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 2025-08-05.

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	13% 9% 3 8% 5% 4 4 4 6% 7% 38%	126265
	Night	12% 9% 4 9% 5% 4 4 4 5% 6% 38%	122074
Transparency	Day	45% 5% 12% 25% 12% 1	129817
	Night	46% 4 12% 24% 13% 1	126598
Seeing	Day	17% 11% 31% 26% 12% 3	17028
	Night	16% 10% 33% 29% 10% 1	26872

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	6% 13% 6% 21% 9% 7% 4 3 4 5% 21%	3617
	11th to 20th	Day	6% 11% 8% 12% 7% 6% 6% 9% 12% 4 19%	3643
	21st to end	Day	9% 15% 8% 13% 6% 4 4 3 5% 5% 29%	4488
February	1st to 10th	Day	8% 12% 5% 11% 8% 6% 5% 5% 8% 6% 27%	1800
	11th to 20th	Day	10% 11% 4 10% 7% 6% 5% 3 6% 5% 32%	1754
	21st to end	Day	14% 11% 5% 9% 6% 4 4 3 5% 6% 32%	1598
March	1st to 10th	Day	13% 7% 3 6% 3 3 3 3 4 7% 48%	4749
	11th to 20th	Day	11% 8% 3 7% 4 4 4 4 5% 8% 42%	4980
	21st to end	Day	24% 12% 4 9% 5% 4 4 3 4 5% 27%	5592
April	1st to 10th	Day	18% 14% 3 8% 5% 4 4 3 6% 6% 31%	2400
	11th to 20th	Day	19% 14% 4 8% 5% 4 3 3 4 5% 31%	2374
	21st to end	Day	21% 13% 3 7% 4 5% 3 4 5% 4 30%	2532
May	1st to 10th	Day	15% 7% 2 5% 4 3 4 3 5% 6% 46%	6138
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 6% 42%	6319
	21st to end	Day	19% 8% 2 6% 4 4 4 3 5% 7% 38%	6935
June	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 46%	2800
	11th to 20th	Day	13% 7% 2 7% 5% 4 5% 5% 6% 9% 38%	2800
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 6% 8% 39%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	6360
	11th to 20th	Day	8% 4 1 4 4 3 4 4 5% 8% 54%	6360
	21st to end	Day	9% 5% 2 7% 4 5% 5% 4 7% 8% 43%	6865
August	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 42%	4157
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 8% 43%	4041
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 6% 8% 46%	4245
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 7% 47%	2280
	11th to 20th	Day	11% 6% 2 5% 4 3 4 4 6% 8% 48%	2280
	21st to end	Day	14% 7% 2 6% 4 4 5% 3 5% 8% 41%	1989
October	1st to 10th	Day	15% 7% 2 5% 4 3 4 3 5% 5% 46%	1900
	11th to 20th	Day	15% 10% 2 7% 4 4 4 3 4 7% 40%	1900
	21st to end	Day	19% 11% 3 6% 4 3 4 2 5% 6% 36%	2048
November	1st to 10th	Day	12% 10% 4 6% 4 3 4 4 5% 7% 40%	1721
	11th to 20th	Day	13% 13% 5% 11% 8% 4 3 3 5% 6% 29%	1600
	21st to end	Day	11% 13% 5% 9% 6% 4 4 3 5% 6% 37%	1600
December	1st to 10th	Day	7% 9% 4 13% 10% 7% 6% 4 5% 6% 29%	2610
	11th to 20th	Day	12% 15% 6% 11% 9% 7% 4 4 5% 6% 21%	3008
	21st to end	Day	17% 20% 9% 20% 5% 4 4 3 3 4 14%	3982
January	1st to 10th	Night	7% 12% 6% 17% 7% 7% 5% 3 4 5% 26%	6348
	11th to 20th	Night	7% 13% 5% 12% 8% 7% 8% 9% 9% 5% 18%	6334
	21st to end	Night	10% 14% 7% 10% 6% 5% 4 3 5% 5% 32%	6490
February	1st to 10th	Night	7% 15% 5% 10% 6% 6% 5% 3 5% 6% 32%	2600
	11th to 20th	Night	8% 10% 4 10% 6% 6% 5% 4 5% 6% 37%	2533
