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 2026-01-17.

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% 6% 5% 5% 4 6% 6% 30%	127372
	Night	15% 10% 4 10% 6% 5% 5% 4 5% 6% 29%	124343
Transparency	Day	49% 8% 12% 19% 11% 1	130853
	Night	48% 9% 13% 18% 11% 1	128936
Seeing	Day	25% 20% 29% 17% 7% 2	16257
	Night	22% 19% 27% 23% 8% 1	25302

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	9% 11% 6% 16% 13% 7% 5% 4 5% 4 21%	3677
	11th to 20th	Day	10% 13% 6% 16% 11% 5% 5% 4 5% 4 22%	3618
	21st to end	Day	11% 13% 7% 13% 6% 5% 6% 4 6% 5% 24%	3960
February	1st to 10th	Day	13% 13% 4 11% 7% 7% 4 4 5% 5% 25%	1778
	11th to 20th	Day	10% 10% 6% 13% 9% 6% 5% 4 6% 6% 24%	1948
	21st to end	Day	15% 14% 4 12% 6% 5% 4 3 4 5% 27%	1650
March	1st to 10th	Day	17% 14% 6% 13% 6% 5% 4 3 4 3 25%	4476
	11th to 20th	Day	13% 13% 6% 9% 5% 5% 4 4 5% 5% 31%	4661
	21st to end	Day	21% 11% 5% 8% 5% 4 4 3 4 5% 29%	5970
April	1st to 10th	Day	19% 13% 4 8% 5% 4 4 4 4 6% 30%	2400
	11th to 20th	Day	16% 12% 3 7% 5% 5% 6% 3 5% 6% 32%	2374
	21st to end	Day	21% 12% 4 9% 4 4 4 3 5% 5% 30%	2570
May	1st to 10th	Day	32% 12% 3 8% 5% 4 4 3 4 5% 20%	5920
	11th to 20th	Day	17% 13% 4 9% 6% 5% 5% 4 6% 6% 26%	6318
	21st to end	Day	19% 8% 2 8% 5% 6% 5% 4 6% 6% 30%	6949
June	1st to 10th	Day	20% 8% 3 7% 6% 6% 6% 5% 6% 6% 27%	2800
	11th to 20th	Day	18% 8% 3 7% 5% 5% 4 4 6% 8% 32%	2800
	21st to end	Day	19% 8% 4 9% 5% 6% 5% 4 6% 7% 28%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 4 7% 10% 33%	6360
	11th to 20th	Day	10% 6% 3 8% 6% 6% 6% 6% 7% 10% 33%	6360
	21st to end	Day	14% 6% 2 6% 4 5% 5% 4 7% 8% 38%	6926
August	1st to 10th	Day	15% 7% 3 7% 6% 6% 6% 4 7% 8% 32%	4499
	11th to 20th	Day	15% 7% 3 7% 5% 6% 6% 6% 8% 9% 28%	4019
	21st to end	Day	11% 5% 2 7% 5% 5% 6% 5% 6% 9% 39%	4312
September	1st to 10th	Day	13% 6% 2 5% 5% 5% 5% 4 6% 7% 43%	2400
	11th to 20th	Day	12% 6% 2 7% 3 4 4 3 5% 7% 46%	2264
	21st to end	Day	17% 7% 2 6% 4 4 4 2 6% 8% 40%	2200
October	1st to 10th	Day	21% 8% 3 8% 5% 4 4 3 5% 5% 35%	2153
	11th to 20th	Day	15% 10% 3 7% 6% 4 5% 3 5% 6% 36%	1939
	21st to end	Day	23% 14% 3 9% 4 4 4 2 4 5% 28%	1980
November	1st to 10th	Day	12% 11% 3 8% 5% 4 5% 4 6% 7% 35%	1851
	11th to 20th	Day	15% 12% 5% 7% 4 5% 5% 4 5% 5% 33%	1890
	21st to end	Day	17% 13% 5% 10% 5% 5% 5% 4 4 5% 26%	1756
December	1st to 10th	Day	14% 14% 5% 16% 8% 7% 5% 4 6% 6% 16%	2680
	11th to 20th	Day	17% 12% 8% 11% 6% 5% 4 3 4 5% 25%	3066
	21st to end	Day	16% 13% 5% 12% 6% 5% 4 4 5% 5% 26%	4048
January	1st to 10th	Night	11% 9% 5% 16% 11% 6% 5% 5% 5% 4 23%	6489
	11th to 20th	Night	9% 12% 7% 17% 11% 8% 7% 5% 6% 5% 15%	6400
	21st to end	Night	12% 11% 4 12% 6% 7% 6% 5% 7% 6% 24%	6996
February	1st to 10th	Night	13% 12% 5% 10% 8% 6% 6% 5% 6% 6% 23%	2619
	11th to 20th	Night	10% 10% 4 10% 9% 7% 6% 5% 7% 7% 26%	2338
	21st to end	Night	13% 11% 4 10% 7% 6% 4 4 6% 6% 28%	1980
