Bolingbroke
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bolingbroke clear sky chart from 2005-11-06 to 2024-11-25.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	101166
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 43%	98267
Transparency	Day	37% 7% 11% 19% 18% 7%	104985
	Night	36% 7% 11% 18% 19% 8%	102587
Seeing	Day	15% 10% 22% 24% 20% 9%	15052
	Night	14% 9% 21% 30% 22% 4	25081

By Month

Percentage Cloud Cover SE (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	17% 6% 1 5% 4 3 4 3 5% 7% 45%	2836
	11th to 20th	Day	12% 6% 2 4 3 2 3 3 5% 6% 56%	2840
	21st to end	Day	22% 8% 3 6% 3 2 3 2 4 5% 41%	3252
February	1st to 10th	Day	17% 6% 3 7% 5% 4 5% 4 6% 5% 39%	1900
	11th to 20th	Day	19% 7% 2 5% 4 3 4 3 6% 6% 42%	1848
	21st to end	Day	16% 7% 2 7% 4 4 4 3 5% 6% 39%	1570
March	1st to 10th	Day	21% 10% 3 6% 4 3 3 3 5% 6% 36%	3275
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 49%	3480
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 5% 43%	3815
April	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 5% 7% 47%	2090
	11th to 20th	Day	12% 5% 2 4 4 4 4 3 5% 7% 49%	2236
	21st to end	Day	8% 4 2 5% 5% 4 4 3 5% 7% 53%	2441
May	1st to 10th	Day	8% 6% 3 7% 4 5% 5% 4 6% 8% 46%	4082
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	4081
	21st to end	Day	6% 3 2 5% 5% 5% 5% 5% 7% 10% 47%	4476
June	1st to 10th	Day	10% 7% 2 7% 6% 6% 7% 5% 8% 10% 32%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 6% 9% 12% 40%	2470
	21st to end	Day	5% 5% 2 7% 6% 7% 6% 5% 9% 10% 37%	2470
July	1st to 10th	Day	12% 7% 3 9% 6% 6% 7% 6% 9% 11% 23%	4120
	11th to 20th	Day	5% 4 3 8% 7% 7% 7% 7% 9% 12% 31%	4120
	21st to end	Day	7% 8% 3 10% 8% 8% 9% 7% 10% 12% 19%	4466
August	1st to 10th	Day	11% 5% 2 6% 5% 6% 7% 6% 9% 11% 30%	4104
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 4 6% 9% 47%	3799
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 6% 10% 47%	4120
September	1st to 10th	Day	11% 7% 3 6% 6% 5% 5% 5% 7% 9% 37%	2090
	11th to 20th	Day	14% 6% 2 7% 4 4 4 5% 6% 8% 39%	2090
	21st to end	Day	14% 5% 2 6% 5% 4 5% 4 6% 6% 44%	2090
October	1st to 10th	Day	13% 5% 1 4 3 4 4 2 4 7% 52%	2090
	11th to 20th	Day	11% 3 2 4 3 3 3 3 5% 6% 58%	1970
	21st to end	Day	11% 3 1 5% 3 3 4 3 4 4 57%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 3 3 4 4 5% 52%	1852
	11th to 20th	Day	14% 5% 2 4 3 3 2 3 5% 6% 54%	1800
	21st to end	Day	13% 6% 2 5% 3 4 3 4 5% 6% 48%	1746
December	1st to 10th	Day	16% 7% 2 6% 5% 4 3 5% 5% 7% 41%	1710
	11th to 20th	Day	19% 6% 2 6% 3 3 3 3 4 5% 46%	2153
	21st to end	Day	20% 7% 3 6% 4 3 3 3 4 6% 41%	3124
January	1st to 10th	Night	18% 6% 2 6% 3 5% 4 4 6% 6% 41%	4109
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 5% 7% 49%	4120
	21st to end	Night	21% 10% 3 6% 4 3 3 3 4 4 40%	4400
February	1st to 10th	Night	16% 6% 3 6% 4 3 4 3 5% 6% 44%	2280
	11th to 20th	Night	19% 7% 2 5% 4 3 4 2 4 5% 45%	2218
