Bolton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bolton clear sky chart from 2005-11-06 to 2026-10-14.

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% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	150365
	Night	14% 10% 4 8% 5% 5% 5% 4 5% 7% 34%	141319
Transparency	Day	49% 6% 12% 21% 11% 1	153961
	Night	49% 6% 12% 20% 12% 1	145812
Seeing	Day	20% 18% 31% 19% 8% 3	17256
	Night	19% 16% 31% 24% 9% 1	26235

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	16% 13% 5% 13% 7% 5% 5% 4 5% 7% 21%	4661
	11th to 20th	Day	11% 20% 8% 16% 9% 7% 5% 3 5% 4 11%	5126
	21st to end	Day	14% 15% 6% 16% 7% 7% 5% 4 5% 6% 16%	5780
February	1st to 10th	Day	13% 16% 5% 13% 7% 6% 4 5% 5% 5% 20%	1890
	11th to 20th	Day	12% 14% 7% 14% 8% 5% 5% 4 5% 7% 21%	1843
	21st to end	Day	17% 16% 4 11% 5% 4 5% 4 5% 5% 24%	1619
March	1st to 10th	Day	18% 12% 5% 9% 6% 4 3 3 4 5% 31%	5042
	11th to 20th	Day	12% 13% 4 9% 5% 5% 5% 4 6% 7% 31%	5090
	21st to end	Day	17% 13% 4 8% 5% 4 4 3 4 4 33%	5586
April	1st to 10th	Day	16% 13% 4 9% 5% 5% 4 3 5% 6% 29%	2467
	11th to 20th	Day	18% 10% 2 8% 5% 4 4 3 4 6% 35%	2597
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 6% 34%	2730
May	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 4 5% 7% 34%	7642
	11th to 20th	Day	15% 7% 2 8% 5% 5% 4 4 5% 7% 36%	8139
	21st to end	Day	18% 5% 2 5% 4 4 4 3 5% 6% 44%	9060
June	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 5% 6% 8% 39%	2940
	11th to 20th	Day	11% 6% 2 5% 5% 4 5% 4 7% 10% 40%	2984
	21st to end	Day	14% 5% 2 6% 5% 5% 4 4 6% 9% 38%	3150
July	1st to 10th	Day	11% 5% 2 7% 5% 6% 6% 4 6% 9% 38%	8607
	11th to 20th	Day	7% 3 2 5% 4 5% 5% 4 7% 10% 49%	8280
	21st to end	Day	10% 4 2 6% 5% 5% 5% 5% 6% 8% 45%	8367
August	1st to 10th	Day	17% 7% 2 7% 5% 4 5% 4 6% 6% 37%	6014
	11th to 20th	Day	10% 5% 3 7% 6% 6% 6% 5% 7% 10% 37%	5951
	21st to end	Day	13% 5% 2 7% 6% 5% 6% 5% 6% 8% 38%	6368
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 7% 41%	2520
	11th to 20th	Day	11% 6% 2 6% 5% 5% 4 3 6% 8% 45%	2377
	21st to end	Day	14% 7% 3 8% 7% 5% 5% 4 6% 7% 35%	2279
October	1st to 10th	Day	18% 8% 3 7% 5% 5% 4 3 6% 7% 36%	2100
	11th to 20th	Day	20% 9% 3 9% 5% 5% 5% 4 5% 6% 29%	2030
	21st to end	Day	25% 13% 3 8% 5% 5% 5% 4 5% 6% 22%	2200
November	1st to 10th	Day	15% 12% 4 7% 4 5% 5% 4 6% 8% 30%	1772
	11th to 20th	Day	13% 15% 4 10% 5% 5% 6% 4 6% 7% 25%	1680
	21st to end	Day	16% 17% 5% 11% 7% 4 5% 4 6% 6% 19%	1680
December	1st to 10th	Day	19% 19% 6% 10% 7% 5% 4 4 4 6% 15%	2680
	11th to 20th	Day	19% 20% 5% 11% 5% 4 4 3 4 5% 20%	3066
	21st to end	Day	26% 18% 7% 14% 5% 4 3 3 4 4 12%	4048
January	1st to 10th	Night	13% 13% 6% 11% 6% 6% 6% 4 6% 7% 24%	8234
	11th to 20th	Night	11% 17% 7% 15% 10% 7% 5% 3 5% 5% 14%	7741
	21st to end	Night	12% 13% 6% 12% 8% 6% 6% 4 6% 6% 22%	8401
February	1st to 10th	Night	12% 15% 4 10% 6% 5% 5% 4 5% 6% 26%	2730
