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 2025-07-06.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	119627
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	119178
Transparency	Day	39% 7% 13% 22% 17% 4	123261
	Night	39% 6% 11% 22% 18% 4	123681
Seeing	Day	17% 12% 27% 25% 15% 5%	16141
	Night	17% 9% 28% 31% 14% 2	25736

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	14% 5% 1 4 3 3 3 3 4 4 56%	3827
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 4 4 53%	4060
	21st to end	Day	10% 9% 3 8% 4 4 4 3 5% 5% 44%	4466
February	1st to 10th	Day	13% 9% 2 6% 4 4 4 4 5% 7% 41%	1800
	11th to 20th	Day	20% 6% 2 6% 4 3 4 2 4 5% 43%	1922
	21st to end	Day	15% 8% 2 5% 5% 4 4 4 5% 9% 38%	1650
March	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 4 6% 39%	4750
	11th to 20th	Day	10% 6% 2 5% 3 3 4 3 5% 6% 52%	4980
	21st to end	Day	13% 6% 2 5% 4 5% 4 3 5% 6% 47%	5643
April	1st to 10th	Day	13% 5% 2 5% 4 3 3 3 5% 7% 49%	2400
	11th to 20th	Day	10% 5% 2 6% 5% 4 4 3 5% 7% 50%	2374
	21st to end	Day	13% 6% 2 6% 5% 4 4 4 6% 6% 44%	2400
May	1st to 10th	Day	13% 7% 3 8% 5% 5% 6% 5% 6% 10% 34%	5405
	11th to 20th	Day	17% 8% 3 8% 5% 5% 6% 4 6% 8% 31%	6022
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 41%	6934
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 10% 39%	2800
	11th to 20th	Day	10% 5% 2 7% 4 5% 5% 5% 8% 10% 40%	2800
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 8% 11% 39%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 6% 10% 12% 35%	4510
	11th to 20th	Day	11% 6% 3 9% 6% 5% 6% 5% 7% 9% 32%	4197
	21st to end	Day	9% 7% 3 9% 7% 7% 6% 6% 8% 9% 29%	4466
August	1st to 10th	Day	16% 9% 3 9% 6% 6% 7% 4 8% 10% 22%	4051
	11th to 20th	Day	9% 6% 3 8% 6% 6% 5% 6% 8% 9% 34%	3730
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 9% 37%	2280
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 5% 7% 38%	2280
	21st to end	Day	21% 7% 2 6% 5% 4 4 3 6% 7% 35%	2038
October	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 4 5% 44%	1900
	11th to 20th	Day	14% 7% 2 4 4 3 5% 2 5% 5% 50%	1900
	21st to end	Day	12% 4 2 7% 3 3 3 3 5% 7% 51%	2090
November	1st to 10th	Day	16% 6% 2 5% 2 3 4 3 4 6% 48%	1862
	11th to 20th	Day	16% 6% 2 4 4 3 3 3 5% 6% 48%	1800
	21st to end	Day	14% 4 2 5% 4 3 4 3 5% 6% 50%	1760
December	1st to 10th	Day	10% 5% 2 6% 4 4 3 3 4 6% 53%	2600
	11th to 20th	Day	14% 8% 2 8% 4 4 4 3 5% 5% 44%	2990
	21st to end	Day	13% 5% 2 5% 3 3 3 3 5% 7% 49%	3960
January	1st to 10th	Night	12% 7% 3 5% 3 3 3 3 4 5% 53%	6101
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 5% 51%	5900
	21st to end	Night	14% 7% 3 6% 4 4 4 3 4 6% 46%	6490
February	1st to 10th	Night	17% 7% 2 6% 4 3 4 4 5% 5% 43%	2600
	11th to 20th	Night	21% 8% 2 6% 4 3 4 3 4 6% 40%	2363
	21st to end	Night	15% 7% 3 6% 4 4 3 3 4 5% 47%	1980
March	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 4 5% 42%	5098
