Bellvale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bellvale Observatory clear sky chart from 2005-11-06 to 2024-11-12.

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	16% 7% 3 7% 5% 4 4 4 6% 7% 38%	101407
	Night	17% 8% 3 6% 4 4 4 3 5% 7% 40%	97749
Transparency	Day	44% 5% 11% 23% 15% 2	104939
	Night	43% 5% 12% 24% 15% 2	102306
Seeing	Day	22% 16% 30% 20% 9% 3	14958
	Night	20% 14% 31% 25% 8% 1	24091

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	14% 8% 2 6% 4 4 4 3 4 6% 44%	2517
	11th to 20th	Day	14% 10% 3 9% 6% 5% 5% 4 5% 4 35%	2520
	21st to end	Day	22% 12% 5% 8% 4 4 3 3 4 5% 31%	3000
February	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 3 5% 6% 36%	1710
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 4 7% 40%	1831
	21st to end	Day	17% 10% 3 7% 4 4 5% 3 4 6% 38%	1570
March	1st to 10th	Day	26% 9% 3 7% 4 3 3 2 4 5% 33%	3116
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	3352
	21st to end	Day	20% 8% 2 6% 3 3 3 3 4 6% 40%	4124
April	1st to 10th	Day	17% 9% 3 6% 5% 4 4 3 5% 7% 37%	2280
	11th to 20th	Day	17% 7% 2 7% 5% 4 4 3 5% 7% 40%	2254
	21st to end	Day	18% 6% 3 7% 6% 4 4 3 5% 7% 37%	2304
May	1st to 10th	Day	26% 9% 3 6% 3 4 4 3 6% 7% 30%	4135
	11th to 20th	Day	21% 9% 3 7% 5% 5% 5% 4 5% 7% 30%	4398
	21st to end	Day	12% 5% 2 6% 4 4 4 5% 7% 10% 42%	4837
June	1st to 10th	Day	16% 6% 2 5% 5% 5% 5% 4 6% 9% 36%	2660
	11th to 20th	Day	14% 6% 2 7% 4 5% 6% 5% 8% 8% 35%	2660
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 5% 9% 38%	2660
July	1st to 10th	Day	10% 6% 3 8% 6% 7% 6% 5% 7% 10% 33%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 4 7% 10% 37%	4440
	21st to end	Day	14% 8% 3 8% 6% 5% 5% 5% 6% 7% 33%	4814
August	1st to 10th	Day	20% 7% 2 8% 5% 5% 4 4 6% 7% 31%	4216
	11th to 20th	Day	12% 7% 3 6% 5% 4 4 5% 6% 8% 40%	3730
	21st to end	Day	12% 5% 3 5% 3 3 4 4 6% 8% 46%	4180
September	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 3 6% 5% 43%	2280
	11th to 20th	Day	14% 4 2 5% 4 4 4 4 6% 7% 48%	2209
	21st to end	Day	17% 6% 3 7% 4 5% 4 4 5% 6% 40%	2090
October	1st to 10th	Day	20% 6% 3 6% 4 4 4 3 4 7% 40%	1965
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 3 6% 6% 46%	1900
	21st to end	Day	20% 6% 2 5% 5% 3 4 4 5% 7% 38%	1941
November	1st to 10th	Day	14% 7% 2 5% 4 3 3 2 4 7% 51%	1761
	11th to 20th	Day	17% 7% 3 4 4 4 4 4 5% 7% 43%	1719
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 5% 7% 40%	1596
December	1st to 10th	Day	18% 9% 4 7% 5% 4 4 3 6% 6% 36%	1520
	11th to 20th	Day	21% 7% 3 5% 3 4 3 3 5% 6% 41%	1906
	21st to end	Day	17% 9% 4 9% 6% 5% 5% 4 5% 6% 29%	2772
January	1st to 10th	Night	16% 7% 2 5% 4 4 4 3 5% 7% 43%	4429
	11th to 20th	Night	14% 8% 3 9% 5% 4 4 3 5% 6% 39%	4440
	21st to end	Night	21% 12% 3 6% 4 3 3 3 5% 6% 33%	4651
February	1st to 10th	Night	16% 10% 2 5% 4 4 4 3 5% 6% 40%	2470
	11th to 20th	Night	15% 8% 3 7% 4 4 5% 3 5% 6% 39%	2233
