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 2024-11-03.

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% 40%	100958
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	97987
Transparency	Day	40% 8% 13% 21% 15% 3	100756
	Night	40% 7% 12% 21% 16% 3	97859
Seeing	Day	16% 11% 27% 26% 15% 5%	15297
	Night	17% 9% 27% 31% 14% 2	24862

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	16% 6% 2 4 3 3 3 3 5% 4 50%	2677
	11th to 20th	Day	12% 7% 3 6% 3 4 4 4 5% 5% 48%	2840
	21st to end	Day	13% 11% 4 8% 4 4 4 3 4 6% 40%	3124
February	1st to 10th	Day	13% 9% 2 6% 4 4 4 4 5% 7% 41%	1710
	11th to 20th	Day	19% 6% 2 6% 4 3 4 2 4 5% 44%	1823
	21st to end	Day	16% 9% 2 5% 5% 4 4 4 5% 9% 37%	1570
March	1st to 10th	Day	23% 10% 3 5% 3 3 3 3 4 6% 36%	3306
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 5% 6% 54%	3480
	21st to end	Day	17% 7% 2 5% 4 4 3 3 5% 6% 44%	3937
April	1st to 10th	Day	13% 5% 2 5% 4 3 3 3 5% 7% 50%	2280
	11th to 20th	Day	10% 4 2 6% 5% 4 4 3 5% 7% 50%	2254
	21st to end	Day	13% 6% 2 6% 5% 4 4 4 6% 6% 44%	2280
May	1st to 10th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 10% 34%	3765
	11th to 20th	Day	20% 7% 3 7% 5% 5% 6% 4 5% 7% 31%	4186
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 10% 45%	4823
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 10% 39%	2660
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 8% 10% 40%	2660
	21st to end	Day	7% 6% 2 6% 6% 6% 6% 5% 8% 11% 37%	2660
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 6% 10% 12% 35%	4440
	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% 5% 2 5% 2 3 4 3 4 6% 49%	1787
	11th to 20th	Day	16% 6% 2 4 4 3 3 3 4 6% 49%	1710
	21st to end	Day	14% 4 2 5% 4 3 4 3 5% 6% 50%	1672
December	1st to 10th	Day	16% 7% 3 7% 4 4 4 3 5% 7% 41%	1520
	11th to 20th	Day	14% 7% 1 5% 3 3 3 3 4 5% 51%	1910
	21st to end	Day	16% 6% 2 6% 3 3 3 3 5% 7% 44%	2772
January	1st to 10th	Night	12% 8% 2 6% 4 3 4 4 5% 6% 46%	4265
	11th to 20th	Night	16% 7% 2 7% 4 4 3 3 5% 6% 42%	4120
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 5% 42%	4532
February	1st to 10th	Night	16% 7% 2 6% 4 3 4 4 5% 5% 43%	2470
	11th to 20th	Night	20% 8% 2 6% 4 3 3 3 4 6% 41%	2242
	21st to end	Night	15% 8% 3 6% 4 4 3 3 4 5% 45%	1884
March	1st to 10th	Night	25% 8% 3 6% 4 4 4 3 5% 5% 33%	3542
	11th to 20th	Night	14% 6% 2 5% 3 4 4 3 4 5% 50%	3480
	21st to end	Night	18% 8% 2 5% 3 3 3 2 4 4 47%	3695
April	1st to 10th	Night	14% 6% 2 6% 3 4 5% 4 6% 6% 45%	1900
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 8% 49%	1875
	21st to end	Night	15% 5% 1 6% 4 4 3 3 4 7% 47%	1900
May	1st to 10th	Night	14% 8% 3 8% 6% 4 5% 4 5% 8% 34%	3130
	11th to 20th	Night	21% 8% 2 6% 4 4 4 4 5% 7% 34%	2732
	21st to end	Night	10% 6% 3 6% 5% 5% 5% 4 6% 10% 41%	2732
June	1st to 10th	Night	10% 6% 2 5% 5% 4 5% 4 8% 11% 40%	1520
	11th to 20th	Night	7% 5% 3 6% 5% 4 6% 5% 8% 11% 41%	1520
	21st to end	Night	7% 5% 2 6% 5% 4 5% 6% 7% 9% 44%	1520
July	1st to 10th	Night	7% 6% 2 7% 5% 5% 6% 5% 8% 12% 37%	2520
	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	16% 6% 1 5% 4 3 4 3 5% 6% 47%	2333
	11th to 20th	Night	15% 6% 3 6% 3 3 3 3 5% 7% 47%	2470
	21st to end	Night	13% 5% 1 5% 4 4 4 3 5% 6% 50%	2504
