East Coast Video Astronomy Rendezvous
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the East Coast Video Astronomy Rendezvous 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	12% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	127107
	Night	11% 7% 2 7% 4 4 4 4 6% 7% 43%	124562
Transparency	Day	41% 5% 11% 23% 16% 4	130710
	Night	39% 5% 12% 23% 17% 4	129028
Seeing	Day	21% 14% 26% 23% 13% 4	16452
	Night	17% 11% 25% 31% 14% 1	26212

By Month

Percentage Cloud Cover SE (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% 6% 3 6% 5% 4 5% 3 5% 6% 45%	4081
	11th to 20th	Day	11% 10% 3 8% 4 4 4 3 4 4 46%	4114
	21st to end	Day	11% 9% 3 6% 5% 5% 4 4 5% 6% 43%	4466
February	1st to 10th	Day	12% 10% 4 8% 4 5% 4 5% 6% 7% 37%	1800
	11th to 20th	Day	16% 8% 3 7% 4 4 5% 3 5% 6% 38%	1753
	21st to end	Day	14% 10% 3 7% 4 3 4 4 5% 5% 40%	1693
March	1st to 10th	Day	15% 9% 3 7% 4 4 4 4 4 5% 41%	4935
	11th to 20th	Day	8% 6% 2 5% 4 3 4 3 5% 7% 51%	4980
	21st to end	Day	14% 8% 3 8% 5% 4 5% 4 5% 6% 38%	5465
April	1st to 10th	Day	11% 8% 2 8% 4 5% 5% 4 6% 7% 41%	2371
	11th to 20th	Day	12% 7% 2 6% 5% 3 4 4 4 6% 46%	2374
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 6% 6% 39%	2446
May	1st to 10th	Day	15% 7% 3 7% 5% 6% 6% 5% 7% 9% 30%	5862
	11th to 20th	Day	19% 9% 2 7% 5% 4 5% 5% 7% 8% 29%	5861
	21st to end	Day	15% 6% 2 7% 4 5% 5% 5% 7% 9% 36%	6907
June	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 8% 40%	2800
	11th to 20th	Day	10% 6% 2 7% 6% 5% 6% 5% 7% 10% 37%	2800
	21st to end	Day	8% 6% 2 5% 5% 5% 6% 5% 7% 11% 41%	2800
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	6231
	11th to 20th	Day	8% 5% 2 8% 6% 6% 7% 6% 9% 12% 29%	5900
	21st to end	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 10% 36%	6424
August	1st to 10th	Day	13% 5% 3 7% 6% 7% 6% 5% 7% 9% 31%	4234
	11th to 20th	Day	8% 5% 2 7% 6% 6% 7% 6% 8% 9% 37%	4109
	21st to end	Day	9% 5% 2 6% 5% 4 5% 5% 6% 8% 46%	4312
September	1st to 10th	Day	12% 7% 2 6% 5% 5% 4 3 5% 6% 45%	2400
	11th to 20th	Day	11% 6% 2 7% 4 4 4 4 6% 7% 45%	2330
	21st to end	Day	20% 8% 3 7% 4 4 5% 3 5% 7% 34%	2125
October	1st to 10th	Day	15% 6% 2 4 3 3 3 3 5% 8% 46%	2000
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 4 5% 48%	2000
	21st to end	Day	13% 6% 3 5% 4 4 4 3 5% 6% 48%	2200
November	1st to 10th	Day	10% 5% 2 5% 4 3 4 3 6% 6% 52%	2057
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 6% 47%	1799
	21st to end	Day	13% 6% 2 5% 4 4 4 4 4 6% 46%	1680
December	1st to 10th	Day	10% 6% 2 6% 4 4 5% 3 5% 6% 50%	2680
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 3 5% 6% 41%	3070
	21st to end	Day	13% 9% 3 7% 5% 4 5% 4 6% 8% 36%	4048
January	1st to 10th	Night	9% 8% 2 7% 5% 4 5% 4 6% 6% 45%	6066
	11th to 20th	Night	11% 8% 2 6% 4 3 4 3 5% 5% 49%	5979
	21st to end	Night	12% 9% 3 6% 4 3 3 3 5% 6% 47%	6490
February	1st to 10th	Night	12% 8% 3 7% 5% 4 5% 4 5% 7% 40%	2600
