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 2024-04-10.

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% 7% 3 5% 5% 3 4 4 5% 7% 40%	82947
	Night	19% 8% 3 5% 5% 3 4 4 4 6% 39%	85816
Transparency	Day	44% 5% 11% 22% 15% 2	82653
	Night	46% 5% 10% 22% 14% 2	85644
Seeing	Day	23% 21% 32% 16% 6% 1	14338
	Night	23% 18% 31% 20% 6% 1	23394

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% 9% 3 5% 4 2 4 4 4 5% 45%	2517
	11th to 20th	Day	20% 8% 2 5% 5% 3 4 4 5% 5% 39%	2520
	21st to end	Day	22% 12% 4 7% 5% 3 3 3 4 5% 34%	2772
February	1st to 10th	Day	20% 8% 3 5% 4 4 5% 5% 5% 6% 37%	1821
	11th to 20th	Day	14% 8% 3 4 5% 3 4 5% 4 7% 42%	1848
	21st to end	Day	19% 9% 3 6% 5% 3 4 4 4 5% 38%	1570
March	1st to 10th	Day	26% 15% 3 6% 4 2 3 3 3 5% 30%	3116
	11th to 20th	Day	12% 6% 3 5% 4 3 4 4 6% 8% 45%	3350
	21st to end	Day	24% 10% 3 5% 4 2 4 4 4 5% 35%	3996
April	1st to 10th	Day	20% 9% 3 4 5% 2 4 4 4 7% 37%	2270
	11th to 20th	Day	18% 6% 3 5% 5% 3 4 4 5% 6% 41%	2143
	21st to end	Day	21% 6% 3 6% 5% 3 4 4 4 6% 37%	2340
May	1st to 10th	Day	26% 9% 3 5% 5% 2 3 4 4 6% 33%	2340
	11th to 20th	Day	20% 6% 3 5% 4 3 4 5% 5% 7% 40%	2301
	21st to end	Day	16% 6% 2 4 5% 3 4 5% 5% 8% 43%	2710
June	1st to 10th	Day	20% 6% 3 5% 6% 3 5% 6% 6% 8% 34%	2520
	11th to 20th	Day	18% 6% 2 5% 5% 3 5% 5% 6% 8% 37%	2520
	21st to end	Day	15% 6% 2 5% 5% 3 5% 5% 6% 9% 39%	2520
July	1st to 10th	Day	13% 6% 3 5% 6% 3 5% 5% 6% 9% 41%	2520
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 5% 6% 11% 38%	2520
	21st to end	Day	12% 5% 3 5% 6% 3 5% 5% 7% 9% 40%	2733
August	1st to 10th	Day	11% 5% 3 5% 6% 3 5% 5% 7% 10% 39%	2327
	11th to 20th	Day	13% 5% 3 6% 6% 5% 4 5% 6% 8% 39%	2272
	21st to end	Day	14% 5% 2 5% 4 3 5% 5% 6% 9% 42%	2376
September	1st to 10th	Day	16% 5% 3 5% 5% 3 4 4 5% 7% 45%	2142
	11th to 20th	Day	14% 5% 3 4 4 3 3 3 5% 6% 51%	1980
	21st to end	Day	21% 5% 2 4 4 2 4 4 4 6% 44%	1980
October	1st to 10th	Day	23% 6% 3 5% 4 2 4 3 3 4 44%	1980
	11th to 20th	Day	16% 6% 2 5% 4 2 3 3 6% 7% 46%	1781
	21st to end	Day	19% 7% 2 5% 4 3 4 5% 4 5% 43%	1782
November	1st to 10th	Day	17% 8% 3 3 5% 2 3 3 4 5% 46%	1671
	11th to 20th	Day	18% 6% 2 4 4 3 3 5% 5% 6% 45%	1710
	21st to end	Day	17% 7% 2 4 4 3 4 4 5% 6% 43%	1710
December	1st to 10th	Day	20% 8% 2 5% 4 3 4 4 4 6% 39%	1611
	11th to 20th	Day	21% 7% 2 4 4 2 4 3 3 6% 44%	1906
	21st to end	Day	22% 11% 3 6% 5% 3 4 3 4 5% 34%	2772
January	1st to 10th	Night	18% 7% 2 4 4 2 4 3 3 6% 48%	4429
	11th to 20th	Night	17% 11% 3 6% 5% 3 4 3 4 4 40%	4440
	21st to end	Night	22% 12% 4 7% 5% 3 4 4 4 4 30%	4884
February	1st to 10th	Night	17% 9% 3 6% 4 3 4 3 4 6% 40%	2352
	11th to 20th	Night	18% 7% 4 6% 5% 2 4 5% 4 6% 40%	2218
	21st to end	Night	18% 8% 3 5% 4 3 4 4 4 5% 41%	1884
March	1st to 10th	Night	29% 10% 2 4 4 2 3 3 4 5% 33%	3724