	11th to 20th	Night	13% 12% 6% 10% 8% 6% 5% 4 5% 6% 26%	2663
	21st to end	Night	11% 14% 4 10% 6% 6% 5% 4 6% 6% 27%	2185
March	1st to 10th	Night	15% 10% 5% 9% 6% 6% 5% 4 5% 5% 31%	5018
	11th to 20th	Night	10% 9% 3 7% 5% 5% 6% 5% 7% 9% 35%	5089
	21st to end	Night	19% 8% 3 7% 4 3 4 3 4 6% 39%	5588
April	1st to 10th	Night	18% 10% 4 8% 5% 4 4 4 5% 6% 31%	2152
	11th to 20th	Night	15% 10% 2 6% 5% 4 5% 4 6% 6% 38%	1990
	21st to end	Night	14% 8% 3 7% 4 4 4 4 6% 7% 37%	1890
May	1st to 10th	Night	19% 7% 3 6% 4 4 4 4 5% 6% 38%	5257
	11th to 20th	Night	16% 7% 3 7% 4 4 4 4 5% 7% 41%	4721
	21st to end	Night	17% 5% 2 5% 3 3 3 3 4 7% 48%	5114
June	1st to 10th	Night	12% 5% 2 4 5% 5% 4 4 6% 8% 44%	1680
	11th to 20th	Night	8% 5% 2 7% 4 4 5% 6% 6% 10% 44%	1626
	21st to end	Night	11% 5% 2 6% 4 4 4 3 6% 7% 47%	1470
July	1st to 10th	Night	7% 4 2 6% 4 4 4 4 5% 9% 50%	4338
	11th to 20th	Night	8% 3 1 4 4 4 4 4 6% 8% 53%	4680
	21st to end	Night	9% 4 2 5% 3 4 4 4 6% 8% 51%	5693
August	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 45%	4135
	11th to 20th	Night	9% 4 2 5% 4 4 5% 3 5% 7% 50%	4095
	21st to end	Night	13% 6% 2 5% 3 4 5% 4 5% 8% 45%	4710
September	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 7% 7% 49%	2100
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 7% 55%	2240
	21st to end	Night	15% 6% 2 7% 6% 4 4 3 5% 6% 42%	2341
October	1st to 10th	Night	16% 7% 3 6% 3 4 4 4 6% 7% 41%	2520
	11th to 20th	Night	18% 9% 3 7% 5% 4 5% 4 5% 6% 35%	2437
	21st to end	Night	19% 10% 3 7% 6% 5% 4 4 6% 6% 30%	2640
November	1st to 10th	Night	16% 9% 3 7% 4 5% 4 4 6% 6% 37%	2743
	11th to 20th	Night	15% 12% 4 7% 6% 5% 5% 4 6% 7% 29%	2940
	21st to end	Night	18% 13% 4 9% 6% 4 5% 4 5% 7% 24%	2940
December	1st to 10th	Night	18% 16% 6% 10% 5% 5% 5% 4 6% 7% 17%	4720
	11th to 20th	Night	14% 20% 6% 12% 5% 4 5% 3 5% 5% 21%	5348
	21st to end	Night	23% 18% 7% 14% 5% 4 4 3 4 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	62% 6% 11% 13% 9%	4662
	11th to 20th	Day	76% 4 7% 8% 4	5160
	21st to end	Day	68% 4 9% 10% 8%	5780
February	1st to 10th	Day	63% 9% 8% 15% 5% 1	1890
	11th to 20th	Day	61% 8% 11% 13% 6% 1	1852
	21st to end	Day	61% 7% 10% 14% 7% 1	1619
March	1st to 10th	Day	55% 6% 13% 13% 13% 1	5045
	11th to 20th	Day	50% 7% 15% 19% 7% 2	5090
	21st to end	Day	53% 6% 9% 18% 13% 2	5586
April	1st to 10th	Day	55% 6% 10% 17% 10% 1	2467
	11th to 20th	Day	50% 6% 10% 21% 13% 1	2597
	21st to end	Day	50% 7% 12% 18% 11% 2	2730
May	1st to 10th	Day	49% 5% 13% 18% 11% 4	7651
	11th to 20th	Day	46% 7% 12% 19% 16% 1	8143
	21st to end	Day	40% 5% 9% 27% 16% 2	9061
June	1st to 10th	Day	41% 8% 12% 21% 16% 1	2940
	11th to 20th	Day	37% 8% 15% 23% 15% 1	2984
	21st to end	Day	40% 8% 14% 24% 13% 1	3150
July	1st to 10th	Day	43% 3 15% 25% 13% 1	8538
	11th to 20th	Day	30% 3 12% 38% 16% 1	8205
	21st to end	Day	37% 4 14% 29% 15% 1	8182