	11th to 20th	Night	13% 6% 3 6% 4 4 4 3 4 5% 48%	4980
	21st to end	Night	15% 7% 2 5% 3 4 4 3 5% 5% 47%	5289
April	1st to 10th	Night	14% 6% 2 6% 3 5% 5% 4 6% 6% 44%	2000
	11th to 20th	Night	12% 7% 2 5% 4 3 4 3 5% 8% 49%	1975
	21st to end	Night	14% 5% 1 6% 4 4 3 4 5% 7% 47%	2000
May	1st to 10th	Night	12% 8% 3 8% 6% 5% 5% 5% 6% 9% 33%	4490
	11th to 20th	Night	19% 9% 3 7% 5% 5% 4 4 5% 7% 32%	3910
	21st to end	Night	14% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	3920
June	1st to 10th	Night	10% 6% 2 5% 5% 4 5% 4 8% 11% 40%	1600
	11th to 20th	Night	8% 6% 3 6% 5% 4 6% 5% 7% 10% 40%	1600
	21st to end	Night	7% 4 2 6% 5% 4 6% 5% 7% 10% 45%	1600
July	1st to 10th	Night	7% 6% 2 7% 5% 5% 6% 5% 8% 13% 37%	2560
	11th to 20th	Night	8% 7% 3 8% 5% 5% 6% 5% 7% 9% 36%	2759
	21st to end	Night	14% 9% 4 8% 6% 5% 5% 5% 7% 7% 30%	3087
August	1st to 10th	Night	17% 9% 4 8% 5% 5% 5% 4 7% 8% 29%	2847
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 4 7% 8% 39%	3097
	21st to end	Night	9% 5% 2 6% 4 4 5% 5% 7% 10% 44%	3476
September	1st to 10th	Night	11% 5% 2 5% 5% 4 5% 3 6% 7% 46%	1900
	11th to 20th	Night	13% 7% 3 6% 4 4 4 4 5% 7% 44%	1900
	21st to end	Night	19% 7% 4 8% 5% 4 4 3 5% 7% 36%	2138
October	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 4 5% 48%	2280
	11th to 20th	Night	15% 6% 2 5% 4 3 3 3 4 5% 50%	2280
	21st to end	Night	13% 5% 2 5% 4 4 3 3 5% 6% 52%	2508
November	1st to 10th	Night	15% 6% 1 6% 4 3 3 3 5% 6% 48%	2433
	11th to 20th	Night	15% 6% 3 6% 3 3 3 3 5% 6% 47%	2600
	21st to end	Night	12% 5% 1 5% 4 4 4 3 5% 6% 51%	2636
December	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 5% 53%	4580
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 3 5% 5% 38%	5205
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 7% 49%	6996

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 6% 26% 21% 12%	3828
	11th to 20th	Day	34% 4 6% 19% 32% 6%	4060
	21st to end	Day	41% 5% 9% 25% 17% 3	4466
February	1st to 10th	Day	42% 5% 12% 24% 13% 3	1800
	11th to 20th	Day	44% 5% 9% 18% 19% 5%	1931
	21st to end	Day	41% 7% 12% 19% 17% 5%	1650
March	1st to 10th	Day	45% 5% 7% 20% 16% 7%	4750
	11th to 20th	Day	33% 4 8% 18% 26% 10%	4980
	21st to end	Day	38% 5% 14% 19% 20% 5%	5643
April	1st to 10th	Day	34% 5% 14% 21% 21% 5%	2400
	11th to 20th	Day	34% 5% 12% 24% 20% 5%	2374
	21st to end	Day	37% 7% 12% 22% 17% 4	2400
May	1st to 10th	Day	44% 9% 17% 18% 10% 1	5437
	11th to 20th	Day	48% 8% 15% 19% 9% 1	6026
	21st to end	Day	38% 8% 14% 25% 13% 2	6949
June	1st to 10th	Day	35% 10% 17% 23% 14% 1	2800
	11th to 20th	Day	36% 9% 18% 22% 14% 1	2800
	21st to end	Day	34% 10% 17% 24% 14% 1	2800
July	1st to 10th	Day	35% 12% 20% 20% 10% 2	4440
	11th to 20th	Day	46% 9% 23% 17% 5% 1	4125
	21st to end	Day	46% 10% 15% 17% 11% 1	4324
August	1st to 10th	Day	55% 8% 17% 16% 4	4038
	11th to 20th	Day	42% 8% 15% 23% 12%	3766
