Bridgewater
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bridgewater clear sky chart from 2005-11-06 to 2025-03-12.

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	11% 7% 3 7% 4 4 5% 4 6% 7% 43%	112413
	Night	9% 7% 3 7% 5% 4 4 4 5% 7% 45%	115327
Transparency	Day	39% 6% 15% 24% 14% 2	112163
	Night	37% 6% 14% 25% 16% 2	115167
Seeing	Day	14% 9% 31% 30% 13% 2	16320
	Night	12% 9% 35% 34% 9% 1	26421

By Month

Percentage Cloud Cover SW (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	10% 12% 4 17% 9% 7% 7% 4 6% 5% 20%	3620
	11th to 20th	Day	5% 10% 5% 11% 7% 5% 7% 4 8% 7% 30%	3640
	21st to end	Day	4 9% 6% 13% 6% 6% 6% 4 5% 5% 37%	4463
February	1st to 10th	Day	9% 12% 4 10% 6% 7% 7% 4 6% 6% 29%	1800
	11th to 20th	Day	8% 10% 5% 10% 6% 5% 5% 5% 5% 7% 34%	1817
	21st to end	Day	9% 10% 3 9% 5% 6% 5% 3 6% 6% 37%	1650
March	1st to 10th	Day	10% 9% 4 10% 3 3 3 3 4 8% 44%	3358
	11th to 20th	Day	9% 7% 3 7% 4 4 4 3 5% 7% 47%	3491
	21st to end	Day	24% 11% 4 11% 5% 4 4 3 5% 6% 24%	3814
April	1st to 10th	Day	16% 7% 2 7% 5% 3 4 3 5% 7% 39%	2167
	11th to 20th	Day	18% 9% 3 6% 4 4 4 4 5% 6% 38%	2439
	21st to end	Day	18% 9% 3 6% 4 3 4 3 5% 7% 39%	2470
May	1st to 10th	Day	15% 7% 2 6% 4 5% 5% 3 6% 7% 39%	4120
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	4082
	21st to end	Day	17% 6% 2 7% 4 3 4 3 6% 8% 40%	4706
June	1st to 10th	Day	11% 5% 2 4 4 4 5% 4 6% 9% 48%	2836
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 8% 45%	2850
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 9% 49%	2850
July	1st to 10th	Day	11% 5% 2 6% 4 4 4 5% 6% 9% 45%	4623
	11th to 20th	Day	6% 3 1 4 3 4 4 3 5% 7% 59%	4430
	21st to end	Day	4 3 1 5% 4 4 4 5% 7% 9% 54%	4510
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 47%	4103
	11th to 20th	Day	17% 8% 2 6% 4 4 4 4 6% 7% 37%	4041
	21st to end	Day	7% 5% 2 5% 4 3 4 3 6% 7% 54%	4532
September	1st to 10th	Day	12% 5% 1 4 3 3 3 2 5% 7% 54%	2195
	11th to 20th	Day	8% 4 1 5% 3 3 3 4 5% 6% 56%	2090
	21st to end	Day	10% 5% 2 5% 3 3 3 3 5% 5% 56%	2090
October	1st to 10th	Day	8% 4 2 4 2 2 3 3 4 5% 64%	3302
	11th to 20th	Day	8% 4 2 5% 3 3 4 3 5% 7% 55%	3140
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 6% 7% 44%	3222
November	1st to 10th	Day	8% 8% 3 6% 3 4 5% 4 7% 7% 45%	1763
	11th to 20th	Day	7% 8% 2 10% 6% 3 4 3 6% 7% 44%	1715
