Battlefield Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Battlefield Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	13% 6% 2 6% 5% 4 5% 4 6% 8% 41%	148838
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 43%	143214
Transparency	Day	39% 6% 13% 22% 16% 3	152431
	Night	39% 6% 12% 22% 17% 4	147692
Seeing	Day	17% 12% 27% 25% 14% 5%	17925
	Night	17% 9% 28% 31% 14% 2	27465

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% 5% 2 5% 3 3 3 3 4 5% 55%	4961
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 4 55%	5280
	21st to end	Day	12% 9% 3 8% 4 4 4 3 4 5% 43%	5780
February	1st to 10th	Day	13% 9% 2 7% 4 4 4 4 5% 7% 41%	1890
	11th to 20th	Day	20% 6% 2 6% 4 3 4 2 4 5% 43%	2021
	21st to end	Day	16% 8% 2 5% 5% 4 4 4 5% 8% 38%	1730
March	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 4 6% 39%	4856
	11th to 20th	Day	10% 6% 2 5% 3 3 4 3 5% 6% 52%	5090
	21st to end	Day	13% 6% 2 5% 4 5% 4 3 5% 7% 47%	5768
April	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 8% 50%	2520
	11th to 20th	Day	9% 4 2 6% 5% 4 4 3 5% 7% 50%	2494
	21st to end	Day	13% 6% 2 6% 5% 4 4 4 6% 6% 44%	2520
May	1st to 10th	Day	14% 6% 3 7% 5% 4 5% 4 6% 9% 37%	7021
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 3 5% 7% 41%	7830
	21st to end	Day	15% 8% 3 7% 5% 4 5% 4 5% 8% 37%	9045
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 10% 40%	2940
	11th to 20th	Day	10% 5% 2 7% 4 5% 5% 5% 8% 10% 39%	2940
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 8% 11% 39%	2940
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 9% 12% 37%	8280
	11th to 20th	Day	11% 7% 3 9% 6% 6% 6% 6% 8% 10% 28%	7827
	21st to end	Day	11% 6% 3 8% 6% 6% 6% 6% 8% 10% 30%	8340
August	1st to 10th	Day	13% 9% 3 9% 6% 6% 7% 5% 8% 12% 22%	5931
	11th to 20th	Day	8% 6% 3 8% 6% 6% 6% 6% 9% 11% 31%	5488
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 5% 8% 10% 37%	6080
September	1st to 10th	Day	13% 6% 2 5% 5% 4 5% 5% 7% 9% 39%	2520
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 3 5% 7% 39%	2520
	21st to end	Day	22% 7% 2 7% 5% 4 4 3 6% 7% 33%	2253
October	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 4 5% 45%	2100
	11th to 20th	Day	15% 7% 2 4 3 3 4 2 5% 5% 49%	2100
	21st to end	Day	12% 4 2 6% 4 3 3 3 5% 7% 52%	2280
November	1st to 10th	Day	15% 6% 2 5% 2 3 4 3 4 6% 49%	1957
	11th to 20th	Day	15% 6% 2 4 4 3 3 3 5% 6% 49%	1890
	21st to end	Day	13% 4 2 6% 4 4 4 3 5% 6% 50%	1848
December	1st to 10th	Day	11% 5% 2 5% 4 4 3 3 4 6% 52%	2680
	11th to 20th	Day	14% 8% 2 8% 4 4 4 3 4 5% 44%	3070
	21st to end	Day	13% 5% 2 5% 3 3 3 3 5% 7% 49%	4048
January	1st to 10th	Night	10% 7% 2 6% 4 3 3 3 5% 6% 50%	7915
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 5% 53%	7680
	21st to end	Night	16% 7% 3 7% 4 4 4 3 4 6% 44%	8401
February	1st to 10th	Night	17% 8% 2 6% 4 3 4 3 5% 5% 43%	2730
	11th to 20th	Night	21% 8% 2 6% 4 3 4 3 4 5% 41%	2484