	21st to end	Day	36% 8% 14% 25% 16% 2	4180
September	1st to 10th	Day	40% 10% 15% 16% 16% 3	2280
	11th to 20th	Day	44% 7% 10% 21% 16% 2	2280
	21st to end	Day	48% 7% 11% 18% 14% 2	2038
October	1st to 10th	Day	33% 4 9% 25% 25% 3	3160
	11th to 20th	Day	27% 4 6% 21% 33% 9%	3140
	21st to end	Day	22% 4 9% 28% 28% 9%	3428
November	1st to 10th	Day	37% 5% 11% 25% 19% 3	1862
	11th to 20th	Day	38% 4 13% 20% 20% 5%	1800
	21st to end	Day	36% 4 10% 25% 21% 4	1760
December	1st to 10th	Day	34% 4 9% 28% 22% 3	2600
	11th to 20th	Day	43% 4 12% 20% 14% 5%	2986
	21st to end	Day	35% 4 13% 30% 17% 1	3960
January	1st to 10th	Night	33% 5% 6% 21% 26% 9%	6101
	11th to 20th	Night	36% 4 5% 20% 29% 5%	5900
	21st to end	Night	39% 5% 8% 24% 20% 4	6490
February	1st to 10th	Night	42% 5% 11% 23% 16% 3	2600
	11th to 20th	Night	46% 4 9% 20% 17% 5%	2365
	21st to end	Night	39% 5% 9% 21% 19% 7%	1980
March	1st to 10th	Night	44% 5% 8% 18% 17% 8%	5098
	11th to 20th	Night	38% 5% 9% 15% 22% 10%	4980
	21st to end	Night	40% 5% 9% 24% 16% 6%	5289
April	1st to 10th	Night	39% 7% 12% 17% 22% 4	2000
	11th to 20th	Night	34% 6% 11% 24% 19% 6%	1975
	21st to end	Night	37% 6% 12% 24% 18% 3	2000
May	1st to 10th	Night	45% 10% 16% 18% 9% 1	4520
	11th to 20th	Night	51% 8% 12% 21% 7% 1	3911
	21st to end	Night	43% 10% 15% 21% 11% 1	3927
June	1st to 10th	Night	35% 10% 16% 25% 14% 1	1600
	11th to 20th	Night	37% 12% 17% 21% 13% 1	1600
	21st to end	Night	33% 10% 17% 26% 14% 2	1600
July	1st to 10th	Night	37% 13% 19% 21% 9% 2	2520
	11th to 20th	Night	40% 10% 25% 19% 6% 1	2708
	21st to end	Night	49% 11% 12% 17% 11%	2988
August	1st to 10th	Night	51% 7% 19% 19% 4	2836
	11th to 20th	Night	41% 8% 15% 24% 12%	3127
	21st to end	Night	35% 7% 12% 27% 19% 1	3476
September	1st to 10th	Night	35% 9% 17% 21% 15% 3	1900
	11th to 20th	Night	41% 6% 11% 21% 18% 2	1900
	21st to end	Night	47% 7% 12% 18% 14% 2	2138
October	1st to 10th	Night	38% 5% 8% 24% 24% 2	3800
	11th to 20th	Night	28% 4 6% 20% 33% 10%	3779
	21st to end	Night	21% 3 7% 30% 27% 12%	4120
November	1st to 10th	Night	38% 5% 9% 22% 22% 3	2433
	11th to 20th	Night	39% 5% 10% 20% 18% 8%	2600
	21st to end	Night	33% 6% 10% 26% 22% 4	2636
December	1st to 10th	Night	33% 4 8% 24% 26% 5%	4580
	11th to 20th	Night	48% 4 11% 16% 15% 5%	5208
	21st to end	Night	34% 6% 15% 27% 16% 1	6996

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	20% 28% 34% 13% 4	250
	11th to 20th	Day	20% 24% 36% 18% 2	300
	21st to end	Day	17% 22% 41% 17% 2 1	330
February	1st to 10th	Day	19% 23% 39% 14% 5%	300
	11th to 20th	Day	28% 23% 34% 13% 3	470
	21st to end	Day	21% 20% 38% 18% 3 1	414
March	1st to 10th	Day	19% 21% 37% 17% 5%	500
	11th to 20th	Day	20% 22% 35% 17% 4 2	500
	21st to end	Day	17% 19% 39% 19% 5% 1	550
April	1st to 10th	Day	16% 22% 39% 16% 6% 1	500
	11th to 20th	Day	13% 17% 36% 23% 9% 2	492
	21st to end	Day	17% 14% 32% 27% 8% 2	500
