Bridgewater Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bridgewater Hill Observatory 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	16% 10% 4 9% 5% 5% 5% 4 6% 7% 30%	101575
	Night	16% 10% 4 9% 6% 5% 5% 4 5% 6% 31%	97925
Transparency	Day	48% 8% 12% 19% 10% 1	105074
	Night	48% 9% 13% 19% 10% 1	102502
Seeing	Day	24% 20% 29% 17% 7% 2	14876
	Night	21% 18% 27% 24% 9% 1	23839

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% 14% 8% 9% 7% 5% 5% 4 4 5% 27%	2517
	11th to 20th	Day	13% 13% 7% 15% 10% 4 4 3 4 5% 21%	2520
	21st to end	Day	15% 15% 8% 13% 6% 4 4 4 5% 4 20%	2772
February	1st to 10th	Day	13% 13% 4 10% 8% 7% 4 4 5% 5% 26%	1688
	11th to 20th	Day	10% 11% 6% 13% 9% 6% 5% 4 6% 6% 25%	1848
	21st to end	Day	15% 14% 4 12% 6% 4 4 3 4 5% 28%	1570
March	1st to 10th	Day	19% 16% 7% 11% 5% 5% 4 3 4 4 23%	3116
	11th to 20th	Day	13% 15% 7% 11% 6% 5% 4 4 4 4 25%	3259
	21st to end	Day	18% 12% 6% 9% 6% 4 4 3 3 5% 31%	4166
April	1st to 10th	Day	19% 14% 4 8% 4 4 4 4 4 6% 31%	2280
	11th to 20th	Day	16% 11% 3 7% 4 5% 6% 3 6% 6% 32%	2254
	21st to end	Day	22% 12% 4 9% 4 4 4 3 5% 5% 29%	2441
May	1st to 10th	Day	24% 11% 3 9% 5% 4 4 3 4 6% 25%	4126
	11th to 20th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	4398
	21st to end	Day	13% 6% 2 7% 4 5% 5% 4 7% 8% 39%	4837
June	1st to 10th	Day	20% 8% 3 7% 6% 6% 6% 5% 7% 6% 27%	2660
	11th to 20th	Day	18% 8% 3 7% 5% 5% 4 4 7% 8% 32%	2660
	21st to end	Day	19% 9% 4 9% 5% 6% 5% 5% 5% 6% 27%	2660
July	1st to 10th	Day	13% 7% 2 6% 5% 6% 6% 5% 7% 10% 34%	4440
	11th to 20th	Day	11% 5% 3 7% 6% 6% 6% 6% 8% 11% 32%	4440
	21st to end	Day	16% 8% 2 7% 5% 5% 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	15% 7% 3 7% 7% 6% 6% 4 7% 8% 31%	4361
	11th to 20th	Day	16% 8% 3 7% 6% 6% 6% 5% 8% 9% 27%	3894
	21st to end	Day	11% 5% 2 7% 5% 5% 6% 5% 6% 9% 39%	4180
September	1st to 10th	Day	13% 6% 2 5% 5% 5% 5% 4 6% 7% 42%	2280
	11th to 20th	Day	13% 7% 2 7% 3 4 4 3 5% 7% 46%	2151
	21st to end	Day	16% 7% 2 6% 4 4 4 2 6% 8% 40%	2090
October	1st to 10th	Day	22% 9% 3 8% 5% 4 4 3 5% 5% 33%	2046
	11th to 20th	Day	15% 11% 3 7% 6% 4 5% 3 5% 6% 36%	1842
	21st to end	Day	23% 14% 3 9% 4 4 4 2 4 5% 28%	1881
November	1st to 10th	Day	11% 11% 3 8% 5% 4 5% 4 6% 7% 36%	1761
	11th to 20th	Day	16% 12% 5% 7% 4 4 5% 4 5% 5% 34%	1800
	21st to end	Day	17% 13% 5% 10% 5% 5% 5% 4 4 5% 27%	1625
December	1st to 10th	Day	18% 15% 5% 12% 6% 5% 4 4 5% 5% 20%	1520
	11th to 20th	Day	20% 14% 6% 8% 4 3 4 4 4 5% 28%	1906
	21st to end	Day	19% 16% 5% 11% 5% 5% 4 3 5% 5% 21%	2772
January	1st to 10th	Night	15% 10% 5% 11% 6% 5% 6% 4 6% 5% 28%	4429
	11th to 20th	Night	11% 11% 9% 16% 10% 6% 5% 4 6% 5% 17%	4440
	21st to end	Night	17% 15% 5% 14% 5% 6% 5% 4 4 4 21%	4884
February	1st to 10th	Night	13% 12% 5% 10% 8% 6% 6% 5% 6% 6% 23%	2489
	11th to 20th	Night	9% 10% 4 10% 8% 7% 6% 5% 7% 6% 26%	2218