	21st to end	Night	15% 7% 2 7% 5% 4 3 4 6% 6% 42%	1884
March	1st to 10th	Night	23% 8% 3 6% 4 3 3 3 4 6% 37%	3576
	11th to 20th	Night	11% 5% 1 4 3 4 4 4 6% 8% 49%	3480
	21st to end	Night	17% 6% 2 6% 4 4 4 4 5% 6% 42%	3817
April	1st to 10th	Night	10% 5% 2 4 4 4 4 5% 6% 8% 48%	2090
	11th to 20th	Night	12% 4 1 4 3 3 4 4 6% 7% 51%	1908
	21st to end	Night	7% 4 2 5% 5% 4 3 3 6% 8% 54%	1739
May	1st to 10th	Night	6% 5% 2 6% 4 4 5% 4 8% 12% 43%	2813
	11th to 20th	Night	14% 5% 2 6% 4 5% 5% 4 6% 7% 41%	2812
	21st to end	Night	7% 4 2 4 3 4 4 4 6% 11% 52%	3079
June	1st to 10th	Night	8% 5% 3 6% 5% 6% 6% 6% 8% 11% 36%	1710
	11th to 20th	Night	5% 3 3 5% 4 5% 5% 4 8% 13% 45%	1710
	21st to end	Night	6% 5% 2 5% 6% 5% 7% 5% 9% 13% 38%	1710
July	1st to 10th	Night	11% 8% 3 9% 6% 6% 6% 6% 8% 10% 26%	2840
	11th to 20th	Night	5% 5% 2 7% 7% 6% 8% 7% 11% 14% 28%	2840
	21st to end	Night	9% 8% 3 8% 7% 7% 8% 6% 8% 12% 24%	3087
August	1st to 10th	Night	10% 6% 3 9% 6% 7% 7% 5% 9% 11% 28%	2825
	11th to 20th	Night	6% 4 2 6% 4 5% 5% 4 7% 10% 46%	3026
	21st to end	Night	7% 5% 1 6% 4 3 4 4 7% 9% 51%	3503
September	1st to 10th	Night	8% 6% 3 8% 6% 6% 6% 5% 7% 9% 36%	2090
	11th to 20th	Night	12% 6% 2 6% 5% 4 5% 4 5% 7% 43%	2090
	21st to end	Night	11% 5% 2 7% 4 5% 4 4 6% 7% 44%	2090
October	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 3 7% 52%	2090
	11th to 20th	Night	11% 4 2 4 3 3 3 2 4 5% 59%	2209
	21st to end	Night	11% 4 1 4 3 4 3 3 4 6% 56%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 4 3 2 5% 6% 53%	2441
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 4 6% 54%	2600
	21st to end	Night	13% 5% 2 6% 4 4 3 3 4 6% 50%	2529
December	1st to 10th	Night	15% 6% 2 5% 3 4 4 4 5% 7% 45%	2470
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 4 5% 49%	3042
	21st to end	Night	19% 9% 3 7% 4 4 4 3 5% 6% 35%	4532

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	38% 5% 7% 18% 25% 7%	2837
	11th to 20th	Day	30% 4 5% 20% 29% 12%	2840
	21st to end	Day	45% 4 4 14% 22% 10%	3252
February	1st to 10th	Day	44% 4 8% 17% 19% 8%	1900
	11th to 20th	Day	42% 3 5% 18% 23% 7%	1858
	21st to end	Day	44% 6% 7% 15% 19% 9%	1570
March	1st to 10th	Day	48% 5% 9% 14% 16% 8%	3275
	11th to 20th	Day	32% 7% 10% 16% 24% 12%	3480
	21st to end	Day	42% 6% 7% 20% 16% 9%	3815
April	1st to 10th	Day	36% 6% 10% 20% 19% 10%	2090
	11th to 20th	Day	34% 5% 8% 19% 22% 12%	2236
	21st to end	Day	29% 5% 11% 23% 26% 5%	2441
May	1st to 10th	Day	33% 9% 12% 23% 19% 3	4120
	11th to 20th	Day	42% 9% 12% 22% 14% 1	4085
	21st to end	Day	30% 8% 16% 24% 20% 3	4489
June	1st to 10th	Day	43% 10% 15% 19% 12% 2	2470
	11th to 20th	Day	31% 10% 17% 25% 15% 2	2470
	21st to end	Day	36% 11% 18% 19% 12% 3	2470
July	1st to 10th	Day	48% 14% 20% 13% 4 1	4055
	11th to 20th	Day	39% 14% 20% 22% 4 1	4051
	21st to end	Day	51% 16% 17% 12% 4	4324
August	1st to 10th	Day	41% 11% 18% 27% 4	4091