	21st to end	Night	18% 8% 3 7% 4 3 4 3 4 5% 41%	1884
March	1st to 10th	Night	23% 11% 3 6% 4 3 3 3 4 5% 33%	3724
	11th to 20th	Night	11% 7% 3 7% 5% 4 5% 4 6% 8% 42%	3590
	21st to end	Night	17% 9% 4 6% 4 3 4 3 4 6% 39%	3508
April	1st to 10th	Night	17% 8% 2 7% 5% 4 5% 4 4 8% 34%	1900
	11th to 20th	Night	18% 7% 3 6% 4 4 4 2 5% 7% 39%	1875
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 6% 40%	1876
May	1st to 10th	Night	26% 7% 3 6% 4 4 3 3 6% 7% 30%	2820
	11th to 20th	Night	23% 8% 3 8% 4 4 4 3 4 6% 33%	2495
	21st to end	Night	12% 7% 2 7% 4 4 4 3 7% 8% 42%	2738
June	1st to 10th	Night	16% 5% 2 5% 5% 4 4 4 6% 8% 41%	1520
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 5% 10% 40%	1520
	21st to end	Night	11% 5% 2 6% 4 4 6% 3 6% 9% 44%	1520
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	2520
	11th to 20th	Night	11% 5% 2 7% 5% 5% 6% 4 6% 9% 39%	2520
	21st to end	Night	15% 8% 2 6% 4 4 5% 4 6% 7% 37%	2739
August	1st to 10th	Night	20% 6% 2 8% 5% 5% 4 4 6% 9% 32%	2665
	11th to 20th	Night	14% 8% 3 7% 5% 5% 5% 4 4 6% 40%	3095
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	3476
September	1st to 10th	Night	14% 6% 3 6% 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Night	13% 6% 2 5% 3 2 3 3 4 7% 53%	1971
	21st to end	Night	16% 6% 2 6% 3 3 4 3 4 7% 45%	2090
October	1st to 10th	Night	19% 6% 2 7% 4 3 3 3 4 6% 41%	2213
	11th to 20th	Night	14% 6% 2 6% 4 3 3 4 4 6% 48%	2280
	21st to end	Night	21% 7% 2 4 4 3 3 3 5% 7% 42%	2655
November	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 6% 6% 48%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 3 2 5% 6% 44%	2484
	21st to end	Night	15% 6% 2 5% 4 4 5% 3 6% 7% 43%	2579
December	1st to 10th	Night	19% 8% 2 7% 4 4 4 3 6% 7% 37%	2660
	11th to 20th	Night	18% 8% 3 5% 4 4 5% 4 5% 6% 38%	3288
	21st to end	Night	20% 10% 3 7% 5% 4 4 3 5% 6% 34%	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	39% 6% 10% 33% 9% 3	2518
	11th to 20th	Day	51% 5% 10% 22% 11% 2	2520
	21st to end	Day	56% 4 9% 21% 9% 1	3000
February	1st to 10th	Day	48% 5% 11% 21% 13% 1	1710
	11th to 20th	Day	44% 5% 8% 24% 17% 3	1840
	21st to end	Day	48% 4 11% 21% 13% 3	1570
March	1st to 10th	Day	53% 4 10% 21% 10% 2	3119
	11th to 20th	Day	38% 7% 12% 29% 12% 1	3352
	21st to end	Day	45% 4 6% 17% 23% 5%	4124
April	1st to 10th	Day	46% 5% 9% 21% 15% 4	2280
	11th to 20th	Day	44% 5% 10% 21% 17% 3	2254
	21st to end	Day	46% 5% 9% 22% 14% 4	2304
May	1st to 10th	Day	53% 5% 13% 16% 12% 2	4149
	11th to 20th	Day	53% 5% 12% 19% 8% 2	4402
	21st to end	Day	35% 7% 13% 27% 17% 2	4837
June	1st to 10th	Day	44% 7% 12% 21% 15% 1	2660
	11th to 20th	Day	43% 7% 12% 26% 11% 2	2660
	21st to end	Day	42% 6% 12% 24% 15% 1	2660
July	1st to 10th	Day	44% 7% 12% 21% 15% 1	4370
	11th to 20th	Day	40% 7% 16% 23% 14% 1	4365
	21st to end	Day	49% 6% 10% 19% 15% 2	4661
August	1st to 10th	Day	51% 6% 12% 21% 9%	4202
