Vincent Court Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vincent Court Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	15% 7% 2 6% 4 4 4 4 5% 7% 41%	108918
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	110417
Transparency	Day	42% 4 10% 24% 17% 3	112493
	Night	41% 3 10% 24% 18% 3	115006
Seeing	Day	21% 14% 29% 22% 10% 3	15448
	Night	19% 12% 31% 28% 9% 1	24806

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	13% 7% 2 5% 3 3 2 3 4 5% 52%	3597
	11th to 20th	Day	14% 8% 3 8% 5% 4 4 3 4 5% 43%	3587
	21st to end	Day	19% 8% 3 7% 4 4 4 2 4 4 42%	4466
February	1st to 10th	Day	15% 7% 3 7% 5% 5% 3 3 5% 7% 40%	1800
	11th to 20th	Day	16% 9% 2 6% 4 4 4 4 4 5% 42%	1937
	21st to end	Day	18% 8% 3 7% 4 4 3 3 5% 5% 39%	1650
March	1st to 10th	Day	25% 7% 2 6% 4 4 3 2 4 5% 38%	3216
	11th to 20th	Day	10% 4 2 6% 5% 4 5% 4 6% 7% 46%	3519
	21st to end	Day	22% 5% 2 5% 3 3 4 3 3 5% 43%	4160
April	1st to 10th	Day	16% 8% 3 6% 4 4 5% 3 5% 7% 40%	2280
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 6% 7% 42%	2254
	21st to end	Day	16% 5% 2 6% 5% 5% 4 4 6% 6% 40%	2280
May	1st to 10th	Day	26% 8% 3 7% 4 4 4 3 5% 7% 29%	4057
	11th to 20th	Day	23% 8% 3 7% 5% 5% 5% 4 5% 6% 30%	4393
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 9% 43%	4837
June	1st to 10th	Day	13% 5% 2 5% 5% 5% 5% 4 8% 10% 38%	2660
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 4 7% 8% 39%	2660
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 7% 9% 41%	2660
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 6% 8% 10% 35%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 10% 36%	4440
	21st to end	Day	12% 8% 3 8% 6% 6% 6% 5% 6% 8% 32%	4814
August	1st to 10th	Day	18% 6% 3 8% 5% 5% 6% 5% 6% 7% 31%	4057
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 40%	3730
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 9% 45%	4180
September	1st to 10th	Day	13% 7% 2 7% 4 4 4 4 5% 7% 44%	2280
	11th to 20th	Day	11% 6% 2 6% 4 3 4 3 5% 7% 50%	2223
	21st to end	Day	18% 7% 2 7% 4 4 4 3 5% 7% 39%	2090
October	1st to 10th	Day	17% 7% 2 6% 3 3 4 3 5% 6% 44%	1945
	11th to 20th	Day	14% 6% 1 4 4 4 3 3 5% 6% 50%	1900
	21st to end	Day	17% 6% 3 6% 2 3 3 3 4 5% 47%	1989
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 2 5% 7% 51%	1761
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 5% 6% 48%	1800
	21st to end	Day	15% 6% 2 6% 4 3 3 3 5% 6% 47%	1710
December	1st to 10th	Day	14% 7% 2 6% 3 3 3 3 6% 7% 45%	2600
	11th to 20th	Day	21% 9% 2 4 3 3 3 3 3 5% 44%	2986
	21st to end	Day	16% 6% 2 5% 3 4 4 4 6% 7% 41%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	6349
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 5% 49%	6297
	21st to end	Night	17% 9% 2 6% 3 3 4 4 5% 6% 41%	6490
February	1st to 10th	Night	16% 9% 3 6% 4 4 3 3 5% 7% 41%	2600
	11th to 20th	Night	18% 9% 3 5% 4 3 4 3 4 6% 42%	2348
