Bancroft
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bancroft clear sky chart from 2005-11-06 to 2025-02-25.

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	15% 12% 4 9% 5% 5% 4 3 5% 6% 30%	108840
	Night	15% 13% 5% 10% 6% 5% 4 3 5% 6% 29%	109824
Transparency	Day	52% 6% 11% 20% 10% 1	112429
	Night	54% 6% 11% 18% 10% 1	114398
Seeing	Day	24% 17% 31% 18% 8% 2	14664
	Night	22% 14% 31% 23% 9% 1	24057

By Month

Percentage Cloud Cover NE (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% 16% 5% 13% 6% 5% 6% 4 5% 5% 23%	3597
	11th to 20th	Day	10% 20% 10% 18% 8% 6% 4 2 4 4 15%	3570
	21st to end	Day	14% 18% 8% 17% 7% 5% 4 3 3 4 18%	4438
February	1st to 10th	Day	12% 18% 5% 16% 7% 5% 4 3 5% 4 20%	1800
	11th to 20th	Day	12% 17% 6% 13% 6% 5% 4 3 4 5% 26%	1753
	21st to end	Day	17% 16% 6% 11% 6% 5% 5% 3 5% 5% 23%	1522
March	1st to 10th	Day	18% 17% 4 9% 6% 3 4 3 4 7% 26%	3325
	11th to 20th	Day	10% 19% 6% 11% 6% 5% 4 3 5% 5% 26%	3480
	21st to end	Day	12% 10% 4 7% 4 4 4 3 4 5% 43%	3892
April	1st to 10th	Day	20% 15% 3 8% 5% 4 4 3 4 5% 28%	2280
	11th to 20th	Day	22% 12% 2 7% 4 3 4 3 4 5% 36%	2260
	21st to end	Day	21% 11% 4 8% 5% 4 3 2 4 5% 32%	2470
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 4 4 4 6% 33%	4325
	11th to 20th	Day	16% 10% 4 9% 5% 5% 5% 3 5% 8% 32%	4398
	21st to end	Day	15% 7% 3 6% 4 4 4 4 5% 7% 40%	4837
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 6% 7% 29%	2660
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 5% 6% 8% 35%	2660
	21st to end	Day	17% 8% 3 7% 4 4 5% 5% 6% 7% 34%	2660
July	1st to 10th	Day	12% 6% 2 8% 5% 5% 5% 4 7% 9% 34%	4440
	11th to 20th	Day	12% 6% 3 6% 4 5% 5% 4 6% 8% 41%	4440
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	4814
August	1st to 10th	Day	15% 9% 4 8% 5% 5% 5% 4 6% 7% 32%	4107
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 8% 8% 32%	4044
	21st to end	Day	10% 7% 3 6% 5% 5% 5% 4 7% 9% 39%	4276
September	1st to 10th	Day	15% 7% 3 6% 4 5% 4 3 6% 9% 39%	2280
	11th to 20th	Day	12% 6% 3 6% 5% 4 5% 3 5% 7% 45%	2280
	21st to end	Day	19% 7% 3 6% 5% 5% 5% 4 5% 6% 36%	1982
October	1st to 10th	Day	20% 11% 3 7% 4 4 4 4 5% 5% 33%	1900
	11th to 20th	Day	23% 12% 3 8% 5% 4 5% 4 4 5% 25%	1900
	21st to end	Day	27% 13% 4 8% 5% 4 4 2 4 6% 22%	2043
November	1st to 10th	Day	16% 12% 3 9% 5% 4 4 4 4 6% 31%	1661
	11th to 20th	Day	15% 16% 5% 11% 6% 4 3 3 5% 7% 25%	1600
	21st to end	Day	21% 20% 6% 10% 6% 5% 4 3 3 4 18%	1600
December	1st to 10th	Day	17% 25% 7% 13% 7% 5% 3 3 4 5% 13%	2600
	11th to 20th	Day	17% 21% 6% 14% 5% 4 3 2 3 4 20%	2986
	21st to end	Day	21% 22% 7% 12% 5% 3 2 2 3 4 19%	3960
January	1st to 10th	Night	14% 16% 7% 13% 7% 6% 4 3 5% 5% 18%	6349
	11th to 20th	Night	10% 16% 7% 19% 10% 7% 5% 3 4 5% 13%	6315
	21st to end	Night	13% 16% 9% 16% 6% 6% 4 3 4 5% 18%	6514
February	1st to 10th	Night	11% 18% 6% 11% 8% 5% 6% 4 6% 6% 21%	2600