	11th to 20th	Day	39% 6% 15% 29% 11% 1	3766
	21st to end	Day	35% 4 11% 32% 16% 2	4180
September	1st to 10th	Day	41% 5% 11% 24% 18% 2	2280
	11th to 20th	Day	35% 5% 9% 24% 25% 2	2209
	21st to end	Day	45% 5% 10% 21% 17% 3	2090
October	1st to 10th	Day	41% 5% 7% 19% 24% 5%	3264
	11th to 20th	Day	29% 4 10% 27% 25% 4	3140
	21st to end	Day	29% 4 16% 25% 22% 4	3179
November	1st to 10th	Day	36% 4 12% 26% 19% 3	1761
	11th to 20th	Day	40% 4 11% 23% 19% 3	1719
	21st to end	Day	41% 6% 13% 28% 11% 2	1596
December	1st to 10th	Day	49% 5% 14% 19% 13% 1	1520
	11th to 20th	Day	43% 5% 16% 23% 11% 2	1906
	21st to end	Day	53% 8% 10% 21% 7%	2772
January	1st to 10th	Night	40% 5% 11% 30% 11% 3	4429
	11th to 20th	Night	46% 5% 11% 24% 12% 2	4440
	21st to end	Night	52% 4 9% 22% 11% 1	4651
February	1st to 10th	Night	44% 4 14% 24% 12% 2	2470
	11th to 20th	Night	45% 5% 9% 19% 18% 4	2235
	21st to end	Night	46% 4 7% 25% 15% 4	1884
March	1st to 10th	Night	52% 4 10% 20% 11% 2	3734
	11th to 20th	Night	37% 8% 14% 26% 13% 2	3591
	21st to end	Night	46% 6% 7% 17% 20% 6%	3508
April	1st to 10th	Night	47% 5% 9% 20% 15% 3	1900
	11th to 20th	Night	45% 5% 9% 21% 19% 1	1875
	21st to end	Night	44% 6% 8% 21% 17% 4	1876
May	1st to 10th	Night	53% 5% 12% 19% 11% 1	2826
	11th to 20th	Night	53% 5% 14% 16% 11% 1	2496
	21st to end	Night	39% 7% 16% 25% 12% 2	2739
June	1st to 10th	Night	40% 6% 11% 24% 18% 1	1520
	11th to 20th	Night	38% 7% 16% 28% 10% 1	1520
	21st to end	Night	34% 8% 15% 27% 15% 1	1520
July	1st to 10th	Night	39% 7% 17% 20% 15% 2	2480
	11th to 20th	Night	41% 7% 13% 26% 12% 1	2472
	21st to end	Night	44% 6% 14% 16% 18% 2	2651
August	1st to 10th	Night	50% 7% 16% 17% 9% 1	2655
	11th to 20th	Night	44% 6% 13% 28% 9%	3125
	21st to end	Night	32% 4 12% 34% 17% 1	3476
September	1st to 10th	Night	39% 5% 11% 26% 19% 1	1900
	11th to 20th	Night	32% 4 11% 26% 25% 2	1971
	21st to end	Night	41% 4 9% 22% 23% 1	2090
October	1st to 10th	Night	41% 4 11% 22% 20% 1	3694
	11th to 20th	Night	27% 4 10% 28% 27% 4	3779
	21st to end	Night	32% 3 14% 30% 18% 2	4367
November	1st to 10th	Night	36% 5% 12% 23% 22% 2	2537
	11th to 20th	Night	42% 4 9% 24% 17% 4	2484
	21st to end	Night	39% 5% 14% 29% 12% 1	2579
December	1st to 10th	Night	46% 4 12% 24% 13% 1	2660
	11th to 20th	Night	45% 6% 12% 20% 15% 3	3288
	21st to end	Night	51% 6% 11% 22% 10%	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	32% 23% 27% 16% 2	180
	11th to 20th	Day	36% 21% 32% 11% 1 1	180
	21st to end	Day	24% 29% 32% 12% 2 3	263
February	1st to 10th	Day	31% 27% 32% 9% 1	270
	11th to 20th	Day	30% 28% 33% 9% 1	439
	21st to end	Day	30% 22% 35% 10% 3	382
March	1st to 10th	Day	29% 23% 29% 16% 3	460
	11th to 20th	Day	26% 26% 31% 13% 3 1	460
	21st to end	Day	31% 21% 30% 11% 4 2	506
April	1st to 10th	Day	31% 20% 30% 12% 6% 1	460
	11th to 20th	Day	26% 24% 30% 12% 7% 1	452