	21st to end	Night	18% 8% 2 7% 3 3 3 3 5% 6% 42%	1980
March	1st to 10th	Night	22% 10% 2 6% 3 3 3 2 4 5% 39%	3844
	11th to 20th	Night	11% 6% 3 7% 4 4 3 3 4 5% 49%	3655
	21st to end	Night	19% 8% 3 6% 4 3 3 2 3 5% 44%	3610
April	1st to 10th	Night	16% 7% 2 7% 4 3 4 4 5% 6% 41%	1900
	11th to 20th	Night	16% 6% 2 7% 5% 4 4 3 5% 6% 42%	1875
	21st to end	Night	18% 6% 2 5% 3 3 5% 3 5% 6% 44%	1900
May	1st to 10th	Night	22% 10% 3 8% 4 3 4 3 5% 7% 30%	2880
	11th to 20th	Night	24% 9% 3 8% 4 3 4 3 5% 6% 31%	2527
	21st to end	Night	13% 7% 2 5% 4 4 4 3 6% 9% 41%	2738
June	1st to 10th	Night	14% 7% 2 5% 5% 3 4 4 7% 8% 42%	1520
	11th to 20th	Night	11% 6% 3 7% 4 4 5% 4 6% 7% 43%	1520
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 5% 7% 11% 41%	1520
July	1st to 10th	Night	7% 6% 3 7% 6% 6% 6% 5% 9% 10% 36%	2520
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 5% 7% 10% 38%	2520
	21st to end	Night	15% 7% 3 7% 5% 4 4 4 7% 8% 35%	2739
August	1st to 10th	Night	22% 7% 2 6% 4 5% 4 4 6% 8% 32%	2837
	11th to 20th	Night	12% 7% 2 7% 4 5% 5% 4 6% 7% 41%	3097
	21st to end	Night	9% 5% 2 5% 4 4 3 3 6% 8% 50%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 5% 4 3 5% 6% 45%	1900
	11th to 20th	Night	11% 6% 2 5% 2 4 4 3 5% 6% 53%	1956
	21st to end	Night	18% 7% 2 7% 3 3 4 3 5% 7% 41%	2090
October	1st to 10th	Night	18% 7% 2 5% 4 4 4 3 4 6% 44%	2232
	11th to 20th	Night	14% 4 2 5% 3 4 4 3 4 6% 51%	2280
	21st to end	Night	18% 7% 2 5% 3 3 3 3 5% 5% 47%	2608
November	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 6% 53%	2537
	11th to 20th	Night	17% 6% 2 6% 4 3 4 2 4 5% 48%	2600
	21st to end	Night	13% 6% 2 6% 4 4 4 4 5% 6% 45%	2688
December	1st to 10th	Night	14% 7% 2 6% 3 3 3 3 5% 6% 48%	4580
	11th to 20th	Night	23% 7% 2 5% 4 3 3 3 4 6% 41%	5208
	21st to end	Night	19% 8% 3 6% 4 4 3 3 5% 5% 42%	6996

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	36% 2 8% 27% 23% 3	3598
	11th to 20th	Day	45% 2 7% 24% 17% 5%	3617
	21st to end	Day	48% 1 10% 24% 15% 2	4466
February	1st to 10th	Day	46% 3 12% 26% 12% 2	1800
	11th to 20th	Day	46% 3 8% 24% 16% 3	1946
	21st to end	Day	46% 3 8% 23% 15% 4	1650
March	1st to 10th	Day	50% 3 7% 27% 10% 3	3219
	11th to 20th	Day	36% 3 9% 31% 18% 3	3519
	21st to end	Day	45% 2 7% 18% 23% 5%	4160
April	1st to 10th	Day	45% 4 8% 22% 16% 5%	2280
	11th to 20th	Day	40% 3 12% 23% 17% 3	2254
	21st to end	Day	43% 4 9% 21% 19% 4	2280
May	1st to 10th	Day	56% 2 11% 18% 10% 2	4071
	11th to 20th	Day	56% 2 12% 20% 9% 1	4397
	21st to end	Day	38% 4 11% 28% 17% 2	4837
June	1st to 10th	Day	41% 5% 13% 24% 16% 2	2660
	11th to 20th	Day	41% 4 13% 26% 13% 3	2660
	21st to end	Day	39% 5% 12% 25% 17% 1	2660
July	1st to 10th	Day	41% 9% 14% 21% 13% 2	4370
	11th to 20th	Day	41% 6% 15% 22% 14% 1	4365
	21st to end	Day	48% 6% 9% 20% 16% 1	4661
August	1st to 10th	Day	51% 5% 12% 21% 10% 1	4043