	21st to end	Day	9% 10% 3 7% 4 4 4 3 7% 7% 43%	1600
December	1st to 10th	Day	4 8% 4 10% 7% 7% 6% 5% 6% 7% 37%	2610
	11th to 20th	Day	8% 11% 5% 11% 6% 6% 5% 5% 6% 7% 30%	2776
	21st to end	Day	14% 8% 4 15% 6% 4 4 4 7% 6% 27%	3498
January	1st to 10th	Night	7% 12% 5% 13% 10% 8% 7% 4 5% 5% 24%	6327
	11th to 20th	Night	3 8% 4 11% 7% 5% 7% 4 8% 8% 35%	6340
	21st to end	Night	3 8% 5% 10% 5% 5% 5% 4 6% 6% 43%	6512
February	1st to 10th	Night	9% 11% 4 8% 6% 5% 5% 3 6% 7% 37%	2600
	11th to 20th	Night	7% 10% 4 8% 5% 4 4 5% 6% 6% 41%	2488
	21st to end	Night	8% 8% 3 7% 5% 4 4 4 6% 8% 43%	1980
March	1st to 10th	Night	8% 8% 3 6% 4 3 4 3 5% 7% 49%	3722
	11th to 20th	Night	8% 7% 2 6% 4 4 4 3 5% 7% 50%	3497
	21st to end	Night	20% 12% 4 8% 6% 4 4 3 4 6% 30%	3817
April	1st to 10th	Night	14% 8% 3 6% 5% 4 4 4 6% 7% 41%	2007
	11th to 20th	Night	16% 8% 3 7% 5% 3 3 2 4 6% 43%	1689
	21st to end	Night	16% 8% 2 5% 3 4 5% 4 5% 7% 42%	1710
May	1st to 10th	Night	17% 5% 2 5% 3 4 3 4 5% 7% 45%	2850
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 5% 8% 52%	2823
	21st to end	Night	15% 6% 2 6% 4 4 3 4 6% 8% 43%	2876
June	1st to 10th	Night	6% 6% 2 5% 3 3 4 3 5% 8% 55%	1341
	11th to 20th	Night	8% 6% 2 5% 4 3 4 3 6% 8% 52%	1330
	21st to end	Night	7% 3 1 3 3 4 4 3 6% 10% 57%	1330
July	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 6% 8% 51%	2342
	11th to 20th	Night	4 2 1 3 3 2 3 4 5% 7% 65%	2540
	21st to end	Night	2 2 1 3 4 3 3 4 6% 10% 61%	3036
August	1st to 10th	Night	6% 5% 2 5% 4 3 5% 4 7% 9% 50%	2836
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 6% 49%	2798
	21st to end	Night	8% 3 2 4 4 3 3 4 6% 9% 56%	3135
September	1st to 10th	Night	8% 4 2 3 3 4 3 2 4 6% 60%	1981
	11th to 20th	Night	7% 3 2 4 3 3 4 4 5% 6% 58%	2090
	21st to end	Night	8% 5% 1 4 3 3 3 3 4 7% 60%	2090
October	1st to 10th	Night	9% 3 1 2 2 2 1 2 3 5% 70%	3626
	11th to 20th	Night	8% 4 1 5% 3 2 2 2 5% 8% 60%	3779
	21st to end	Night	11% 6% 2 7% 3 3 4 4 5% 9% 46%	4331
November	1st to 10th	Night	8% 6% 2 5% 4 3 4 3 5% 7% 52%	2535
	11th to 20th	Night	7% 7% 2 7% 5% 4 4 3 6% 8% 46%	2683
	21st to end	Night	8% 7% 2 7% 4 4 4 4 5% 7% 47%	2800
December	1st to 10th	Night	4 6% 4 9% 6% 5% 5% 4 5% 6% 46%	4580
	11th to 20th	Night	8% 9% 3 9% 6% 7% 5% 5% 6% 6% 36%	5426
	21st to end	Night	14% 12% 5% 13% 5% 4 4 3 5% 6% 30%	7480