May	1st to 10th	Day	17% 13% 36% 22% 9% 2	503
	11th to 20th	Day	19% 8% 31% 29% 9% 3	527
	21st to end	Day	20% 6% 24% 27% 18% 6%	573
June	1st to 10th	Day	11% 5% 24% 32% 23% 4	530
	11th to 20th	Day	13% 8% 20% 32% 20% 7%	530
	21st to end	Day	9% 3 22% 35% 23% 7%	530
July	1st to 10th	Day	10% 1 13% 38% 30% 9%	505
	11th to 20th	Day	10% 3 10% 34% 35% 10%	484
	21st to end	Day	10% 3 17% 35% 29% 7%	532
August	1st to 10th	Day	15% 3 14% 34% 25% 8%	486
	11th to 20th	Day	13% 2 16% 30% 26% 13%	479
	21st to end	Day	13% 4 14% 30% 28% 12%	539
September	1st to 10th	Day	14% 3 21% 28% 19% 14%	490
	11th to 20th	Day	18% 5% 16% 31% 22% 9%	490
	21st to end	Day	24% 8% 22% 23% 17% 5%	490
October	1st to 10th	Day	22% 6% 20% 25% 20% 8%	490
	11th to 20th	Day	16% 12% 26% 27% 17% 3	490
	21st to end	Day	15% 11% 31% 24% 13% 5%	539
November	1st to 10th	Day	19% 10% 30% 25% 13% 4	448
	11th to 20th	Day	19% 16% 30% 25% 8% 2	400
	21st to end	Day	15% 19% 28% 23% 13% 2	360
December	1st to 10th	Day	22% 15% 43% 15% 5% 1	200
	11th to 20th	Day	18% 19% 35% 20% 8% 1	200
	21st to end	Day	21% 16% 34% 20% 8% 1	220
January	1st to 10th	Night	18% 20% 33% 24% 5%	800
	11th to 20th	Night	21% 16% 32% 26% 5%	800
	21st to end	Night	18% 15% 39% 25% 2	880
February	1st to 10th	Night	21% 13% 40% 23% 3	800
	11th to 20th	Night	26% 19% 37% 17% 2	783
	21st to end	Night	20% 16% 38% 23% 4	660
March	1st to 10th	Night	19% 13% 36% 26% 6%	800
	11th to 20th	Night	20% 13% 40% 21% 6% 1	800
	21st to end	Night	19% 14% 38% 26% 4	880
April	1st to 10th	Night	18% 14% 40% 22% 5%	800
	11th to 20th	Night	14% 13% 38% 26% 9%	791
	21st to end	Night	18% 7% 35% 33% 7% 1	800
May	1st to 10th	Night	17% 10% 35% 31% 7% 1	800
	11th to 20th	Night	18% 4 28% 38% 11% 1	547
	21st to end	Night	16% 3 17% 43% 19% 2	433
June	1st to 10th	Night	12% 3 19% 46% 19% 2	400
	11th to 20th	Night	8% 4 21% 44% 22% 3	400
	21st to end	Night	7% 2 18% 50% 23% 1	400
July	1st to 10th	Night	8% 1 16% 47% 27% 2	380
	11th to 20th	Night	8% 1 10% 44% 34% 4	500
	21st to end	Night	11% 2 14% 44% 26% 3	624
August	1st to 10th	Night	11% 3 14% 40% 29% 4	576
	11th to 20th	Night	12% 2 13% 40% 29% 4	750
	21st to end	Night	13% 2 13% 32% 35% 5%	836
September	1st to 10th	Night	13% 2 14% 37% 27% 7%	760
	11th to 20th	Night	15% 4 15% 34% 27% 5%	760
	21st to end	Night	21% 6% 19% 31% 23% 1	760
October	1st to 10th	Night	16% 6% 18% 32% 24% 4	760
	11th to 20th	Night	18% 8% 24% 31% 17% 2	760
	21st to end	Night	16% 8% 24% 32% 17% 3	836
November	1st to 10th	Night	17% 9% 26% 30% 15% 2	780
	11th to 20th	Night	17% 12% 31% 28% 11% 1	800
	21st to end	Night	14% 12% 30% 30% 13% 1	800
December	1st to 10th	Night	20% 12% 33% 26% 8%	800
	11th to 20th	Night	18% 12% 34% 29% 7%	800
	21st to end	Night	18% 12% 31% 31% 7%	880

Page generated at 2025-07-03 01:30:12. Version ac07

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