	11th to 20th	Day	41% 6% 11% 27% 14% 1	3766
	21st to end	Day	36% 5% 10% 27% 20% 2	4180
September	1st to 10th	Day	41% 3 11% 20% 22% 3	2280
	11th to 20th	Day	35% 4 8% 24% 26% 3	2223
	21st to end	Day	47% 2 8% 20% 21% 2	2090
October	1st to 10th	Day	37% 3 10% 19% 27% 5%	3231
	11th to 20th	Day	25% 2 8% 25% 32% 7%	3140
	21st to end	Day	26% 2 12% 27% 28% 5%	3253
November	1st to 10th	Day	35% 2 11% 27% 22% 3	1761
	11th to 20th	Day	39% 3 8% 25% 23% 3	1800
	21st to end	Day	39% 2 13% 31% 13% 3	1710
December	1st to 10th	Day	39% 2 15% 27% 14% 2	2600
	11th to 20th	Day	45% 2 9% 29% 12% 2	2986
	21st to end	Day	42% 3 11% 24% 18% 2	3960
January	1st to 10th	Night	36% 2 7% 26% 22% 6%	6349
	11th to 20th	Night	40% 2 9% 24% 17% 7%	6342
	21st to end	Night	46% 2 8% 23% 19% 3	6490
February	1st to 10th	Night	45% 2 13% 25% 13% 2	2600
	11th to 20th	Night	46% 2 8% 21% 19% 5%	2350
	21st to end	Night	45% 2 6% 24% 18% 5%	1980
March	1st to 10th	Night	50% 1 7% 25% 13% 4	3854
	11th to 20th	Night	38% 3 12% 27% 17% 3	3656
	21st to end	Night	46% 2 6% 18% 23% 6%	3610
April	1st to 10th	Night	44% 2 9% 21% 18% 5%	1900
	11th to 20th	Night	44% 3 9% 22% 20% 3	1875
	21st to end	Night	41% 3 9% 21% 21% 5%	1900
May	1st to 10th	Night	54% 3 10% 21% 11% 1	2886
	11th to 20th	Night	54% 3 14% 16% 10% 2	2528
	21st to end	Night	39% 5% 15% 22% 16% 3	2739
June	1st to 10th	Night	40% 5% 11% 23% 19% 2	1520
	11th to 20th	Night	40% 5% 13% 28% 13% 2	1520
	21st to end	Night	37% 5% 15% 26% 16% 2	1520
July	1st to 10th	Night	42% 8% 18% 19% 12% 2	2480
	11th to 20th	Night	39% 6% 17% 26% 11% 1	2472
	21st to end	Night	45% 5% 14% 19% 15% 2	2651
August	1st to 10th	Night	50% 6% 18% 17% 9% 1	2827
	11th to 20th	Night	41% 4 12% 30% 12% 1	3127
	21st to end	Night	33% 4 11% 29% 22% 2	3476
September	1st to 10th	Night	40% 4 11% 22% 21% 2	1900
	11th to 20th	Night	32% 3 10% 25% 26% 4	1956
	21st to end	Night	44% 4 7% 23% 21% 2	2090
October	1st to 10th	Night	38% 4 9% 24% 22% 3	3726
	11th to 20th	Night	25% 2 9% 28% 30% 6%	3779
	21st to end	Night	27% 2 10% 31% 25% 4	4294
November	1st to 10th	Night	34% 3 11% 25% 23% 5%	2537
	11th to 20th	Night	41% 3 7% 23% 21% 5%	2600
	21st to end	Night	39% 2 12% 27% 17% 2	2688
December	1st to 10th	Night	39% 2 11% 31% 14% 3	4580
	11th to 20th	Night	46% 2 10% 24% 14% 4	5208
	21st to end	Night	46% 2 10% 21% 18% 3	6996

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% 23% 32% 14% 4 1	200
	11th to 20th	Day	35% 26% 28% 11% 1	203
	21st to end	Day	26% 22% 33% 14% 4 1	330
February	1st to 10th	Day	27% 28% 31% 12% 2	300
	11th to 20th	Day	32% 23% 33% 11% 2	485
	21st to end	Day	29% 20% 33% 15% 2	414
March	1st to 10th	Day	28% 19% 31% 17% 4	500
	11th to 20th	Day	27% 21% 32% 14% 5% 1	500
	21st to end	Day	29% 20% 32% 15% 4 1	530
April	1st to 10th	Day	26% 23% 32% 12% 6% 1	460
	11th to 20th	Day	24% 17% 34% 15% 9% 2	452
