Rochester
Clear Sky Chart
History

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

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 5% 7% 31%	127088
	Night	16% 12% 4 10% 5% 4 4 4 5% 6% 28%	122925
Transparency	Day	52% 6% 12% 19% 10% 1	130656
	Night	55% 6% 12% 18% 9% 1	127442
Seeing	Day	20% 17% 30% 21% 9% 3	15847
	Night	22% 16% 29% 23% 9% 1	25101

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% 18% 8% 17% 8% 5% 5% 4 5% 4 14%	3597
	11th to 20th	Day	12% 22% 8% 16% 9% 6% 4 4 4 5% 10%	3708
	21st to end	Day	17% 17% 7% 13% 9% 6% 5% 4 5% 5% 12%	4466
February	1st to 10th	Day	16% 17% 6% 14% 7% 6% 4 5% 4 6% 15%	1800
	11th to 20th	Day	12% 16% 7% 15% 9% 8% 5% 4 4 4 16%	1753
	21st to end	Day	19% 14% 3 14% 9% 5% 4 3 6% 5% 17%	1605
March	1st to 10th	Day	16% 13% 5% 11% 7% 5% 4 3 4 4 28%	4789
	11th to 20th	Day	11% 15% 5% 9% 5% 4 5% 3 4 7% 32%	4980
	21st to end	Day	16% 13% 4 10% 5% 5% 4 3 5% 6% 28%	5556
April	1st to 10th	Day	18% 13% 4 8% 5% 4 3 4 5% 6% 30%	2400
	11th to 20th	Day	19% 10% 3 6% 4 4 4 4 5% 5% 36%	2374
	21st to end	Day	20% 11% 3 7% 5% 4 5% 4 5% 6% 31%	2535
May	1st to 10th	Day	26% 10% 2 7% 4 4 4 3 5% 6% 29%	6072
	11th to 20th	Day	16% 9% 3 9% 5% 4 4 3 5% 6% 34%	6318
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 4 5% 7% 38%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 10% 38%	2800
	21st to end	Day	14% 6% 3 6% 5% 4 5% 4 7% 8% 40%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 5% 4 4 7% 9% 43%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 4 7% 10% 40%	6360
	21st to end	Day	10% 4 2 6% 4 4 5% 4 6% 9% 45%	6747
August	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 5% 7% 40%	4234
	11th to 20th	Day	14% 6% 3 8% 6% 5% 5% 4 6% 8% 35%	4174
	21st to end	Day	15% 5% 2 5% 4 4 5% 3 5% 9% 42%	4324
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 4 6% 7% 42%	2400
	11th to 20th	Day	15% 5% 2 5% 4 5% 4 3 5% 7% 45%	2400
	21st to end	Day	16% 7% 3 7% 6% 5% 5% 4 5% 7% 35%	2090
October	1st to 10th	Day	20% 8% 3 6% 4 4 4 4 5% 7% 37%	2000
	11th to 20th	Day	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	2000
	21st to end	Day	27% 11% 2 7% 6% 5% 4 4 5% 5% 24%	2190
November	1st to 10th	Day	15% 13% 2 9% 6% 5% 5% 4 5% 6% 32%	1761
	11th to 20th	Day	14% 15% 5% 9% 6% 6% 5% 3 5% 6% 26%	1600
	21st to end	Day	20% 16% 4 9% 5% 5% 6% 4 5% 7% 19%	1600
December	1st to 10th	Day	19% 17% 7% 12% 7% 6% 6% 4 6% 5% 12%	2600
	11th to 20th	Day	20% 19% 6% 12% 6% 4 4 3 4 4 19%	2986
	21st to end	Day	21% 17% 6% 15% 7% 4 4 4 4 5% 11%	3960
January	1st to 10th	Night	15% 18% 8% 21% 6% 5% 4 3 4 5% 13%	6349
	11th to 20th	Night	10% 18% 9% 19% 8% 6% 4 4 5% 5% 11%	6164
	21st to end	Night	18% 20% 7% 13% 7% 5% 5% 3 4 5% 14%	6490
February	1st to 10th	Night	17% 17% 5% 13% 7% 5% 3 4 5% 5% 19%	2600
	11th to 20th	Night	14% 13% 6% 15% 8% 6% 5% 3 5% 6% 19%	2533