	11th to 20th	Night	12% 14% 6% 12% 6% 5% 5% 4 4 5% 26%	2533
	21st to end	Night	13% 16% 4 11% 6% 5% 4 4 5% 6% 26%	2025
March	1st to 10th	Night	17% 14% 5% 11% 5% 4 3 3 5% 4 30%	3514
	11th to 20th	Night	13% 18% 6% 10% 5% 4 4 3 6% 6% 26%	3479
	21st to end	Night	12% 11% 4 8% 5% 5% 4 4 6% 6% 35%	3739
April	1st to 10th	Night	19% 13% 4 7% 5% 4 3 4 4 5% 31%	1900
	11th to 20th	Night	18% 10% 3 7% 4 4 4 3 4 6% 37%	1875
	21st to end	Night	18% 11% 4 6% 5% 4 5% 4 5% 6% 32%	1720
May	1st to 10th	Night	20% 9% 2 6% 4 3 3 3 6% 7% 36%	2615
	11th to 20th	Night	17% 8% 2 7% 5% 5% 4 4 6% 8% 34%	2495
	21st to end	Night	12% 7% 2 6% 5% 4 4 3 5% 7% 43%	2738
June	1st to 10th	Night	16% 8% 3 7% 5% 4 5% 5% 6% 8% 34%	1520
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 4 5% 8% 44%	1520
	21st to end	Night	15% 9% 3 7% 4 5% 4 4 5% 8% 36%	1520
July	1st to 10th	Night	9% 6% 3 8% 6% 7% 5% 4 6% 7% 41%	2520
	11th to 20th	Night	14% 6% 2 5% 4 3 4 4 4 7% 45%	2520
	21st to end	Night	9% 5% 2 6% 5% 4 4 3 6% 9% 47%	2739
August	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 7% 8% 44%	2818
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 4 5% 7% 43%	2785
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 6% 8% 44%	3347
September	1st to 10th	Night	11% 5% 2 6% 4 4 4 4 7% 7% 48%	1900
	11th to 20th	Night	13% 5% 1 5% 4 4 4 3 5% 7% 49%	1900
	21st to end	Night	18% 8% 3 6% 3 4 4 3 5% 5% 41%	2194
October	1st to 10th	Night	19% 9% 3 6% 5% 4 4 3 4 7% 38%	2280
	11th to 20th	Night	20% 12% 3 8% 6% 4 4 3 4 5% 29%	2280
	21st to end	Night	22% 13% 3 9% 5% 5% 4 4 4 6% 25%	2554
November	1st to 10th	Night	16% 12% 3 8% 6% 4 4 3 5% 6% 33%	2632
	11th to 20th	Night	17% 14% 5% 9% 5% 4 4 3 5% 6% 28%	2800
	21st to end	Night	19% 17% 5% 10% 6% 5% 5% 4 5% 5% 20%	2800
December	1st to 10th	Night	19% 20% 7% 12% 6% 5% 4 3 5% 5% 14%	4580
	11th to 20th	Night	18% 21% 6% 14% 5% 4 4 3 3 4 19%	5208
	21st to end	Night	24% 19% 6% 12% 6% 4 3 3 3 4 15%	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	61% 4 11% 15% 8% 1	3598
	11th to 20th	Day	73% 5% 7% 8% 6% 1	3600
	21st to end	Day	71% 4 6% 10% 8% 2	4438
February	1st to 10th	Day	66% 6% 8% 14% 6% 1	1800
	11th to 20th	Day	61% 5% 10% 13% 10% 2	1762
	21st to end	Day	63% 5% 8% 13% 10% 1	1522
March	1st to 10th	Day	58% 4 13% 12% 11% 1	3328
	11th to 20th	Day	57% 6% 12% 15% 9% 2	3480
	21st to end	Day	42% 5% 9% 23% 16% 4	3892
April	1st to 10th	Day	57% 4 7% 18% 10% 3	2280
	11th to 20th	Day	51% 5% 8% 21% 14% 1	2260
	21st to end	Day	55% 5% 7% 17% 13% 3	2470
May	1st to 10th	Day	49% 6% 10% 22% 11% 2	4338
	11th to 20th	Day	48% 8% 11% 22% 10% 1	4402
	21st to end	Day	41% 8% 12% 20% 16% 3	4837
June	1st to 10th	Day	52% 7% 10% 19% 11% 1	2660
	11th to 20th	Day	44% 7% 12% 22% 12% 2	2660
	21st to end	Day	47% 7% 14% 20% 12% 1	2660
July	1st to 10th	Day	43% 7% 14% 23% 12% 1	4370
	11th to 20th	Day	37% 8% 15% 30% 10%	4365