March	1st to 10th	Night	18% 11% 4 8% 8% 7% 5% 4 5% 6% 25%	5364
	11th to 20th	Night	13% 11% 5% 9% 6% 5% 5% 3 5% 6% 32%	5295
	21st to end	Night	18% 15% 4 10% 4 5% 4 3 4 4 27%	4962
April	1st to 10th	Night	17% 10% 3 10% 5% 5% 5% 4 5% 6% 30%	2000
	11th to 20th	Night	17% 11% 4 7% 6% 5% 4 3 5% 6% 32%	1975
	21st to end	Night	19% 10% 4 7% 5% 4 5% 3 6% 5% 32%	1830
May	1st to 10th	Night	34% 12% 3 7% 4 4 4 3 4 5% 20%	4054
	11th to 20th	Night	14% 13% 4 11% 6% 6% 4 3 5% 6% 28%	3585
	21st to end	Night	18% 9% 2 9% 6% 6% 5% 4 5% 7% 29%	3926
June	1st to 10th	Night	20% 7% 3 7% 5% 4 5% 4 6% 7% 33%	1600
	11th to 20th	Night	19% 6% 2 7% 4 4 5% 4 5% 5% 38%	1600
	21st to end	Night	16% 8% 2 7% 5% 5% 5% 4 6% 7% 36%	1600
July	1st to 10th	Night	12% 7% 3 7% 6% 5% 6% 5% 6% 9% 36%	3600
	11th to 20th	Night	15% 7% 3 9% 6% 5% 5% 4 6% 7% 34%	3600
	21st to end	Night	11% 6% 2 6% 5% 5% 5% 4 5% 6% 46%	3926
August	1st to 10th	Night	19% 9% 3 8% 5% 5% 4 3 5% 6% 34%	2617
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 3 6% 7% 39%	3025
	21st to end	Night	8% 5% 2 5% 5% 5% 5% 5% 7% 9% 45%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 6% 47%	2000
	11th to 20th	Night	12% 7% 1 5% 4 4 3 3 5% 6% 50%	2135
	21st to end	Night	16% 7% 2 5% 4 4 4 4 5% 5% 44%	2200
October	1st to 10th	Night	18% 8% 3 7% 4 5% 4 3 5% 6% 36%	2245
	11th to 20th	Night	16% 9% 3 6% 5% 6% 5% 4 6% 7% 35%	2460
	21st to end	Night	21% 11% 3 8% 5% 5% 5% 4 5% 6% 28%	2860
November	1st to 10th	Night	14% 11% 3 8% 5% 4 5% 4 5% 5% 35%	2667
	11th to 20th	Night	14% 10% 4 8% 6% 6% 5% 4 5% 5% 32%	2730
	21st to end	Night	14% 11% 4 10% 6% 5% 5% 3 6% 6% 30%	2861
December	1st to 10th	Night	14% 14% 6% 14% 6% 6% 7% 5% 6% 5% 17%	4720
	11th to 20th	Night	21% 14% 5% 10% 5% 5% 4 3 4 4 24%	5348
	21st to end	Night	16% 15% 5% 12% 6% 6% 5% 3 4 5% 23%	7150

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	57% 9% 11% 17% 5%	3678
	11th to 20th	Day	57% 8% 8% 14% 9% 2	3648
	21st to end	Day	54% 8% 10% 15% 11% 2	3960
February	1st to 10th	Day	57% 9% 12% 14% 8% 2	1778
	11th to 20th	Day	54% 9% 12% 15% 9% 2	1957
	21st to end	Day	54% 8% 10% 15% 11% 1	1650
March	1st to 10th	Day	59% 7% 9% 10% 10% 5%	4479
	11th to 20th	Day	48% 8% 17% 16% 8% 3	4661
	21st to end	Day	53% 5% 9% 12% 17% 3	5970
April	1st to 10th	Day	53% 7% 8% 18% 11% 3	2400
	11th to 20th	Day	49% 8% 9% 19% 14% 1	2374
	21st to end	Day	54% 7% 9% 16% 12% 3	2570
May	1st to 10th	Day	65% 7% 7% 14% 7% 1	5931
	11th to 20th	Day	53% 10% 13% 15% 8% 1	6322
	21st to end	Day	49% 10% 12% 21% 8% 1	6949
June	1st to 10th	Day	52% 8% 14% 16% 9% 1	2800
	11th to 20th	Day	48% 8% 12% 19% 12% 2	2800
	21st to end	Day	53% 9% 10% 18% 10% 1	2800
July	1st to 10th	Day	42% 8% 15% 23% 12%	6290
	11th to 20th	Day	38% 11% 15% 23% 12% 1	6285
	21st to end	Day	39% 8% 13% 30% 10%	6728
August	1st to 10th	Day	45% 10% 15% 19% 11%	4485