	21st to end	Night	16% 15% 5% 15% 7% 5% 4 4 4 6% 20%	2025
March	1st to 10th	Night	16% 10% 6% 12% 8% 4 4 3 5% 5% 26%	5047
	11th to 20th	Night	14% 15% 3 8% 5% 4 4 3 5% 7% 33%	4979
	21st to end	Night	17% 11% 5% 12% 5% 4 4 3 4 5% 30%	5361
April	1st to 10th	Night	19% 14% 4 8% 6% 4 4 3 5% 6% 28%	2000
	11th to 20th	Night	19% 9% 2 7% 4 4 5% 4 5% 7% 34%	1975
	21st to end	Night	20% 7% 2 6% 4 4 4 4 6% 7% 36%	1882
May	1st to 10th	Night	30% 10% 3 7% 5% 4 4 3 5% 5% 26%	3869
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 4 6% 7% 37%	3575
	21st to end	Night	18% 6% 3 7% 4 4 4 4 5% 7% 38%	3926
June	1st to 10th	Night	16% 6% 3 6% 4 5% 4 5% 7% 8% 37%	1600
	11th to 20th	Night	12% 6% 2 7% 5% 4 6% 4 7% 8% 41%	1600
	21st to end	Night	11% 6% 3 7% 4 5% 5% 5% 6% 8% 40%	1600
July	1st to 10th	Night	12% 5% 2 7% 4 5% 4 4 6% 10% 41%	3600
	11th to 20th	Night	10% 6% 3 6% 4 5% 5% 3 6% 8% 45%	3600
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 7% 49%	4102
August	1st to 10th	Night	13% 5% 2 6% 5% 4 4 3 6% 9% 42%	2915
	11th to 20th	Night	15% 5% 3 7% 5% 4 5% 4 6% 8% 39%	2875
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 6% 8% 46%	3540
September	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 5% 8% 46%	2000
	11th to 20th	Night	15% 5% 2 6% 3 4 4 3 6% 6% 48%	2000
	21st to end	Night	17% 8% 2 6% 4 4 4 4 7% 7% 37%	2305
October	1st to 10th	Night	20% 6% 3 7% 5% 4 4 4 5% 6% 37%	2400
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 4 5% 7% 31%	2400
	21st to end	Night	27% 10% 3 7% 4 4 3 3 5% 6% 28%	2650
November	1st to 10th	Night	16% 11% 3 7% 5% 5% 4 4 5% 5% 35%	2579
	11th to 20th	Night	16% 13% 3 9% 5% 5% 5% 4 6% 6% 28%	2800
	21st to end	Night	20% 14% 4 8% 6% 5% 4 3 6% 7% 22%	2800
December	1st to 10th	Night	17% 19% 8% 13% 7% 5% 5% 4 5% 5% 13%	4580
	11th to 20th	Night	19% 19% 6% 11% 5% 4 4 3 4 4 20%	5208
	21st to end	Night	25% 17% 7% 11% 7% 5% 4 3 4 4 13%	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	74% 6% 9% 10% 2	3598
	11th to 20th	Day	75% 8% 8% 4 3 1	3738
	21st to end	Day	74% 6% 12% 7% 2	4466
February	1st to 10th	Day	72% 5% 10% 11% 3	1800
	11th to 20th	Day	72% 6% 8% 11% 3	1762
	21st to end	Day	68% 5% 10% 11% 5%	1605
March	1st to 10th	Day	59% 5% 11% 16% 8%	4792
	11th to 20th	Day	52% 6% 18% 14% 9% 1	4980
	21st to end	Day	56% 6% 8% 20% 9% 1	5556
April	1st to 10th	Day	56% 4 10% 18% 10% 1	2400
	11th to 20th	Day	49% 6% 9% 20% 14% 2	2374
	21st to end	Day	53% 6% 8% 18% 13% 2	2535
May	1st to 10th	Day	56% 5% 10% 16% 11% 2	6081
	11th to 20th	Day	51% 6% 11% 18% 13% 2	6322
	21st to end	Day	47% 6% 12% 21% 12% 2	6949
June	1st to 10th	Day	45% 7% 10% 20% 17% 1	2800
	11th to 20th	Day	41% 7% 13% 24% 13% 1	2800
	21st to end	Day	41% 6% 14% 22% 16% 1	2800
July	1st to 10th	Day	36% 7% 12% 28% 15% 1	6290
	11th to 20th	Day	38% 8% 14% 25% 14% 1	6285
	21st to end	Day	35% 8% 13% 29% 15%	6552