	21st to end	Night	13% 11% 4 10% 7% 5% 4 4 6% 6% 29%	1884
March	1st to 10th	Night	21% 13% 4 8% 6% 5% 4 4 5% 5% 25%	3724
	11th to 20th	Night	12% 12% 6% 9% 7% 6% 5% 3 4 5% 30%	3697
	21st to end	Night	17% 15% 4 10% 4 4 3 2 4 4 31%	3466
April	1st to 10th	Night	17% 10% 3 9% 5% 5% 5% 4 5% 6% 31%	1900
	11th to 20th	Night	18% 11% 4 6% 6% 5% 4 3 6% 7% 32%	1875
	21st to end	Night	19% 10% 4 7% 4 4 5% 3 6% 5% 31%	1739
May	1st to 10th	Night	26% 12% 3 7% 4 4 5% 4 5% 6% 25%	2834
	11th to 20th	Night	17% 11% 4 10% 5% 6% 4 4 5% 5% 29%	2505
	21st to end	Night	15% 7% 2 6% 4 4 5% 4 6% 9% 37%	2738
June	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	1520
	11th to 20th	Night	19% 6% 2 7% 4 4 5% 4 5% 5% 39%	1520
	21st to end	Night	17% 8% 2 6% 5% 5% 4 4 6% 7% 35%	1520
July	1st to 10th	Night	12% 6% 3 7% 6% 6% 6% 5% 6% 8% 34%	2520
	11th to 20th	Night	15% 6% 3 7% 5% 4 4 4 6% 8% 37%	2520
	21st to end	Night	15% 7% 3 6% 5% 5% 5% 4 6% 6% 39%	2739
August	1st to 10th	Night	19% 9% 3 8% 5% 5% 4 3 5% 6% 34%	2536
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 3 6% 7% 38%	2930
	21st to end	Night	9% 5% 2 5% 5% 5% 4 5% 7% 9% 45%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 6% 46%	1900
	11th to 20th	Night	13% 7% 1 5% 4 4 3 3 5% 5% 49%	2028
	21st to end	Night	16% 8% 2 5% 4 4 4 3 5% 5% 44%	2090
October	1st to 10th	Night	19% 8% 3 8% 5% 5% 4 3 5% 6% 34%	2133
	11th to 20th	Night	16% 9% 3 6% 5% 6% 5% 4 5% 6% 35%	2337
	21st to end	Night	22% 11% 3 8% 5% 4 5% 4 5% 6% 28%	2717
November	1st to 10th	Night	14% 10% 3 8% 5% 4 5% 4 5% 6% 35%	2537
	11th to 20th	Night	15% 10% 4 8% 6% 6% 5% 5% 5% 5% 32%	2600
	21st to end	Night	14% 11% 4 10% 5% 5% 4 3 6% 6% 31%	2648
December	1st to 10th	Night	17% 14% 6% 13% 7% 5% 5% 4 5% 5% 19%	2660
	11th to 20th	Night	19% 11% 5% 10% 5% 4 5% 4 5% 6% 25%	3288
	21st to end	Night	19% 14% 6% 13% 5% 5% 4 4 5% 4 20%	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	52% 10% 13% 20% 5%	2518
	11th to 20th	Day	62% 8% 10% 15% 4 1	2520
	21st to end	Day	63% 7% 11% 12% 6% 1	2772
February	1st to 10th	Day	56% 9% 12% 14% 7% 1	1688
	11th to 20th	Day	54% 9% 12% 14% 9% 2	1857
	21st to end	Day	54% 8% 11% 15% 11% 1	1570
March	1st to 10th	Day	63% 7% 9% 9% 10% 2	3119
	11th to 20th	Day	54% 9% 15% 12% 9% 2	3259
	21st to end	Day	54% 5% 9% 12% 18% 3	4166
April	1st to 10th	Day	52% 7% 8% 19% 11% 3	2280
	11th to 20th	Day	48% 8% 9% 19% 13% 1	2254
	21st to end	Day	55% 7% 9% 16% 11% 2	2441
May	1st to 10th	Day	58% 8% 9% 17% 7% 1	4140
	11th to 20th	Day	50% 11% 13% 16% 8% 1	4402
	21st to end	Day	39% 10% 13% 26% 12% 1	4837
June	1st to 10th	Day	52% 8% 14% 15% 9%	2660
	11th to 20th	Day	47% 8% 12% 18% 13% 2	2660
	21st to end	Day	54% 9% 10% 17% 9% 1	2660
July	1st to 10th	Day	40% 9% 15% 23% 13%	4370
	11th to 20th	Day	39% 12% 16% 25% 8%	4365
	21st to end	Day	44% 8% 13% 23% 11% 1	4661
August	1st to 10th	Day	45% 10% 15% 19% 10%	4347