August	1st to 10th	Day	47% 5% 13% 17% 14% 3	6001
	11th to 20th	Day	41% 5% 17% 28% 8%	5990
	21st to end	Day	44% 4 14% 30% 8%	6368
September	1st to 10th	Day	40% 8% 15% 26% 10% 1	2520
	11th to 20th	Day	35% 8% 15% 29% 13%	2377
	21st to end	Day	46% 8% 12% 24% 10% 1	2279
October	1st to 10th	Day	42% 5% 12% 27% 14% 1	3360
	11th to 20th	Day	41% 5% 14% 25% 15%	3270
	21st to end	Day	56% 5% 12% 19% 7%	3538
November	1st to 10th	Day	50% 9% 13% 19% 8% 1	1772
	11th to 20th	Day	55% 9% 14% 14% 8%	1680
	21st to end	Day	63% 9% 14% 12% 3	1680
December	1st to 10th	Day	70% 6% 13% 8% 3	2680
	11th to 20th	Day	65% 8% 11% 8% 7%	3066
	21st to end	Day	75% 6% 6% 7% 6%	4048
January	1st to 10th	Night	59% 6% 12% 18% 5%	8234
	11th to 20th	Night	70% 8% 7% 10% 5% 1	7782
	21st to end	Night	61% 7% 10% 10% 12% 1	8401
February	1st to 10th	Night	56% 8% 12% 18% 6% 1	2730
	11th to 20th	Night	57% 8% 11% 16% 8%	2665
	21st to end	Night	54% 7% 14% 17% 8%	2185
March	1st to 10th	Night	53% 6% 12% 16% 11% 2	5028
	11th to 20th	Night	41% 7% 19% 22% 9% 2	5090
	21st to end	Night	47% 5% 10% 20% 16% 2	5588
April	1st to 10th	Night	53% 6% 11% 17% 12% 2	2152
	11th to 20th	Night	46% 7% 10% 21% 15% 2	1990
	21st to end	Night	44% 7% 13% 19% 15% 2	1890
May	1st to 10th	Night	47% 5% 11% 17% 15% 5%	5264
	11th to 20th	Night	44% 7% 11% 22% 14% 2	4722
	21st to end	Night	38% 5% 12% 26% 17% 2	5115
June	1st to 10th	Night	36% 7% 12% 23% 19% 2	1680
	11th to 20th	Night	35% 7% 13% 24% 20% 2	1626
	21st to end	Night	36% 7% 13% 26% 17% 1	1470
July	1st to 10th	Night	33% 3 17% 28% 17% 2	4299
	11th to 20th	Night	30% 3 11% 35% 21% 1	4632
	21st to end	Night	33% 4 12% 30% 20% 1	5563
August	1st to 10th	Night	42% 3 12% 24% 15% 4	4124
	11th to 20th	Night	34% 4 16% 35% 11% 1	4122
	21st to end	Night	39% 3 16% 31% 11% 1	4710
September	1st to 10th	Night	33% 8% 15% 30% 13% 1	2100
	11th to 20th	Night	29% 7% 13% 33% 18% 1	2240
	21st to end	Night	42% 7% 12% 26% 12% 1	2341
October	1st to 10th	Night	33% 5% 8% 29% 24% 1	4040
	11th to 20th	Night	42% 5% 14% 26% 13%	3936
	21st to end	Night	42% 6% 16% 25% 11% 1	4252
November	1st to 10th	Night	45% 7% 15% 21% 11% 1	2743
	11th to 20th	Night	51% 9% 14% 16% 9% 1	2940
	21st to end	Night	58% 9% 16% 13% 3	2940
December	1st to 10th	Night	66% 5% 14% 11% 3	4720
	11th to 20th	Night	64% 7% 10% 11% 8%	5348
	21st to end	Night	71% 8% 8% 8% 5%	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% 28% 29% 15% 4	220
	11th to 20th	Day	24% 30% 29% 12% 3 1	308
	21st to end	Day	28% 26% 30% 13% 2	363
February	1st to 10th	Day	30% 29% 27% 11% 3	330
	11th to 20th	Day	25% 32% 37% 5% 1	330
	21st to end	Day	34% 32% 27% 7%	278
March	1st to 10th	Day	24% 24% 34% 14% 4	540
	11th to 20th	Day	27% 21% 35% 13% 4	540
	21st to end	Day	31% 22% 29% 12% 5%	594
April	1st to 10th	Day	26% 25% 32% 11% 5%	540
	11th to 20th	Day	32% 21% 25% 13% 6% 2	532
