Bristol Community College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bristol Community College Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	18% 8% 2 6% 4 4 4 3 5% 7% 39%	119660
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	119450
Transparency	Day	45% 5% 11% 21% 15% 3	123252
	Night	45% 5% 10% 21% 16% 3	124019
Seeing	Day	24% 21% 32% 16% 5% 1	16035
	Night	24% 18% 31% 20% 6% 1	24914

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% 9% 3 6% 3 3 3 3 5% 6% 47%	3597
	11th to 20th	Day	16% 9% 2 5% 3 3 3 3 4 4 48%	3570
	21st to end	Day	18% 9% 3 6% 4 4 4 2 4 4 42%	3960
February	1st to 10th	Day	20% 8% 3 5% 4 5% 4 4 5% 6% 36%	1918
	11th to 20th	Day	15% 9% 2 5% 4 4 4 4 5% 7% 42%	1948
	21st to end	Day	18% 10% 2 6% 4 4 4 3 4 5% 39%	1650
March	1st to 10th	Day	22% 13% 2 6% 3 2 3 2 4 5% 37%	4476
	11th to 20th	Day	15% 10% 2 6% 3 4 4 3 5% 7% 41%	4794
	21st to end	Day	25% 10% 3 6% 4 4 4 3 5% 5% 31%	5730
April	1st to 10th	Day	21% 9% 3 5% 4 3 4 3 5% 7% 37%	2400
	11th to 20th	Day	19% 8% 3 5% 4 3 4 3 5% 6% 40%	2385
	21st to end	Day	20% 7% 2 7% 5% 4 4 3 5% 6% 38%	2600
May	1st to 10th	Day	28% 11% 3 6% 4 3 3 3 5% 7% 27%	5886
	11th to 20th	Day	22% 10% 3 7% 4 4 4 3 4 6% 32%	5861
	21st to end	Day	21% 7% 2 6% 4 4 4 4 5% 7% 35%	6920
June	1st to 10th	Day	19% 6% 2 6% 5% 4 5% 5% 6% 8% 34%	2800
	11th to 20th	Day	17% 6% 2 5% 5% 4 4 4 6% 8% 38%	2800
	21st to end	Day	15% 7% 2 5% 4 4 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 10% 34%	4580
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 7% 11% 37%	4580
	21st to end	Day	20% 8% 3 6% 5% 5% 5% 4 7% 7% 31%	4790
August	1st to 10th	Day	18% 7% 3 7% 6% 5% 5% 4 6% 9% 31%	4105
	11th to 20th	Day	13% 6% 3 7% 5% 6% 5% 4 6% 8% 37%	3985
	21st to end	Day	8% 4 1 5% 4 4 6% 4 7% 9% 47%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 3 4 3 5% 7% 46%	2261
	11th to 20th	Day	14% 6% 2 5% 3 4 3 2 5% 6% 50%	2090
	21st to end	Day	21% 6% 2 6% 4 3 4 3 4 6% 42%	2090
October	1st to 10th	Day	22% 6% 3 5% 3 3 3 2 3 5% 44%	2090
	11th to 20th	Day	16% 6% 2 5% 3 3 2 3 6% 7% 47%	1879
	21st to end	Day	18% 7% 2 5% 3 4 3 4 4 5% 44%	1881
November	1st to 10th	Day	16% 8% 2 4 4 3 3 3 5% 4 47%	1761
	11th to 20th	Day	17% 6% 2 4 3 3 3 4 5% 5% 47%	1800
	21st to end	Day	17% 7% 2 5% 3 4 3 3 5% 6% 43%	1800
December	1st to 10th	Day	15% 9% 2 5% 3 3 3 2 4 6% 49%	2747
	11th to 20th	Day	20% 7% 2 4 3 3 3 2 3 5% 49%	2986
	21st to end	Day	20% 11% 3 8% 4 4 3 3 4 4 37%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 4 3 4 6% 51%	6349
	11th to 20th	Night	15% 10% 3 7% 4 4 4 3 4 4 42%	6315
	21st to end	Night	17% 10% 3 7% 5% 4 4 4 5% 5% 37%	6996
February	1st to 10th	Night	17% 9% 4 7% 4 4 4 3 4 6% 40%	2475
	11th to 20th	Night	19% 8% 3 6% 4 3 3 4 5% 5% 39%	2338
	21st to end	Night	18% 9% 2 6% 4 4 4 3 4 4 41%	1980