	11th to 20th	Day	45% 11% 16% 22% 5%	3933
	21st to end	Day	34% 9% 18% 27% 12% 1	4180
September	1st to 10th	Day	37% 9% 13% 28% 13% 1	2280
	11th to 20th	Day	36% 6% 12% 28% 17% 1	2151
	21st to end	Day	40% 8% 13% 25% 12% 2	2090
October	1st to 10th	Day	45% 8% 10% 21% 15% 1	3397
	11th to 20th	Day	42% 6% 14% 23% 12% 3	3032
	21st to end	Day	41% 7% 10% 29% 12% 2	3081
November	1st to 10th	Day	43% 9% 15% 20% 13%	1761
	11th to 20th	Day	46% 8% 13% 15% 16% 2	1800
	21st to end	Day	54% 7% 17% 15% 6% 1	1625
December	1st to 10th	Day	61% 9% 13% 11% 6% 1	1520
	11th to 20th	Day	56% 7% 12% 15% 9% 1	1906
	21st to end	Day	63% 8% 9% 12% 8%	2772
January	1st to 10th	Night	49% 13% 14% 16% 7% 1	4429
	11th to 20th	Night	59% 12% 13% 10% 6% 1	4440
	21st to end	Night	62% 9% 9% 11% 7% 1	4884
February	1st to 10th	Night	54% 12% 11% 13% 8% 1	2489
	11th to 20th	Night	49% 11% 13% 16% 11% 1	2220
	21st to end	Night	49% 9% 13% 17% 8% 3	1884
March	1st to 10th	Night	57% 7% 11% 15% 9% 1	3734
	11th to 20th	Night	50% 9% 13% 16% 10% 1	3698
	21st to end	Night	52% 5% 10% 12% 17% 3	3466
April	1st to 10th	Night	49% 8% 12% 18% 12% 2	1900
	11th to 20th	Night	48% 7% 11% 20% 13% 1	1875
	21st to end	Night	49% 8% 11% 17% 12% 3	1739
May	1st to 10th	Night	55% 9% 10% 16% 8% 1	2840
	11th to 20th	Night	52% 12% 11% 16% 7% 1	2506
	21st to end	Night	39% 12% 15% 20% 13% 2	2739
June	1st to 10th	Night	46% 11% 14% 18% 11%	1520
	11th to 20th	Night	44% 9% 14% 19% 13% 1	1520
	21st to end	Night	46% 10% 12% 23% 10% 1	1520
July	1st to 10th	Night	40% 11% 15% 21% 12% 1	2480
	11th to 20th	Night	37% 12% 13% 28% 10%	2472
	21st to end	Night	41% 8% 11% 29% 10% 1	2651
August	1st to 10th	Night	48% 10% 12% 20% 9%	2526
	11th to 20th	Night	40% 10% 19% 23% 9%	2957
	21st to end	Night	26% 9% 21% 31% 12% 1	3476
September	1st to 10th	Night	35% 9% 15% 27% 14%	1900
	11th to 20th	Night	34% 7% 12% 30% 17% 1	2028
	21st to end	Night	39% 7% 12% 25% 14% 2	2090
October	1st to 10th	Night	44% 9% 12% 24% 10% 1	3562
	11th to 20th	Night	36% 7% 13% 26% 14% 4	3873
	21st to end	Night	43% 7% 12% 27% 9% 1	4467
November	1st to 10th	Night	45% 8% 14% 19% 12% 2	2537
	11th to 20th	Night	47% 8% 14% 16% 13% 2	2600
	21st to end	Night	49% 10% 16% 18% 7%	2648
December	1st to 10th	Night	59% 11% 13% 13% 4	2660
	11th to 20th	Night	55% 9% 13% 12% 11%	3288
	21st to end	Night	62% 9% 10% 14% 5%	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	28% 27% 24% 17% 3 1	180
	11th to 20th	Day	32% 26% 30% 9% 2 1	180
	21st to end	Day	21% 26% 34% 15% 4	198
February	1st to 10th	Day	33% 35% 24% 6% 1	266
	11th to 20th	Day	30% 29% 28% 12% 2	456
	21st to end	Day	29% 26% 29% 14% 2	382
March	1st to 10th	Day	27% 25% 33% 13% 3	460
	11th to 20th	Day	31% 26% 27% 12% 3 1	460
	21st to end	Day	32% 25% 25% 11% 5% 2	506
April	1st to 10th	Day	37% 19% 26% 11% 6% 1	460
	11th to 20th	Day	34% 25% 21% 12% 5% 2	452
	21st to end	Day	33% 22% 27% 11% 5% 2	460