August	1st to 10th	Day	43% 6% 14% 22% 14% 1	4221
	11th to 20th	Day	45% 7% 14% 26% 7%	4213
	21st to end	Day	39% 6% 12% 33% 9% 1	4324
September	1st to 10th	Day	39% 8% 12% 29% 11% 1	2400
	11th to 20th	Day	38% 5% 13% 29% 15%	2400
	21st to end	Day	48% 6% 12% 21% 11% 1	2090
October	1st to 10th	Day	40% 6% 15% 26% 12%	3260
	11th to 20th	Day	47% 5% 16% 20% 12%	3240
	21st to end	Day	49% 7% 13% 24% 7% 1	3516
November	1st to 10th	Day	53% 7% 11% 19% 9% 2	1761
	11th to 20th	Day	59% 6% 14% 14% 6% 1	1600
	21st to end	Day	64% 6% 15% 13% 2	1600
December	1st to 10th	Day	71% 7% 14% 7% 1	2600
	11th to 20th	Day	70% 4 11% 8% 7%	2986
	21st to end	Day	75% 5% 11% 6% 2	3960
January	1st to 10th	Night	75% 6% 10% 7% 2	6349
	11th to 20th	Night	73% 9% 9% 6% 3	6208
	21st to end	Night	72% 4 11% 6% 6%	6490
February	1st to 10th	Night	66% 5% 11% 13% 4	2600
	11th to 20th	Night	67% 5% 11% 13% 3	2535
	21st to end	Night	64% 6% 12% 13% 6%	2025
March	1st to 10th	Night	59% 5% 13% 17% 6%	5057
	11th to 20th	Night	51% 4 17% 18% 7% 2	4980
	21st to end	Night	54% 6% 9% 20% 10% 1	5361
April	1st to 10th	Night	57% 6% 10% 19% 8% 1	2000
	11th to 20th	Night	50% 7% 11% 18% 14% 1	1975
	21st to end	Night	47% 6% 11% 20% 14% 2	1882
May	1st to 10th	Night	61% 6% 8% 13% 10% 3	3876
	11th to 20th	Night	45% 7% 15% 20% 11% 2	3576
	21st to end	Night	45% 6% 13% 19% 15% 1	3927
June	1st to 10th	Night	42% 7% 12% 23% 14% 2	1600
	11th to 20th	Night	40% 6% 13% 23% 17% 2	1600
	21st to end	Night	40% 9% 11% 23% 16% 1	1600
July	1st to 10th	Night	38% 6% 15% 25% 15% 1	3560
	11th to 20th	Night	35% 7% 14% 26% 16% 2	3552
	21st to end	Night	33% 6% 13% 27% 20% 1	3982
August	1st to 10th	Night	38% 6% 15% 27% 13% 1	2904
	11th to 20th	Night	41% 8% 15% 27% 9%	2902
	21st to end	Night	33% 6% 13% 35% 12% 1	3540
September	1st to 10th	Night	36% 6% 14% 30% 13% 2	2000
	11th to 20th	Night	36% 4 13% 31% 16% 1	2000
	21st to end	Night	43% 7% 13% 24% 12% 1	2305
October	1st to 10th	Night	44% 5% 11% 26% 13% 1	3920
	11th to 20th	Night	47% 5% 15% 20% 13% 1	3899
	21st to end	Night	46% 5% 13% 27% 8% 1	4274
November	1st to 10th	Night	49% 7% 13% 20% 9% 2	2579
	11th to 20th	Night	55% 6% 16% 15% 8% 1	2800
	21st to end	Night	60% 7% 16% 14% 3	2800
December	1st to 10th	Night	72% 7% 13% 7% 1	4580
	11th to 20th	Night	67% 5% 10% 11% 6% 1	5208
	21st to end	Night	74% 6% 9% 8% 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	27% 31% 29% 10% 4	200
	11th to 20th	Day	25% 28% 35% 10% 2	230
	21st to end	Day	25% 25% 33% 13% 3	330
February	1st to 10th	Day	28% 31% 31% 9% 1	300
	11th to 20th	Day	27% 34% 32% 5% 1	300
	21st to end	Day	34% 24% 34% 8% 1	369
March	1st to 10th	Day	20% 29% 32% 15% 3 1	500
	11th to 20th	Day	28% 24% 31% 14% 3 1	500
	21st to end	Day	29% 22% 29% 14% 5%	550
April	1st to 10th	Day	26% 25% 29% 14% 4 1	500
	11th to 20th	Day	29% 24% 26% 13% 6% 3	492
	21st to end	Day	24% 22% 31% 16% 6% 1	500
