Observatoire Bennett
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Bennett clear sky chart from 2005-11-06 to 2025-04-11.

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% 13% 5% 11% 7% 5% 5% 4 5% 6% 23%	112718
	Night	14% 14% 6% 13% 8% 6% 5% 3 4 5% 22%	115533
Transparency	Day	59% 7% 11% 15% 7% 1	116234
	Night	63% 7% 10% 13% 7% 1	120182
Seeing	Day	28% 21% 28% 15% 6% 1	15422
	Night	26% 19% 27% 20% 7% 1	24401

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% 17% 11% 17% 13% 9% 7% 3 3 3 8%	3597
	11th to 20th	Day	5% 13% 7% 27% 17% 8% 4 3 3 3 11%	3570
	21st to end	Day	7% 13% 7% 20% 9% 9% 9% 4 4 4 15%	3960
February	1st to 10th	Day	10% 17% 7% 16% 14% 7% 4 3 3 3 16%	1669
	11th to 20th	Day	9% 18% 9% 19% 9% 7% 4 3 3 4 14%	1948
	21st to end	Day	9% 19% 5% 14% 9% 8% 5% 4 5% 5% 18%	1650
March	1st to 10th	Day	14% 16% 7% 16% 8% 6% 5% 3 4 4 18%	4476
	11th to 20th	Day	11% 17% 6% 11% 6% 5% 5% 4 5% 6% 24%	4786
	21st to end	Day	18% 16% 7% 14% 6% 4 4 2 4 4 22%	5774
April	1st to 10th	Day	18% 22% 5% 11% 6% 3 2 3 3 4 24%	2400
	11th to 20th	Day	15% 17% 6% 8% 4 4 4 3 4 5% 31%	2333
	21st to end	Day	21% 17% 4 10% 4 3 4 2 4 4 25%	2470
May	1st to 10th	Day	23% 13% 3 9% 5% 5% 5% 3 4 5% 25%	4106
	11th to 20th	Day	20% 10% 3 9% 6% 6% 5% 4 6% 6% 25%	4365
	21st to end	Day	15% 9% 3 7% 5% 6% 6% 5% 7% 9% 28%	4837
June	1st to 10th	Day	27% 11% 3 7% 4 4 5% 4 6% 6% 23%	2679
	11th to 20th	Day	20% 8% 3 7% 5% 5% 5% 4 7% 7% 30%	2850
	21st to end	Day	23% 10% 5% 8% 6% 5% 5% 4 6% 7% 24%	2670
July	1st to 10th	Day	14% 7% 3 8% 6% 6% 6% 5% 8% 9% 28%	4440
	11th to 20th	Day	15% 9% 3 8% 6% 6% 6% 5% 7% 8% 27%	4440
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	19% 9% 3 9% 6% 6% 6% 5% 6% 8% 24%	4307
	11th to 20th	Day	18% 10% 4 9% 6% 5% 6% 6% 8% 7% 21%	4045
	21st to end	Day	19% 9% 3 7% 4 4 4 4 6% 8% 33%	4292
September	1st to 10th	Day	18% 7% 3 7% 4 4 5% 4 5% 6% 38%	2280
	11th to 20th	Day	18% 9% 2 6% 4 4 4 4 4 6% 40%	2095
	21st to end	Day	18% 11% 3 7% 4 4 5% 3 6% 8% 32%	2090
October	1st to 10th	Day	20% 13% 4 6% 4 4 4 4 6% 7% 27%	2090
	11th to 20th	Day	16% 18% 5% 10% 6% 4 4 4 5% 5% 25%	1774
	21st to end	Day	19% 24% 5% 8% 5% 3 4 3 4 4 21%	1881
November	1st to 10th	Day	15% 19% 7% 9% 6% 4 4 4 4 4 25%	1761
	11th to 20th	Day	16% 17% 8% 12% 6% 4 3 3 5% 5% 20%	1800
	21st to end	Day	15% 18% 7% 15% 7% 4 4 3 4 7% 16%	1640
December	1st to 10th	Day	12% 19% 10% 24% 11% 5% 3 2 3 3 10%	2280
	11th to 20th	Day	13% 17% 10% 14% 8% 5% 4 2 4 4 19%	2618
	21st to end	Day	13% 20% 7% 14% 8% 4 6% 3 4 5% 17%	3931
January	1st to 10th	Night	9% 16% 10% 23% 13% 9% 6% 3 3 3 7%	6349
	11th to 20th	Night	7% 15% 8% 25% 17% 9% 5% 3 3 2 7%	6315
	21st to end	Night	7% 12% 8% 17% 9% 11% 8% 4 5% 5% 14%	6996
February	1st to 10th	Night	9% 15% 7% 17% 12% 7% 6% 3 4 4 16%	2726
	11th to 20th	Night	8% 16% 8% 19% 11% 8% 6% 3 3 4 14%	2338
