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 2026-07-01.

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% 31%	138387
	Night	15% 12% 5% 10% 6% 5% 4 3 5% 6% 29%	133946
Transparency	Day	52% 6% 11% 20% 11% 1	141929
	Night	54% 6% 11% 18% 11% 1	138491
Seeing	Day	24% 17% 31% 18% 8% 2	16432
	Night	22% 14% 31% 23% 9% 1	25675

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	13% 16% 4 13% 7% 6% 6% 4 5% 5% 21%	4661
	11th to 20th	Day	10% 24% 11% 18% 7% 6% 4 2 3 4 12%	4650
	21st to end	Day	11% 15% 8% 18% 8% 7% 6% 3 4 3 17%	5738
February	1st to 10th	Day	12% 17% 6% 16% 6% 5% 4 3 5% 4 21%	1890
	11th to 20th	Day	12% 16% 6% 13% 6% 4 4 3 5% 6% 25%	1843
	21st to end	Day	16% 16% 6% 11% 6% 5% 5% 4 4 5% 23%	1638
March	1st to 10th	Day	17% 15% 4 11% 6% 3 4 3 4 6% 26%	4875
	11th to 20th	Day	11% 15% 5% 11% 6% 5% 5% 3 5% 7% 26%	5090
	21st to end	Day	17% 14% 4 8% 4 4 3 2 3 4 37%	5693
April	1st to 10th	Day	20% 15% 4 8% 5% 4 4 3 4 5% 28%	2520
	11th to 20th	Day	21% 13% 3 7% 4 3 4 3 4 4 35%	2501
	21st to end	Day	21% 12% 3 8% 5% 4 3 2 4 5% 33%	2730
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 5% 4 5% 7% 32%	8053
	11th to 20th	Day	17% 9% 3 8% 5% 4 5% 3 5% 7% 34%	8221
	21st to end	Day	19% 9% 3 6% 4 4 5% 3 5% 6% 36%	9060
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 4 4 6% 7% 30%	2940
	11th to 20th	Day	15% 6% 3 7% 5% 5% 5% 5% 6% 8% 35%	2940
	21st to end	Day	16% 8% 3 7% 4 4 5% 5% 6% 8% 33%	2940
July	1st to 10th	Day	13% 7% 2 8% 5% 5% 5% 4 7% 9% 35%	6360
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 6% 8% 44%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 45%	6926
August	1st to 10th	Day	15% 8% 4 8% 5% 5% 5% 4 6% 7% 33%	4237
	11th to 20th	Day	16% 8% 2 7% 5% 5% 5% 4 8% 7% 32%	4174
	21st to end	Day	10% 7% 3 6% 5% 5% 5% 5% 7% 9% 39%	4411
September	1st to 10th	Day	15% 7% 3 6% 4 5% 4 3 6% 9% 39%	2400
	11th to 20th	Day	12% 6% 3 6% 5% 3 5% 3 5% 7% 46%	2400
	21st to end	Day	19% 7% 3 6% 5% 5% 4 4 5% 6% 36%	2086
October	1st to 10th	Day	19% 10% 3 7% 4 4 4 4 5% 6% 35%	2000
	11th to 20th	Day	23% 12% 3 8% 5% 4 5% 4 5% 5% 26%	2000
	21st to end	Day	27% 13% 4 8% 5% 4 4 2 4 6% 23%	2150
November	1st to 10th	Day	16% 13% 4 9% 5% 4 4 4 4 6% 31%	1746
	11th to 20th	Day	15% 16% 6% 11% 6% 4 3 3 6% 6% 25%	1680
	21st to end	Day	20% 21% 6% 10% 6% 5% 3 3 4 4 18%	1680
December	1st to 10th	Day	17% 25% 7% 13% 7% 5% 3 3 4 5% 13%	2680
	11th to 20th	Day	17% 21% 6% 14% 5% 4 3 2 3 4 20%	3066
	21st to end	Day	21% 22% 7% 12% 5% 3 2 2 3 4 19%	4048
January	1st to 10th	Night	13% 16% 8% 16% 8% 6% 4 3 4 5% 17%	8234
	11th to 20th	Night	9% 19% 8% 19% 9% 7% 5% 3 4 4 13%	8235
	21st to end	Night	10% 14% 8% 15% 8% 7% 5% 3 5% 5% 20%	8425
February	1st to 10th	Night	11% 17% 6% 11% 7% 5% 6% 4 6% 6% 22%	2730
	11th to 20th	Night	12% 14% 6% 12% 6% 5% 5% 4 4 5% 26%	2663