	21st to end	Day	38% 8% 11% 32% 12%	4661
August	1st to 10th	Day	49% 6% 13% 21% 10%	4093
	11th to 20th	Day	47% 7% 14% 25% 7%	4083
	21st to end	Day	39% 7% 13% 31% 10% 1	4276
September	1st to 10th	Day	42% 6% 12% 28% 12% 1	2280
	11th to 20th	Day	38% 6% 12% 29% 15% 1	2280
	21st to end	Day	47% 7% 13% 22% 11%	1982
October	1st to 10th	Day	47% 6% 11% 27% 10% 1	3160
	11th to 20th	Day	50% 5% 12% 20% 11% 1	3140
	21st to end	Day	59% 6% 12% 17% 6%	3345
November	1st to 10th	Day	53% 7% 13% 19% 7% 1	1661
	11th to 20th	Day	59% 6% 12% 14% 9% 1	1600
	21st to end	Day	69% 6% 9% 9% 6% 1	1600
December	1st to 10th	Day	75% 5% 11% 7% 3	2600
	11th to 20th	Day	68% 4 8% 12% 7%	2986
	21st to end	Day	70% 4 6% 13% 7%	3960
January	1st to 10th	Night	67% 4 10% 13% 6%	6349
	11th to 20th	Night	72% 7% 7% 7% 6% 1	6360
	21st to end	Night	68% 5% 7% 7% 11% 1	6514
February	1st to 10th	Night	61% 6% 11% 14% 7% 1	2600
	11th to 20th	Night	59% 5% 10% 17% 9% 1	2535
	21st to end	Night	57% 5% 9% 19% 9% 1	2025
March	1st to 10th	Night	56% 5% 10% 17% 11% 1	3524
	11th to 20th	Night	56% 7% 13% 16% 7% 1	3480
	21st to end	Night	46% 6% 11% 21% 15% 2	3739
April	1st to 10th	Night	54% 5% 8% 19% 12% 2	1900
	11th to 20th	Night	48% 5% 8% 22% 15% 1	1875
	21st to end	Night	51% 7% 9% 16% 15% 3	1720
May	1st to 10th	Night	46% 7% 11% 20% 13% 2	2621
	11th to 20th	Night	44% 10% 13% 22% 10% 1	2496
	21st to end	Night	38% 7% 12% 23% 18% 2	2739
June	1st to 10th	Night	45% 8% 11% 23% 12% 1	1520
	11th to 20th	Night	37% 8% 12% 24% 18% 1	1520
	21st to end	Night	45% 7% 14% 21% 12% 1	1520
July	1st to 10th	Night	40% 7% 15% 21% 16% 1	2480
	11th to 20th	Night	38% 6% 11% 33% 12%	2472
	21st to end	Night	33% 8% 15% 29% 14%	2651
August	1st to 10th	Night	36% 7% 15% 26% 15% 2	2808
	11th to 20th	Night	39% 8% 17% 26% 10% 1	2812
	21st to end	Night	35% 7% 14% 31% 11% 2	3347
September	1st to 10th	Night	33% 9% 16% 27% 14% 1	1900
	11th to 20th	Night	34% 6% 13% 33% 13% 1	1900
	21st to end	Night	44% 7% 11% 28% 11%	2194
October	1st to 10th	Night	39% 5% 11% 26% 18%	3800
	11th to 20th	Night	49% 5% 14% 22% 10%	3779
	21st to end	Night	52% 6% 16% 20% 6%	4202
November	1st to 10th	Night	51% 6% 13% 20% 9% 1	2632
	11th to 20th	Night	57% 6% 11% 16% 8% 1	2800
	21st to end	Night	65% 6% 12% 13% 4	2800
December	1st to 10th	Night	71% 6% 11% 10% 2	4580
	11th to 20th	Night	69% 4 8% 10% 7%	5208
	21st to end	Night	73% 4 6% 10% 7%	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	24% 20% 39% 14% 4 1	200
	11th to 20th	Day	28% 21% 33% 13% 5% 1	200
	21st to end	Day	27% 21% 36% 14% 3	323
February	1st to 10th	Day	31% 28% 30% 9% 2	300
	11th to 20th	Day	32% 29% 32% 7% 1	300
	21st to end	Day	35% 22% 32% 10% 1	310
March	1st to 10th	Day	25% 21% 34% 17% 3	460
	11th to 20th	Day	27% 22% 31% 15% 5% 1	460
	21st to end	Day	32% 20% 29% 12% 5% 1	506
April	1st to 10th	Day	30% 19% 33% 13% 6%	460
	11th to 20th	Day	35% 22% 21% 13% 8% 2	452