	11th to 20th	Day	30% 9% 15% 32% 13% 1	3837
	21st to end	Day	31% 8% 16% 30% 14% 2	4120
September	1st to 10th	Day	41% 8% 16% 16% 15% 4	2090
	11th to 20th	Day	40% 7% 11% 19% 18% 4	2090
	21st to end	Day	39% 6% 9% 21% 19% 6%	2090
October	1st to 10th	Day	29% 3 7% 14% 33% 13%	3480
	11th to 20th	Day	16% 3 5% 17% 28% 32%	3249
	21st to end	Day	22% 3 5% 18% 37% 16%	3428
November	1st to 10th	Day	35% 5% 7% 13% 27% 14%	1852
	11th to 20th	Day	32% 4 6% 19% 26% 14%	1800
	21st to end	Day	37% 5% 8% 16% 24% 10%	1746
December	1st to 10th	Day	43% 6% 9% 16% 19% 8%	1710
	11th to 20th	Day	41% 3 7% 15% 27% 8%	2150
	21st to end	Day	43% 6% 7% 17% 23% 4	3124
January	1st to 10th	Night	39% 7% 6% 15% 22% 10%	4109
	11th to 20th	Night	32% 6% 7% 23% 24% 9%	4120
	21st to end	Night	47% 5% 4 11% 24% 9%	4400
February	1st to 10th	Night	41% 4 7% 17% 21% 10%	2280
	11th to 20th	Night	42% 4 6% 16% 22% 11%	2219
	21st to end	Night	41% 5% 8% 15% 18% 13%	1884
March	1st to 10th	Night	49% 6% 10% 14% 14% 8%	3576
	11th to 20th	Night	32% 6% 10% 13% 26% 12%	3480
	21st to end	Night	41% 6% 7% 17% 18% 11%	3817
April	1st to 10th	Night	33% 6% 11% 19% 20% 12%	2090
	11th to 20th	Night	31% 5% 7% 23% 23% 11%	1908
	21st to end	Night	27% 6% 12% 23% 24% 8%	1739
May	1st to 10th	Night	28% 11% 15% 23% 18% 4	2840
	11th to 20th	Night	38% 8% 14% 23% 14% 2	2813
	21st to end	Night	25% 8% 15% 27% 21% 4	3087
June	1st to 10th	Night	37% 12% 18% 19% 14% 1	1710
	11th to 20th	Night	28% 8% 19% 30% 13% 2	1710
	21st to end	Night	33% 12% 23% 16% 12% 4	1710
July	1st to 10th	Night	50% 14% 19% 12% 3 2	2795
	11th to 20th	Night	38% 15% 26% 16% 5% 1	2786
	21st to end	Night	49% 16% 18% 12% 4 1	2988
August	1st to 10th	Night	44% 10% 20% 22% 4	2814
	11th to 20th	Night	31% 9% 18% 26% 14% 1	3054
	21st to end	Night	28% 8% 17% 29% 16% 2	3503
September	1st to 10th	Night	40% 11% 16% 15% 15% 4	2090
	11th to 20th	Night	39% 9% 12% 18% 18% 5%	2090
	21st to end	Night	38% 7% 9% 17% 21% 8%	2090
October	1st to 10th	Night	33% 5% 7% 12% 28% 15%	3480
	11th to 20th	Night	17% 2 5% 13% 30% 33%	3669
	21st to end	Night	21% 3 5% 19% 33% 20%	4120
November	1st to 10th	Night	33% 5% 8% 14% 26% 14%	2441
	11th to 20th	Night	33% 3 6% 17% 28% 14%	2600
	21st to end	Night	36% 4 7% 18% 24% 11%	2529
December	1st to 10th	Night	39% 5% 9% 19% 22% 7%	2470
	11th to 20th	Night	38% 4 6% 14% 29% 8%	3044
	21st to end	Night	47% 6% 6% 20% 18% 4	4532

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	19% 23% 43% 13% 2	270
	11th to 20th	Day	20% 23% 33% 17% 6% 1	270
	21st to end	Day	24% 26% 37% 10% 3 1	297
February	1st to 10th	Day	21% 23% 39% 13% 4	270
	11th to 20th	Day	32% 21% 31% 14% 1	270
	21st to end	Day	25% 23% 40% 11% 2	225
March	1st to 10th	Day	19% 22% 31% 21% 6% 1	360
	11th to 20th	Day	19% 15% 35% 22% 8% 1	460
	21st to end	Day	20% 16% 35% 21% 8%	506
April	1st to 10th	Day	19% 17% 28% 24% 11% 2	460
