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 2024-03-18.

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	13% 6% 2 5% 5% 3 5% 5% 6% 8% 42%	78747
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 44%	81039
Transparency	Day	39% 8% 13% 22% 16% 3	78460
	Night	39% 7% 12% 22% 17% 3	80845
Seeing	Day	17% 11% 27% 25% 15% 5%	14378
	Night	17% 9% 27% 31% 14% 2	24092

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	17% 6% 2 4 3 2 3 4 4 5% 50%	2677
	11th to 20th	Day	12% 6% 3 6% 4 3 4 4 5% 5% 48%	2840
	21st to end	Day	13% 10% 4 7% 5% 3 4 4 4 6% 40%	3124
February	1st to 10th	Day	13% 9% 2 5% 5% 3 4 5% 5% 7% 41%	1710
	11th to 20th	Day	19% 5% 2 5% 4 3 4 3 4 6% 44%	1823
	21st to end	Day	16% 8% 3 5% 5% 2 4 6% 5% 9% 37%	1570
March	1st to 10th	Day	14% 6% 3 5% 4 2 3 3 4 6% 50%	2009
	11th to 20th	Day	16% 6% 3 4 4 3 5% 4 5% 6% 43%	2058
	21st to end	Day	17% 6% 2 4 5% 3 5% 4 5% 7% 42%	2245
April	1st to 10th	Day	13% 5% 2 4 4 3 3 4 5% 7% 51%	2160
	11th to 20th	Day	10% 4 2 5% 5% 3 4 4 5% 7% 50%	2134
	21st to end	Day	14% 5% 3 5% 5% 2 5% 5% 6% 7% 44%	2160
May	1st to 10th	Day	14% 6% 2 6% 4 2 5% 5% 5% 9% 42%	2160
	11th to 20th	Day	16% 6% 3 6% 5% 3 5% 5% 5% 7% 39%	2350
	21st to end	Day	13% 6% 2 5% 5% 3 5% 5% 6% 9% 40%	2725
June	1st to 10th	Day	11% 5% 2 4 5% 3 6% 6% 8% 11% 38%	2520
	11th to 20th	Day	11% 5% 2 6% 5% 4 6% 6% 8% 10% 39%	2520
	21st to end	Day	8% 5% 2 5% 6% 4 6% 6% 8% 11% 37%	2520
July	1st to 10th	Day	9% 4 2 5% 6% 4 5% 7% 9% 11% 38%	2520
	11th to 20th	Day	9% 5% 2 6% 6% 3 6% 8% 9% 11% 36%	2389
	21st to end	Day	7% 4 2 6% 8% 5% 8% 7% 8% 12% 34%	2547
August	1st to 10th	Day	11% 6% 3 6% 7% 4 8% 6% 8% 9% 33%	2301
	11th to 20th	Day	11% 7% 3 7% 7% 3 6% 7% 7% 10% 32%	2126
	21st to end	Day	11% 6% 3 6% 6% 4 5% 7% 8% 11% 33%	2376
September	1st to 10th	Day	13% 6% 3 5% 6% 3 5% 7% 7% 9% 37%	2160
	11th to 20th	Day	14% 6% 3 6% 6% 4 5% 5% 5% 7% 40%	2160
	21st to end	Day	21% 7% 2 5% 5% 3 5% 4 5% 6% 35%	1932
October	1st to 10th	Day	18% 6% 2 4 5% 3 4 4 4 5% 44%	1800
	11th to 20th	Day	15% 7% 2 4 4 2 4 4 4 5% 48%	1800
	21st to end	Day	12% 5% 2 6% 5% 2 4 3 5% 7% 49%	1980
November	1st to 10th	Day	16% 5% 2 4 3 2 4 4 4 6% 50%	1767
	11th to 20th	Day	16% 6% 2 3 4 2 3 4 4 6% 49%	1710
	21st to end	Day	14% 4 2 5% 4 3 4 4 5% 6% 50%	1672
December	1st to 10th	Day	16% 6% 3 6% 5% 3 4 4 5% 7% 42%	1520
	11th to 20th	Day	14% 7% 2 5% 4 2 4 3 4 5% 51%	1910
	21st to end	Day	17% 6% 2 5% 4 2 3 4 5% 7% 44%	2772
January	1st to 10th	Night	13% 7% 2 5% 5% 3 4 4 5% 6% 46%	4265
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 4 4 6% 42%	4120
	21st to end	Night	17% 7% 3 6% 5% 4 4 4 4 6% 42%	4532
February	1st to 10th	Night	17% 7% 2 5% 5% 2 4 4 5% 6% 43%	2470
	11th to 20th	Night	21% 7% 2 6% 4 2 4 4 4 6% 41%	2242
	21st to end	Night	16% 7% 3 5% 5% 3 4 3 4 6% 45%	1884