December	1st to 10th	Night	15% 8% 4 7% 5% 4 4 3 5% 6% 40%	2660
	11th to 20th	Night	17% 6% 2 5% 3 4 4 3 5% 6% 46%	3285
	21st to end	Night	14% 6% 2 6% 4 4 4 4 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	48% 6% 10% 17% 14% 5%	3306
	11th to 20th	Day	30% 6% 9% 20% 25% 9%	3480
	21st to end	Day	41% 7% 14% 18% 18% 3	3937
April	1st to 10th	Day	33% 6% 14% 21% 22% 5%	2280
	11th to 20th	Day	33% 6% 11% 24% 20% 5%	2254
	21st to end	Day	38% 7% 12% 22% 16% 4	2280
May	1st to 10th	Day	44% 11% 16% 19% 8% 2	3800
	11th to 20th	Day	48% 10% 13% 20% 7% 1	4190
	21st to end	Day	31% 10% 15% 26% 14% 2	4837
June	1st to 10th	Day	35% 11% 17% 23% 14% 1	2660
	11th to 20th	Day	35% 10% 17% 22% 14% 1	2660
	21st to end	Day	35% 11% 17% 23% 13% 1	2660
July	1st to 10th	Day	36% 12% 20% 20% 10% 2	4370
	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	41% 5% 9% 21% 20% 3	1900
	11th to 20th	Day	37% 5% 8% 25% 21% 5%	1900
	21st to end	Day	33% 6% 9% 22% 24% 6%	2090
November	1st to 10th	Day	37% 5% 11% 25% 19% 3	1787
	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	52% 6% 10% 17% 13% 2	3542
	11th to 20th	Night	35% 6% 9% 17% 23% 9%	3480
	21st to end	Night	41% 6% 7% 26% 16% 4	3695
April	1st to 10th	Night	38% 7% 12% 17% 22% 4	1900
	11th to 20th	Night	34% 6% 11% 23% 20% 6%	1875
	21st to end	Night	37% 6% 12% 24% 17% 4	1900
May	1st to 10th	Night	45% 12% 14% 20% 8% 2	3160
	11th to 20th	Night	47% 10% 13% 23% 7% 1	2733
	21st to end	Night	34% 12% 18% 22% 12% 1	2739
June	1st to 10th	Night	35% 10% 16% 25% 13% 1	1520
	11th to 20th	Night	35% 12% 17% 21% 14% 1	1520
	21st to end	Night	33% 10% 17% 24% 14% 2	1520
July	1st to 10th	Night	37% 13% 19% 21% 9% 2	2480
	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% 6% 9% 22% 22% 3	2280
	11th to 20th	Night	36% 6% 8% 22% 23% 5%	2280
	21st to end	Night	33% 5% 9% 24% 22% 7%	2508
November	1st to 10th	Night	38% 6% 9% 21% 22% 3	2333
	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% 22% 35% 17% 4 2	460
	21st to end	Day	17% 18% 38% 21% 5% 1	506
April	1st to 10th	Day	16% 22% 39% 16% 6% 1	460
	11th to 20th	Day	12% 17% 35% 23% 10% 2	452
	21st to end	Day	18% 15% 32% 26% 8% 2	460
May	1st to 10th	Day	17% 13% 35% 22% 10% 2	463
	11th to 20th	Day	19% 8% 29% 31% 10% 2	487
	21st to end	Day	18% 5% 23% 29% 19% 6%	529
June	1st to 10th	Day	12% 6% 23% 32% 24% 4	490
	11th to 20th	Day	12% 9% 20% 32% 20% 7%	490
	21st to end	Day	10% 3 24% 36% 21% 6%	490
July	1st to 10th	Day	10% 1 13% 37% 29% 9%	485
	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% 29% 25% 13% 4	421
	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% 21% 6% 1	760
	21st to end	Night	20% 13% 36% 27% 4	836
April	1st to 10th	Night	18% 14% 40% 22% 5%	760
	11th to 20th	Night	14% 12% 38% 26% 9%	751
	21st to end	Night	18% 7% 35% 33% 6% 1	760
May	1st to 10th	Night	18% 9% 34% 31% 6% 1	760
	11th to 20th	Night	17% 4 28% 39% 11% 1	520
	21st to end	Night	15% 2 17% 45% 20% 2	411
June	1st to 10th	Night	12% 3 18% 46% 19% 2	380
	11th to 20th	Night	7% 3 20% 46% 22% 3	380
	21st to end	Night	7% 2 18% 49% 22% 1	380
July	1st to 10th	Night	8% 1 16% 46% 27% 2	370
	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% 29% 16% 2	749
	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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