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 2024-11-02.

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% 3 8% 5% 5% 5% 4 6% 7% 32%	101840
	Night	15% 10% 4 8% 5% 5% 5% 4 5% 6% 34%	97115
Transparency	Day	48% 8% 13% 19% 10% 1	101607
	Night	47% 9% 13% 19% 10% 1	96968
Seeing	Day	20% 18% 31% 19% 9% 3	14752
	Night	19% 15% 31% 24% 9% 1	23798

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% 15% 6% 9% 7% 5% 5% 3 4 7% 24%	2517
	11th to 20th	Day	11% 20% 7% 16% 9% 6% 5% 3 5% 5% 14%	2776
	21st to end	Day	21% 20% 6% 12% 6% 5% 4 2 3 5% 16%	3124
February	1st to 10th	Day	14% 17% 5% 12% 8% 6% 5% 5% 5% 5% 19%	1710
	11th to 20th	Day	12% 14% 6% 13% 7% 5% 5% 4 5% 7% 22%	1663
	21st to end	Day	17% 16% 4 11% 5% 4 5% 4 5% 5% 25%	1469
March	1st to 10th	Day	18% 13% 6% 8% 5% 5% 4 3 4 5% 29%	3432
	11th to 20th	Day	11% 15% 5% 10% 6% 5% 5% 4 5% 5% 29%	3480
	21st to end	Day	15% 11% 3 8% 5% 5% 4 4 5% 5% 36%	3815
April	1st to 10th	Day	16% 13% 4 9% 5% 5% 4 4 5% 6% 29%	2232
	11th to 20th	Day	18% 10% 2 8% 5% 4 4 3 5% 6% 36%	2347
	21st to end	Day	20% 9% 3 8% 4 4 4 3 5% 6% 34%	2470
May	1st to 10th	Day	18% 8% 3 7% 4 4 4 4 6% 8% 34%	4108
	11th to 20th	Day	17% 8% 2 8% 6% 6% 4 4 5% 7% 32%	4356
	21st to end	Day	12% 5% 2 6% 4 5% 4 4 6% 7% 46%	4837
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% 14% 3 8% 5% 5% 5% 4 5% 6% 22%	2090
November	1st to 10th	Day	14% 13% 3 7% 4 5% 5% 4 6% 9% 30%	1611
	11th to 20th	Day	13% 14% 4 10% 4 5% 6% 5% 6% 8% 24%	1520
	21st to end	Day	17% 16% 5% 10% 6% 4 4 4 6% 7% 20%	1520
December	1st to 10th	Day	16% 18% 6% 13% 7% 6% 4 4 4 6% 16%	1520
	11th to 20th	Day	17% 16% 5% 12% 6% 5% 5% 4 5% 6% 19%	1906
	21st to end	Day	28% 19% 6% 13% 6% 4 4 3 4 4 10%	2772
January	1st to 10th	Night	17% 16% 6% 11% 6% 6% 5% 3 4 6% 21%	4429
	11th to 20th	Night	14% 14% 6% 13% 9% 8% 7% 4 5% 5% 14%	4180
	21st to end	Night	19% 13% 6% 11% 8% 6% 6% 3 5% 6% 17%	4532
February	1st to 10th	Night	13% 16% 5% 10% 6% 6% 5% 4 5% 6% 25%	2470
	11th to 20th	Night	12% 12% 5% 10% 8% 6% 5% 4 5% 6% 26%	2403
	21st to end	Night	12% 13% 4 10% 6% 6% 5% 4 6% 6% 28%	1983
March	1st to 10th	Night	17% 10% 5% 8% 6% 5% 4 4 5% 5% 31%	3408
	11th to 20th	Night	12% 11% 4 7% 5% 5% 5% 4 5% 7% 37%	3479
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 6% 44%	3817
April	1st to 10th	Night	18% 11% 3 8% 5% 4 4 4 5% 6% 32%	1948
	11th to 20th	Night	15% 9% 2 6% 5% 4 5% 4 6% 6% 38%	1798
	21st to end	Night	15% 9% 4 7% 4 4 4 4 6% 8% 35%	1710
May	1st to 10th	Night	17% 7% 2 6% 4 4 5% 4 5% 6% 40%	2833
