Ithaca
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ithaca 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	15% 11% 4 10% 6% 5% 5% 4 5% 7% 29%	127132
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 28%	122877
Transparency	Day	54% 4 11% 20% 10% 1	130675
	Night	57% 4 10% 18% 9% 1	127420
Seeing	Day	26% 14% 29% 18% 9% 3	15970
	Night	25% 13% 24% 26% 10% 1	25181

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	12% 18% 10% 24% 7% 3 3 2 3 4 13%	3597
	11th to 20th	Day	13% 18% 10% 20% 8% 6% 3 3 4 4 11%	3616
	21st to end	Day	14% 15% 9% 13% 9% 6% 5% 3 5% 4 17%	4466
February	1st to 10th	Day	16% 15% 5% 16% 8% 5% 5% 3 5% 4 18%	1800
	11th to 20th	Day	14% 16% 7% 17% 8% 6% 5% 3 3 5% 17%	1778
	21st to end	Day	17% 15% 4 14% 9% 4 3 3 4 4 22%	1650
March	1st to 10th	Day	18% 16% 6% 12% 7% 4 3 2 3 4 25%	4647
	11th to 20th	Day	10% 17% 6% 9% 5% 4 4 2 5% 6% 34%	4980
	21st to end	Day	14% 13% 5% 13% 6% 5% 5% 3 5% 5% 26%	5683
April	1st to 10th	Day	16% 15% 4 9% 5% 5% 4 3 5% 6% 28%	2400
	11th to 20th	Day	18% 11% 3 8% 4 5% 3 3 5% 6% 33%	2374
	21st to end	Day	18% 13% 3 7% 5% 4 4 3 4 7% 33%	2431
May	1st to 10th	Day	28% 11% 3 6% 4 4 4 3 5% 6% 26%	6181
	11th to 20th	Day	16% 10% 4 9% 5% 5% 5% 4 6% 7% 30%	6318
	21st to end	Day	20% 10% 3 7% 4 5% 4 3 6% 7% 31%	6949
June	1st to 10th	Day	17% 8% 2 7% 5% 4 4 4 6% 8% 34%	2800
	11th to 20th	Day	15% 7% 2 6% 5% 4 6% 5% 7% 10% 34%	2800
	21st to end	Day	11% 7% 3 7% 5% 5% 6% 5% 7% 9% 34%	2800
July	1st to 10th	Day	9% 6% 2 7% 4 5% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	8% 5% 2 7% 6% 6% 7% 6% 9% 11% 33%	6360
	21st to end	Day	11% 5% 3 6% 5% 5% 6% 4 6% 8% 42%	6848
August	1st to 10th	Day	16% 6% 3 7% 5% 6% 6% 4 6% 9% 33%	4234
	11th to 20th	Day	15% 7% 3 8% 6% 6% 6% 5% 6% 9% 30%	4075
	21st to end	Day	13% 8% 3 7% 5% 4 5% 4 5% 7% 38%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 5% 8% 41%	2400
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 3 5% 8% 44%	2400
	21st to end	Day	18% 7% 2 7% 5% 5% 6% 4 6% 7% 32%	2124
October	1st to 10th	Day	17% 9% 2 8% 4 5% 4 4 6% 6% 35%	2000
	11th to 20th	Day	19% 10% 3 7% 5% 6% 5% 4 5% 7% 30%	2000
	21st to end	Day	21% 15% 3 8% 5% 4 4 4 4 5% 26%	2144
November	1st to 10th	Day	14% 13% 4 9% 4 4 4 4 6% 5% 33%	1779
	11th to 20th	Day	14% 15% 5% 11% 5% 4 4 4 5% 7% 25%	1680
	21st to end	Day	17% 19% 6% 11% 6% 5% 4 3 5% 7% 19%	1600
December	1st to 10th	Day	15% 19% 9% 17% 7% 6% 4 3 4 5% 11%	2600
	11th to 20th	Day	18% 15% 9% 11% 7% 5% 4 3 4 3 22%	2986
	21st to end	Day	18% 18% 6% 13% 7% 5% 4 3 4 5% 15%	3960