March	1st to 10th	Night	24% 8% 2 5% 3 3 4 3 4 6% 37%	5364
	11th to 20th	Night	17% 11% 3 8% 4 3 4 3 4 6% 38%	5163
	21st to end	Night	23% 11% 2 6% 4 4 4 3 5% 4 33%	5197
April	1st to 10th	Night	19% 9% 4 7% 4 4 4 3 4 6% 36%	2000
	11th to 20th	Night	20% 8% 2 5% 5% 4 3 3 5% 7% 37%	1962
	21st to end	Night	20% 8% 3 6% 4 4 3 3 5% 5% 39%	1800
May	1st to 10th	Night	28% 10% 3 8% 4 3 4 3 4 7% 26%	4054
	11th to 20th	Night	20% 11% 3 8% 5% 5% 4 4 5% 7% 27%	4033
	21st to end	Night	22% 8% 3 8% 4 5% 5% 4 6% 6% 30%	3983
June	1st to 10th	Night	18% 8% 3 7% 4 4 5% 3 6% 7% 36%	1600
	11th to 20th	Night	19% 7% 1 6% 6% 4 4 3 5% 8% 38%	1600
	21st to end	Night	18% 8% 2 4 4 4 4 4 6% 8% 39%	1600
July	1st to 10th	Night	18% 7% 4 7% 5% 4 5% 4 6% 8% 31%	2600
	11th to 20th	Night	18% 7% 2 5% 4 5% 4 4 7% 8% 36%	2600
	21st to end	Night	18% 9% 3 7% 5% 5% 5% 4 6% 8% 30%	2739
August	1st to 10th	Night	23% 8% 2 6% 4 5% 5% 4 6% 8% 28%	2815
	11th to 20th	Night	9% 8% 3 9% 5% 5% 5% 5% 6% 9% 36%	2839
	21st to end	Night	9% 4 1 5% 4 3 4 3 5% 9% 54%	3476
September	1st to 10th	Night	14% 6% 2 7% 4 4 5% 4 5% 6% 44%	1918
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 6% 51%	2090
	21st to end	Night	20% 7% 2 6% 4 4 5% 3 4 5% 41%	2090
October	1st to 10th	Night	20% 8% 2 6% 4 3 4 2 4 6% 41%	2090
	11th to 20th	Night	16% 5% 2 4 3 4 4 3 5% 7% 48%	2295
	21st to end	Night	20% 9% 2 6% 3 4 4 3 4 6% 39%	2717
November	1st to 10th	Night	15% 6% 2 6% 3 4 4 4 4 5% 47%	2537
	11th to 20th	Night	17% 8% 2 5% 3 3 3 3 4 5% 46%	2600
	21st to end	Night	15% 7% 3 5% 3 3 3 3 6% 7% 45%	2600
December	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 3 4 47%	4431
	11th to 20th	Night	22% 9% 2 5% 3 3 3 3 3 5% 41%	5208
	21st to end	Night	17% 11% 3 7% 4 3 3 2 4 5% 40%	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	38% 3 7% 27% 22% 3	3598
	11th to 20th	Day	40% 4 10% 21% 18% 8%	3600
	21st to end	Day	48% 3 8% 20% 19% 2	3960
February	1st to 10th	Day	48% 4 13% 18% 14% 3	1918
	11th to 20th	Day	43% 5% 11% 23% 15% 3	1957
	21st to end	Day	48% 4 10% 20% 16% 3	1650
March	1st to 10th	Day	50% 3 11% 14% 13% 10%	4479
	11th to 20th	Day	42% 5% 14% 27% 11% 2	4794
	21st to end	Day	54% 4 7% 14% 15% 6%	5730
April	1st to 10th	Day	47% 4 10% 19% 17% 3	2400
	11th to 20th	Day	45% 5% 9% 21% 18% 3	2385
	21st to end	Day	46% 6% 9% 19% 16% 4	2600
May	1st to 10th	Day	58% 7% 13% 14% 7% 1	5897
	11th to 20th	Day	52% 6% 14% 16% 10% 1	5865
	21st to end	Day	48% 7% 10% 21% 13% 1	6920
June	1st to 10th	Day	46% 7% 12% 22% 12% 1	2800
	11th to 20th	Day	43% 6% 13% 23% 14% 1	2800
	21st to end	Day	40% 7% 15% 23% 13% 1	2800
July	1st to 10th	Day	46% 8% 11% 20% 13% 1	4510
	11th to 20th	Day	42% 6% 16% 23% 12% 1	4505
	21st to end	Day	50% 7% 12% 18% 12% 2	4637
August	1st to 10th	Day	49% 6% 17% 20% 7%	4092
	11th to 20th	Day	42% 8% 15% 27% 9% 1	4024