	11th to 20th	Night	17% 10% 2 6% 4 4 4 4 5% 6% 38%	2533
	21st to end	Night	13% 9% 3 7% 5% 5% 5% 4 5% 6% 40%	1934
March	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 4 6% 46%	4918
	11th to 20th	Night	8% 7% 2 5% 4 4 4 4 5% 8% 50%	4980
	21st to end	Night	10% 6% 2 8% 5% 5% 4 3 5% 7% 44%	5467
April	1st to 10th	Night	12% 7% 2 7% 5% 5% 5% 4 6% 6% 39%	2028
	11th to 20th	Night	11% 6% 2 7% 5% 3 4 4 7% 7% 44%	1975
	21st to end	Night	13% 6% 3 6% 4 4 5% 4 6% 7% 43%	1970
May	1st to 10th	Night	15% 9% 3 8% 5% 5% 5% 4 6% 9% 31%	4033
	11th to 20th	Night	18% 9% 3 8% 5% 4 4 3 6% 8% 32%	4032
	21st to end	Night	14% 7% 3 8% 5% 5% 5% 4 6% 9% 35%	3933
June	1st to 10th	Night	10% 5% 3 6% 5% 4 4 5% 6% 10% 44%	1600
	11th to 20th	Night	10% 6% 2 6% 6% 5% 5% 4 7% 9% 41%	1600
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 8% 10% 43%	1600
July	1st to 10th	Night	6% 5% 2 6% 6% 5% 5% 6% 8% 10% 41%	3723
	11th to 20th	Night	7% 6% 3 9% 7% 7% 8% 6% 8% 10% 30%	4057
	21st to end	Night	6% 6% 3 8% 6% 5% 5% 5% 8% 10% 39%	4427
August	1st to 10th	Night	11% 7% 2 8% 6% 5% 5% 4 8% 9% 34%	2915
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 5% 6% 9% 38%	2904
	21st to end	Night	8% 6% 2 6% 4 4 4 4 6% 9% 48%	3586
September	1st to 10th	Night	12% 6% 2 6% 3 3 5% 4 6% 7% 47%	2000
	11th to 20th	Night	10% 5% 3 6% 4 5% 4 3 5% 7% 49%	2067
	21st to end	Night	17% 8% 3 6% 5% 4 4 3 4 6% 40%	2275
October	1st to 10th	Night	16% 5% 2 5% 3 4 3 3 4 6% 49%	2400
	11th to 20th	Night	12% 6% 2 5% 2 3 4 3 6% 6% 51%	2400
	21st to end	Night	13% 6% 2 5% 3 4 4 3 5% 6% 50%	2640
November	1st to 10th	Night	9% 8% 2 6% 4 3 3 3 5% 6% 51%	2461
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 6% 47%	2814
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 6% 45%	2940
December	1st to 10th	Night	9% 6% 2 6% 4 3 4 3 5% 7% 51%	4720
	11th to 20th	Night	16% 6% 3 7% 4 4 4 4 6% 6% 39%	5345
	21st to end	Night	12% 7% 3 8% 5% 5% 5% 5% 7% 8% 36%	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	39% 3 6% 22% 22% 7%	4082
	11th to 20th	Day	41% 4 6% 19% 20% 10%	4114
	21st to end	Day	42% 3 7% 26% 17% 4	4466
February	1st to 10th	Day	45% 5% 11% 23% 13% 2	1800
	11th to 20th	Day	46% 5% 8% 21% 17% 3	1762
	21st to end	Day	45% 4 12% 18% 17% 4	1693
March	1st to 10th	Day	44% 5% 7% 19% 17% 8%	4935
	11th to 20th	Day	31% 4 9% 28% 21% 6%	4980
	21st to end	Day	46% 4 10% 16% 18% 5%	5465
April	1st to 10th	Day	41% 5% 11% 19% 17% 6%	2371
	11th to 20th	Day	38% 5% 10% 21% 20% 5%	2374
	21st to end	Day	45% 5% 10% 22% 14% 3	2446
May	1st to 10th	Day	48% 6% 14% 20% 9% 2	5897
	11th to 20th	Day	50% 6% 11% 24% 8% 1	5865
	21st to end	Day	42% 7% 13% 21% 15% 2	6922
June	1st to 10th	Day	40% 7% 12% 24% 16% 1	2800
	11th to 20th	Day	41% 8% 14% 22% 13% 2	2800
	21st to end	Day	36% 9% 14% 25% 15% 2	2800
July	1st to 10th	Day	36% 7% 18% 22% 13% 4	6163
	11th to 20th	Day	46% 7% 18% 22% 7% 1	5831
	21st to end	Day	42% 5% 10% 26% 16%	6240