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	64% 6% 13% 12% 4	3621
	11th to 20th	Day	51% 5% 13% 17% 14%	3640
	21st to end	Day	47% 6% 17% 19% 10%	4463
February	1st to 10th	Day	53% 7% 14% 16% 9%	1800
	11th to 20th	Day	48% 7% 14% 19% 11% 2	1826
	21st to end	Day	46% 6% 14% 21% 11% 1	1650
March	1st to 10th	Day	39% 6% 13% 28% 12% 1	3358
	11th to 20th	Day	35% 7% 11% 29% 14% 3	3491
	21st to end	Day	61% 7% 15% 13% 3 1	3814
April	1st to 10th	Day	44% 6% 14% 25% 11% 1	2167
	11th to 20th	Day	46% 7% 11% 21% 12% 2	2442
	21st to end	Day	45% 6% 13% 24% 11%	2470
May	1st to 10th	Day	40% 8% 13% 23% 16%	4120
	11th to 20th	Day	34% 7% 12% 28% 16% 3	4086
	21st to end	Day	40% 7% 13% 25% 13% 1	4706
June	1st to 10th	Day	32% 8% 17% 29% 12% 2	2836
	11th to 20th	Day	36% 7% 16% 23% 15% 3	2850
	21st to end	Day	30% 8% 16% 29% 14% 2	2850
July	1st to 10th	Day	35% 6% 20% 27% 11% 1	4552
	11th to 20th	Day	25% 5% 14% 34% 19% 4	4355
	21st to end	Day	24% 7% 17% 28% 22% 2	4367
August	1st to 10th	Day	33% 7% 16% 29% 14% 2	4090
	11th to 20th	Day	45% 5% 16% 24% 9% 1	4080
	21st to end	Day	29% 6% 18% 27% 16% 3	4531
September	1st to 10th	Day	31% 5% 12% 29% 20% 2	2195
	11th to 20th	Day	28% 5% 18% 32% 16% 1	2090
	21st to end	Day	30% 6% 10% 31% 21% 2	2090
October	1st to 10th	Day	24% 3 9% 26% 34% 4	3302
	11th to 20th	Day	30% 3 13% 29% 20% 4	3140
	21st to end	Day	40% 4 14% 22% 19% 2	3222
November	1st to 10th	Day	35% 7% 17% 25% 13% 3	1763
	11th to 20th	Day	40% 6% 15% 25% 13% 1	1715
	21st to end	Day	40% 5% 16% 25% 13% 1	1600
December	1st to 10th	Day	46% 6% 16% 24% 8% 1	2610
	11th to 20th	Day	53% 5% 19% 18% 6%	2773
	21st to end	Day	54% 6% 14% 20% 5%	3498
January	1st to 10th	Night	60% 5% 11% 16% 8%	6327
	11th to 20th	Night	44% 5% 15% 18% 16% 1	6340
	21st to end	Night	40% 7% 15% 25% 13%	6512
February	1st to 10th	Night	46% 7% 14% 21% 12% 1	2600
	11th to 20th	Night	43% 6% 13% 22% 13% 3	2489
	21st to end	Night	38% 6% 14% 25% 13% 4	1980
March	1st to 10th	Night	33% 7% 12% 26% 21% 2	3722
	11th to 20th	Night	32% 6% 11% 29% 19% 2	3497
	21st to end	Night	55% 7% 15% 17% 6% 1	3817
April	1st to 10th	Night	41% 8% 15% 23% 13% 1	2007
	11th to 20th	Night	44% 4 11% 21% 18% 2	1689
	21st to end	Night	40% 8% 12% 26% 12% 2	1710
May	1st to 10th	Night	36% 7% 15% 22% 17% 3	2850
	11th to 20th	Night	29% 8% 13% 24% 23% 3	2824
	21st to end	Night	38% 7% 13% 26% 15% 1	2877
June	1st to 10th	Night	28% 6% 16% 29% 17% 4	1341
	11th to 20th	Night	28% 7% 16% 25% 19% 5%	1330
	21st to end	Night	23% 6% 17% 30% 21% 3	1330
July	1st to 10th	Night	31% 5% 22% 26% 13% 4	2304
	11th to 20th	Night	18% 5% 15% 38% 20% 4	2492
	21st to end	Night	18% 6% 19% 32% 22% 4	2940
August	1st to 10th	Night	30% 6% 16% 30% 16% 2	2826
	11th to 20th	Night	35% 6% 16% 28% 13% 1	2825
	21st to end	Night	26% 6% 18% 24% 22% 4	3135
September	1st to 10th	Night	26% 6% 13% 28% 23% 4	1981
	11th to 20th	Night	24% 7% 15% 33% 19% 2	2090
	21st to end	Night	26% 5% 10% 31% 24% 4	2090
October	1st to 10th	Night	20% 3 7% 22% 44% 5%	3626
	11th to 20th	Night	25% 3 12% 32% 25% 4	3779
	21st to end	Night	37% 4 13% 27% 18% 1	4331
November	1st to 10th	Night	31% 5% 15% 25% 19% 4	2535
	11th to 20th	Night	35% 6% 15% 24% 17% 2	2683
	21st to end	Night	36% 5% 16% 27% 16% 1	2800
December	1st to 10th	Night	39% 5% 16% 31% 9%	4580
	11th to 20th	Night	47% 5% 18% 23% 7%	5428
	21st to end	Night	55% 5% 12% 21% 6%	7480