	21st to end	Day	30% 7% 15% 27% 19% 2	4180
September	1st to 10th	Day	39% 6% 9% 25% 20% 1	2261
	11th to 20th	Day	35% 4 11% 24% 23% 3	2090
	21st to end	Day	45% 5% 6% 20% 21% 2	2090
October	1st to 10th	Day	39% 4 11% 24% 18% 4	3480
	11th to 20th	Day	31% 3 7% 28% 28% 2	3095
	21st to end	Day	31% 3 14% 29% 20% 3	3081
November	1st to 10th	Day	40% 4 13% 21% 19% 2	1761
	11th to 20th	Day	38% 4 10% 22% 22% 4	1800
	21st to end	Day	43% 5% 11% 27% 13% 3	1800
December	1st to 10th	Day	40% 3 13% 28% 15% 1	2747
	11th to 20th	Day	41% 3 13% 17% 19% 7%	2986
	21st to end	Day	51% 4 7% 20% 13% 4	3960
January	1st to 10th	Night	34% 3 9% 34% 18% 3	6349
	11th to 20th	Night	46% 3 10% 21% 15% 5%	6360
	21st to end	Night	50% 4 9% 18% 15% 4	6996
February	1st to 10th	Night	48% 3 11% 22% 13% 2	2475
	11th to 20th	Night	46% 4 9% 20% 17% 4	2340
	21st to end	Night	47% 4 7% 24% 15% 4	1980
March	1st to 10th	Night	49% 3 10% 16% 15% 7%	5374
	11th to 20th	Night	48% 5% 14% 18% 12% 3	5164
	21st to end	Night	55% 3 5% 11% 20% 6%	5197
April	1st to 10th	Night	49% 4 9% 19% 16% 3	2000
	11th to 20th	Night	47% 5% 8% 19% 19% 2	1962
	21st to end	Night	48% 5% 8% 18% 16% 5%	1800
May	1st to 10th	Night	59% 8% 12% 12% 8% 1	4060
	11th to 20th	Night	55% 7% 12% 15% 8% 3	4034
	21st to end	Night	53% 8% 10% 18% 10%	3984
June	1st to 10th	Night	47% 7% 12% 20% 14%	1600
	11th to 20th	Night	45% 5% 12% 24% 13% 1	1600
	21st to end	Night	44% 7% 13% 23% 13% 1	1600
July	1st to 10th	Night	52% 8% 9% 17% 15%	2560
	11th to 20th	Night	45% 7% 16% 23% 9% 1	2552
	21st to end	Night	52% 5% 13% 16% 12% 2	2651
August	1st to 10th	Night	53% 9% 11% 17% 9% 1	2804
	11th to 20th	Night	42% 8% 15% 27% 8%	2866
	21st to end	Night	29% 5% 16% 28% 22% 1	3476
September	1st to 10th	Night	41% 5% 10% 24% 19% 2	1918
	11th to 20th	Night	36% 4 10% 23% 25% 2	2090
	21st to end	Night	47% 5% 6% 20% 19% 3	2090
October	1st to 10th	Night	41% 5% 10% 26% 16% 3	3480
	11th to 20th	Night	26% 4 8% 25% 33% 3	3818
	21st to end	Night	34% 4 12% 27% 22% 2	4467
November	1st to 10th	Night	39% 6% 11% 22% 19% 3	2537
	11th to 20th	Night	41% 3 9% 22% 20% 4	2600
	21st to end	Night	39% 4 13% 28% 13% 2	2600
December	1st to 10th	Night	42% 3 11% 27% 16% 1	4431
	11th to 20th	Night	47% 3 13% 15% 20% 2	5208
	21st to end	Night	47% 4 8% 21% 18% 2	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	30% 29% 28% 14% 1	200
	11th to 20th	Day	31% 31% 31% 7% 2	200
	21st to end	Day	26% 31% 28% 11% 2 1	220
February	1st to 10th	Day	31% 27% 30% 11% 1	453
	11th to 20th	Day	33% 33% 27% 7%	496
	21st to end	Day	29% 28% 31% 9% 3	414
March	1st to 10th	Day	31% 29% 26% 11% 2	500
	11th to 20th	Day	27% 31% 30% 9% 3	500
	21st to end	Day	31% 29% 26% 10% 3 1	550
April	1st to 10th	Day	32% 28% 25% 10% 3 1	500
	11th to 20th	Day	30% 26% 32% 9% 2	492
	21st to end	Day	27% 26% 30% 13% 4 2	500