	11th to 20th	Night	11% 9% 3 7% 5% 3 5% 5% 5% 7% 41%	3607
	21st to end	Night	25% 9% 3 5% 5% 3 4 4 4 4 33%	3631
April	1st to 10th	Night	19% 9% 4 6% 5% 3 4 4 4 6% 37%	1900
	11th to 20th	Night	20% 7% 3 5% 4 3 4 4 5% 7% 38%	1764
	21st to end	Night	22% 7% 3 5% 5% 3 4 4 4 5% 38%	1620
May	1st to 10th	Night	25% 9% 3 6% 5% 3 4 4 5% 6% 30%	1620
	11th to 20th	Night	19% 7% 3 6% 6% 3 4 5% 5% 8% 35%	1593
	21st to end	Night	19% 7% 2 6% 5% 3 5% 5% 6% 7% 35%	1579
June	1st to 10th	Night	19% 7% 3 6% 5% 3 5% 4 5% 7% 36%	1440
	11th to 20th	Night	19% 6% 2 5% 7% 3 4 4 5% 8% 37%	1440
	21st to end	Night	18% 8% 2 4 3 3 4 4 6% 9% 40%	1440
July	1st to 10th	Night	15% 6% 3 6% 6% 3 6% 5% 5% 8% 38%	1440
	11th to 20th	Night	14% 7% 2 5% 5% 4 5% 5% 6% 9% 36%	1440
	21st to end	Night	13% 6% 3 5% 6% 4 6% 5% 6% 9% 38%	1574
August	1st to 10th	Night	9% 8% 3 5% 5% 3 5% 6% 7% 9% 40%	1602
	11th to 20th	Night	11% 6% 3 6% 6% 4 5% 6% 6% 10% 37%	1625
	21st to end	Night	14% 5% 2 4 5% 3 4 4 5% 8% 46%	1980
September	1st to 10th	Night	15% 6% 2 6% 4 3 5% 5% 5% 6% 43%	1817
	11th to 20th	Night	15% 5% 3 4 4 2 3 4 4 6% 51%	1980
	21st to end	Night	19% 6% 2 5% 4 3 4 4 4 5% 43%	1980
October	1st to 10th	Night	21% 8% 2 5% 4 3 3 3 3 5% 41%	1980
	11th to 20th	Night	17% 5% 2 4 4 3 4 4 4 7% 46%	2173
	21st to end	Night	20% 9% 2 5% 4 3 4 4 5% 6% 38%	2574
November	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 4 6% 46%	2407
	11th to 20th	Night	17% 8% 2 5% 4 2 4 4 4 5% 45%	2470
	21st to end	Night	15% 6% 3 4 4 2 4 4 5% 7% 45%	2470
December	1st to 10th	Night	21% 7% 3 5% 5% 3 4 4 3 5% 40%	2567
	11th to 20th	Night	20% 7% 2 5% 4 3 4 4 4 6% 41%	3288
	21st to end	Night	21% 11% 4 6% 5% 3 3 3 4 5% 36%	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	41% 4 8% 29% 15% 2	2518
	11th to 20th	Day	47% 6% 9% 24% 12% 2	2520
	21st to end	Day	55% 3 8% 22% 11% 1	2772
February	1st to 10th	Day	48% 4 13% 18% 14% 3	1821
	11th to 20th	Day	43% 5% 11% 23% 15% 3	1857
	21st to end	Day	49% 4 10% 20% 15% 3	1570
March	1st to 10th	Day	58% 3 11% 15% 11% 3	3119
	11th to 20th	Day	34% 6% 15% 29% 14% 2	3350
	21st to end	Day	53% 3 6% 16% 16% 6%	3996
April	1st to 10th	Day	47% 4 10% 20% 16% 3	2270
	11th to 20th	Day	44% 5% 9% 21% 18% 3	2143
	21st to end	Day	47% 6% 9% 19% 15% 4	2340
May	1st to 10th	Day	53% 5% 8% 19% 13% 3	2340
	11th to 20th	Day	44% 5% 11% 20% 17% 3	2305
	21st to end	Day	42% 7% 13% 23% 12% 2	2710
June	1st to 10th	Day	47% 7% 12% 21% 11% 1	2520
	11th to 20th	Day	45% 6% 12% 22% 14% 1	2520
	21st to end	Day	40% 8% 15% 24% 12% 1	2520
July	1st to 10th	Day	40% 7% 12% 24% 14% 2	2450
	11th to 20th	Day	41% 6% 14% 26% 12% 2	2445
	21st to end	Day	38% 8% 13% 23% 15% 3	2580
August	1st to 10th	Day	39% 8% 16% 24% 13% 1	2314
	11th to 20th	Day	41% 7% 13% 25% 12% 1	2272
	21st to end	Day	38% 7% 12% 22% 19% 2	2376