	21st to end	Night	15% 8% 3 6% 4 4 3 3 4 5% 46%	2076
March	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 4 5% 42%	5212
	11th to 20th	Night	13% 5% 3 6% 4 4 4 3 4 5% 48%	5090
	21st to end	Night	15% 7% 2 5% 3 3 4 3 5% 5% 47%	5406
April	1st to 10th	Night	13% 5% 2 6% 3 4 5% 4 6% 7% 45%	2100
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 5% 8% 49%	2075
	21st to end	Night	14% 6% 1 6% 4 4 3 4 5% 7% 47%	2100
May	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 5% 6% 9% 35%	5833
	11th to 20th	Night	16% 8% 2 6% 4 4 4 3 5% 7% 42%	5068
	21st to end	Night	18% 9% 3 7% 5% 5% 4 4 5% 7% 34%	5108
June	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 4 7% 11% 41%	1680
	11th to 20th	Night	8% 6% 3 7% 5% 4 6% 5% 7% 10% 40%	1680
	21st to end	Night	7% 4 2 6% 5% 4 6% 5% 6% 9% 45%	1680
July	1st to 10th	Night	7% 5% 2 6% 4 5% 5% 5% 8% 13% 38%	4678
	11th to 20th	Night	9% 8% 3 9% 5% 5% 6% 5% 8% 11% 30%	5112
	21st to end	Night	14% 8% 4 8% 5% 5% 5% 4 7% 9% 31%	5736
August	1st to 10th	Night	14% 9% 4 8% 5% 5% 5% 4 7% 9% 29%	4158
	11th to 20th	Night	9% 6% 3 8% 5% 5% 5% 4 7% 10% 38%	4557
	21st to end	Night	10% 6% 2 7% 5% 4 5% 4 6% 9% 41%	5045
September	1st to 10th	Night	11% 5% 2 5% 5% 4 5% 3 6% 8% 47%	2100
	11th to 20th	Night	13% 6% 3 6% 4 4 4 4 5% 7% 44%	2100
	21st to end	Night	20% 7% 4 8% 5% 4 4 3 5% 7% 34%	2362
October	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 4 5% 49%	2520
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 5% 50%	2520
	21st to end	Night	13% 5% 2 5% 3 3 3 3 4 5% 53%	2737
November	1st to 10th	Night	15% 6% 1 6% 4 3 3 3 4 6% 48%	2558
	11th to 20th	Night	15% 7% 3 6% 3 3 3 3 5% 6% 47%	2730
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 6% 51%	2768
December	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 5% 5% 53%	4720
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 3 5% 5% 39%	5345
	21st to end	Night	11% 5% 2 6% 4 4 4 4 5% 7% 49%	7150

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	32% 3 12% 26% 18% 9%	4962
	11th to 20th	Day	34% 3 5% 16% 35% 8%	5280
	21st to end	Day	43% 4 8% 25% 17% 4	5780
February	1st to 10th	Day	42% 4 13% 24% 14% 3	1890
	11th to 20th	Day	44% 5% 9% 18% 19% 5%	2030
	21st to end	Day	42% 7% 11% 19% 16% 5%	1730
March	1st to 10th	Day	44% 5% 8% 20% 16% 7%	4856
	11th to 20th	Day	34% 4 9% 19% 25% 10%	5090
	21st to end	Day	37% 5% 14% 19% 20% 5%	5768
April	1st to 10th	Day	33% 5% 15% 21% 21% 5%	2520
	11th to 20th	Day	33% 5% 12% 25% 19% 5%	2494
	21st to end	Day	37% 7% 13% 23% 16% 4	2520
May	1st to 10th	Day	43% 7% 16% 19% 13% 2	7053
	11th to 20th	Day	41% 6% 14% 24% 14% 1	7834
	21st to end	Day	44% 7% 12% 21% 14% 2	9061
June	1st to 10th	Day	35% 10% 16% 23% 15% 2	2940
	11th to 20th	Day	36% 9% 18% 23% 13% 1	2940
	21st to end	Day	34% 10% 17% 24% 13% 1	2940
July	1st to 10th	Day	36% 9% 18% 27% 8% 1	8210
	11th to 20th	Day	47% 9% 20% 17% 6%	7755
	21st to end	Day	44% 8% 13% 22% 12%	8156