May	1st to 10th	Day	33% 25% 30% 9% 3	503
	11th to 20th	Day	24% 23% 37% 12% 4	527
	21st to end	Day	21% 18% 35% 20% 4 2	573
June	1st to 10th	Day	20% 19% 36% 19% 6% 1	530
	11th to 20th	Day	18% 22% 38% 16% 5% 1	530
	21st to end	Day	18% 19% 36% 21% 6% 1	530
July	1st to 10th	Day	19% 10% 36% 23% 9% 2	525
	11th to 20th	Day	16% 10% 37% 25% 9% 2	524
	21st to end	Day	17% 14% 41% 20% 7% 1	532
August	1st to 10th	Day	14% 11% 39% 26% 9% 1	486
	11th to 20th	Day	17% 10% 36% 24% 10% 4	479
	21st to end	Day	14% 12% 35% 24% 11% 4	539
September	1st to 10th	Day	20% 10% 33% 23% 10% 3	490
	11th to 20th	Day	16% 14% 32% 23% 10% 6%	490
	21st to end	Day	27% 14% 33% 17% 6% 4	490
October	1st to 10th	Day	25% 15% 29% 19% 7% 4	490
	11th to 20th	Day	20% 19% 33% 18% 9% 2	433
	21st to end	Day	26% 21% 28% 16% 7% 2	429
November	1st to 10th	Day	21% 21% 32% 17% 9% 1	395
	11th to 20th	Day	26% 28% 29% 13% 4	400
	21st to end	Day	24% 28% 27% 14% 6%	400
December	1st to 10th	Day	23% 30% 31% 13% 3	295
	11th to 20th	Day	34% 31% 27% 6% 3	200
	21st to end	Day	31% 24% 30% 10% 3 2	220
January	1st to 10th	Night	23% 29% 32% 14% 3	800
	11th to 20th	Night	32% 25% 28% 13% 2	800
	21st to end	Night	24% 27% 31% 14% 3	880
February	1st to 10th	Night	26% 23% 35% 15% 1	800
	11th to 20th	Night	28% 28% 32% 10% 1	783
	21st to end	Night	26% 28% 30% 14% 2	660
March	1st to 10th	Night	27% 23% 32% 15% 3	800
	11th to 20th	Night	23% 25% 32% 16% 3 1	800
	21st to end	Night	28% 22% 31% 15% 3	880
April	1st to 10th	Night	25% 23% 31% 18% 3	800
	11th to 20th	Night	25% 21% 33% 18% 3	778
	21st to end	Night	26% 19% 34% 17% 4	600
May	1st to 10th	Night	30% 15% 32% 19% 5%	600
	11th to 20th	Night	22% 19% 38% 16% 5% 1	592
	21st to end	Night	23% 9% 31% 29% 6% 1	464
June	1st to 10th	Night	22% 10% 30% 30% 8% 1	400
	11th to 20th	Night	21% 8% 32% 32% 7%	400
	21st to end	Night	24% 4 30% 35% 8%	400
July	1st to 10th	Night	19% 7% 29% 34% 10% 1	390
	11th to 20th	Night	17% 6% 28% 34% 14%	388
	21st to end	Night	17% 5% 30% 38% 10%	416
August	1st to 10th	Night	15% 7% 35% 33% 10%	558
	11th to 20th	Night	13% 8% 31% 35% 12% 1	593
	21st to end	Night	16% 6% 25% 34% 16% 3	836
September	1st to 10th	Night	17% 6% 32% 31% 13% 2	760
	11th to 20th	Night	18% 8% 27% 26% 17% 3	760
	21st to end	Night	26% 9% 26% 25% 11% 2	760
October	1st to 10th	Night	25% 11% 26% 23% 11% 3	760
	11th to 20th	Night	20% 17% 30% 20% 12% 1	760
	21st to end	Night	26% 17% 33% 16% 8%	836
November	1st to 10th	Night	20% 18% 32% 19% 10% 1	780
	11th to 20th	Night	24% 25% 30% 16% 6% 1	800
	21st to end	Night	24% 23% 31% 17% 5% 1	800
December	1st to 10th	Night	28% 25% 28% 15% 5% 1	800
	11th to 20th	Night	28% 23% 30% 15% 3 1	800
	21st to end	Night	27% 25% 31% 14% 3	880

Page generated at 2025-07-18 01:09:59. Version ac07

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