	21st to end	Night	9% 16% 7% 14% 9% 8% 5% 3 3 4 21%	1980
March	1st to 10th	Night	15% 18% 7% 16% 7% 6% 7% 2 3 3 17%	5364
	11th to 20th	Night	11% 18% 7% 12% 6% 6% 5% 3 5% 5% 22%	5173
	21st to end	Night	15% 16% 6% 13% 7% 4 4 3 4 4 24%	5156
April	1st to 10th	Night	18% 19% 5% 10% 4 4 4 3 4 4 25%	2000
	11th to 20th	Night	18% 15% 4 7% 4 4 4 4 4 5% 30%	1803
	21st to end	Night	20% 17% 3 7% 5% 4 4 3 4 6% 27%	1710
May	1st to 10th	Night	22% 12% 3 7% 4 4 5% 4 6% 7% 24%	2834
	11th to 20th	Night	17% 10% 3 10% 6% 4 4 4 5% 5% 31%	2544
	21st to end	Night	18% 8% 2 7% 5% 5% 5% 4 6% 8% 34%	2738
June	1st to 10th	Night	26% 10% 3 8% 4 3 4 3 5% 7% 27%	1489
	11th to 20th	Night	19% 9% 3 7% 4 4 4 4 6% 7% 34%	1330
	21st to end	Night	20% 9% 3 8% 6% 5% 5% 4 6% 6% 29%	1497
July	1st to 10th	Night	16% 10% 3 8% 5% 5% 6% 5% 5% 7% 30%	2520
	11th to 20th	Night	19% 10% 4 7% 4 5% 4 4 5% 6% 30%	2520
	21st to end	Night	15% 7% 3 7% 5% 4 6% 4 6% 7% 36%	2739
August	1st to 10th	Night	23% 9% 4 8% 5% 4 5% 3 5% 6% 29%	2592
	11th to 20th	Night	16% 9% 3 9% 6% 5% 6% 5% 6% 7% 28%	2782
	21st to end	Night	17% 9% 3 5% 4 4 4 3 5% 7% 39%	3348
September	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 5% 6% 40%	1900
	11th to 20th	Night	17% 9% 2 5% 4 4 4 4 5% 6% 43%	2085
	21st to end	Night	19% 9% 2 6% 4 3 4 3 4 7% 39%	2090
October	1st to 10th	Night	18% 11% 3 8% 5% 4 4 4 5% 5% 33%	2090
	11th to 20th	Night	20% 17% 3 8% 5% 5% 4 3 5% 5% 26%	2401
	21st to end	Night	22% 18% 4 9% 4 4 4 3 5% 4 22%	2717
November	1st to 10th	Night	18% 18% 6% 10% 6% 4 4 2 4 5% 23%	2537
	11th to 20th	Night	14% 17% 8% 14% 6% 3 3 3 4 5% 24%	2600
	21st to end	Night	14% 17% 8% 16% 9% 5% 3 2 4 4 17%	2750
December	1st to 10th	Night	12% 18% 10% 25% 12% 5% 3 2 2 3 7%	4910
	11th to 20th	Night	12% 21% 7% 17% 11% 5% 5% 3 3 3 13%	5591
	21st to end	Night	14% 18% 7% 13% 7% 5% 5% 3 4 4 19%	7019

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	80% 6% 7% 5% 1	3598
	11th to 20th	Day	78% 5% 4 9% 3 1	3600
	21st to end	Day	71% 5% 8% 6% 8% 2	3960
February	1st to 10th	Day	75% 3 5% 9% 7% 1	1669
	11th to 20th	Day	74% 4 7% 9% 5% 1	1957
	21st to end	Day	67% 5% 8% 12% 6% 2	1650
March	1st to 10th	Day	69% 6% 7% 8% 7% 4	4479
	11th to 20th	Day	60% 5% 13% 12% 8% 1	4786
	21st to end	Day	64% 5% 6% 12% 12% 1	5774
April	1st to 10th	Day	65% 4 8% 13% 8% 2	2400
	11th to 20th	Day	56% 5% 8% 19% 11% 1	2333
	21st to end	Day	61% 6% 8% 14% 9% 3	2470
May	1st to 10th	Day	58% 10% 10% 15% 7% 1	4120
	11th to 20th	Day	54% 12% 15% 13% 5% 1	4369
	21st to end	Day	46% 12% 14% 19% 9%	4837
June	1st to 10th	Day	58% 9% 10% 18% 4	2679
	11th to 20th	Day	50% 8% 14% 19% 8% 1	2850
	21st to end	Day	58% 9% 11% 16% 6%	2670
July	1st to 10th	Day	48% 8% 15% 21% 7%	4370
	11th to 20th	Day	50% 8% 15% 24% 3	4365
	21st to end	Day	48% 8% 12% 26% 6%	4661