August	1st to 10th	Day	47% 6% 17% 23% 7%	4221
	11th to 20th	Day	42% 6% 12% 28% 12% 1	4148
	21st to end	Day	35% 5% 15% 30% 13% 1	4312
September	1st to 10th	Day	40% 6% 13% 18% 20% 4	2400
	11th to 20th	Day	38% 5% 8% 26% 19% 3	2330
	21st to end	Day	50% 6% 9% 18% 15% 2	2125
October	1st to 10th	Day	33% 3 8% 22% 28% 5%	3260
	11th to 20th	Day	29% 3 7% 28% 21% 12%	3240
	21st to end	Day	25% 3 9% 28% 26% 9%	3538
November	1st to 10th	Day	33% 4 10% 25% 25% 4	2057
	11th to 20th	Day	38% 3 10% 21% 22% 5%	1799
	21st to end	Day	38% 4 10% 25% 20% 2	1680
December	1st to 10th	Day	35% 3 11% 25% 20% 5%	2680
	11th to 20th	Day	43% 3 14% 24% 13% 4	3066
	21st to end	Day	44% 5% 14% 21% 15% 1	4048
January	1st to 10th	Night	35% 6% 7% 22% 26% 4	6066
	11th to 20th	Night	35% 5% 7% 23% 22% 8%	5979
	21st to end	Night	39% 3 8% 27% 20% 3	6490
February	1st to 10th	Night	43% 5% 12% 24% 14% 2	2600
	11th to 20th	Night	46% 5% 8% 21% 15% 5%	2535
	21st to end	Night	43% 6% 10% 21% 15% 5%	1934
March	1st to 10th	Night	38% 5% 8% 25% 17% 8%	4918
	11th to 20th	Night	31% 5% 10% 27% 22% 5%	4980
	21st to end	Night	42% 3 11% 21% 19% 4	5467
April	1st to 10th	Night	42% 6% 10% 20% 17% 3	2028
	11th to 20th	Night	38% 5% 13% 21% 19% 5%	1975
	21st to end	Night	39% 5% 12% 27% 14% 3	1970
May	1st to 10th	Night	49% 7% 14% 18% 8% 3	4060
	11th to 20th	Night	50% 6% 12% 22% 9%	4033
	21st to end	Night	46% 6% 16% 23% 7% 2	3940
June	1st to 10th	Night	35% 8% 14% 28% 14% 2	1600
	11th to 20th	Night	38% 9% 16% 24% 13% 1	1600
	21st to end	Night	34% 8% 17% 22% 18% 2	1600
July	1st to 10th	Night	37% 9% 19% 20% 13% 2	3683
	11th to 20th	Night	48% 9% 19% 18% 6% 1	4006
	21st to end	Night	38% 6% 17% 24% 15%	4297
August	1st to 10th	Night	44% 6% 18% 24% 6%	2904
	11th to 20th	Night	43% 4 14% 25% 13%	2931
	21st to end	Night	34% 5% 16% 32% 12% 1	3586
September	1st to 10th	Night	36% 7% 13% 23% 18% 4	2000
	11th to 20th	Night	36% 4 10% 29% 19% 2	2067
	21st to end	Night	46% 5% 10% 20% 16% 2	2275
October	1st to 10th	Night	40% 3 10% 22% 21% 4	3920
	11th to 20th	Night	26% 4 10% 22% 30% 9%	3899
	21st to end	Night	23% 3 9% 32% 25% 8%	4252
November	1st to 10th	Night	34% 4 11% 23% 23% 5%	2461
	11th to 20th	Night	38% 4 7% 23% 22% 5%	2814
	21st to end	Night	38% 5% 10% 24% 20% 4	2940
December	1st to 10th	Night	33% 3 11% 30% 21% 2	4720
	11th to 20th	Night	43% 4 12% 19% 18% 4	5348
	21st to end	Night	42% 6% 14% 21% 15% 1	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% 26% 12% 4	316
	11th to 20th	Day	35% 24% 26% 13% 2	318
	21st to end	Day	27% 29% 29% 12% 2 1	330
February	1st to 10th	Day	30% 28% 30% 9% 2	300
	11th to 20th	Day	46% 23% 26% 4 1	300
	21st to end	Day	33% 20% 29% 16% 3	399
March	1st to 10th	Day	28% 22% 27% 20% 3	500
	11th to 20th	Day	27% 24% 28% 15% 6% 1	500
	21st to end	Day	26% 21% 30% 17% 5% 1	550
April	1st to 10th	Day	27% 24% 26% 16% 6% 2	500
	11th to 20th	Day	23% 19% 26% 20% 10% 3	492