September	1st to 10th	Day	39% 6% 10% 25% 19% 1	2142
	11th to 20th	Day	35% 4 11% 23% 24% 3	1980
	21st to end	Day	43% 5% 6% 21% 22% 2	1980
October	1st to 10th	Day	45% 3 10% 22% 16% 3	1980
	11th to 20th	Day	38% 4 11% 29% 17% 1	1781
	21st to end	Day	44% 4 12% 20% 17% 4	1782
November	1st to 10th	Day	41% 5% 13% 21% 18% 2	1671
	11th to 20th	Day	40% 4 10% 22% 20% 3	1710
	21st to end	Day	43% 5% 11% 26% 12% 3	1710
December	1st to 10th	Day	48% 4 12% 17% 17% 2	1611
	11th to 20th	Day	45% 3 13% 17% 16% 6%	1906
	21st to end	Day	53% 5% 9% 21% 11% 1	2772
January	1st to 10th	Night	39% 3 11% 32% 12% 3	4429
	11th to 20th	Night	48% 4 10% 23% 12% 3	4440
	21st to end	Night	58% 4 8% 17% 11% 2	4884
February	1st to 10th	Night	48% 3 12% 22% 13% 2	2352
	11th to 20th	Night	47% 4 9% 20% 17% 4	2220
	21st to end	Night	47% 4 7% 24% 15% 4	1884
March	1st to 10th	Night	55% 3 9% 19% 12% 3	3734
	11th to 20th	Night	41% 7% 14% 20% 16% 3	3608
	21st to end	Night	55% 4 5% 12% 18% 6%	3631
April	1st to 10th	Night	49% 5% 9% 18% 16% 3	1900
	11th to 20th	Night	46% 5% 8% 20% 18% 3	1764
	21st to end	Night	49% 5% 8% 19% 14% 5%	1620
May	1st to 10th	Night	56% 5% 9% 16% 12% 3	1620
	11th to 20th	Night	48% 6% 11% 19% 13% 4	1594
	21st to end	Night	50% 9% 12% 18% 11% 1	1580
June	1st to 10th	Night	48% 6% 12% 19% 14%	1440
	11th to 20th	Night	45% 4 12% 23% 13% 1	1440
	21st to end	Night	43% 7% 14% 23% 13% 1	1440
July	1st to 10th	Night	45% 8% 11% 21% 14% 1	1400
	11th to 20th	Night	43% 7% 14% 24% 10% 1	1392
	21st to end	Night	43% 5% 13% 25% 11% 2	1486
August	1st to 10th	Night	38% 9% 14% 24% 13% 2	1591
	11th to 20th	Night	41% 9% 15% 24% 11% 1	1625
	21st to end	Night	37% 6% 12% 25% 20% 1	1980
September	1st to 10th	Night	42% 5% 11% 24% 17% 2	1817
	11th to 20th	Night	36% 5% 10% 23% 25% 2	1980
	21st to end	Night	44% 5% 7% 21% 20% 3	1980
October	1st to 10th	Night	48% 5% 9% 21% 14% 4	1980
	11th to 20th	Night	38% 6% 12% 28% 15% 1	2173
	21st to end	Night	48% 4 11% 20% 14% 3	2574
November	1st to 10th	Night	40% 6% 11% 22% 18% 3	2407
	11th to 20th	Night	42% 3 10% 23% 18% 4	2470
	21st to end	Night	39% 4 13% 28% 13% 2	2470
December	1st to 10th	Night	48% 4 12% 20% 15% 1	2567
	11th to 20th	Night	45% 4 13% 18% 18% 2	3288
	21st to end	Night	52% 5% 8% 23% 12% 1	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	27% 29% 28% 15% 1	180
	11th to 20th	Day	29% 31% 31% 8% 2	180
	21st to end	Day	24% 31% 29% 12% 3 1	198
February	1st to 10th	Day	31% 27% 30% 12% 1	416
	11th to 20th	Day	30% 34% 29% 7%	456
	21st to end	Day	30% 26% 31% 10% 3	382
March	1st to 10th	Day	29% 30% 28% 11% 2	460
	11th to 20th	Day	26% 32% 30% 9% 3	460
	21st to end	Day	28% 29% 27% 10% 4 1	506
April	1st to 10th	Day	31% 27% 26% 11% 4 1	457
	11th to 20th	Day	26% 27% 33% 11% 3	412
	21st to end	Day	29% 26% 28% 12% 4 2	420