May	1st to 10th	Day	27% 15% 30% 18% 7% 3	503
	11th to 20th	Day	18% 13% 32% 27% 8% 2	527
	21st to end	Day	16% 13% 32% 23% 13% 3	573
June	1st to 10th	Day	16% 15% 36% 24% 8% 1	530
	11th to 20th	Day	15% 13% 32% 27% 11% 2	530
	21st to end	Day	14% 13% 36% 26% 10% 1	530
July	1st to 10th	Day	11% 8% 27% 34% 16% 3	525
	11th to 20th	Day	11% 9% 30% 31% 16% 2	524
	21st to end	Day	10% 9% 35% 31% 13% 2	576
August	1st to 10th	Day	11% 7% 26% 34% 16% 6%	526
	11th to 20th	Day	14% 8% 24% 32% 16% 6%	519
	21st to end	Day	13% 10% 28% 31% 13% 6%	583
September	1st to 10th	Day	14% 12% 24% 28% 15% 7%	530
	11th to 20th	Day	12% 8% 22% 25% 22% 11%	530
	21st to end	Day	17% 15% 26% 23% 15% 4	530
October	1st to 10th	Day	21% 13% 26% 21% 15% 4	530
	11th to 20th	Day	25% 19% 33% 16% 6% 2	530
	21st to end	Day	30% 19% 31% 17% 3	577
November	1st to 10th	Day	22% 18% 32% 19% 8% 1	383
	11th to 20th	Day	24% 28% 31% 14% 3 1	200
	21st to end	Day	31% 24% 34% 10% 2	200
December	1st to 10th	Day	31% 24% 29% 14% 3 1	200
	11th to 20th	Day	32% 26% 30% 11% 3	200
	21st to end	Day	28% 27% 32% 10% 2 1	220
January	1st to 10th	Night	26% 26% 33% 12% 3	800
	11th to 20th	Night	26% 25% 32% 15% 3	800
	21st to end	Night	21% 24% 35% 17% 3	880
February	1st to 10th	Night	27% 26% 31% 16% 1	800
	11th to 20th	Night	25% 25% 34% 14% 2	783
	21st to end	Night	26% 23% 33% 16% 2	660
March	1st to 10th	Night	26% 20% 33% 18% 4	800
	11th to 20th	Night	25% 22% 31% 18% 4	800
	21st to end	Night	24% 17% 33% 20% 6%	880
April	1st to 10th	Night	26% 16% 35% 20% 3	800
	11th to 20th	Night	24% 17% 29% 23% 6% 1	791
	21st to end	Night	23% 12% 29% 27% 8% 1	682
May	1st to 10th	Night	25% 8% 25% 28% 13% 1	520
	11th to 20th	Night	15% 7% 31% 37% 10%	394
	21st to end	Night	12% 6% 34% 29% 18% 1	433
June	1st to 10th	Night	15% 8% 31% 32% 13% 1	400
	11th to 20th	Night	14% 9% 28% 32% 16% 1	400
	21st to end	Night	12% 7% 31% 40% 9%	400
July	1st to 10th	Night	9% 4 23% 44% 18% 3	390
	11th to 20th	Night	11% 3 28% 38% 19% 2	388
	21st to end	Night	11% 3 24% 42% 17% 3	513
August	1st to 10th	Night	10% 3 25% 40% 21% 3	597
	11th to 20th	Night	11% 6% 20% 37% 23% 3	594
	21st to end	Night	10% 6% 25% 36% 22% 2	847
September	1st to 10th	Night	16% 7% 24% 28% 22% 3	800
	11th to 20th	Night	15% 6% 18% 30% 26% 5%	800
	21st to end	Night	17% 11% 23% 28% 17% 4	800
October	1st to 10th	Night	21% 10% 28% 23% 17% 2	800
	11th to 20th	Night	24% 18% 31% 21% 6% 1	800
	21st to end	Night	32% 12% 32% 21% 2	880
November	1st to 10th	Night	23% 16% 32% 20% 7%	789
	11th to 20th	Night	22% 22% 33% 16% 7% 1	800
	21st to end	Night	28% 22% 29% 16% 5%	800
December	1st to 10th	Night	30% 25% 28% 14% 3	800
	11th to 20th	Night	26% 27% 28% 16% 2 1	800
	21st to end	Night	33% 22% 29% 14% 2	880

Page generated at 2025-10-31 02:02:38. Version ac07

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