January	1st to 10th	Night	12% 17% 7% 25% 7% 5% 4 3 5% 4 12%	6349
	11th to 20th	Night	9% 17% 8% 18% 9% 6% 5% 3 4 5% 14%	6252
	21st to end	Night	15% 16% 6% 16% 8% 6% 6% 3 4 4 16%	6490
February	1st to 10th	Night	15% 16% 6% 13% 8% 5% 5% 3 5% 5% 20%	2600
	11th to 20th	Night	13% 15% 6% 15% 9% 5% 5% 4 4 5% 19%	2507
	21st to end	Night	16% 14% 5% 16% 7% 4 4 3 5% 5% 21%	1980
March	1st to 10th	Night	15% 13% 7% 12% 7% 5% 4 3 5% 5% 24%	5190
	11th to 20th	Night	15% 17% 5% 8% 4 3 3 2 4 5% 33%	4979
	21st to end	Night	17% 13% 5% 12% 5% 6% 5% 3 5% 4 26%	5248
April	1st to 10th	Night	17% 15% 4 7% 5% 4 4 4 6% 6% 28%	2000
	11th to 20th	Night	20% 12% 4 7% 5% 3 4 3 5% 5% 33%	1975
	21st to end	Night	18% 9% 3 6% 5% 3 4 4 4 6% 36%	1985
May	1st to 10th	Night	29% 11% 3 7% 5% 4 4 3 5% 5% 24%	3760
	11th to 20th	Night	16% 8% 3 7% 5% 4 5% 4 6% 7% 35%	3575
	21st to end	Night	19% 10% 3 7% 5% 4 4 4 6% 7% 31%	3926
June	1st to 10th	Night	17% 9% 2 6% 5% 3 4 4 6% 7% 37%	1600
	11th to 20th	Night	13% 7% 3 7% 5% 4 5% 4 6% 8% 39%	1600
	21st to end	Night	12% 6% 2 5% 5% 5% 5% 4 6% 8% 41%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 4 4 6% 10% 39%	3600
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	3600
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 8% 46%	3987
August	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 5% 8% 37%	2915
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 3 4 7% 40%	2941
	21st to end	Night	11% 9% 3 7% 4 4 4 4 6% 7% 42%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 5% 6% 3 6% 6% 43%	2000
	11th to 20th	Night	13% 6% 2 5% 4 4 4 4 4 7% 49%	2000
	21st to end	Night	16% 8% 3 7% 4 5% 5% 4 4 7% 36%	2273
October	1st to 10th	Night	17% 9% 3 8% 4 4 5% 3 6% 5% 36%	2400
	11th to 20th	Night	19% 9% 3 7% 5% 4 3 3 5% 7% 34%	2400
	21st to end	Night	24% 14% 3 7% 4 4 4 3 4 6% 26%	2695
November	1st to 10th	Night	13% 14% 3 7% 5% 4 4 3 5% 6% 36%	2564
	11th to 20th	Night	16% 14% 4 8% 5% 5% 5% 3 5% 6% 29%	2716
	21st to end	Night	15% 16% 5% 10% 6% 5% 4 4 6% 6% 23%	2800
December	1st to 10th	Night	14% 18% 8% 14% 7% 6% 5% 3 6% 5% 14%	4580
	11th to 20th	Night	19% 15% 6% 12% 6% 4 4 3 4 4 23%	5208
	21st to end	Night	20% 19% 6% 11% 7% 4 4 3 4 4 16%	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	78% 2 6% 11% 2	3598
	11th to 20th	Day	77% 3 6% 9% 4	3646
	21st to end	Day	69% 4 10% 13% 4	4466
February	1st to 10th	Day	72% 2 8% 14% 4 1	1800
	11th to 20th	Day	72% 3 4 16% 5%	1787
	21st to end	Day	67% 2 7% 16% 6% 1	1650
March	1st to 10th	Day	65% 3 9% 16% 7%	4650
	11th to 20th	Day	54% 2 18% 16% 9% 1	4980