August	1st to 10th	Day	53% 9% 22% 13% 3	5918
	11th to 20th	Day	41% 8% 21% 20% 9%	5524
	21st to end	Day	39% 7% 15% 24% 14% 1	6080
September	1st to 10th	Day	39% 9% 15% 19% 16% 3	2520
	11th to 20th	Day	44% 6% 10% 22% 15% 2	2520
	21st to end	Day	49% 7% 11% 18% 13% 2	2253
October	1st to 10th	Day	34% 4 9% 25% 26% 3	3360
	11th to 20th	Day	28% 4 6% 20% 33% 9%	3340
	21st to end	Day	22% 4 9% 27% 27% 10%	3618
November	1st to 10th	Day	36% 5% 11% 26% 19% 3	1957
	11th to 20th	Day	37% 4 13% 21% 20% 5%	1890
	21st to end	Day	36% 4 10% 25% 21% 4	1848
December	1st to 10th	Day	35% 4 9% 28% 21% 3	2680
	11th to 20th	Day	43% 4 13% 20% 15% 5%	3066
	21st to end	Day	35% 4 13% 30% 17% 1	4048
January	1st to 10th	Night	34% 4 11% 23% 21% 7%	7916
	11th to 20th	Night	35% 4 5% 19% 30% 8%	7680
	21st to end	Night	42% 4 7% 24% 19% 5%	8401
February	1st to 10th	Night	42% 5% 10% 22% 17% 3	2730
	11th to 20th	Night	46% 4 9% 20% 16% 5%	2486
	21st to end	Night	41% 5% 9% 21% 18% 7%	2076
March	1st to 10th	Night	44% 5% 9% 18% 17% 8%	5212
	11th to 20th	Night	38% 5% 9% 16% 22% 10%	5090
	21st to end	Night	40% 5% 9% 24% 16% 6%	5406
April	1st to 10th	Night	38% 6% 12% 19% 22% 4	2100
	11th to 20th	Night	33% 6% 12% 25% 19% 5%	2075
	21st to end	Night	37% 6% 12% 24% 18% 3	2100
May	1st to 10th	Night	43% 8% 15% 22% 10% 1	5863
	11th to 20th	Night	42% 7% 13% 26% 11% 1	5069
	21st to end	Night	47% 8% 12% 18% 12% 2	5115
June	1st to 10th	Night	34% 9% 15% 24% 16% 1	1680
	11th to 20th	Night	37% 11% 17% 21% 13% 1	1680
	21st to end	Night	33% 10% 16% 25% 14% 2	1680
July	1st to 10th	Night	35% 10% 19% 25% 10% 1	4640
	11th to 20th	Night	45% 10% 21% 16% 8%	5064
	21st to end	Night	48% 8% 13% 18% 12%	5606
August	1st to 10th	Night	49% 9% 22% 17% 3	4147
	11th to 20th	Night	40% 7% 19% 25% 9%	4587
	21st to end	Night	40% 6% 12% 25% 16% 1	5045
September	1st to 10th	Night	34% 8% 17% 23% 15% 3	2100
	11th to 20th	Night	41% 6% 11% 21% 18% 2	2100
	21st to end	Night	49% 7% 12% 17% 13% 2	2362
October	1st to 10th	Night	37% 4 8% 24% 24% 2	4040
	11th to 20th	Night	29% 4 5% 19% 32% 10%	4019
	21st to end	Night	22% 3 7% 29% 27% 13%	4349
November	1st to 10th	Night	38% 5% 9% 22% 23% 3	2558
	11th to 20th	Night	39% 5% 10% 20% 18% 7%	2730
	21st to end	Night	34% 6% 10% 25% 22% 4	2768
December	1st to 10th	Night	34% 4 8% 24% 25% 4	4720
	11th to 20th	Night	48% 4 11% 16% 15% 6%	5348
	21st to end	Night	34% 6% 15% 27% 16% 1	7150

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	19% 29% 36% 13% 4	275
	11th to 20th	Day	21% 25% 36% 16% 2	330
	21st to end	Day	18% 22% 41% 16% 2 1	363
February	1st to 10th	Day	20% 22% 38% 15% 5%	330
	11th to 20th	Day	28% 23% 34% 13% 2	509
	21st to end	Day	22% 21% 37% 17% 3 1	446
March	1st to 10th	Day	18% 21% 38% 18% 5%	540
	11th to 20th	Day	20% 22% 35% 16% 4 2	540
	21st to end	Day	16% 19% 40% 19% 5% 1	594
April	1st to 10th	Day	15% 21% 39% 17% 6% 2	540
	11th to 20th	Day	12% 17% 37% 23% 9% 2	532
