% 13% 30% 26% 8% 1	300
	11th to 20th	Day	16% 16% 47% 17% 4	300
	21st to end	Day	20% 15% 39% 21% 6%	300
December	1st to 10th	Day	12% 9% 44% 25% 9%	285
	11th to 20th	Day	18% 14% 38% 26% 4	270
	21st to end	Day	29% 10% 36% 22% 3	297
January	1st to 10th	Night	10% 11% 39% 30% 9% 1	950
	11th to 20th	Night	14% 11% 36% 30% 8% 1	950
	21st to end	Night	16% 10% 37% 28% 8%	1045
February	1st to 10th	Night	11% 10% 40% 30% 8%	950
	11th to 20th	Night	11% 12% 40% 29% 7% 1	929
	21st to end	Night	15% 10% 34% 31% 9% 1	767
March	1st to 10th	Night	16% 9% 38% 27% 10% 1	580
	11th to 20th	Night	16% 9% 39% 29% 6% 1	570
	21st to end	Night	21% 11% 29% 26% 12% 1	627
April	1st to 10th	Night	16% 11% 39% 28% 6%	570
	11th to 20th	Night	23% 11% 29% 28% 9% 1	563
	21st to end	Night	21% 8% 33% 28% 9% 1	570
May	1st to 10th	Night	18% 9% 34% 30% 8% 1	570
	11th to 20th	Night	16% 7% 34% 31% 11% 1	561
	21st to end	Night	17% 6% 26% 38% 12% 1	617
June	1st to 10th	Night	10% 7% 28% 38% 16% 1	570
	11th to 20th	Night	10% 3 34% 42% 11%	570
	21st to end	Night	9% 4 27% 43% 15% 1	570
July	1st to 10th	Night	6% 1 22% 44% 24% 2	555
	11th to 20th	Night	6% 3 26% 43% 21% 1	552
	21st to end	Night	5% 3 23% 46% 21% 2	624
August	1st to 10th	Night	8% 2 20% 47% 21% 2	567
	11th to 20th	Night	11% 2 18% 39% 25% 4	564
	21st to end	Night	8% 4 25% 39% 21% 2	627
September	1st to 10th	Night	9% 6% 27% 39% 18% 2	570
	11th to 20th	Night	11% 6% 30% 34% 19% 1	570
	21st to end	Night	11% 8% 34% 32% 14% 1	582
October	1st to 10th	Night	17% 8% 34% 28% 12% 1	760
	11th to 20th	Night	19% 12% 34% 28% 7% 1	760
	21st to end	Night	23% 12% 31% 28% 6%	845
November	1st to 10th	Night	16% 11% 34% 31% 9%	975
	11th to 20th	Night	17% 13% 37% 25% 8% 1	1000
	21st to end	Night	16% 14% 38% 26% 6%	1000
December	1st to 10th	Night	14% 10% 38% 29% 8% 1	978
	11th to 20th	Night	20% 8% 36% 30% 6% 1	950
	21st to end	Night	24% 10% 34% 26% 5%	1045

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