March	1st to 10th	Night	15% 5% 2 4 4 3 4 4 5% 5% 49%	2166
	11th to 20th	Night	17% 6% 2 5% 4 3 6% 5% 4 6% 41%	2068
	21st to end	Night	17% 6% 3 6% 4 3 5% 3 5% 6% 42%	2111
April	1st to 10th	Night	14% 5% 2 5% 5% 3 5% 5% 6% 6% 45%	1800
	11th to 20th	Night	13% 5% 2 5% 4 2 3 4 4 8% 49%	1775
	21st to end	Night	15% 5% 1 5% 5% 3 3 4 5% 7% 47%	1800
May	1st to 10th	Night	14% 6% 3 6% 5% 3 4 5% 6% 9% 40%	1800
	11th to 20th	Night	14% 6% 2 5% 4 3 5% 4 6% 10% 41%	1554
	21st to end	Night	12% 5% 2 5% 5% 3 5% 5% 6% 9% 43%	1550
June	1st to 10th	Night	11% 5% 3 4 5% 3 5% 5% 7% 12% 41%	1440
	11th to 20th	Night	8% 5% 3 6% 5% 3 5% 6% 8% 10% 40%	1440
	21st to end	Night	7% 4 3 5% 5% 3 6% 7% 6% 10% 43%	1440
July	1st to 10th	Night	7% 6% 3 6% 5% 3 5% 6% 7% 12% 41%	1440
	11th to 20th	Night	8% 4 2 5% 5% 4 6% 6% 7% 10% 44%	1567
	21st to end	Night	8% 5% 3 5% 6% 4 5% 7% 9% 10% 37%	1771
August	1st to 10th	Night	9% 6% 3 7% 6% 4 5% 6% 7% 9% 38%	1619
	11th to 20th	Night	9% 6% 3 7% 7% 4 5% 6% 7% 9% 36%	1774
	21st to end	Night	11% 6% 2 7% 6% 3 6% 5% 6% 9% 39%	1980
September	1st to 10th	Night	11% 5% 3 5% 5% 3 5% 4 6% 8% 45%	1800
	11th to 20th	Night	12% 6% 2 5% 5% 3 4 4 5% 7% 45%	1800
	21st to end	Night	19% 6% 3 7% 6% 3 4 4 5% 7% 36%	2024
October	1st to 10th	Night	15% 5% 2 4 4 3 4 4 4 5% 48%	2160
	11th to 20th	Night	15% 6% 2 5% 4 2 3 4 4 6% 49%	2160
	21st to end	Night	14% 5% 2 4 4 3 3 4 5% 6% 50%	2376
November	1st to 10th	Night	16% 6% 1 5% 4 2 3 4 4 7% 48%	2308
	11th to 20th	Night	15% 6% 3 5% 3 2 3 4 4 7% 47%	2470
	21st to end	Night	13% 5% 2 4 4 3 4 4 5% 6% 50%	2504
December	1st to 10th	Night	16% 8% 4 6% 5% 3 5% 3 4 6% 40%	2660
	11th to 20th	Night	17% 6% 2 4 4 2 4 3 4 6% 46%	3285
	21st to end	Night	14% 6% 2 5% 5% 3 4 5% 5% 7% 45%	4884

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	36% 5% 9% 30% 19% 3	2678
	11th to 20th	Day	38% 5% 8% 19% 26% 5%	2840
	21st to end	Day	44% 6% 11% 19% 17% 3	3124
February	1st to 10th	Day	42% 5% 12% 25% 13% 3	1710
	11th to 20th	Day	43% 5% 9% 18% 19% 6%	1832
	21st to end	Day	42% 7% 12% 17% 15% 5%	1570
March	1st to 10th	Day	37% 4 11% 24% 20% 4	2009
	11th to 20th	Day	40% 6% 11% 19% 18% 6%	2058
	21st to end	Day	42% 6% 13% 19% 18% 3	2245
April	1st to 10th	Day	33% 5% 14% 22% 21% 5%	2160
	11th to 20th	Day	33% 6% 12% 23% 20% 5%	2134
	21st to end	Day	38% 7% 12% 21% 17% 5%	2160
May	1st to 10th	Day	38% 9% 15% 21% 14% 3	2160
	11th to 20th	Day	44% 7% 14% 23% 9% 2	2354
	21st to end	Day	39% 9% 15% 24% 12% 1	2725
June	1st to 10th	Day	35% 11% 17% 22% 13% 1	2520
	11th to 20th	Day	37% 10% 18% 21% 14% 2	2520
	21st to end	Day	36% 11% 17% 22% 13% 1	2520
July	1st to 10th	Day	34% 14% 16% 22% 12% 2	2450
	11th to 20th	Day	36% 12% 21% 22% 9% 1	2317
	21st to end	Day	37% 12% 19% 21% 11% 1	2405
August	1st to 10th	Day	42% 12% 18% 21% 7%	2288
	11th to 20th	Day	44% 10% 15% 20% 10% 1	2126