	21st to end	Night	13% 17% 4 11% 6% 5% 4 4 5% 6% 26%	2166
March	1st to 10th	Night	17% 15% 5% 11% 5% 4 3 2 4 4 29%	5184
	11th to 20th	Night	10% 14% 4 10% 6% 5% 5% 4 7% 8% 27%	5089
	21st to end	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 30%	5480
April	1st to 10th	Night	19% 13% 4 8% 5% 5% 3 4 5% 5% 30%	2100
	11th to 20th	Night	18% 11% 3 8% 4 4 4 3 4 6% 36%	2075
	21st to end	Night	18% 10% 4 6% 5% 5% 5% 4 5% 6% 32%	1901
May	1st to 10th	Night	20% 9% 3 6% 4 4 4 4 6% 7% 33%	4845
	11th to 20th	Night	15% 8% 2 7% 4 4 4 3 5% 7% 40%	4639
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 7% 39%	5114
June	1st to 10th	Night	15% 8% 2 7% 5% 4 4 5% 6% 7% 36%	1680
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 5% 8% 44%	1680
	21st to end	Night	14% 8% 3 7% 4 5% 4 4 5% 8% 37%	1680
July	1st to 10th	Night	12% 6% 2 7% 5% 6% 6% 4 7% 8% 38%	3600
	11th to 20th	Night	13% 6% 2 5% 4 3 4 4 5% 7% 47%	3600
	21st to end	Night	8% 5% 2 6% 4 4 4 3 5% 8% 51%	3926
August	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	2908
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 4 5% 7% 44%	2875
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	3453
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 7% 7% 48%	2000
	11th to 20th	Night	13% 5% 1 5% 4 4 4 3 5% 7% 50%	2000
	21st to end	Night	18% 8% 2 6% 3 4 4 3 5% 5% 41%	2310
October	1st to 10th	Night	18% 8% 3 6% 4 3 4 3 4 7% 39%	2400
	11th to 20th	Night	20% 12% 3 9% 6% 4 4 3 4 5% 30%	2400
	21st to end	Night	22% 13% 3 9% 5% 4 4 4 5% 6% 27%	2689
November	1st to 10th	Night	16% 12% 3 8% 6% 4 4 3 5% 6% 32%	2767
	11th to 20th	Night	17% 14% 5% 9% 5% 4 4 3 5% 6% 28%	2940
	21st to end	Night	19% 18% 5% 10% 6% 5% 5% 4 5% 5% 19%	2940
December	1st to 10th	Night	18% 20% 7% 12% 6% 5% 4 3 5% 5% 14%	4720
	11th to 20th	Night	17% 20% 6% 14% 5% 4 4 3 3 5% 19%	5348
	21st to end	Night	23% 19% 6% 12% 6% 4 3 3 3 4 15%	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	63% 4 11% 14% 7% 1	4662
	11th to 20th	Day	77% 4 6% 6% 7% 1	4680
	21st to end	Day	71% 4 7% 10% 7% 1	5738
February	1st to 10th	Day	65% 5% 8% 14% 7% 1	1890
	11th to 20th	Day	61% 5% 10% 13% 10% 1	1852
	21st to end	Day	63% 4 8% 13% 10% 1	1638
March	1st to 10th	Day	59% 4 12% 9% 14% 2	4878
	11th to 20th	Day	56% 5% 14% 15% 8% 2	5090
	21st to end	Day	52% 4 7% 19% 15% 3	5693
April	1st to 10th	Day	58% 4 7% 17% 11% 3	2520
	11th to 20th	Day	53% 4 8% 20% 14% 1	2501
	21st to end	Day	54% 5% 7% 18% 13% 2	2730
May	1st to 10th	Day	50% 6% 10% 20% 11% 4	8063
	11th to 20th	Day	49% 6% 10% 20% 15% 1	8225
	21st to end	Day	48% 6% 10% 22% 13% 2	9061
June	1st to 10th	Day	51% 6% 9% 19% 13% 1	2940
	11th to 20th	Day	44% 7% 12% 23% 12% 2	2940
	21st to end	Day	46% 7% 14% 20% 11% 1	2940
July	1st to 10th	Day	43% 6% 14% 25% 11% 1	6290
	11th to 20th	Day	36% 7% 13% 28% 16%	6285
	21st to end	Day	34% 7% 13% 35% 11%	6728