	21st to end	Night	13% 9% 4 9% 5% 5% 5% 4 4 5% 36%	2031
March	1st to 10th	Night	12% 8% 3 8% 3 3 3 3 4 6% 47%	5148
	11th to 20th	Night	9% 7% 3 8% 4 4 4 4 6% 8% 42%	4980
	21st to end	Night	24% 12% 3 10% 4 3 3 2 3 5% 29%	5336
April	1st to 10th	Night	16% 11% 3 7% 4 3 4 4 5% 6% 37%	2000
	11th to 20th	Night	20% 11% 3 6% 4 3 4 3 5% 6% 36%	1975
	21st to end	Night	20% 10% 3 7% 4 4 4 3 4 5% 37%	1865
May	1st to 10th	Night	14% 7% 3 6% 3 3 4 3 4 6% 49%	3763
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 4 7% 51%	3596
	21st to end	Night	17% 8% 3 7% 4 3 4 3 5% 7% 40%	3942
June	1st to 10th	Night	9% 4 1 6% 4 4 5% 4 6% 9% 49%	1600
	11th to 20th	Night	11% 5% 2 6% 6% 5% 4 4 7% 8% 42%	1600
	21st to end	Night	10% 4 2 5% 4 3 4 3 5% 8% 53%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 4 4 7% 9% 48%	3618
	11th to 20th	Night	7% 5% 1 4 3 4 4 4 6% 9% 55%	3617
	21st to end	Night	8% 5% 2 5% 3 3 4 5% 6% 8% 51%	3990
August	1st to 10th	Night	11% 6% 2 6% 4 3 4 4 5% 6% 48%	2872
	11th to 20th	Night	13% 5% 1 5% 3 3 4 3 5% 6% 52%	2798
	21st to end	Night	10% 5% 2 7% 5% 4 4 3 5% 7% 50%	3411
September	1st to 10th	Night	9% 4 2 5% 3 3 3 4 5% 6% 57%	1900
	11th to 20th	Night	12% 4 2 5% 4 3 3 3 5% 7% 52%	1900
	21st to end	Night	12% 7% 2 4 3 3 3 3 5% 6% 53%	2169
October	1st to 10th	Night	13% 7% 2 6% 3 3 4 3 4 6% 50%	2280
	11th to 20th	Night	14% 8% 3 6% 4 4 4 4 5% 7% 43%	2280
	21st to end	Night	16% 10% 3 7% 4 4 4 4 5% 6% 38%	2548
November	1st to 10th	Night	12% 9% 3 6% 5% 3 4 3 4 6% 44%	2575
	11th to 20th	Night	11% 12% 5% 9% 5% 4 4 3 5% 5% 36%	2800
	21st to end	Night	13% 12% 4 10% 5% 4 3 3 4 6% 36%	2800
December	1st to 10th	Night	7% 9% 3 14% 7% 8% 6% 4 5% 7% 30%	4580
	11th to 20th	Night	11% 16% 7% 15% 6% 4 5% 4 4 5% 24%	5199
	21st to end	Night	20% 16% 8% 16% 6% 4 4 3 4 4 15%	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	66% 4 9% 16% 5%	3618
	11th to 20th	Day	57% 6% 8% 16% 13%	3643
	21st to end	Day	58% 4 13% 15% 9%	4488
February	1st to 10th	Day	54% 5% 13% 17% 10% 1	1800
	11th to 20th	Day	52% 5% 11% 24% 7% 1	1763
	21st to end	Day	53% 4 12% 20% 10% 1	1598
March	1st to 10th	Day	38% 3 16% 28% 13% 3	4749
	11th to 20th	Day	39% 4 13% 31% 11% 1	4980
	21st to end	Day	61% 4 10% 16% 8% 1	5594
April	1st to 10th	Day	53% 4 11% 20% 10% 1	2400
	11th to 20th	Day	55% 5% 10% 18% 10% 1	2374
	21st to end	Day	55% 6% 12% 18% 9% 1	2532
May	1st to 10th	Day	38% 5% 11% 28% 17% 1	6176
	11th to 20th	Day	40% 6% 10% 26% 15% 2	6323
	21st to end	Day	46% 5% 9% 29% 11% 1	6949
June	1st to 10th	Day	36% 7% 13% 30% 14% 1	2800
	11th to 20th	Day	42% 7% 14% 21% 14% 1	2800
	21st to end	Day	41% 7% 15% 24% 12% 2	2800
July	1st to 10th	Day	37% 4 17% 33% 8% 1	6291
	11th to 20th	Day	29% 4 15% 35% 16% 1	6285
	21st to end	Day	38% 5% 13% 28% 15% 1	6670
August	1st to 10th	Day	39% 6% 11% 29% 13% 2	4143
	11th to 20th	Day	39% 4 14% 32% 10%	4080