	21st to end	Day	41% 11% 16% 16% 12% 3	2376
September	1st to 10th	Day	41% 10% 15% 17% 15% 3	2160
	11th to 20th	Day	43% 7% 10% 21% 17% 2	2160
	21st to end	Day	47% 7% 11% 19% 14% 2	1932
October	1st to 10th	Day	42% 5% 9% 21% 20% 3	1800
	11th to 20th	Day	38% 6% 8% 26% 19% 4	1800
	21st to end	Day	35% 6% 8% 22% 24% 5%	1980
November	1st to 10th	Day	37% 5% 11% 25% 19% 3	1767
	11th to 20th	Day	37% 5% 13% 20% 20% 5%	1710
	21st to end	Day	37% 4 10% 25% 20% 4	1672
December	1st to 10th	Day	43% 5% 13% 21% 15% 3	1520
	11th to 20th	Day	36% 5% 9% 27% 16% 7%	1906
	21st to end	Day	39% 5% 10% 28% 16% 2	2772
January	1st to 10th	Night	36% 6% 9% 25% 19% 4	4265
	11th to 20th	Night	43% 6% 8% 20% 21% 3	4120
	21st to end	Night	43% 6% 9% 18% 18% 5%	4532
February	1st to 10th	Night	41% 6% 10% 24% 16% 3	2470
	11th to 20th	Night	46% 4 8% 20% 17% 5%	2244
	21st to end	Night	41% 5% 9% 20% 18% 6%	1884
March	1st to 10th	Night	38% 5% 11% 23% 19% 4	2166
	11th to 20th	Night	43% 7% 12% 17% 16% 5%	2068
	21st to end	Night	42% 7% 10% 21% 16% 4	2111
April	1st to 10th	Night	38% 7% 12% 18% 22% 4	1800
	11th to 20th	Night	34% 6% 11% 24% 20% 6%	1775
	21st to end	Night	38% 6% 12% 23% 17% 4	1800
May	1st to 10th	Night	39% 10% 13% 23% 13% 3	1800
	11th to 20th	Night	40% 10% 15% 21% 12% 2	1555
	21st to end	Night	35% 12% 16% 24% 11% 2	1551
June	1st to 10th	Night	35% 9% 16% 26% 13% 1	1440
	11th to 20th	Night	36% 12% 18% 20% 13% 1	1440
	21st to end	Night	34% 11% 18% 23% 13% 2	1440
July	1st to 10th	Night	35% 12% 18% 22% 10% 3	1400
	11th to 20th	Night	30% 12% 21% 24% 11% 1	1516
	21st to end	Night	33% 15% 17% 24% 9% 1	1672
August	1st to 10th	Night	39% 11% 21% 21% 7%	1608
	11th to 20th	Night	40% 12% 17% 21% 10%	1774
	21st to end	Night	41% 10% 16% 21% 10% 2	1980
September	1st to 10th	Night	36% 9% 17% 21% 14% 3	1800
	11th to 20th	Night	40% 7% 11% 22% 18% 2	1800
	21st to end	Night	47% 7% 12% 18% 14% 2	2024
October	1st to 10th	Night	38% 7% 9% 22% 21% 3	2160
	11th to 20th	Night	36% 6% 9% 23% 22% 4	2160
	21st to end	Night	35% 5% 9% 23% 22% 6%	2376
November	1st to 10th	Night	38% 6% 9% 21% 22% 4	2308
	11th to 20th	Night	38% 5% 10% 20% 19% 8%	2470
	21st to end	Night	34% 6% 10% 25% 22% 4	2504
December	1st to 10th	Night	45% 6% 11% 20% 15% 3	2660
	11th to 20th	Night	39% 5% 11% 20% 19% 6%	3288
	21st to end	Night	37% 7% 14% 24% 17% 2	4884

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% 28% 35% 14% 4	225
	11th to 20th	Day	21% 24% 36% 17% 2	270
	21st to end	Day	18% 21% 40% 18% 3 1	297
February	1st to 10th	Day	19% 21% 39% 15% 6%	270
	11th to 20th	Day	26% 23% 34% 13% 3	431
	21st to end	Day	22% 21% 38% 16% 3 1	382
March	1st to 10th	Day	19% 20% 38% 17% 5%	460
	11th to 20th	Day	20% 21% 35% 18% 4 2	449
	21st to end	Day	16% 18% 37% 21% 6% 1	462
April	1st to 10th	Day	17% 21% 37% 17% 6% 1	420
	11th to 20th	Day	12% 17% 35% 23% 11% 2	412