August	1st to 10th	Day	55% 9% 15% 18% 4	4293
	11th to 20th	Day	55% 11% 16% 15% 4	4084
	21st to end	Day	48% 8% 14% 17% 11% 3	4292
September	1st to 10th	Day	45% 7% 15% 23% 10%	2280
	11th to 20th	Day	44% 6% 12% 25% 12% 1	2095
	21st to end	Day	49% 8% 11% 21% 10% 1	2090
October	1st to 10th	Day	53% 6% 14% 19% 8%	3480
	11th to 20th	Day	57% 5% 10% 20% 7%	2912
	21st to end	Day	55% 5% 14% 17% 8%	3081
November	1st to 10th	Day	61% 6% 10% 15% 7% 1	1761
	11th to 20th	Day	65% 7% 8% 11% 7% 2	1800
	21st to end	Day	68% 7% 10% 12% 3	1640
December	1st to 10th	Day	82% 4 6% 5% 3	2280
	11th to 20th	Day	66% 5% 8% 9% 11% 1	2618
	21st to end	Day	69% 5% 6% 9% 8% 3	3931
January	1st to 10th	Night	83% 5% 6% 5% 2	6349
	11th to 20th	Night	84% 5% 4 5% 2	6360
	21st to end	Night	69% 7% 8% 8% 8% 1	6996
February	1st to 10th	Night	71% 6% 7% 11% 5% 1	2726
	11th to 20th	Night	74% 6% 7% 9% 4 1	2340
	21st to end	Night	65% 6% 6% 13% 8% 1	1980
March	1st to 10th	Night	71% 5% 7% 10% 5% 2	5374
	11th to 20th	Night	62% 7% 12% 11% 7% 1	5174
	21st to end	Night	63% 5% 6% 12% 13% 1	5156
April	1st to 10th	Night	62% 5% 8% 15% 10% 1	2000
	11th to 20th	Night	54% 8% 9% 18% 11% 1	1803
	21st to end	Night	58% 6% 11% 14% 9% 2	1710
May	1st to 10th	Night	53% 11% 15% 13% 6% 1	2840
	11th to 20th	Night	51% 11% 15% 17% 6% 1	2545
	21st to end	Night	45% 10% 16% 17% 12%	2739
June	1st to 10th	Night	55% 7% 12% 19% 6%	1489
	11th to 20th	Night	47% 8% 12% 22% 9% 2	1330
	21st to end	Night	54% 10% 11% 18% 7%	1497
July	1st to 10th	Night	51% 9% 14% 19% 7%	2480
	11th to 20th	Night	51% 8% 14% 22% 4	2472
	21st to end	Night	46% 9% 14% 25% 6%	2651
August	1st to 10th	Night	55% 7% 14% 20% 3	2582
	11th to 20th	Night	52% 11% 17% 15% 5%	2809
	21st to end	Night	45% 6% 15% 20% 13% 1	3348
September	1st to 10th	Night	44% 7% 14% 24% 10%	1900
	11th to 20th	Night	42% 7% 15% 25% 11% 1	2085
	21st to end	Night	45% 8% 13% 23% 11% 1	2090
October	1st to 10th	Night	49% 9% 12% 19% 11%	3480
	11th to 20th	Night	57% 6% 9% 19% 9%	4002
	21st to end	Night	55% 6% 14% 15% 8% 2	4467
November	1st to 10th	Night	63% 6% 11% 14% 6% 1	2537
	11th to 20th	Night	63% 5% 10% 12% 8% 2	2600
	21st to end	Night	70% 6% 10% 9% 4	2750
December	1st to 10th	Night	82% 6% 5% 5% 2	4910
	11th to 20th	Night	75% 6% 5% 7% 7%	5592
	21st to end	Night	67% 6% 7% 10% 7% 2	7019

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% 32% 31% 10%	200
	11th to 20th	Day	33% 28% 27% 12% 2 1	200
	21st to end	Day	28% 25% 33% 12% 2	220
February	1st to 10th	Day	36% 31% 25% 6% 1	261
	11th to 20th	Day	32% 30% 30% 6% 1	496
	21st to end	Day	31% 30% 25% 13% 1	414
March	1st to 10th	Day	30% 28% 28% 10% 3	500
	11th to 20th	Day	30% 31% 23% 12% 4	500
	21st to end	Day	35% 23% 24% 12% 6%	550
April	1st to 10th	Day	40% 20% 25% 9% 5%	500
	11th to 20th	Day	31% 27% 24% 12% 5% 1	452
	21st to end	Day	37% 22% 26% 10% 4 2	460