	21st to end	Day	61% 3 6% 18% 11% 1	5683
April	1st to 10th	Day	58% 3 7% 21% 10% 2	2400
	11th to 20th	Day	52% 3 11% 19% 13% 1	2374
	21st to end	Day	54% 2 8% 21% 13% 2	2431
May	1st to 10th	Day	60% 3 9% 16% 10% 1	6191
	11th to 20th	Day	55% 3 9% 22% 10% 1	6322
	21st to end	Day	53% 3 11% 21% 11% 1	6949
June	1st to 10th	Day	49% 3 10% 23% 15% 1	2800
	11th to 20th	Day	45% 5% 14% 24% 11% 1	2800
	21st to end	Day	45% 5% 14% 23% 13% 1	2800
July	1st to 10th	Day	38% 7% 14% 29% 11% 1	6290
	11th to 20th	Day	39% 9% 16% 25% 11% 1	6285
	21st to end	Day	39% 6% 12% 27% 15%	6651
August	1st to 10th	Day	48% 5% 13% 22% 12% 1	4221
	11th to 20th	Day	49% 6% 14% 25% 6%	4114
	21st to end	Day	44% 4 9% 33% 9% 1	4312
September	1st to 10th	Day	43% 3 10% 30% 14% 1	2400
	11th to 20th	Day	40% 3 10% 28% 18% 1	2400
	21st to end	Day	51% 3 11% 22% 11% 2	2124
October	1st to 10th	Day	43% 5% 11% 27% 13% 1	3260
	11th to 20th	Day	48% 4 12% 21% 15%	3240
	21st to end	Day	46% 4 15% 26% 9% 1	3446
November	1st to 10th	Day	51% 4 12% 20% 11% 2	1779
	11th to 20th	Day	58% 2 12% 17% 9% 2	1680
	21st to end	Day	67% 3 12% 15% 3	1600
December	1st to 10th	Day	76% 5% 8% 9% 2	2600
	11th to 20th	Day	66% 6% 9% 11% 7%	2986
	21st to end	Day	72% 4 10% 11% 3	3960
January	1st to 10th	Night	78% 4 8% 7% 3	6349
	11th to 20th	Night	72% 4 8% 10% 4 1	6297
	21st to end	Night	70% 4 7% 10% 8% 1	6490
February	1st to 10th	Night	67% 3 10% 14% 7%	2600
	11th to 20th	Night	70% 2 9% 13% 5% 1	2509
	21st to end	Night	66% 2 9% 14% 8% 1	1980
March	1st to 10th	Night	64% 4 10% 15% 7%	5200
	11th to 20th	Night	56% 2 16% 17% 6% 3	4980
	21st to end	Night	62% 3 5% 18% 10% 1	5248
April	1st to 10th	Night	57% 3 10% 19% 9% 1	2000
	11th to 20th	Night	54% 3 10% 19% 14% 1	1975
	21st to end	Night	50% 3 10% 21% 14% 3	1985
May	1st to 10th	Night	64% 3 8% 15% 8% 2	3766
	11th to 20th	Night	49% 5% 14% 21% 11% 1	3576
	21st to end	Night	54% 3 11% 19% 12% 1	3927
June	1st to 10th	Night	46% 2 9% 26% 15% 1	1600
	11th to 20th	Night	43% 4 14% 25% 12% 2	1600
	21st to end	Night	40% 5% 12% 29% 14% 1	1600
July	1st to 10th	Night	40% 7% 15% 25% 13% 1	3560
	11th to 20th	Night	38% 7% 14% 26% 14% 1	3552
	21st to end	Night	35% 6% 12% 27% 18% 1	3869
August	1st to 10th	Night	46% 5% 14% 25% 9% 1	2904
	11th to 20th	Night	43% 5% 15% 28% 10%	2968
	21st to end	Night	41% 4 12% 32% 10% 1	3586
September	1st to 10th	Night	41% 4 13% 26% 15% 2	2000
	11th to 20th	Night	36% 3 12% 31% 16% 1	2000
	21st to end	Night	48% 4 11% 23% 13% 1	2273
October	1st to 10th	Night	50% 5% 9% 24% 11%	3920
	11th to 20th	Night	45% 4 12% 21% 14% 3	3899