	21st to end	Day	16% 14% 32% 27% 8% 2	540
May	1st to 10th	Day	18% 15% 36% 22% 9% 2	543
	11th to 20th	Day	19% 8% 31% 29% 10% 3	567
	21st to end	Day	20% 7% 24% 27% 17% 5%	617
June	1st to 10th	Day	11% 6% 26% 31% 22% 4	570
	11th to 20th	Day	12% 8% 22% 31% 19% 7%	570
	21st to end	Day	9% 4 22% 37% 22% 6%	570
July	1st to 10th	Day	10% 2 13% 36% 30% 9%	565
	11th to 20th	Day	10% 3 10% 34% 34% 10%	564
	21st to end	Day	10% 4 16% 35% 28% 8%	620
August	1st to 10th	Day	15% 4 16% 33% 25% 7%	566
	11th to 20th	Day	12% 2 16% 32% 25% 13%	559
	21st to end	Day	13% 4 17% 29% 25% 11%	627
September	1st to 10th	Day	14% 3 22% 29% 20% 13%	570
	11th to 20th	Day	18% 6% 15% 31% 21% 9%	570
	21st to end	Day	25% 8% 22% 23% 16% 5%	570
October	1st to 10th	Day	22% 5% 20% 26% 19% 8%	570
	11th to 20th	Day	18% 13% 26% 25% 15% 3	570
	21st to end	Day	15% 12% 30% 25% 13% 5%	615
November	1st to 10th	Day	18% 10% 30% 25% 12% 4	483
	11th to 20th	Day	19% 17% 31% 24% 8% 2	430
	21st to end	Day	15% 19% 29% 24% 12% 2	388
December	1st to 10th	Day	23% 14% 42% 15% 5% 1	220
	11th to 20th	Day	18% 18% 36% 20% 7% 1	220
	21st to end	Day	20% 19% 34% 18% 8% 1	242
January	1st to 10th	Night	18% 20% 34% 24% 5%	840
	11th to 20th	Night	21% 17% 33% 25% 5%	840
	21st to end	Night	19% 15% 40% 25% 2	924
February	1st to 10th	Night	21% 13% 39% 23% 3	840
	11th to 20th	Night	26% 19% 37% 17% 2	823
	21st to end	Night	20% 17% 38% 22% 3	692
March	1st to 10th	Night	19% 13% 36% 26% 5%	840
	11th to 20th	Night	20% 13% 39% 22% 5%	840
	21st to end	Night	19% 14% 38% 25% 3	924
April	1st to 10th	Night	18% 14% 40% 22% 5%	840
	11th to 20th	Night	13% 12% 39% 26% 8%	831
	21st to end	Night	18% 7% 35% 33% 7% 1	840
May	1st to 10th	Night	17% 10% 35% 31% 6% 1	840
	11th to 20th	Night	17% 4 29% 39% 10% 1	574
	21st to end	Night	17% 4 18% 42% 18% 2	455
June	1st to 10th	Night	12% 3 20% 46% 18% 2	420
	11th to 20th	Night	8% 4 21% 43% 21% 3	420
	21st to end	Night	7% 2 18% 49% 22% 1	420
July	1st to 10th	Night	8% 1 16% 48% 26% 2	410
	11th to 20th	Night	8% 1 10% 43% 33% 5%	554
	21st to end	Night	11% 3 13% 42% 28% 3	690
August	1st to 10th	Night	12% 3 14% 40% 28% 4	637
	11th to 20th	Night	12% 3 12% 40% 29% 4	830
	21st to end	Night	12% 3 14% 34% 33% 5%	924
September	1st to 10th	Night	13% 2 15% 38% 27% 6%	840
	11th to 20th	Night	15% 4 15% 34% 27% 5%	840
	21st to end	Night	22% 6% 18% 30% 22% 2	840
October	1st to 10th	Night	16% 5% 18% 32% 24% 5%	840
	11th to 20th	Night	20% 8% 24% 30% 16% 2	840
	21st to end	Night	16% 8% 24% 33% 17% 3	913
November	1st to 10th	Night	17% 10% 27% 30% 15% 2	820
	11th to 20th	Night	17% 13% 32% 28% 10% 1	840
	21st to end	Night	14% 12% 30% 30% 13% 1	840
December	1st to 10th	Night	20% 12% 33% 27% 8%	840
	11th to 20th	Night	18% 12% 34% 29% 7%	840
	21st to end	Night	18% 12% 32% 31% 6%	924

Page generated at 2026-10-26 04:37:52. Version ac07

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