	21st to end	Day	28% 18% 32% 14% 7% 1	460
May	1st to 10th	Day	35% 18% 24% 16% 6% 1	463
	11th to 20th	Day	22% 15% 36% 18% 8% 1	487
	21st to end	Day	18% 12% 28% 28% 12% 2	529
June	1st to 10th	Day	17% 14% 33% 23% 11% 2	490
	11th to 20th	Day	17% 13% 31% 28% 11% 1	490
	21st to end	Day	15% 14% 31% 27% 11% 2	490
July	1st to 10th	Day	14% 9% 30% 28% 14% 5%	485
	11th to 20th	Day	12% 6% 30% 32% 16% 3	484
	21st to end	Day	13% 9% 33% 32% 12% 2	532
August	1st to 10th	Day	14% 7% 29% 30% 14% 7%	486
	11th to 20th	Day	14% 9% 25% 32% 14% 6%	479
	21st to end	Day	14% 11% 26% 27% 16% 6%	539
September	1st to 10th	Day	19% 13% 23% 22% 16% 7%	490
	11th to 20th	Day	15% 9% 30% 22% 13% 11%	490
	21st to end	Day	22% 12% 28% 23% 10% 5%	490
October	1st to 10th	Day	26% 11% 26% 20% 13% 5%	490
	11th to 20th	Day	18% 19% 30% 21% 11% 2	490
	21st to end	Day	25% 15% 32% 17% 10% 1	460
November	1st to 10th	Day	18% 19% 29% 22% 9% 3	395
	11th to 20th	Day	26% 22% 29% 17% 5% 1	373
	21st to end	Day	25% 23% 29% 17% 4 2	256
December	1st to 10th	Day	27% 29% 28% 9% 6% 1	180
	11th to 20th	Day	37% 22% 29% 11% 2	180
	21st to end	Day	24% 23% 37% 12% 4 1	198
January	1st to 10th	Night	19% 22% 39% 17% 3	760
	11th to 20th	Night	25% 19% 33% 19% 4	760
	21st to end	Night	20% 21% 38% 19% 2	836
February	1st to 10th	Night	23% 18% 38% 20% 1	760
	11th to 20th	Night	24% 24% 37% 14% 1	743
	21st to end	Night	28% 20% 33% 16% 3	628
March	1st to 10th	Night	24% 22% 33% 18% 4	760
	11th to 20th	Night	24% 22% 33% 17% 3 1	760
	21st to end	Night	23% 22% 33% 17% 5%	836
April	1st to 10th	Night	22% 22% 35% 18% 4	760
	11th to 20th	Night	22% 18% 33% 22% 5%	751
	21st to end	Night	23% 16% 33% 22% 5%	736
May	1st to 10th	Night	26% 16% 31% 23% 4	569
	11th to 20th	Night	21% 13% 34% 25% 7% 1	374
	21st to end	Night	15% 9% 26% 38% 11% 1	411
June	1st to 10th	Night	18% 6% 29% 36% 11%	380
	11th to 20th	Night	13% 7% 32% 34% 13% 1	380
	21st to end	Night	12% 5% 31% 41% 10%	380
July	1st to 10th	Night	14% 6% 21% 40% 19% 1	370
	11th to 20th	Night	14% 2 25% 43% 15% 1	368
	21st to end	Night	11% 5% 27% 38% 17% 1	416
August	1st to 10th	Night	12% 2 25% 43% 17% 2	472
	11th to 20th	Night	14% 5% 22% 36% 22% 1	748
	21st to end	Night	15% 5% 19% 39% 21% 2	836
September	1st to 10th	Night	16% 6% 24% 36% 16% 2	760
	11th to 20th	Night	16% 5% 24% 32% 19% 4	760
	21st to end	Night	19% 6% 26% 31% 16% 2	760
October	1st to 10th	Night	23% 9% 27% 25% 14% 2	760
	11th to 20th	Night	17% 13% 33% 28% 9%	760
	21st to end	Night	24% 13% 34% 22% 7%	836
November	1st to 10th	Night	16% 14% 36% 24% 9% 1	780
	11th to 20th	Night	23% 17% 31% 21% 7% 1	765
	21st to end	Night	19% 19% 34% 20% 7%	760
December	1st to 10th	Night	27% 18% 33% 18% 5%	760
	11th to 20th	Night	23% 18% 35% 20% 3	760
	21st to end	Night	26% 18% 37% 17% 2	836

Page generated at 2024-11-08 22:45:36. Version ac07

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