May	1st to 10th	Day	39% 17% 27% 12% 3 1	463
	11th to 20th	Day	28% 20% 29% 17% 6% 1	487
	21st to end	Day	23% 22% 34% 15% 5% 1	529
June	1st to 10th	Day	34% 18% 29% 13% 4 1	509
	11th to 20th	Day	23% 17% 28% 23% 9%	680
	21st to end	Day	29% 18% 32% 13% 7% 1	500
July	1st to 10th	Day	20% 16% 36% 21% 6% 1	485
	11th to 20th	Day	20% 15% 39% 18% 6% 1	484
	21st to end	Day	23% 15% 33% 23% 5% 1	532
August	1st to 10th	Day	21% 16% 29% 23% 10% 1	486
	11th to 20th	Day	19% 15% 29% 23% 11% 3	479
	21st to end	Day	23% 18% 24% 20% 10% 4	539
September	1st to 10th	Day	22% 17% 27% 20% 11% 3	490
	11th to 20th	Day	20% 13% 26% 20% 13% 7%	490
	21st to end	Day	21% 15% 25% 22% 12% 4	490
October	1st to 10th	Day	26% 20% 24% 16% 11% 4	490
	11th to 20th	Day	29% 22% 25% 14% 8% 1	407
	21st to end	Day	36% 25% 21% 15% 3	429
November	1st to 10th	Day	35% 22% 26% 14% 3	395
	11th to 20th	Day	30% 23% 31% 10% 5% 2	400
	21st to end	Day	33% 26% 27% 9% 3 1	285
December	1st to 10th	Day	28% 28% 30% 12% 3	200
	11th to 20th	Day	37% 27% 27% 7% 3 1	200
	21st to end	Day	33% 31% 24% 7% 4	220
January	1st to 10th	Night	24% 30% 33% 11% 2	800
	11th to 20th	Night	23% 30% 29% 16% 3	800
	21st to end	Night	18% 27% 36% 14% 4 1	880
February	1st to 10th	Night	20% 31% 32% 14% 2	800
	11th to 20th	Night	23% 29% 32% 13% 2	783
	21st to end	Night	24% 26% 32% 16% 2	660
March	1st to 10th	Night	28% 22% 32% 16% 2	800
	11th to 20th	Night	26% 26% 26% 16% 4 1	800
	21st to end	Night	28% 22% 25% 19% 6% 1	880
April	1st to 10th	Night	34% 22% 25% 16% 4	800
	11th to 20th	Night	28% 20% 25% 21% 5% 1	675
	21st to end	Night	31% 21% 25% 18% 4 1	570
May	1st to 10th	Night	28% 14% 30% 19% 7% 1	570
	11th to 20th	Night	23% 13% 30% 27% 6% 1	410
	21st to end	Night	25% 11% 30% 26% 8%	411
June	1st to 10th	Night	30% 7% 29% 24% 8% 1	380
	11th to 20th	Night	23% 9% 22% 31% 15%	380
	21st to end	Night	21% 9% 28% 32% 10% 1	380
July	1st to 10th	Night	18% 7% 30% 33% 11% 1	370
	11th to 20th	Night	17% 10% 26% 32% 14% 1	368
	21st to end	Night	19% 7% 21% 39% 14%	416
August	1st to 10th	Night	20% 5% 19% 36% 19%	425
	11th to 20th	Night	17% 9% 22% 34% 17% 1	562
	21st to end	Night	21% 9% 23% 29% 16% 2	760
September	1st to 10th	Night	21% 9% 24% 32% 13% 1	760
	11th to 20th	Night	22% 9% 19% 27% 21% 2	760
	21st to end	Night	24% 12% 23% 24% 16% 2	760
October	1st to 10th	Night	25% 13% 23% 26% 12% 1	760
	11th to 20th	Night	30% 17% 22% 21% 9% 2	760
	21st to end	Night	36% 19% 25% 16% 4	836
November	1st to 10th	Night	33% 20% 24% 18% 5% 1	780
	11th to 20th	Night	25% 23% 29% 14% 7% 2	800
	21st to end	Night	30% 23% 28% 15% 4 1	820
December	1st to 10th	Night	29% 26% 27% 14% 3	900
	11th to 20th	Night	30% 27% 23% 15% 4	900
	21st to end	Night	33% 25% 26% 14% 2	885

Page generated at 2025-04-08 09:17:28. Version ac07

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