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 2024-11-04.

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% 38%	101151
	Night	19% 8% 3 6% 4 4 4 3 5% 6% 38%	97871
Transparency	Day	44% 5% 12% 22% 15% 2	104716
	Night	46% 5% 11% 22% 15% 2	102395
Seeing	Day	23% 21% 32% 16% 6% 2	15149
	Night	23% 18% 31% 21% 7% 1	24033

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% 2 6% 3 3 3 3 4 4 45%	2517
	11th to 20th	Day	20% 8% 2 6% 4 4 4 3 5% 5% 39%	2520
	21st to end	Day	21% 12% 4 7% 4 3 3 2 4 5% 34%	2772
February	1st to 10th	Day	20% 8% 2 5% 4 5% 4 4 5% 6% 36%	1821
	11th to 20th	Day	14% 9% 2 5% 4 4 4 4 5% 7% 42%	1848
	21st to end	Day	19% 11% 2 7% 4 4 4 3 4 5% 37%	1570
March	1st to 10th	Day	25% 15% 3 7% 3 3 3 2 3 5% 30%	3116
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 6% 8% 45%	3350
	21st to end	Day	24% 10% 3 6% 4 3 4 3 4 5% 35%	3996
April	1st to 10th	Day	20% 10% 2 5% 4 4 4 3 5% 7% 37%	2280
	11th to 20th	Day	19% 8% 3 5% 4 4 4 4 5% 6% 40%	2264
	21st to end	Day	20% 7% 2 7% 5% 4 4 3 5% 6% 37%	2470
May	1st to 10th	Day	27% 12% 3 7% 4 3 3 3 4 7% 29%	4106
	11th to 20th	Day	24% 10% 3 6% 4 4 3 3 4 6% 32%	4081
	21st to end	Day	16% 6% 2 4 4 3 3 3 5% 8% 44%	4822
June	1st to 10th	Day	19% 6% 2 6% 5% 4 5% 5% 6% 8% 34%	2660
	11th to 20th	Day	16% 6% 2 5% 5% 5% 4 4 6% 8% 38%	2660
	21st to end	Day	15% 7% 2 6% 4 4 5% 4 6% 9% 38%	2660
July	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 10% 34%	4440
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 10% 37%	4440
	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	17% 8% 2 4 4 3 3 3 4 4 47%	1698
	11th to 20th	Day	18% 7% 2 4 3 3 3 4 5% 6% 45%	1710
	21st to end	Day	17% 7% 2 5% 3 5% 3 3 5% 6% 43%	1710
December	1st to 10th	Day	20% 9% 2 6% 4 4 4 2 4 6% 39%	1611
	11th to 20th	Day	21% 7% 2 4 4 3 3 2 4 6% 44%	1906
	21st to end	Day	22% 11% 3 8% 4 4 3 3 4 5% 34%	2772
January	1st to 10th	Night	17% 7% 2 4 3 3 3 2 3 6% 48%	4429
	11th to 20th	Night	17% 11% 3 7% 4 4 4 2 4 4 40%	4440
	21st to end	Night	22% 13% 4 8% 4 4 4 3 4 4 30%	4884
February	1st to 10th	Night	17% 9% 3 7% 4 4 3 3 4 5% 40%	2352
	11th to 20th	Night	18% 8% 3 6% 4 3 3 4 4 5% 40%	2218
	21st to end	Night	18% 10% 2 6% 4 4 4 3 4 4 41%	1884
March	1st to 10th	Night	29% 10% 2 4 3 3 3 3 4 5% 32%	3724
	11th to 20th	Night	10% 10% 2 8% 4 4 5% 4 5% 6% 41%	3607
	21st to end	Night	25% 10% 2 6% 4 4 4 3 4 4 33%	3631
April	1st to 10th	Night	18% 9% 4 7% 4 4 4 3 4 6% 37%	1900
	11th to 20th	Night	20% 8% 2 5% 5% 4 3 3 5% 6% 37%	1863
	21st to end	Night	21% 7% 3 6% 4 4 3 3 5% 5% 39%	1710
May	1st to 10th	Night	26% 10% 4 9% 4 4 4 3 5% 6% 26%	2834
	11th to 20th	Night	23% 11% 3 7% 5% 4 4 4 6% 7% 25%	2813
	21st to end	Night	18% 7% 2 6% 4 4 4 3 6% 7% 37%	2781
June	1st to 10th	Night	18% 8% 3 7% 4 4 4 3 6% 8% 36%	1520
	11th to 20th	Night	19% 7% 1 5% 6% 4 4 3 5% 8% 38%	1520
	21st to end	Night	18% 8% 2 4 4 4 4 4 6% 9% 38%	1520
July	1st to 10th	Night	18% 7% 4 7% 5% 4 5% 4 6% 8% 32%	2520