	21st to end	Day	27% 19% 33% 15% 5% 1	540
May	1st to 10th	Day	24% 17% 33% 18% 7% 2	543
	11th to 20th	Day	18% 19% 31% 19% 9% 3	567
	21st to end	Day	16% 17% 31% 24% 8% 4	617
June	1st to 10th	Day	14% 18% 35% 23% 8% 2	570
	11th to 20th	Day	15% 16% 32% 24% 10% 2	614
	21st to end	Day	17% 13% 38% 23% 7% 3	780
July	1st to 10th	Day	11% 10% 31% 29% 15% 3	685
	11th to 20th	Day	11% 10% 32% 27% 14% 6%	564
	21st to end	Day	10% 8% 36% 29% 11% 6%	620
August	1st to 10th	Day	12% 9% 27% 30% 15% 8%	566
	11th to 20th	Day	9% 9% 31% 25% 17% 8%	559
	21st to end	Day	14% 10% 30% 26% 15% 6%	627
September	1st to 10th	Day	14% 12% 28% 26% 12% 8%	570
	11th to 20th	Day	10% 9% 27% 23% 19% 12%	570
	21st to end	Day	17% 13% 29% 24% 13% 5%	570
October	1st to 10th	Day	22% 16% 25% 21% 12% 4	570
	11th to 20th	Day	23% 20% 34% 15% 6% 2	542
	21st to end	Day	30% 21% 29% 15% 4	583
November	1st to 10th	Day	27% 25% 27% 17% 4 1	302
	11th to 20th	Day	27% 24% 33% 10% 5%	220
	21st to end	Day	27% 24% 38% 10% 1	220
December	1st to 10th	Day	31% 25% 30% 12% 1	220
	11th to 20th	Day	30% 29% 28% 11% 1	220
	21st to end	Day	30% 30% 29% 8% 3	242
January	1st to 10th	Night	23% 26% 33% 14% 4	840
	11th to 20th	Night	26% 23% 31% 16% 3	840
	21st to end	Night	19% 20% 36% 19% 6%	924
February	1st to 10th	Night	21% 24% 37% 18% 1	840
	11th to 20th	Night	22% 22% 37% 16% 3	823
	21st to end	Night	22% 24% 34% 17% 3	692
March	1st to 10th	Night	20% 24% 34% 19% 3	840
	11th to 20th	Night	23% 22% 31% 18% 5%	840
	21st to end	Night	23% 18% 33% 20% 6%	924
April	1st to 10th	Night	25% 18% 35% 20% 2	840
	11th to 20th	Night	20% 18% 34% 19% 8% 1	746
	21st to end	Night	20% 14% 32% 26% 7%	630
May	1st to 10th	Night	18% 11% 30% 30% 10% 2	630
	11th to 20th	Night	17% 9% 33% 31% 9% 1	446
	21st to end	Night	12% 9% 33% 32% 13%	455
June	1st to 10th	Night	14% 8% 34% 32% 12%	420
	11th to 20th	Night	11% 9% 30% 35% 14%	420
	21st to end	Night	12% 7% 30% 38% 14%	420
July	1st to 10th	Night	6% 4 26% 44% 18% 1	410
	11th to 20th	Night	11% 6% 27% 35% 19% 1	408
	21st to end	Night	8% 5% 29% 39% 18% 2	672
August	1st to 10th	Night	11% 2 22% 39% 25% 1	627
	11th to 20th	Night	9% 5% 22% 40% 22% 3	624
	21st to end	Night	12% 5% 29% 30% 22% 2	767
September	1st to 10th	Night	10% 8% 27% 31% 19% 4	840
	11th to 20th	Night	12% 7% 23% 30% 24% 5%	840
	21st to end	Night	18% 10% 21% 31% 16% 4	840
October	1st to 10th	Night	19% 10% 31% 24% 15% 2	840
	11th to 20th	Night	21% 17% 32% 21% 8% 1	813
	21st to end	Night	25% 17% 32% 22% 4	880
November	1st to 10th	Night	22% 18% 30% 22% 7% 1	820
	11th to 20th	Night	22% 23% 29% 20% 6% 1	840
	21st to end	Night	26% 19% 32% 19% 4	840
December	1st to 10th	Night	28% 21% 33% 15% 3	840
	11th to 20th	Night	24% 22% 35% 16% 3	840
	21st to end	Night	31% 18% 32% 16% 3	924

Page generated at 2026-10-11 05:06:28. Version ac07

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