	11th to 20th	Day	45% 10% 16% 23% 6%	4058
	21st to end	Day	34% 9% 17% 27% 12% 1	4312
September	1st to 10th	Day	37% 9% 13% 28% 14% 1	2400
	11th to 20th	Day	36% 6% 11% 28% 18% 1	2264
	21st to end	Day	40% 8% 13% 25% 13% 2	2200
October	1st to 10th	Day	44% 8% 9% 21% 16% 1	3504
	11th to 20th	Day	42% 5% 13% 23% 13% 3	3129
	21st to end	Day	41% 7% 10% 28% 12% 2	3180
November	1st to 10th	Day	43% 9% 14% 20% 14%	1851
	11th to 20th	Day	46% 8% 13% 15% 15% 2	1890
	21st to end	Day	55% 7% 16% 16% 5% 1	1756
December	1st to 10th	Day	59% 9% 14% 13% 4	2680
	11th to 20th	Day	55% 8% 10% 15% 10% 1	3066
	21st to end	Day	57% 7% 8% 13% 12% 4	4048
January	1st to 10th	Night	54% 10% 13% 15% 7% 1	6489
	11th to 20th	Night	58% 11% 12% 11% 7% 1	6445
	21st to end	Night	51% 9% 11% 12% 13% 3	6996
February	1st to 10th	Night	54% 12% 11% 14% 9% 1	2619
	11th to 20th	Night	48% 11% 14% 15% 11% 1	2340
	21st to end	Night	49% 9% 13% 18% 7% 3	1980
March	1st to 10th	Night	52% 7% 12% 14% 12% 3	5374
	11th to 20th	Night	48% 8% 18% 15% 10% 2	5296
	21st to end	Night	54% 5% 9% 13% 17% 2	4962
April	1st to 10th	Night	50% 7% 12% 18% 11% 1	2000
	11th to 20th	Night	49% 7% 11% 19% 13% 1	1975
	21st to end	Night	48% 8% 10% 17% 12% 3	1830
May	1st to 10th	Night	62% 8% 9% 13% 7% 1	4060
	11th to 20th	Night	52% 11% 11% 18% 7% 1	3586
	21st to end	Night	49% 12% 12% 16% 10% 1	3927
June	1st to 10th	Night	46% 10% 13% 18% 12%	1600
	11th to 20th	Night	44% 9% 14% 18% 12% 1	1600
	21st to end	Night	45% 10% 12% 23% 10% 1	1600
July	1st to 10th	Night	39% 9% 17% 24% 11%	3560
	11th to 20th	Night	41% 12% 12% 24% 11%	3552
	21st to end	Night	34% 8% 14% 33% 10% 1	3809
August	1st to 10th	Night	47% 10% 12% 21% 10% 1	2607
	11th to 20th	Night	40% 10% 18% 23% 9%	3052
	21st to end	Night	26% 9% 21% 32% 12% 1	3586
September	1st to 10th	Night	35% 8% 15% 28% 14% 1	2000
	11th to 20th	Night	33% 7% 11% 30% 18% 1	2135
	21st to end	Night	39% 7% 12% 25% 15% 2	2200
October	1st to 10th	Night	43% 9% 12% 24% 11% 1	3674
	11th to 20th	Night	36% 7% 13% 26% 14% 4	3996
	21st to end	Night	43% 8% 12% 28% 9% 1	4610
November	1st to 10th	Night	45% 8% 14% 19% 12% 2	2667
	11th to 20th	Night	46% 8% 14% 17% 12% 2	2730
	21st to end	Night	49% 10% 15% 18% 7%	2861
December	1st to 10th	Night	56% 10% 14% 16% 3	4720
	11th to 20th	Night	58% 7% 9% 11% 14%	5348
	21st to end	Night	58% 8% 9% 12% 9% 4	7150

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	29% 29% 24% 16% 2	220
	11th to 20th	Day	34% 27% 29% 8% 1 1	212
	21st to end	Day	26% 26% 31% 13% 4	220
February	1st to 10th	Day	36% 35% 22% 6% 1	296
	11th to 20th	Day	33% 27% 27% 11% 1	496
	21st to end	Day	30% 26% 29% 13% 2	414
March	1st to 10th	Day	29% 25% 31% 12% 2	500
	11th to 20th	Day	32% 26% 27% 12% 3 1	500
	21st to end	Day	33% 25% 24% 10% 5% 2	550
April	1st to 10th	Day	39% 20% 24% 10% 5% 1	500
	11th to 20th	Day	34% 27% 21% 11% 5% 2	492