May	1st to 10th	Day	32% 26% 28% 11% 4	423
	11th to 20th	Day	23% 23% 37% 12% 4	447
	21st to end	Day	20% 17% 35% 22% 5% 2	485
June	1st to 10th	Day	19% 18% 36% 20% 6% 1	450
	11th to 20th	Day	19% 21% 38% 16% 4 1	450
	21st to end	Day	18% 18% 35% 22% 6% 1	450
July	1st to 10th	Day	18% 10% 35% 26% 10% 2	445
	11th to 20th	Day	16% 9% 36% 27% 9% 2	444
	21st to end	Day	16% 13% 42% 21% 7% 1	488
August	1st to 10th	Day	14% 11% 38% 27% 9% 1	446
	11th to 20th	Day	17% 10% 36% 24% 10% 3	439
	21st to end	Day	16% 12% 34% 23% 12% 4	495
September	1st to 10th	Day	21% 10% 32% 23% 11% 3	450
	11th to 20th	Day	16% 15% 34% 21% 9% 5%	450
	21st to end	Day	26% 14% 31% 18% 6% 4	450
October	1st to 10th	Day	27% 15% 28% 19% 7% 4	450
	11th to 20th	Day	20% 20% 32% 19% 9% 1	399
	21st to end	Day	27% 22% 27% 16% 7% 2	396
November	1st to 10th	Day	21% 20% 32% 17% 9% 1	365
	11th to 20th	Day	26% 28% 28% 13% 4	370
	21st to end	Day	23% 28% 28% 15% 6%	370
December	1st to 10th	Day	23% 32% 29% 12% 3	271
	11th to 20th	Day	32% 33% 27% 6% 2	180
	21st to end	Day	31% 25% 30% 9% 4 2	198
January	1st to 10th	Night	23% 28% 32% 14% 3	760
	11th to 20th	Night	32% 25% 28% 13% 2	760
	21st to end	Night	25% 27% 31% 14% 3	836
February	1st to 10th	Night	25% 23% 36% 15% 1	760
	11th to 20th	Night	27% 29% 33% 10% 1	743
	21st to end	Night	27% 27% 30% 14% 2	628
March	1st to 10th	Night	26% 23% 33% 15% 3	760
	11th to 20th	Night	22% 25% 33% 17% 3	760
	21st to end	Night	27% 22% 31% 15% 4	836
April	1st to 10th	Night	24% 24% 31% 19% 3	760
	11th to 20th	Night	24% 20% 33% 19% 3	700
	21st to end	Night	26% 18% 33% 18% 4	540
May	1st to 10th	Night	29% 16% 31% 19% 5%	540
	11th to 20th	Night	22% 19% 38% 16% 4 1	532
	21st to end	Night	23% 8% 32% 29% 7% 1	418
June	1st to 10th	Night	22% 9% 28% 31% 9% 1	360
	11th to 20th	Night	22% 8% 29% 33% 8%	360
	21st to end	Night	24% 3 31% 35% 7%	360
July	1st to 10th	Night	19% 7% 30% 34% 10% 1	350
	11th to 20th	Night	17% 7% 27% 36% 14%	348
	21st to end	Night	15% 5% 31% 38% 10% 1	394
August	1st to 10th	Night	13% 6% 36% 34% 11%	528
	11th to 20th	Night	13% 8% 31% 35% 12% 1	561
	21st to end	Night	16% 6% 25% 33% 16% 3	792
September	1st to 10th	Night	17% 6% 31% 31% 13% 2	720
	11th to 20th	Night	19% 8% 28% 26% 16% 3	720
	21st to end	Night	25% 9% 27% 26% 12% 2	720
October	1st to 10th	Night	26% 12% 27% 22% 11% 3	720
	11th to 20th	Night	21% 16% 30% 21% 11% 1	720
	21st to end	Night	27% 18% 32% 16% 7%	792
November	1st to 10th	Night	21% 18% 31% 19% 10% 1	740
	11th to 20th	Night	24% 25% 29% 15% 6% 1	760
	21st to end	Night	23% 23% 31% 17% 5% 1	760
December	1st to 10th	Night	28% 24% 28% 14% 4 1	760
	11th to 20th	Night	28% 24% 30% 15% 3 1	760
	21st to end	Night	28% 25% 31% 13% 3	836

Page generated at 2024-04-07 09:28:13. Version ac07

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