	11th to 20th	Night	16% 7% 3 6% 4 4 4 3 6% 8% 39%	2537
	21st to end	Night	9% 4 2 6% 3 4 3 4 5% 8% 52%	2738
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% 5% 4 6% 6% 28%	2508
November	1st to 10th	Night	16% 9% 3 6% 4 5% 5% 4 6% 6% 37%	2487
	11th to 20th	Night	16% 12% 3 8% 5% 5% 5% 4 6% 7% 28%	2660
	21st to end	Night	18% 12% 4 10% 6% 4 5% 4 5% 7% 25%	2660
December	1st to 10th	Night	20% 15% 4 8% 5% 5% 5% 4 6% 7% 21%	2660
	11th to 20th	Night	14% 18% 6% 11% 6% 5% 5% 4 5% 7% 20%	3288
	21st to end	Night	27% 18% 6% 12% 5% 5% 4 3 4 4 12%	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	56% 11% 16% 11% 6%	2518
	11th to 20th	Day	70% 8% 11% 8% 3	2776
	21st to end	Day	70% 8% 10% 8% 4 1	3124
February	1st to 10th	Day	63% 10% 9% 14% 3 1	1710
	11th to 20th	Day	58% 9% 12% 13% 6% 1	1672
	21st to end	Day	60% 8% 11% 14% 7% 1	1469
March	1st to 10th	Day	54% 9% 13% 15% 9%	3435
	11th to 20th	Day	51% 10% 11% 18% 8% 1	3480
	21st to end	Day	46% 9% 12% 18% 13% 2	3815
April	1st to 10th	Day	55% 7% 11% 17% 10% 1	2232
	11th to 20th	Day	48% 6% 11% 20% 14% 1	2347
	21st to end	Day	50% 7% 12% 17% 11% 2	2470
May	1st to 10th	Day	45% 9% 13% 18% 13% 1	4120
	11th to 20th	Day	47% 12% 14% 19% 8% 1	4360
	21st to end	Day	34% 10% 14% 22% 18% 3	4837
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	49% 8% 13% 18% 11% 1	1900
	11th to 20th	Day	52% 9% 13% 18% 7% 1	1900
	21st to end	Day	62% 8% 12% 13% 5%	2090
November	1st to 10th	Day	49% 10% 14% 18% 8% 1	1611
	11th to 20th	Day	54% 10% 14% 13% 8%	1520
	21st to end	Day	62% 10% 15% 12% 2	1520
December	1st to 10th	Day	67% 10% 12% 9% 2	1520
	11th to 20th	Day	61% 12% 14% 10% 4	1906
	21st to end	Day	76% 8% 8% 7% 1	2772
January	1st to 10th	Night	60% 10% 14% 12% 3	4429
	11th to 20th	Night	64% 14% 11% 8% 3	4180
	21st to end	Night	63% 12% 12% 8% 4 1	4532
February	1st to 10th	Night	57% 9% 12% 17% 5%	2470
	11th to 20th	Night	56% 9% 11% 16% 8%	2405
	21st to end	Night	53% 8% 15% 16% 8%	1983
March	1st to 10th	Night	50% 9% 13% 20% 6% 1	3418
	11th to 20th	Night	42% 10% 16% 21% 10% 2	3480
	21st to end	Night	40% 7% 12% 23% 17% 2	3817
April	1st to 10th	Night	52% 6% 11% 17% 12% 1	1948
	11th to 20th	Night	45% 8% 10% 20% 15% 2	1798
	21st to end	Night	45% 8% 13% 17% 15% 2	1710
May	1st to 10th	Night	41% 9% 14% 17% 16% 3	2840
	11th to 20th	Night	41% 11% 14% 21% 10% 2	2538
	21st to end	Night	29% 8% 16% 23% 22% 2	2739
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	44% 9% 11% 20% 16% 1	2280