	21st to end	Day	35% 4 16% 29% 14% 1	4244
September	1st to 10th	Day	37% 6% 12% 29% 16% 1	2280
	11th to 20th	Day	34% 7% 17% 31% 11% 1	2280
	21st to end	Day	41% 5% 12% 30% 12% 1	1989
October	1st to 10th	Day	27% 3 7% 39% 23%	3160
	11th to 20th	Day	39% 3 14% 23% 19% 2	3140
	21st to end	Day	42% 3 13% 31% 10%	3350
November	1st to 10th	Day	43% 6% 14% 21% 14% 2	1721
	11th to 20th	Day	57% 5% 11% 17% 9% 1	1600
	21st to end	Day	49% 5% 17% 20% 7% 1	1600
December	1st to 10th	Day	56% 5% 15% 16% 7% 1	2610
	11th to 20th	Day	64% 4 13% 13% 5%	3005
	21st to end	Day	77% 4 6% 10% 3	3982
January	1st to 10th	Night	61% 3 9% 19% 7%	6348
	11th to 20th	Night	61% 5% 9% 13% 12%	6334
	21st to end	Night	55% 4 11% 19% 11% 1	6490
February	1st to 10th	Night	52% 4 12% 20% 10% 2	2600
	11th to 20th	Night	48% 5% 12% 24% 9% 1	2534
	21st to end	Night	49% 5% 11% 21% 12% 2	2031
March	1st to 10th	Night	40% 4 11% 29% 13% 3	5148
	11th to 20th	Night	38% 4 13% 28% 16% 1	4980
	21st to end	Night	60% 3 10% 16% 10% 1	5336
April	1st to 10th	Night	47% 6% 9% 21% 14% 2	2000
	11th to 20th	Night	48% 5% 10% 19% 15% 2	1975
	21st to end	Night	49% 5% 10% 21% 14% 2	1865
May	1st to 10th	Night	37% 5% 9% 25% 22% 1	3789
	11th to 20th	Night	34% 5% 11% 24% 20% 7%	3597
	21st to end	Night	45% 5% 9% 30% 12%	3950
June	1st to 10th	Night	30% 8% 14% 31% 15% 2	1600
	11th to 20th	Night	38% 6% 13% 22% 17% 3	1600
	21st to end	Night	30% 7% 15% 30% 16% 3	1600
July	1st to 10th	Night	32% 3 20% 31% 12% 1	3580
	11th to 20th	Night	27% 4 16% 37% 14% 1	3572
	21st to end	Night	31% 4 17% 29% 17% 2	3875
August	1st to 10th	Night	36% 5% 11% 29% 16% 3	2862
	11th to 20th	Night	34% 4 14% 31% 15% 1	2825
	21st to end	Night	35% 4 17% 27% 15% 2	3411
September	1st to 10th	Night	28% 5% 11% 33% 21% 1	1900
	11th to 20th	Night	32% 5% 15% 31% 16% 1	1900
	21st to end	Night	32% 5% 13% 30% 19% 1	2169
October	1st to 10th	Night	26% 3 6% 35% 27% 3	3800
	11th to 20th	Night	30% 4 11% 29% 23% 3	3779
	21st to end	Night	46% 3 10% 26% 15% 1	4196
November	1st to 10th	Night	41% 5% 13% 24% 15% 2	2575
	11th to 20th	Night	51% 5% 11% 20% 11% 2	2800
	21st to end	Night	50% 5% 15% 22% 7% 1	2800
December	1st to 10th	Night	54% 3 17% 19% 6%	4580
	11th to 20th	Night	63% 4 10% 16% 6%	5201
	21st to end	Night	74% 4 7% 11% 5%	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	13% 15% 39% 20% 9% 3	300
	11th to 20th	Day	18% 15% 40% 20% 5% 2	300
	21st to end	Day	16% 13% 37% 24% 8% 1	330
February	1st to 10th	Day	16% 17% 43% 18% 5% 1	300
	11th to 20th	Day	17% 21% 36% 22% 4 1	300
	21st to end	Day	20% 17% 40% 22% 2	249
March	1st to 10th	Day	17% 15% 44% 18% 6%	300
	11th to 20th	Day	24% 14% 34% 20% 6% 2	300
	21st to end	Day	28% 15% 28% 19% 5% 3	383
April	1st to 10th	Day	28% 14% 35% 17% 6%	500
	11th to 20th	Day	28% 21% 22% 22% 7% 1	492
	21st to end	Day	29% 14% 28% 22% 6% 2	632