August	1st to 10th	Day	48% 6% 13% 22% 10%	4223
	11th to 20th	Day	46% 7% 13% 25% 8%	4213
	21st to end	Day	38% 7% 14% 32% 10% 1	4411
September	1st to 10th	Day	41% 6% 12% 29% 11% 1	2400
	11th to 20th	Day	37% 6% 12% 30% 15% 1	2400
	21st to end	Day	48% 7% 12% 22% 11%	2086
October	1st to 10th	Day	46% 6% 11% 27% 10%	3260
	11th to 20th	Day	50% 5% 12% 20% 12% 1	3240
	21st to end	Day	59% 6% 12% 18% 6%	3452
November	1st to 10th	Day	54% 6% 13% 19% 7% 1	1746
	11th to 20th	Day	59% 6% 12% 13% 9% 1	1680
	21st to end	Day	69% 6% 9% 10% 6% 1	1680
December	1st to 10th	Day	74% 5% 10% 7% 3	2680
	11th to 20th	Day	68% 4 8% 12% 7%	3066
	21st to end	Day	70% 3 6% 13% 7%	4048
January	1st to 10th	Night	69% 4 9% 12% 6%	8234
	11th to 20th	Night	74% 6% 6% 7% 7% 1	8280
	21st to end	Night	64% 5% 8% 10% 12% 1	8425
February	1st to 10th	Night	60% 6% 11% 15% 7% 1	2730
	11th to 20th	Night	59% 5% 9% 18% 9%	2665
	21st to end	Night	57% 5% 9% 19% 9% 1	2166
March	1st to 10th	Night	58% 4 8% 14% 12% 5%	5194
	11th to 20th	Night	52% 7% 16% 17% 7% 1	5090
	21st to end	Night	52% 5% 10% 18% 13% 2	5480
April	1st to 10th	Night	55% 5% 9% 18% 12% 2	2100
	11th to 20th	Night	49% 5% 8% 23% 14% 1	2075
	21st to end	Night	51% 6% 9% 18% 14% 2	1901
May	1st to 10th	Night	48% 6% 10% 21% 12% 3	4851
	11th to 20th	Night	42% 8% 12% 22% 15% 2	4640
	21st to end	Night	43% 6% 11% 23% 16% 1	5115
June	1st to 10th	Night	44% 7% 11% 23% 14% 1	1680
	11th to 20th	Night	38% 7% 12% 25% 17% 1	1680
	21st to end	Night	45% 7% 13% 21% 12% 1	1680
July	1st to 10th	Night	42% 6% 15% 21% 15% 1	3560
	11th to 20th	Night	35% 6% 12% 32% 15%	3552
	21st to end	Night	31% 6% 12% 36% 14%	3809
August	1st to 10th	Night	35% 7% 15% 26% 15% 2	2898
	11th to 20th	Night	39% 8% 16% 26% 10% 1	2902
	21st to end	Night	35% 7% 14% 31% 11% 2	3453
September	1st to 10th	Night	33% 9% 16% 28% 13% 1	2000
	11th to 20th	Night	33% 6% 13% 34% 13% 1	2000
	21st to end	Night	44% 6% 10% 27% 12%	2310
October	1st to 10th	Night	38% 5% 11% 27% 18%	3920
	11th to 20th	Night	48% 5% 14% 22% 10%	3899
	21st to end	Night	51% 6% 16% 20% 7%	4337
November	1st to 10th	Night	52% 6% 13% 20% 9% 1	2767
	11th to 20th	Night	57% 6% 11% 16% 8% 1	2940
	21st to end	Night	65% 6% 11% 13% 4	2940
December	1st to 10th	Night	71% 6% 11% 10% 2	4720
	11th to 20th	Night	69% 4 9% 11% 8%	5348
	21st to end	Night	73% 4 6% 10% 7%	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	25% 20% 39% 13% 3	220
	11th to 20th	Day	30% 20% 33% 12% 4	220
	21st to end	Day	26% 21% 35% 14% 3	355
February	1st to 10th	Day	30% 26% 31% 10% 2 1	330
	11th to 20th	Day	30% 28% 34% 7% 1	330
	21st to end	Day	33% 22% 34% 10% 1	354
March	1st to 10th	Day	26% 21% 34% 16% 3	540
	11th to 20th	Day	31% 21% 29% 14% 4 1	540
	21st to end	Day	33% 20% 29% 12% 5% 1	594
April	1st to 10th	Day	32% 20% 31% 12% 5%	540
	11th to 20th	Day	36% 22% 21% 13% 7% 2	532