May	1st to 10th	Day	30% 24% 27% 12% 6% 2	463
	11th to 20th	Day	20% 21% 35% 19% 3 2	487
	21st to end	Day	19% 19% 33% 22% 6% 1	529
June	1st to 10th	Day	23% 18% 36% 15% 7% 1	490
	11th to 20th	Day	20% 19% 33% 19% 8% 1	490
	21st to end	Day	23% 19% 32% 17% 6% 2	490
July	1st to 10th	Day	17% 14% 34% 22% 11% 3	485
	11th to 20th	Day	15% 9% 39% 24% 10% 2	484
	21st to end	Day	16% 11% 43% 19% 9% 2	532
August	1st to 10th	Day	14% 14% 33% 24% 13% 3	486
	11th to 20th	Day	18% 15% 30% 20% 12% 6%	479
	21st to end	Day	13% 16% 29% 25% 12% 6%	539
September	1st to 10th	Day	15% 16% 29% 21% 13% 6%	490
	11th to 20th	Day	14% 16% 24% 19% 17% 10%	490
	21st to end	Day	18% 14% 31% 20% 12% 5%	490
October	1st to 10th	Day	27% 15% 26% 19% 12% 3	490
	11th to 20th	Day	22% 21% 24% 23% 8% 1	459
	21st to end	Day	34% 19% 23% 17% 6% 1	429
November	1st to 10th	Day	24% 22% 24% 21% 9% 1	395
	11th to 20th	Day	30% 23% 25% 16% 5% 1	400
	21st to end	Day	32% 20% 29% 16% 2 1	261
December	1st to 10th	Day	28% 27% 29% 12% 3 1	180
	11th to 20th	Day	33% 28% 31% 7% 2	180
	21st to end	Day	36% 20% 29% 12% 3 1	198
January	1st to 10th	Night	22% 23% 33% 19% 3	760
	11th to 20th	Night	23% 23% 31% 20% 2 1	760
	21st to end	Night	17% 23% 35% 21% 4	836
February	1st to 10th	Night	19% 26% 36% 17% 2	760
	11th to 20th	Night	21% 25% 34% 18% 2	743
	21st to end	Night	25% 21% 32% 20% 2	628
March	1st to 10th	Night	24% 20% 34% 19% 3	760
	11th to 20th	Night	22% 26% 27% 20% 5%	760
	21st to end	Night	26% 22% 26% 19% 6% 1	836
April	1st to 10th	Night	23% 24% 28% 21% 4	760
	11th to 20th	Night	22% 26% 26% 18% 7% 1	751
	21st to end	Night	28% 19% 27% 21% 6%	599
May	1st to 10th	Night	29% 17% 25% 21% 8%	570
	11th to 20th	Night	19% 17% 29% 29% 6% 1	384
	21st to end	Night	17% 12% 27% 32% 11% 1	411
June	1st to 10th	Night	18% 13% 28% 31% 11%	380
	11th to 20th	Night	19% 11% 24% 33% 12% 1	380
	21st to end	Night	18% 10% 24% 35% 11% 1	380
July	1st to 10th	Night	17% 7% 21% 41% 12% 2	370
	11th to 20th	Night	14% 6% 24% 38% 16% 2	368
	21st to end	Night	13% 7% 24% 39% 16% 1	416
August	1st to 10th	Night	13% 6% 20% 38% 21% 1	395
	11th to 20th	Night	16% 7% 24% 32% 17% 3	645
	21st to end	Night	15% 8% 22% 33% 20% 2	836
September	1st to 10th	Night	15% 11% 24% 29% 19% 2	760
	11th to 20th	Night	15% 10% 22% 26% 21% 5%	760
	21st to end	Night	18% 12% 23% 27% 17% 2	760
October	1st to 10th	Night	24% 14% 20% 25% 15% 2	760
	11th to 20th	Night	20% 17% 25% 26% 11% 1	760
	21st to end	Night	28% 20% 25% 22% 6%	836
November	1st to 10th	Night	20% 22% 27% 23% 8% 1	780
	11th to 20th	Night	22% 21% 29% 20% 8% 1	800
	21st to end	Night	21% 24% 29% 19% 5% 1	779
December	1st to 10th	Night	24% 24% 30% 17% 4	760
	11th to 20th	Night	26% 24% 27% 20% 3	760
	21st to end	Night	28% 23% 31% 16% 3	836

Page generated at 2024-11-22 08:02:29. Version ac07

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