	21st to end	Day	26% 15% 32% 16% 9% 1	500
May	1st to 10th	Day	23% 19% 29% 15% 12% 3	503
	11th to 20th	Day	24% 14% 27% 22% 10% 3	527
	21st to end	Day	19% 9% 26% 24% 16% 6%	573
June	1st to 10th	Day	14% 11% 27% 27% 18% 3	530
	11th to 20th	Day	16% 8% 27% 29% 15% 5%	530
	21st to end	Day	11% 9% 27% 31% 20% 3	530
July	1st to 10th	Day	11% 4 20% 33% 22% 10%	525
	11th to 20th	Day	9% 2 21% 33% 26% 10%	524
	21st to end	Day	12% 2 22% 32% 23% 9%	576
August	1st to 10th	Day	12% 7% 20% 27% 23% 11%	526
	11th to 20th	Day	13% 4 21% 31% 21% 10%	519
	21st to end	Day	10% 4 20% 31% 21% 14%	583
September	1st to 10th	Day	16% 6% 20% 24% 22% 12%	530
	11th to 20th	Day	14% 4 19% 30% 21% 11%	530
	21st to end	Day	28% 8% 23% 25% 13% 4	530
October	1st to 10th	Day	20% 5% 21% 28% 20% 6%	530
	11th to 20th	Day	20% 14% 25% 24% 15% 2	530
	21st to end	Day	20% 15% 32% 22% 8% 3	583
November	1st to 10th	Day	17% 16% 30% 24% 10% 2	535
	11th to 20th	Day	22% 15% 32% 20% 9% 1	331
	21st to end	Day	27% 19% 31% 17% 5%	220
December	1st to 10th	Day	22% 27% 33% 13% 5%	220
	11th to 20th	Day	25% 23% 29% 17% 6%	220
	21st to end	Day	28% 15% 35% 16% 6%	242
January	1st to 10th	Night	19% 22% 33% 23% 3	840
	11th to 20th	Night	24% 17% 29% 27% 4	825
	21st to end	Night	19% 16% 38% 25% 2	880
February	1st to 10th	Night	18% 20% 33% 27% 2	800
	11th to 20th	Night	29% 22% 32% 16% 2	783
	21st to end	Night	22% 19% 36% 20% 2	660
March	1st to 10th	Night	20% 18% 34% 25% 3	800
	11th to 20th	Night	23% 19% 30% 22% 5% 1	800
	21st to end	Night	19% 19% 34% 23% 4 1	880
April	1st to 10th	Night	18% 21% 31% 24% 6%	800
	11th to 20th	Night	16% 15% 32% 28% 9% 1	791
	21st to end	Night	17% 12% 30% 32% 8%	770
May	1st to 10th	Night	22% 8% 23% 31% 16% 2	600
	11th to 20th	Night	17% 6% 24% 37% 15% 2	591
	21st to end	Night	14% 3 20% 41% 20% 2	447
June	1st to 10th	Night	13% 4 20% 40% 23% 1	400
	11th to 20th	Night	14% 4 13% 46% 22% 2	400
	21st to end	Night	7% 3 17% 47% 24% 3	400
July	1st to 10th	Night	13% 3 14% 38% 28% 5%	435
	11th to 20th	Night	7% 2 9% 38% 40% 4	582
	21st to end	Night	7% 2 11% 43% 33% 5%	657
August	1st to 10th	Night	11% 1 8% 45% 31% 3	597
	11th to 20th	Night	12% 2 10% 41% 31% 3	610
	21st to end	Night	11% 3 13% 38% 32% 3	880
September	1st to 10th	Night	14% 3 12% 37% 29% 6%	800
	11th to 20th	Night	12% 3 13% 37% 30% 5%	800
	21st to end	Night	23% 3 19% 33% 20% 2	800
October	1st to 10th	Night	19% 7% 16% 30% 24% 4	800
	11th to 20th	Night	17% 11% 25% 33% 14% 1	800
	21st to end	Night	16% 11% 26% 33% 12% 1	880
November	1st to 10th	Night	15% 12% 27% 33% 11% 1	820
	11th to 20th	Night	19% 15% 28% 29% 8% 1	840
	21st to end	Night	18% 15% 28% 30% 8% 1	840
December	1st to 10th	Night	21% 15% 31% 27% 5% 1	840
	11th to 20th	Night	19% 11% 33% 33% 4	840
	21st to end	Night	18% 16% 30% 34% 2	924

Page generated at 2026-01-14 01:14:28. Version ac07

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