May	1st to 10th	Day	20% 12% 33% 25% 8% 1	703
	11th to 20th	Day	21% 11% 26% 28% 10% 3	725
	21st to end	Day	20% 8% 28% 28% 14% 3	789
June	1st to 10th	Day	11% 10% 27% 30% 16% 5%	730
	11th to 20th	Day	16% 8% 34% 29% 11% 2	730
	21st to end	Day	16% 11% 29% 25% 14% 5%	730
July	1st to 10th	Day	8% 5% 24% 31% 22% 9%	721
	11th to 20th	Day	8% 7% 29% 30% 18% 7%	718
	21st to end	Day	7% 4 28% 37% 19% 5%	794
August	1st to 10th	Day	10% 5% 21% 35% 22% 7%	697
	11th to 20th	Day	14% 4 21% 32% 22% 7%	665
	21st to end	Day	10% 6% 26% 33% 18% 6%	578
September	1st to 10th	Day	16% 7% 26% 29% 15% 6%	490
	11th to 20th	Day	12% 7% 29% 30% 18% 5%	490
	21st to end	Day	14% 15% 30% 25% 13% 2	322
October	1st to 10th	Day	16% 14% 26% 26% 16% 3	300
	11th to 20th	Day	19% 17% 36% 22% 6%	300
	21st to end	Day	28% 16% 35% 14% 7% 2	330
November	1st to 10th	Day	18% 17% 36% 20% 7% 1	300
	11th to 20th	Day	20% 24% 36% 16% 4	300
	21st to end	Day	22% 14% 41% 19% 5%	300
December	1st to 10th	Day	15% 14% 39% 25% 5% 1	300
	11th to 20th	Day	19% 17% 38% 21% 5%	300
	21st to end	Day	29% 13% 39% 15% 3	330
January	1st to 10th	Night	13% 15% 39% 25% 8% 1	1000
	11th to 20th	Night	19% 13% 38% 25% 5% 1	1000
	21st to end	Night	18% 13% 37% 26% 6% 1	1100
February	1st to 10th	Night	14% 15% 41% 25% 5% 1	1000
	11th to 20th	Night	14% 16% 41% 23% 6% 1	979
	21st to end	Night	18% 16% 35% 25% 6%	711
March	1st to 10th	Night	19% 13% 37% 24% 7% 1	705
	11th to 20th	Night	17% 13% 36% 28% 6%	600
	21st to end	Night	22% 9% 31% 27% 10% 2	660
April	1st to 10th	Night	21% 14% 35% 26% 5%	600
	11th to 20th	Night	26% 10% 30% 28% 6%	593
	21st to end	Night	26% 8% 34% 24% 8% 1	600
May	1st to 10th	Night	21% 8% 32% 31% 7% 1	600
	11th to 20th	Night	19% 7% 30% 34% 10% 1	591
	21st to end	Night	16% 7% 26% 37% 13% 1	650
June	1st to 10th	Night	11% 5% 29% 41% 14% 1	600
	11th to 20th	Night	13% 5% 34% 37% 11%	600
	21st to end	Night	11% 4 26% 41% 17% 1	600
July	1st to 10th	Night	6% 2 23% 38% 28% 2	585
	11th to 20th	Night	6% 5% 25% 40% 22% 2	582
	21st to end	Night	6% 3 19% 49% 20% 3	657
August	1st to 10th	Night	7% 2 18% 47% 24% 2	579
	11th to 20th	Night	11% 4 20% 35% 27% 4	564
	21st to end	Night	9% 3 25% 39% 21% 2	627
September	1st to 10th	Night	11% 3 25% 39% 17% 4	570
	11th to 20th	Night	13% 3 31% 33% 16% 3	570
	21st to end	Night	13% 9% 31% 32% 14% 1	674
October	1st to 10th	Night	17% 7% 29% 33% 12% 1	760
	11th to 20th	Night	19% 12% 35% 27% 7% 1	760
	21st to end	Night	22% 11% 32% 28% 6%	836
November	1st to 10th	Night	17% 13% 35% 26% 7% 1	819
	11th to 20th	Night	17% 16% 39% 23% 6% 1	1000
	21st to end	Night	18% 16% 37% 21% 7%	1000
December	1st to 10th	Night	17% 12% 39% 25% 7% 1	1000
	11th to 20th	Night	21% 11% 38% 22% 7% 1	1000
	21st to end	Night	25% 14% 34% 23% 5%	1100

Page generated at 2025-08-02 00:32:26. Version ac07

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