	21st to end	Day	33% 23% 27% 11% 4 2	500
May	1st to 10th	Day	33% 23% 26% 11% 6% 2	503
	11th to 20th	Day	21% 21% 33% 19% 3 2	527
	21st to end	Day	20% 18% 34% 21% 6% 1	573
June	1st to 10th	Day	24% 18% 36% 15% 6% 1	530
	11th to 20th	Day	21% 20% 32% 18% 8% 1	530
	21st to end	Day	23% 19% 33% 17% 6% 2	530
July	1st to 10th	Day	17% 14% 35% 21% 10% 3	525
	11th to 20th	Day	15% 10% 39% 25% 10% 2	524
	21st to end	Day	16% 11% 43% 20% 9% 2	576
August	1st to 10th	Day	13% 13% 33% 25% 13% 3	526
	11th to 20th	Day	17% 14% 29% 20% 13% 7%	519
	21st to end	Day	13% 16% 30% 26% 11% 5%	583
September	1st to 10th	Day	14% 17% 29% 21% 13% 6%	530
	11th to 20th	Day	13% 15% 26% 20% 16% 10%	530
	21st to end	Day	19% 14% 30% 20% 12% 5%	530
October	1st to 10th	Day	26% 15% 25% 19% 13% 3	530
	11th to 20th	Day	22% 23% 24% 23% 8% 1	496
	21st to end	Day	34% 21% 23% 17% 5% 1	462
November	1st to 10th	Day	26% 22% 24% 19% 8% 1	425
	11th to 20th	Day	30% 24% 25% 15% 5% 1	430
	21st to end	Day	35% 21% 27% 14% 2 1	296
December	1st to 10th	Day	29% 27% 29% 11% 3	220
	11th to 20th	Day	35% 29% 28% 6% 1	220
	21st to end	Day	35% 19% 30% 13% 3	242
January	1st to 10th	Night	23% 24% 32% 18% 3	840
	11th to 20th	Night	25% 23% 31% 19% 2 1	825
	21st to end	Night	17% 24% 35% 20% 4	880
February	1st to 10th	Night	21% 26% 36% 16% 2	800
	11th to 20th	Night	23% 25% 33% 17% 2	783
	21st to end	Night	25% 22% 32% 19% 2	660
March	1st to 10th	Night	26% 20% 34% 18% 3	800
	11th to 20th	Night	23% 25% 27% 20% 5%	800
	21st to end	Night	26% 22% 26% 19% 6% 1	880
April	1st to 10th	Night	25% 24% 27% 20% 4	800
	11th to 20th	Night	22% 27% 26% 17% 7% 1	791
	21st to end	Night	28% 19% 27% 20% 5%	630
May	1st to 10th	Night	30% 17% 26% 21% 7%	600
	11th to 20th	Night	19% 18% 28% 29% 6% 1	404
	21st to end	Night	18% 12% 27% 32% 10% 1	433
June	1st to 10th	Night	18% 13% 28% 31% 11%	400
	11th to 20th	Night	19% 12% 23% 34% 11% 1	400
	21st to end	Night	18% 10% 26% 35% 11% 1	400
July	1st to 10th	Night	17% 7% 21% 42% 12% 2	390
	11th to 20th	Night	14% 6% 24% 37% 17% 2	388
	21st to end	Night	12% 7% 26% 39% 16% 1	438
August	1st to 10th	Night	13% 6% 20% 39% 21% 1	416
	11th to 20th	Night	16% 7% 24% 32% 18% 3	680
	21st to end	Night	15% 8% 22% 33% 19% 2	880
September	1st to 10th	Night	15% 11% 24% 30% 19% 2	800
	11th to 20th	Night	14% 10% 22% 28% 22% 5%	800
	21st to end	Night	18% 12% 24% 27% 17% 2	800
October	1st to 10th	Night	23% 14% 20% 25% 16% 2	800
	11th to 20th	Night	20% 17% 25% 26% 11% 1	800
	21st to end	Night	27% 19% 26% 22% 5%	880
November	1st to 10th	Night	22% 22% 26% 22% 8% 1	820
	11th to 20th	Night	22% 22% 28% 19% 8% 1	840
	21st to end	Night	22% 25% 29% 19% 5% 1	840
December	1st to 10th	Night	25% 25% 29% 17% 4	840
	11th to 20th	Night	28% 25% 26% 18% 3	840
	21st to end	Night	28% 23% 31% 16% 3	924

Page generated at 2026-01-14 01:14:24. Version ac07

Copyright © 2026 Attilla Danko.