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 2025-05-26.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 31%	114620
	Night	14% 10% 4 9% 5% 5% 5% 4 5% 6% 33%	114616
Transparency	Day	50% 7% 13% 19% 11% 1	118259
	Night	49% 7% 13% 19% 11% 1	119141
Seeing	Day	20% 18% 31% 19% 8% 3	15384
	Night	20% 16% 31% 23% 9% 1	24566

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% 12% 6% 14% 7% 5% 5% 4 5% 7% 23%	3597
	11th to 20th	Day	10% 18% 7% 17% 10% 7% 6% 3 5% 4 13%	3934
	21st to end	Day	17% 17% 6% 15% 6% 5% 5% 3 4 5% 16%	4466
February	1st to 10th	Day	14% 17% 5% 13% 8% 6% 5% 5% 5% 5% 19%	1800
	11th to 20th	Day	12% 14% 6% 14% 8% 5% 5% 4 5% 7% 21%	1753
	21st to end	Day	17% 16% 4 11% 5% 4 4 4 5% 5% 24%	1544
March	1st to 10th	Day	18% 12% 5% 9% 6% 4 4 3 4 5% 31%	4932
	11th to 20th	Day	12% 13% 4 9% 5% 5% 5% 4 6% 7% 31%	4980
	21st to end	Day	17% 13% 3 8% 5% 4 4 3 4 4 33%	5465
April	1st to 10th	Day	16% 13% 4 8% 5% 5% 4 3 5% 6% 29%	2350
	11th to 20th	Day	18% 11% 2 8% 5% 4 4 3 4 6% 35%	2472
	21st to end	Day	20% 9% 3 8% 4 4 4 3 5% 6% 34%	2600
May	1st to 10th	Day	18% 8% 3 7% 4 4 4 4 6% 8% 34%	4238
	11th to 20th	Day	17% 8% 2 8% 6% 5% 4 4 5% 7% 32%	4495
	21st to end	Day	12% 5% 2 6% 4 5% 4 4 6% 7% 46%	4907
June	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 5% 7% 7% 37%	2660
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 4 7% 10% 40%	2700
	21st to end	Day	14% 5% 2 6% 5% 5% 4 4 6% 10% 38%	2850
July	1st to 10th	Day	11% 4 2 6% 5% 5% 5% 5% 7% 10% 39%	4614
	11th to 20th	Day	10% 4 2 5% 4 5% 5% 5% 7% 9% 44%	4440
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 8% 41%	4479
August	1st to 10th	Day	17% 6% 2 6% 5% 4 5% 4 6% 8% 36%	4104
	11th to 20th	Day	11% 6% 3 8% 6% 6% 5% 5% 7% 9% 34%	4044
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 8% 42%	4381
September	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 7% 40%	2280
	11th to 20th	Day	11% 6% 2 6% 5% 5% 4 3 6% 8% 44%	2151
	21st to end	Day	15% 7% 3 8% 7% 5% 5% 4 6% 7% 34%	2062
October	1st to 10th	Day	19% 9% 3 7% 5% 5% 4 3 6% 7% 34%	1900
	11th to 20th	Day	20% 9% 3 9% 6% 5% 5% 4 5% 6% 29%	1900
	21st to end	Day	25% 13% 3 8% 5% 5% 5% 4 5% 6% 22%	2090
November	1st to 10th	Day	14% 12% 3 7% 4 6% 5% 4 6% 8% 30%	1686
	11th to 20th	Day	14% 15% 4 10% 5% 5% 6% 4 6% 7% 25%	1600
	21st to end	Day	16% 17% 6% 11% 6% 4 5% 4 6% 6% 19%	1600
December	1st to 10th	Day	19% 19% 6% 10% 7% 6% 4 4 4 6% 15%	2600
	11th to 20th	Day	20% 20% 4 11% 5% 4 4 3 4 5% 20%	2986
	21st to end	Day	26% 18% 7% 14% 5% 4 3 3 4 4 12%	3960
January	1st to 10th	Night	14% 13% 5% 11% 7% 6% 6% 4 6% 7% 24%	6349
	11th to 20th	Night	13% 14% 6% 16% 10% 8% 6% 4 5% 5% 14%	5933
	21st to end	Night	15% 14% 6% 12% 7% 6% 6% 4 5% 6% 21%	6490
February	1st to 10th	Night	13% 16% 5% 10% 6% 5% 5% 4 5% 6% 25%	2600
	11th to 20th	Night	13% 12% 6% 10% 8% 6% 5% 4 5% 6% 25%	2533