	11th to 20th	Day	14% 15% 30% 23% 14% 3	452
	21st to end	Day	12% 10% 29% 28% 18% 3	460
May	1st to 10th	Day	11% 13% 27% 27% 16% 6%	463
	11th to 20th	Day	14% 5% 22% 28% 20% 11%	487
	21st to end	Day	11% 3 19% 29% 24% 13%	529
June	1st to 10th	Day	14% 4 16% 26% 27% 13%	490
	11th to 20th	Day	7% 5% 17% 23% 27% 22%	490
	21st to end	Day	8% 3 14% 30% 28% 17%	490
July	1st to 10th	Day	10% 2 9% 19% 36% 26%	485
	11th to 20th	Day	6% 1 6% 23% 33% 31%	484
	21st to end	Day	5% 4 8% 24% 37% 23%	532
August	1st to 10th	Day	13% 4 10% 26% 27% 20%	486
	11th to 20th	Day	8% 3 8% 23% 38% 21%	479
	21st to end	Day	15% 3 6% 30% 31% 16%	539
September	1st to 10th	Day	13% 3 11% 25% 33% 15%	490
	11th to 20th	Day	16% 3 14% 27% 29% 10%	490
	21st to end	Day	16% 2 16% 31% 26% 10%	490
October	1st to 10th	Day	17% 4 25% 26% 19% 8%	490
	11th to 20th	Day	12% 8% 21% 34% 20% 6%	490
	21st to end	Day	14% 7% 25% 31% 19% 5%	539
November	1st to 10th	Day	22% 8% 25% 26% 15% 4	390
	11th to 20th	Day	16% 16% 39% 18% 10% 1	300
	21st to end	Day	14% 17% 32% 25% 12%	282
December	1st to 10th	Day	25% 17% 31% 19% 7% 1	270
	11th to 20th	Day	26% 18% 32% 17% 7%	270
	21st to end	Day	29% 16% 31% 17% 5% 1	297
January	1st to 10th	Night	20% 19% 31% 24% 6%	760
	11th to 20th	Night	19% 15% 28% 28% 9%	760
	21st to end	Night	20% 17% 33% 26% 3	836
February	1st to 10th	Night	18% 13% 31% 31% 7%	760
	11th to 20th	Night	23% 16% 31% 28% 3	743
	21st to end	Night	19% 15% 35% 28% 3	628
March	1st to 10th	Night	15% 14% 33% 31% 6%	760
	11th to 20th	Night	17% 14% 30% 28% 11%	760
	21st to end	Night	16% 15% 29% 31% 9% 1	836
April	1st to 10th	Night	13% 16% 29% 30% 13% 1	760
	11th to 20th	Night	15% 12% 25% 30% 17% 2	596
	21st to end	Night	9% 7% 24% 39% 20% 1	570
May	1st to 10th	Night	8% 7% 25% 38% 21% 1	570
	11th to 20th	Night	11% 4 20% 35% 25% 6%	561
	21st to end	Night	12% 2 13% 36% 31% 6%	617
June	1st to 10th	Night	10% 1 12% 37% 35% 4	570
	11th to 20th	Night	4 2 12% 34% 35% 13%	570
	21st to end	Night	7% 2 7% 33% 43% 8%	570
July	1st to 10th	Night	8% 2 8% 25% 42% 16%	555
	11th to 20th	Night	9% 1 5% 22% 47% 16%	552
	21st to end	Night	6% 2 6% 27% 47% 12%	624
August	1st to 10th	Night	8% 2 5% 28% 44% 13%	567
	11th to 20th	Night	9% 2 7% 26% 46% 10%	564
	21st to end	Night	15% 2 6% 27% 39% 11%	641
September	1st to 10th	Night	10% 3 12% 25% 41% 10%	760
	11th to 20th	Night	15% 3 11% 28% 38% 6%	760
	21st to end	Night	15% 3 14% 29% 35% 4	760
October	1st to 10th	Night	14% 3 20% 32% 25% 6%	760
	11th to 20th	Night	13% 6% 21% 33% 24% 3	760
	21st to end	Night	13% 8% 21% 33% 23% 2	836
November	1st to 10th	Night	15% 7% 22% 35% 21% 1	780
	11th to 20th	Night	17% 11% 25% 32% 14% 2	800
	21st to end	Night	15% 12% 25% 34% 14% 1	779
December	1st to 10th	Night	17% 13% 28% 29% 12% 1	760
	11th to 20th	Night	21% 10% 28% 30% 10% 1	760
	21st to end	Night	27% 12% 24% 29% 7%	836

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