	21st to end	Day	31% 18% 31% 14% 4 2	460
May	1st to 10th	Day	27% 15% 32% 17% 5% 4	463
	11th to 20th	Day	21% 15% 35% 19% 7% 3	487
	21st to end	Day	19% 18% 32% 19% 9% 2	529
June	1st to 10th	Day	24% 14% 34% 20% 7% 1	490
	11th to 20th	Day	17% 14% 34% 22% 11% 2	490
	21st to end	Day	21% 16% 33% 19% 10% 2	490
July	1st to 10th	Day	13% 9% 35% 28% 12% 2	485
	11th to 20th	Day	15% 13% 32% 24% 13% 4	484
	21st to end	Day	14% 11% 36% 25% 12% 3	532
August	1st to 10th	Day	15% 9% 33% 26% 12% 5%	486
	11th to 20th	Day	14% 9% 26% 31% 16% 5%	479
	21st to end	Day	15% 12% 31% 24% 12% 6%	539
September	1st to 10th	Day	17% 10% 33% 20% 14% 6%	490
	11th to 20th	Day	13% 11% 24% 22% 18% 12%	490
	21st to end	Day	21% 15% 24% 23% 12% 5%	490
October	1st to 10th	Day	24% 17% 26% 18% 12% 3	490
	11th to 20th	Day	27% 22% 31% 14% 6% 1	490
	21st to end	Day	33% 19% 30% 14% 4	514
November	1st to 10th	Day	28% 17% 33% 18% 4	295
	11th to 20th	Day	30% 22% 33% 11% 5% 1	200
	21st to end	Day	30% 25% 33% 9% 3 1	200
December	1st to 10th	Day	34% 19% 32% 15% 2	200
	11th to 20th	Day	36% 25% 28% 10% 2 1	200
	21st to end	Day	35% 19% 28% 12% 4 1	220
January	1st to 10th	Night	26% 21% 34% 15% 4	800
	11th to 20th	Night	27% 18% 31% 18% 5%	800
	21st to end	Night	20% 16% 35% 21% 7% 1	880
February	1st to 10th	Night	22% 22% 37% 18% 2	800
	11th to 20th	Night	22% 20% 39% 16% 3	783
	21st to end	Night	24% 20% 35% 18% 3	647
March	1st to 10th	Night	24% 20% 35% 18% 3	760
	11th to 20th	Night	23% 19% 33% 19% 4 1	760
	21st to end	Night	21% 16% 35% 21% 8% 1	836
April	1st to 10th	Night	25% 16% 34% 19% 5%	760
	11th to 20th	Night	24% 19% 29% 19% 8% 1	751
	21st to end	Night	23% 11% 31% 24% 10% 1	580
May	1st to 10th	Night	24% 9% 29% 28% 10% 1	442
	11th to 20th	Night	16% 13% 33% 29% 8%	374
	21st to end	Night	13% 9% 36% 29% 12%	411
June	1st to 10th	Night	18% 8% 34% 30% 9% 1	380
	11th to 20th	Night	14% 11% 31% 32% 12% 1	380
	21st to end	Night	18% 10% 28% 34% 11%	380
July	1st to 10th	Night	7% 6% 34% 37% 14% 2	370
	11th to 20th	Night	12% 5% 29% 36% 17%	368
	21st to end	Night	10% 6% 31% 40% 13% 1	416
August	1st to 10th	Night	13% 4 22% 35% 25% 2	560
	11th to 20th	Night	12% 5% 21% 38% 21% 2	564
	21st to end	Night	13% 7% 30% 29% 19% 2	759
September	1st to 10th	Night	13% 7% 33% 28% 17% 2	760
	11th to 20th	Night	15% 6% 25% 28% 22% 4	760
	21st to end	Night	23% 11% 21% 26% 17% 3	760
October	1st to 10th	Night	23% 11% 28% 23% 13% 3	760
	11th to 20th	Night	27% 15% 31% 18% 8% 1	760
	21st to end	Night	30% 15% 30% 20% 5%	836
November	1st to 10th	Night	24% 16% 30% 24% 7%	780
	11th to 20th	Night	25% 17% 34% 15% 7% 2	800
	21st to end	Night	29% 18% 28% 19% 5%	800
December	1st to 10th	Night	31% 18% 33% 15% 4	800
	11th to 20th	Night	27% 19% 34% 17% 3 1	800
	21st to end	Night	34% 15% 32% 16% 3	880

Page generated at 2025-02-22 06:51:39. Version ac07

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