	21st to end	Night	12% 13% 4 10% 6% 6% 5% 4 6% 6% 28%	2084
March	1st to 10th	Night	15% 9% 5% 9% 6% 6% 5% 4 5% 5% 30%	4908
	11th to 20th	Night	9% 9% 3 7% 5% 5% 6% 5% 7% 9% 36%	4979
	21st to end	Night	19% 8% 3 7% 4 3 4 3 4 6% 39%	5467
April	1st to 10th	Night	18% 11% 4 8% 5% 4 4 4 5% 6% 31%	2050
	11th to 20th	Night	16% 9% 2 6% 5% 4 5% 4 6% 6% 38%	1894
	21st to end	Night	15% 9% 4 7% 4 4 4 4 6% 7% 36%	1800
May	1st to 10th	Night	17% 8% 2 6% 4 4 5% 4 4 6% 40%	2923
	11th to 20th	Night	16% 7% 3 7% 4 4 4 3 6% 8% 39%	2618
	21st to end	Night	10% 4 2 6% 3 4 3 4 5% 8% 51%	2784
June	1st to 10th	Night	12% 6% 2 5% 6% 5% 4 4 6% 8% 42%	1520
	11th to 20th	Night	8% 5% 2 7% 4 3 5% 6% 5% 10% 44%	1472
	21st to end	Night	11% 5% 2 6% 4 4 4 3 6% 7% 47%	1330
July	1st to 10th	Night	5% 4 2 6% 4 3 4 4 6% 9% 52%	2334
	11th to 20th	Night	12% 3 1 4 4 3 4 3 5% 8% 53%	2520
	21st to end	Night	7% 4 2 5% 4 4 4 4 7% 8% 52%	3071
August	1st to 10th	Night	10% 5% 3 5% 4 4 4 3 5% 7% 49%	2825
	11th to 20th	Night	11% 4 3 5% 4 4 4 3 5% 7% 49%	2785
	21st to end	Night	12% 5% 2 5% 3 4 5% 3 5% 7% 49%	3242
September	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 7% 7% 49%	1900
	11th to 20th	Night	11% 4 2 4 3 3 4 3 5% 6% 54%	2026
	21st to end	Night	16% 6% 2 7% 6% 4 4 3 5% 6% 41%	2118
October	1st to 10th	Night	17% 7% 3 6% 4 4 4 4 6% 6% 38%	2280
	11th to 20th	Night	17% 9% 2 7% 5% 5% 5% 4 5% 6% 35%	2280
	21st to end	Night	20% 11% 3 7% 6% 5% 4 4 6% 7% 28%	2508
November	1st to 10th	Night	16% 9% 3 7% 4 5% 5% 4 6% 6% 37%	2609
	11th to 20th	Night	16% 12% 4 7% 5% 5% 5% 4 6% 8% 28%	2800
	21st to end	Night	18% 12% 5% 10% 6% 4 4 4 5% 7% 24%	2800
December	1st to 10th	Night	19% 16% 6% 10% 5% 5% 5% 4 6% 7% 17%	4580
	11th to 20th	Night	14% 20% 6% 12% 5% 4 5% 3 5% 5% 20%	5208
	21st to end	Night	24% 18% 7% 14% 5% 4 4 3 4 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	58% 7% 12% 14% 9%	3598
	11th to 20th	Day	73% 6% 9% 8% 4	3968
	21st to end	Day	70% 6% 10% 8% 6%	4466
February	1st to 10th	Day	63% 10% 9% 14% 4 1	1800
	11th to 20th	Day	60% 9% 11% 13% 6% 1	1762
	21st to end	Day	61% 7% 10% 14% 7% 1	1544
March	1st to 10th	Day	54% 6% 13% 12% 13% 1	4935
	11th to 20th	Day	49% 7% 15% 19% 7% 2	4980
	21st to end	Day	52% 6% 9% 18% 13% 2	5465
April	1st to 10th	Day	55% 6% 10% 17% 10% 1	2350
	11th to 20th	Day	49% 6% 11% 20% 14% 1	2472
	21st to end	Day	50% 7% 12% 17% 12% 2	2600
May	1st to 10th	Day	45% 9% 13% 18% 13% 2	4250
	11th to 20th	Day	47% 11% 14% 18% 9% 1	4499
	21st to end	Day	35% 9% 14% 22% 17% 3	4907
June	1st to 10th	Day	42% 9% 12% 20% 14% 1	2660
	11th to 20th	Day	36% 9% 15% 23% 15% 1	2700
	21st to end	Day	40% 9% 14% 23% 13% 1	2850
July	1st to 10th	Day	39% 5% 15% 24% 15% 2	4544
	11th to 20th	Day	34% 6% 13% 35% 11% 1	4365
	21st to end	Day	38% 7% 15% 21% 17% 1	4336
August	1st to 10th	Day	44% 7% 14% 21% 13% 1	4091