	11th to 20th	Night	47% 9% 14% 22% 8%	2280
	21st to end	Night	53% 8% 13% 16% 9%	2508
November	1st to 10th	Night	45% 7% 16% 20% 11% 1	2487
	11th to 20th	Night	51% 10% 15% 16% 8% 1	2660
	21st to end	Night	56% 10% 17% 13% 3	2660
December	1st to 10th	Night	61% 10% 14% 12% 3	2660
	11th to 20th	Night	61% 10% 14% 12% 3	3288
	21st to end	Night	72% 11% 9% 6% 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	24% 24% 31% 17% 4 1	180
	11th to 20th	Day	23% 30% 29% 13% 4 1	252
	21st to end	Day	28% 25% 32% 13% 2	297
February	1st to 10th	Day	30% 31% 27% 9% 3	270
	11th to 20th	Day	23% 34% 37% 5% 1	270
	21st to end	Day	35% 32% 26% 7%	230
March	1st to 10th	Day	23% 23% 35% 15% 4	460
	11th to 20th	Day	23% 22% 38% 14% 4	460
	21st to end	Day	30% 21% 31% 13% 5%	506
April	1st to 10th	Day	24% 24% 33% 12% 6%	460
	11th to 20th	Day	33% 21% 25% 12% 7% 3	452
	21st to end	Day	27% 20% 32% 15% 5% 2	460
May	1st to 10th	Day	23% 15% 32% 19% 8% 3	463
	11th to 20th	Day	19% 17% 29% 21% 10% 3	487
	21st to end	Day	14% 16% 30% 27% 9% 5%	529
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% 22% 29% 14% 4	539
November	1st to 10th	Day	25% 24% 28% 18% 4 1	255
	11th to 20th	Day	25% 24% 33% 12% 6% 1	180
	21st to end	Day	28% 23% 38% 10% 1	180
December	1st to 10th	Day	31% 28% 29% 11% 1	180
	11th to 20th	Day	26% 32% 29% 12% 1 1	180
	21st to end	Day	28% 30% 32% 7% 3	198
January	1st to 10th	Night	23% 26% 32% 14% 4	760
	11th to 20th	Night	26% 22% 32% 17% 3	760
	21st to end	Night	19% 18% 36% 20% 6%	836
February	1st to 10th	Night	21% 24% 37% 16% 1	760
	11th to 20th	Night	22% 22% 38% 15% 3	743
	21st to end	Night	21% 25% 34% 17% 3	628
March	1st to 10th	Night	20% 23% 35% 19% 3	760
	11th to 20th	Night	22% 23% 31% 18% 5%	760
	21st to end	Night	22% 18% 33% 21% 7%	836
April	1st to 10th	Night	24% 18% 36% 20% 3	760
	11th to 20th	Night	20% 18% 34% 19% 8% 1	674
	21st to end	Night	21% 15% 33% 25% 8%	570
May	1st to 10th	Night	16% 10% 30% 31% 11% 2	570
	11th to 20th	Night	17% 8% 33% 32% 9% 1	403
	21st to end	Night	11% 8% 34% 33% 15%	411
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% 17% 31% 21% 4	836
November	1st to 10th	Night	21% 18% 30% 22% 8% 1	745
	11th to 20th	Night	21% 23% 30% 19% 6% 1	760
	21st to end	Night	25% 19% 33% 20% 4	760
December	1st to 10th	Night	29% 21% 33% 14% 3	760
	11th to 20th	Night	23% 22% 36% 16% 3	760
	21st to end	Night	32% 18% 32% 16% 2	836

Page generated at 2024-10-30 01:23:00. Version ac07

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