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	15% 10% 34% 30% 10% 1	300
	11th to 20th	Day	14% 15% 35% 28% 7%	300
	21st to end	Day	12% 12% 38% 29% 8% 1	330
February	1st to 10th	Day	12% 19% 40% 25% 4	300
	11th to 20th	Day	14% 12% 43% 24% 6% 1	300
	21st to end	Day	16% 12% 42% 26% 4	249
March	1st to 10th	Day	13% 16% 43% 24% 4 1	300
	11th to 20th	Day	18% 9% 38% 23% 10% 2	273
	21st to end	Day	22% 12% 35% 25% 5% 1	297
April	1st to 10th	Day	25% 17% 35% 18% 5%	347
	11th to 20th	Day	23% 14% 33% 23% 6%	640
	21st to end	Day	23% 16% 29% 21% 10% 2	650
May	1st to 10th	Day	18% 13% 35% 25% 9% 1	653
	11th to 20th	Day	19% 9% 31% 24% 15% 2	675
	21st to end	Day	19% 10% 29% 30% 11% 2	734
June	1st to 10th	Day	7% 9% 24% 35% 21% 5%	680
	11th to 20th	Day	15% 6% 35% 30% 12% 3	680
	21st to end	Day	9% 6% 31% 38% 15% 2	680
July	1st to 10th	Day	7% 3 20% 39% 24% 6%	671
	11th to 20th	Day	5% 4 30% 38% 21% 2	668
	21st to end	Day	5% 2 26% 43% 21% 3	739
August	1st to 10th	Day	9% 4 22% 41% 21% 3	676
	11th to 20th	Day	10% 4 26% 35% 22% 3	665
	21st to end	Day	7% 5% 27% 37% 20% 3	748
September	1st to 10th	Day	13% 7% 28% 33% 16% 4	405
	11th to 20th	Day	8% 4 37% 32% 16% 3	300
	21st to end	Day	14% 13% 30% 30% 11% 3	300
October	1st to 10th	Day	18% 8% 30% 26% 15% 3	300
	11th to 20th	Day	15% 15% 34% 25% 10% 1	300
	21st to end	Day	21% 15% 33% 24% 7% 1	330
November	1st to 10th	Day	19% 13% 34% 24% 9% 1	300
	11th to 20th	Day	14% 17% 37% 27% 6%	300
	21st to end	Day	16% 16% 32% 28% 7% 1	300
December	1st to 10th	Day	13% 9% 37% 29% 12% 1	300
	11th to 20th	Day	15% 15% 39% 26% 5%	300
	21st to end	Day	21% 11% 38% 26% 3 1	330
January	1st to 10th	Night	12% 10% 37% 32% 8% 1	1000
	11th to 20th	Night	11% 13% 37% 32% 7% 1	1000
	21st to end	Night	12% 11% 40% 30% 7%	1100
February	1st to 10th	Night	14% 12% 40% 29% 5%	1000
	11th to 20th	Night	13% 12% 37% 32% 6% 1	934
	21st to end	Night	12% 7% 39% 33% 7% 1	660
March	1st to 10th	Night	13% 9% 41% 32% 3 1	720
	11th to 20th	Night	15% 11% 38% 31% 5%	575
	21st to end	Night	19% 10% 37% 26% 8%	627
April	1st to 10th	Night	17% 14% 42% 25% 3	570
	11th to 20th	Night	22% 12% 34% 24% 7%	563
	21st to end	Night	17% 12% 36% 27% 7% 1	570
May	1st to 10th	Night	17% 9% 37% 27% 10%	570
	11th to 20th	Night	16% 8% 35% 30% 11%	561
	21st to end	Night	18% 5% 35% 33% 9%	617
June	1st to 10th	Night	9% 5% 29% 40% 17%	570
	11th to 20th	Night	11% 3 34% 43% 9% 1	570
	21st to end	Night	6% 5% 28% 48% 13%	570
July	1st to 10th	Night	6% 2 23% 46% 22% 1	555
	11th to 20th	Night	4 3 26% 50% 17%	552
	21st to end	Night	4 2 27% 52% 15% 1	624
August	1st to 10th	Night	5% 3 26% 50% 16% 1	567
	11th to 20th	Night	8% 2 27% 44% 19% 1	564
	21st to end	Night	6% 4 29% 44% 17% 1	627
September	1st to 10th	Night	9% 5% 29% 40% 15% 2	570
	11th to 20th	Night	6% 4 36% 38% 13% 2	570
	21st to end	Night	9% 6% 36% 36% 12% 1	570
October	1st to 10th	Night	16% 11% 32% 30% 11%	651
	11th to 20th	Night	15% 11% 37% 28% 9%	760
	21st to end	Night	16% 12% 34% 31% 8%	959
November	1st to 10th	Night	11% 8% 40% 32% 9% 1	975
	11th to 20th	Night	12% 11% 41% 28% 7% 1	1000
	21st to end	Night	12% 10% 37% 35% 7%	1000
December	1st to 10th	Night	12% 11% 40% 30% 6% 1	1000
	11th to 20th	Night	14% 9% 37% 32% 8%	1000
	21st to end	Night	20% 12% 34% 27% 7%	1100

Page generated at 2025-03-09 07:08:43. Version ac07

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