	11th to 20th	Day	43% 7% 16% 23% 10%	4083
	21st to end	Day	38% 6% 13% 32% 10% 1	4381
September	1st to 10th	Day	40% 9% 15% 25% 10% 1	2280
	11th to 20th	Day	35% 9% 15% 28% 13%	2151
	21st to end	Day	46% 9% 13% 24% 8%	2062
October	1st to 10th	Day	43% 5% 12% 26% 14% 1	3160
	11th to 20th	Day	41% 5% 14% 25% 15%	3140
	21st to end	Day	57% 5% 13% 19% 6%	3428
November	1st to 10th	Day	49% 10% 14% 18% 9% 1	1686
	11th to 20th	Day	55% 10% 14% 13% 8%	1600
	21st to end	Day	62% 9% 14% 12% 2	1600
December	1st to 10th	Day	70% 6% 14% 7% 3	2600
	11th to 20th	Day	65% 8% 12% 8% 7%	2986
	21st to end	Day	75% 6% 6% 7% 6%	3960
January	1st to 10th	Night	57% 7% 12% 19% 5%	6349
	11th to 20th	Night	67% 10% 9% 10% 4	5974
	21st to end	Night	62% 9% 10% 9% 10% 1	6490
February	1st to 10th	Night	57% 9% 12% 17% 5%	2600
	11th to 20th	Night	58% 8% 11% 16% 8%	2535
	21st to end	Night	53% 8% 14% 17% 8%	2084
March	1st to 10th	Night	53% 7% 12% 15% 11% 2	4918
	11th to 20th	Night	41% 7% 19% 22% 9% 2	4980
	21st to end	Night	46% 5% 10% 20% 17% 2	5467
April	1st to 10th	Night	53% 6% 11% 17% 12% 2	2050
	11th to 20th	Night	46% 8% 10% 20% 15% 2	1894
	21st to end	Night	45% 7% 13% 17% 16% 2	1800
May	1st to 10th	Night	42% 8% 13% 17% 16% 4	2930
	11th to 20th	Night	41% 10% 14% 21% 11% 2	2619
	21st to end	Night	29% 8% 16% 23% 21% 2	2785
June	1st to 10th	Night	37% 8% 13% 23% 17% 2	1520
	11th to 20th	Night	35% 8% 13% 23% 20% 2	1472
	21st to end	Night	36% 7% 14% 25% 17% 1	1330
July	1st to 10th	Night	29% 5% 15% 25% 21% 4	2295
	11th to 20th	Night	31% 4 13% 35% 16% 1	2472
	21st to end	Night	30% 7% 15% 31% 15% 2	2973
August	1st to 10th	Night	36% 4 14% 29% 15% 2	2814
	11th to 20th	Night	34% 5% 16% 32% 12% 1	2812
	21st to end	Night	36% 5% 12% 33% 13% 1	3242
September	1st to 10th	Night	32% 9% 15% 28% 14% 1	1900
	11th to 20th	Night	29% 8% 12% 33% 18%	2026
	21st to end	Night	42% 8% 13% 26% 11%	2118
October	1st to 10th	Night	34% 5% 8% 28% 24% 1	3800
	11th to 20th	Night	41% 5% 14% 26% 13%	3779
	21st to end	Night	42% 6% 16% 25% 10%	4120
November	1st to 10th	Night	45% 7% 16% 21% 10% 1	2609
	11th to 20th	Night	51% 9% 15% 16% 8% 1	2800
	21st to end	Night	57% 10% 17% 13% 3	2800
December	1st to 10th	Night	66% 6% 14% 11% 3	4580
	11th to 20th	Night	64% 7% 10% 10% 7%	5208
	21st to end	Night	71% 8% 8% 8% 5%	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	23% 27% 30% 16% 4 1	200
	11th to 20th	Day	23% 30% 30% 13% 4 1	280
	21st to end	Day	28% 26% 31% 12% 2	330
February	1st to 10th	Day	31% 32% 26% 9% 3	300
	11th to 20th	Day	25% 33% 36% 5% 1	300
	21st to end	Day	35% 31% 27% 7%	254
March	1st to 10th	Day	24% 23% 35% 14% 4	500
	11th to 20th	Day	24% 21% 37% 14% 4	500
	21st to end	Day	31% 21% 30% 13% 5%	550
April	1st to 10th	Day	26% 25% 32% 12% 6%	500
	11th to 20th	Day	32% 21% 25% 13% 7% 2	492
	21st to end	Day	27% 20% 32% 15% 5% 1	500