	21st to end	Day	32% 18% 31% 14% 4 1	540
May	1st to 10th	Day	28% 16% 33% 15% 5% 3	543
	11th to 20th	Day	21% 17% 36% 18% 6% 3	567
	21st to end	Day	22% 19% 30% 19% 8% 2	617
June	1st to 10th	Day	24% 14% 34% 19% 8% 1	570
	11th to 20th	Day	17% 16% 35% 21% 9% 2	570
	21st to end	Day	20% 16% 33% 20% 9% 2	570
July	1st to 10th	Day	13% 9% 36% 28% 12% 2	525
	11th to 20th	Day	15% 13% 32% 25% 12% 3	524
	21st to end	Day	13% 12% 37% 24% 11% 3	576
August	1st to 10th	Day	14% 9% 33% 28% 13% 5%	526
	11th to 20th	Day	14% 9% 25% 31% 15% 5%	519
	21st to end	Day	15% 11% 32% 23% 12% 6%	583
September	1st to 10th	Day	18% 10% 31% 21% 14% 6%	530
	11th to 20th	Day	13% 10% 24% 23% 19% 12%	530
	21st to end	Day	21% 15% 25% 22% 12% 5%	530
October	1st to 10th	Day	23% 16% 25% 20% 12% 3	530
	11th to 20th	Day	25% 22% 33% 14% 5% 1	530
	21st to end	Day	32% 18% 31% 16% 3	555
November	1st to 10th	Day	29% 17% 33% 17% 4	320
	11th to 20th	Day	30% 22% 33% 10% 5%	220
	21st to end	Day	32% 24% 33% 8% 2 1	220
December	1st to 10th	Day	33% 20% 31% 15% 2	220
	11th to 20th	Day	36% 25% 29% 9% 1 1	220
	21st to end	Day	36% 20% 27% 12% 4 1	242
January	1st to 10th	Night	26% 21% 34% 16% 4	840
	11th to 20th	Night	27% 19% 31% 17% 5%	840
	21st to end	Night	19% 17% 35% 21% 6% 1	924
February	1st to 10th	Night	21% 21% 37% 18% 2	840
	11th to 20th	Night	22% 19% 39% 17% 3	823
	21st to end	Night	24% 21% 34% 18% 3	692
March	1st to 10th	Night	24% 21% 34% 18% 3	840
	11th to 20th	Night	23% 19% 34% 19% 4 1	840
	21st to end	Night	22% 16% 34% 20% 7% 1	924
April	1st to 10th	Night	25% 16% 34% 19% 5%	840
	11th to 20th	Night	24% 19% 30% 19% 7%	831
	21st to end	Night	23% 12% 32% 24% 9%	641
May	1st to 10th	Night	23% 10% 27% 27% 11% 1	488
	11th to 20th	Night	17% 13% 34% 29% 7%	414
	21st to end	Night	15% 9% 36% 29% 11%	455
June	1st to 10th	Night	19% 8% 34% 31% 9%	420
	11th to 20th	Night	15% 11% 31% 31% 11%	420
	21st to end	Night	17% 9% 29% 35% 10%	420
July	1st to 10th	Night	7% 6% 35% 37% 14% 2	390
	11th to 20th	Night	12% 5% 29% 37% 17%	388
	21st to end	Night	10% 6% 32% 40% 12% 1	438
August	1st to 10th	Night	12% 4 21% 37% 24% 2	590
	11th to 20th	Night	12% 4 21% 39% 21% 2	594
	21st to end	Night	13% 7% 30% 29% 19% 2	799
September	1st to 10th	Night	12% 7% 33% 29% 17% 2	800
	11th to 20th	Night	14% 6% 24% 30% 22% 4	800
	21st to end	Night	23% 10% 21% 26% 17% 3	800
October	1st to 10th	Night	22% 11% 28% 24% 13% 2	800
	11th to 20th	Night	26% 15% 32% 18% 8% 1	800
	21st to end	Night	29% 15% 30% 22% 5%	880
November	1st to 10th	Night	24% 16% 29% 23% 7%	820
	11th to 20th	Night	25% 17% 34% 15% 7% 2	840
	21st to end	Night	29% 18% 28% 19% 5%	840
December	1st to 10th	Night	30% 18% 33% 15% 4	840
	11th to 20th	Night	27% 19% 35% 16% 3 1	840
	21st to end	Night	34% 15% 32% 16% 3	924

Page generated at 2026-06-28 02:10:47. Version ac07

Copyright © 2026 Attilla Danko.