	21st to end	Day	18% 15% 31% 25% 9% 2	420
May	1st to 10th	Day	18% 13% 36% 22% 9% 2	423
	11th to 20th	Day	20% 7% 29% 31% 10% 3	447
	21st to end	Day	20% 5% 21% 28% 20% 6%	485
June	1st to 10th	Day	13% 5% 22% 32% 24% 4	450
	11th to 20th	Day	13% 9% 19% 31% 21% 7%	450
	21st to end	Day	10% 3 24% 36% 21% 6%	450
July	1st to 10th	Day	11% 1 13% 37% 30% 8%	445
	11th to 20th	Day	8% 3 9% 33% 36% 11%	444
	21st to end	Day	9% 2 16% 35% 30% 7%	488
August	1st to 10th	Day	13% 2 15% 35% 27% 9%	446
	11th to 20th	Day	13% 2 14% 30% 28% 14%	439
	21st to end	Day	14% 4 14% 29% 28% 12%	495
September	1st to 10th	Day	14% 3 20% 28% 20% 15%	450
	11th to 20th	Day	16% 4 15% 32% 23% 9%	450
	21st to end	Day	24% 8% 21% 24% 17% 6%	450
October	1st to 10th	Day	23% 5% 20% 24% 21% 8%	450
	11th to 20th	Day	17% 10% 26% 26% 18% 3	450
	21st to end	Day	17% 11% 32% 24% 12% 4	495
November	1st to 10th	Day	20% 10% 28% 25% 13% 4	413
	11th to 20th	Day	19% 15% 31% 25% 8% 2	370
	21st to end	Day	16% 17% 28% 23% 13% 2	332
December	1st to 10th	Day	22% 14% 42% 15% 6% 1	180
	11th to 20th	Day	16% 20% 34% 21% 8% 1	180
	21st to end	Day	23% 15% 31% 20% 9% 2	198
January	1st to 10th	Night	18% 19% 33% 25% 5%	760
	11th to 20th	Night	21% 16% 31% 26% 5%	760
	21st to end	Night	19% 15% 38% 25% 2	836
February	1st to 10th	Night	20% 13% 40% 24% 3	760
	11th to 20th	Night	25% 20% 37% 17% 2	743
	21st to end	Night	20% 16% 37% 23% 4	628
March	1st to 10th	Night	19% 13% 36% 27% 6%	760
	11th to 20th	Night	21% 13% 39% 22% 6% 1	752
	21st to end	Night	20% 13% 35% 28% 4	792
April	1st to 10th	Night	19% 13% 39% 23% 6%	720
	11th to 20th	Night	14% 12% 38% 27% 9%	711
	21st to end	Night	19% 7% 34% 33% 7% 1	720
May	1st to 10th	Night	18% 9% 35% 31% 7% 1	720
	11th to 20th	Night	17% 4 28% 39% 11% 1	493
	21st to end	Night	16% 2 16% 45% 19% 2	389
June	1st to 10th	Night	13% 3 18% 45% 20% 2	360
	11th to 20th	Night	7% 3 20% 45% 22% 3	360
	21st to end	Night	8% 1 18% 49% 23% 1	360
July	1st to 10th	Night	8% 1 17% 45% 27% 2	350
	11th to 20th	Night	7% 1 10% 44% 34% 5%	472
	21st to end	Night	10% 2 13% 45% 27% 3	591
August	1st to 10th	Night	10% 2 13% 41% 30% 4	545
	11th to 20th	Night	12% 2 12% 39% 30% 5%	710
	21st to end	Night	13% 2 13% 32% 35% 6%	792
September	1st to 10th	Night	13% 2 13% 37% 28% 7%	720
	11th to 20th	Night	15% 3 15% 35% 27% 5%	720
	21st to end	Night	21% 6% 19% 31% 23% 2	720
October	1st to 10th	Night	16% 6% 18% 31% 24% 4	720
	11th to 20th	Night	18% 8% 24% 30% 18% 2	720
	21st to end	Night	17% 9% 25% 31% 16% 3	792
November	1st to 10th	Night	18% 9% 26% 29% 16% 2	740
	11th to 20th	Night	18% 12% 31% 28% 12% 1	760
	21st to end	Night	14% 12% 29% 30% 13% 1	760
December	1st to 10th	Night	20% 12% 33% 27% 8%	760
	11th to 20th	Night	18% 12% 35% 29% 7%	760
	21st to end	Night	19% 12% 31% 31% 7%	836

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