	21st to end	Day	27% 19% 31% 17% 4 2	460
May	1st to 10th	Day	30% 17% 28% 16% 7% 2	463
	11th to 20th	Day	22% 14% 33% 22% 8% 1	487
	21st to end	Day	19% 9% 23% 29% 16% 3	529
June	1st to 10th	Day	16% 11% 30% 26% 13% 3	490
	11th to 20th	Day	17% 11% 28% 27% 12% 4	490
	21st to end	Day	14% 8% 32% 30% 14% 3	490
July	1st to 10th	Day	10% 6% 24% 32% 21% 6%	485
	11th to 20th	Day	9% 5% 24% 33% 23% 7%	484
	21st to end	Day	12% 8% 30% 31% 15% 4	532
August	1st to 10th	Day	12% 5% 25% 35% 15% 8%	486
	11th to 20th	Day	13% 5% 22% 32% 21% 7%	479
	21st to end	Day	13% 6% 25% 28% 18% 10%	539
September	1st to 10th	Day	17% 8% 24% 24% 18% 9%	490
	11th to 20th	Day	12% 7% 28% 29% 15% 9%	490
	21st to end	Day	24% 12% 25% 24% 10% 5%	490
October	1st to 10th	Day	26% 7% 23% 24% 13% 7%	490
	11th to 20th	Day	18% 14% 27% 28% 11% 3	490
	21st to end	Day	20% 15% 31% 20% 11% 2	485
November	1st to 10th	Day	18% 11% 34% 23% 11% 3	395
	11th to 20th	Day	23% 17% 30% 23% 6% 1	400
	21st to end	Day	21% 16% 36% 17% 8% 1	310
December	1st to 10th	Day	24% 26% 31% 13% 7%	200
	11th to 20th	Day	38% 17% 28% 13% 5%	200
	21st to end	Day	29% 14% 33% 17% 5% 2	220
January	1st to 10th	Night	20% 20% 37% 21% 3	800
	11th to 20th	Night	24% 16% 34% 22% 4	800
	21st to end	Night	19% 19% 39% 22% 1	880
February	1st to 10th	Night	22% 15% 37% 25% 2	800
	11th to 20th	Night	25% 22% 36% 16% 1	783
	21st to end	Night	23% 19% 37% 19% 2	660
March	1st to 10th	Night	23% 17% 36% 20% 4	800
	11th to 20th	Night	23% 18% 35% 19% 5%	800
	21st to end	Night	21% 13% 41% 20% 5%	863
April	1st to 10th	Night	21% 18% 38% 18% 4	760
	11th to 20th	Night	19% 14% 37% 25% 4	751
	21st to end	Night	21% 11% 36% 26% 5%	760
May	1st to 10th	Night	21% 15% 33% 25% 5%	603
	11th to 20th	Night	20% 9% 39% 25% 6%	393
	21st to end	Night	16% 7% 24% 39% 13% 1	411
June	1st to 10th	Night	17% 5% 30% 36% 12%	380
	11th to 20th	Night	11% 6% 27% 44% 13%	380
	21st to end	Night	11% 2 28% 46% 13%	380
July	1st to 10th	Night	10% 6% 19% 45% 18% 1	370
	11th to 20th	Night	7% 4 21% 48% 19% 1	368
	21st to end	Night	10% 3 24% 46% 18%	416
August	1st to 10th	Night	11% 3 18% 45% 20% 3	566
	11th to 20th	Night	13% 4 17% 39% 24% 3	750
	21st to end	Night	14% 4 16% 40% 24% 2	836
September	1st to 10th	Night	16% 4 24% 36% 18% 2	760
	11th to 20th	Night	15% 4 20% 34% 23% 4	760
	21st to end	Night	22% 5% 21% 34% 17% 1	760
October	1st to 10th	Night	20% 7% 26% 27% 16% 3	760
	11th to 20th	Night	16% 10% 29% 34% 12%	760
	21st to end	Night	21% 9% 32% 28% 9%	836
November	1st to 10th	Night	13% 8% 37% 29% 11% 1	780
	11th to 20th	Night	20% 15% 30% 27% 8%	800
	21st to end	Night	18% 14% 36% 26% 7%	800
December	1st to 10th	Night	26% 15% 32% 19% 7% 1	800
	11th to 20th	Night	21% 14% 35% 25% 5%	800
	21st to end	Night	23% 14% 33% 28% 3	880

Page generated at 2025-03-24 09:04:25. Version ac07

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