May	1st to 10th	Day	23% 16% 33% 18% 8% 2	503
	11th to 20th	Day	18% 19% 29% 20% 10% 3	527
	21st to end	Day	15% 15% 30% 26% 9% 4	549
June	1st to 10th	Day	15% 19% 33% 24% 8% 2	490
	11th to 20th	Day	15% 15% 32% 24% 12% 2	530
	21st to end	Day	16% 13% 38% 22% 7% 3	680
July	1st to 10th	Day	10% 9% 32% 28% 15% 4	594
	11th to 20th	Day	11% 9% 32% 27% 14% 6%	484
	21st to end	Day	11% 7% 36% 28% 12% 6%	532
August	1st to 10th	Day	11% 9% 28% 29% 14% 9%	486
	11th to 20th	Day	9% 8% 30% 25% 19% 9%	479
	21st to end	Day	13% 11% 29% 25% 15% 6%	539
September	1st to 10th	Day	14% 12% 29% 25% 12% 8%	490
	11th to 20th	Day	10% 9% 26% 22% 20% 12%	490
	21st to end	Day	17% 14% 29% 22% 13% 5%	490
October	1st to 10th	Day	23% 16% 25% 20% 12% 4	490
	11th to 20th	Day	23% 20% 34% 15% 6% 2	490
	21st to end	Day	31% 21% 29% 14% 4	539
November	1st to 10th	Day	24% 25% 28% 18% 4 1	276
	11th to 20th	Day	26% 25% 33% 11% 5% 1	200
	21st to end	Day	27% 24% 39% 10% 1	200
December	1st to 10th	Day	32% 27% 29% 12% 1	200
	11th to 20th	Day	29% 31% 28% 12% 1 1	200
	21st to end	Day	29% 30% 30% 8% 3	220
January	1st to 10th	Night	23% 27% 33% 14% 4	800
	11th to 20th	Night	26% 22% 32% 17% 3	800
	21st to end	Night	20% 19% 36% 19% 6%	880
February	1st to 10th	Night	22% 24% 36% 17% 1	800
	11th to 20th	Night	22% 22% 38% 15% 3	783
	21st to end	Night	21% 25% 34% 17% 3	660
March	1st to 10th	Night	20% 24% 35% 19% 3	800
	11th to 20th	Night	22% 23% 32% 18% 5%	800
	21st to end	Night	23% 18% 33% 20% 7%	880
April	1st to 10th	Night	24% 18% 36% 20% 2	800
	11th to 20th	Night	20% 18% 34% 19% 8% 1	710
	21st to end	Night	21% 15% 33% 25% 7%	600
May	1st to 10th	Night	17% 10% 30% 30% 10% 2	600
	11th to 20th	Night	17% 9% 32% 32% 9% 1	424
	21st to end	Night	11% 8% 34% 32% 14%	423
June	1st to 10th	Night	14% 8% 34% 32% 12%	380
	11th to 20th	Night	11% 9% 29% 36% 14%	380
	21st to end	Night	12% 7% 30% 36% 15%	380
July	1st to 10th	Night	6% 4 27% 42% 18% 2	370
	11th to 20th	Night	12% 6% 27% 34% 20% 2	368
	21st to end	Night	8% 4 29% 39% 18% 2	608
August	1st to 10th	Night	11% 2 24% 39% 24% 1	567
	11th to 20th	Night	9% 5% 22% 39% 23% 3	564
	21st to end	Night	13% 5% 30% 28% 22% 2	693
September	1st to 10th	Night	11% 8% 28% 30% 19% 5%	760
	11th to 20th	Night	13% 6% 24% 28% 24% 6%	760
	21st to end	Night	19% 10% 21% 30% 16% 4	760
October	1st to 10th	Night	20% 10% 31% 23% 14% 2	760
	11th to 20th	Night	21% 17% 32% 21% 8% 1	760
	21st to end	Night	26% 18% 31% 21% 4	836
November	1st to 10th	Night	21% 18% 30% 22% 8% 1	780
	11th to 20th	Night	21% 23% 30% 20% 6% 1	800
	21st to end	Night	26% 19% 32% 19% 4	800
December	1st to 10th	Night	28% 21% 33% 14% 3	800
	11th to 20th	Night	24% 22% 35% 16% 3	800
	21st to end	Night	31% 18% 32% 16% 3	880

Page generated at 2025-05-23 09:20:56. Version ac07

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