	11th to 20th	Night	18% 7% 2 5% 4 5% 5% 4 7% 8% 36%	2520
	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%	2453
	11th to 20th	Night	17% 8% 2 5% 3 3 3 3 4 5% 45%	2470
	21st to end	Night	15% 7% 3 5% 3 3 3 3 6% 7% 45%	2470
December	1st to 10th	Night	21% 7% 3 6% 4 4 4 3 4 5% 40%	2567
	11th to 20th	Night	20% 8% 2 5% 4 4 3 3 4 6% 41%	3288
	21st to end	Night	20% 12% 3 7% 4 4 3 2 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% 17% 3	2280
	11th to 20th	Day	45% 5% 9% 21% 17% 3	2264
	21st to end	Day	47% 6% 9% 19% 16% 4	2470
May	1st to 10th	Day	57% 6% 12% 14% 9% 1	4120
	11th to 20th	Day	53% 6% 15% 16% 10% 2	4085
	21st to end	Day	39% 7% 10% 24% 18% 1	4822
June	1st to 10th	Day	46% 7% 12% 22% 12% 1	2660
	11th to 20th	Day	43% 7% 13% 23% 14% 1	2660
	21st to end	Day	41% 7% 15% 23% 13% 1	2660
July	1st to 10th	Day	46% 8% 11% 20% 13% 1	4370
	11th to 20th	Day	42% 6% 15% 23% 12% 1	4365
	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	41% 5% 13% 21% 18% 2	1698
	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% 19% 16% 3	1900
	11th to 20th	Night	47% 5% 8% 19% 18% 3	1863
	21st to end	Night	48% 5% 8% 19% 15% 5%	1710
May	1st to 10th	Night	58% 8% 10% 14% 7% 2	2840
	11th to 20th	Night	58% 7% 12% 14% 7% 2	2814
	21st to end	Night	46% 10% 11% 20% 13% 1	2782
June	1st to 10th	Night	47% 6% 12% 20% 14%	1520
	11th to 20th	Night	44% 4 12% 25% 14% 1	1520
	21st to end	Night	44% 7% 13% 23% 12% 1	1520
July	1st to 10th	Night	52% 7% 9% 17% 15%	2480
	11th to 20th	Night	45% 7% 16% 23% 9% 1	2472
	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	40% 6% 11% 22% 18% 3	2453
	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	460
	11th to 20th	Day	27% 27% 33% 10% 3	452
	21st to end	Day	27% 26% 29% 12% 4 2	460
May	1st to 10th	Day	32% 26% 29% 10% 3	463
	11th to 20th	Day	24% 22% 37% 12% 5%	487
	21st to end	Day	19% 18% 35% 21% 5% 2	529
June	1st to 10th	Day	19% 19% 36% 19% 6% 1	490
	11th to 20th	Day	17% 21% 39% 16% 5% 1	490
	21st to end	Day	18% 19% 36% 21% 6% 1	490
July	1st to 10th	Day	18% 10% 36% 24% 10% 1	485
	11th to 20th	Day	17% 10% 37% 25% 9% 2	484
	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% 20% 32% 17% 9% 1	374
	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	23% 24% 31% 19% 3	760
	11th to 20th	Night	24% 21% 33% 19% 3	739
	21st to end	Night	25% 18% 34% 17% 5%	570
May	1st to 10th	Night	29% 15% 31% 19% 5%	570
	11th to 20th	Night	23% 19% 37% 15% 4 1	562
	21st to end	Night	23% 8% 32% 30% 7% 1	441
June	1st to 10th	Night	21% 10% 29% 31% 9% 1	380
	11th to 20th	Night	21% 7% 31% 33% 7%	380
	21st to end	Night	25% 3 31% 34% 7%	380
July	1st to 10th	Night	19% 7% 29% 34% 10% 1	370
	11th to 20th	Night	17% 7% 28% 35% 13%	368
	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	755
	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 2024-11-01 22:47:48. Version ac07

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