	21st to end	Night	46% 4 13% 26% 9%	4343
November	1st to 10th	Night	51% 3 10% 22% 13% 2	2564
	11th to 20th	Night	58% 2 9% 20% 9% 2	2716
	21st to end	Night	61% 3 14% 18% 4	2800
December	1st to 10th	Night	71% 6% 10% 11% 2	4580
	11th to 20th	Night	65% 4 9% 13% 9%	5208
	21st to end	Night	73% 3 7% 12% 4	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	34% 21% 33% 10% 2 1	200
	11th to 20th	Day	40% 24% 26% 7% 1 1	210
	21st to end	Day	30% 25% 33% 10% 2	330
February	1st to 10th	Day	38% 26% 28% 6% 2	300
	11th to 20th	Day	38% 26% 30% 7%	325
	21st to end	Day	37% 22% 29% 10% 2	414
March	1st to 10th	Day	35% 20% 27% 13% 3 1	500
	11th to 20th	Day	36% 19% 28% 13% 4	500
	21st to end	Day	34% 22% 27% 11% 5% 1	550
April	1st to 10th	Day	36% 17% 27% 14% 4 1	500
	11th to 20th	Day	36% 19% 25% 10% 7% 3	492
	21st to end	Day	32% 17% 30% 13% 7% 1	500
May	1st to 10th	Day	30% 16% 30% 15% 6% 3	503
	11th to 20th	Day	24% 12% 33% 19% 10% 2	527
	21st to end	Day	21% 11% 31% 20% 13% 3	573
June	1st to 10th	Day	22% 14% 30% 21% 11% 2	530
	11th to 20th	Day	21% 10% 30% 26% 11% 2	530
	21st to end	Day	18% 12% 34% 24% 10% 2	530
July	1st to 10th	Day	13% 8% 32% 27% 15% 4	525
	11th to 20th	Day	12% 7% 28% 31% 18% 4	524
	21st to end	Day	13% 7% 36% 28% 12% 4	576
August	1st to 10th	Day	14% 6% 28% 27% 19% 5%	526
	11th to 20th	Day	17% 8% 26% 27% 13% 8%	519
	21st to end	Day	17% 9% 27% 22% 16% 8%	583
September	1st to 10th	Day	19% 11% 23% 23% 15% 9%	530
	11th to 20th	Day	15% 6% 25% 23% 21% 11%	530
	21st to end	Day	23% 11% 29% 21% 12% 4	530
October	1st to 10th	Day	25% 9% 26% 20% 15% 5%	530
	11th to 20th	Day	27% 16% 28% 19% 8% 2	530
	21st to end	Day	34% 15% 29% 17% 4	552
November	1st to 10th	Day	27% 19% 23% 20% 10% 1	401
	11th to 20th	Day	33% 22% 28% 14% 4	280
	21st to end	Day	41% 16% 33% 10% 1	200
December	1st to 10th	Day	36% 20% 33% 9% 2 1	200
	11th to 20th	Day	37% 26% 27% 9% 2	200
	21st to end	Day	39% 15% 38% 8%	220
January	1st to 10th	Night	29% 23% 29% 16% 2	800
	11th to 20th	Night	30% 17% 26% 22% 4	800
	21st to end	Night	26% 20% 30% 21% 3	880
February	1st to 10th	Night	27% 21% 28% 23% 2	800
	11th to 20th	Night	30% 22% 29% 16% 2	783
	21st to end	Night	34% 18% 27% 18% 3	660
March	1st to 10th	Night	28% 22% 26% 20% 4	800
	11th to 20th	Night	29% 17% 28% 21% 5%	800
	21st to end	Night	29% 19% 24% 20% 7%	880
April	1st to 10th	Night	31% 16% 28% 23% 4	800
	11th to 20th	Night	31% 15% 24% 22% 8% 1	791
	21st to end	Night	25% 13% 27% 26% 8%	785
May	1st to 10th	Night	29% 7% 19% 33% 11% 1	475
	11th to 20th	Night	15% 10% 29% 34% 11% 1	394
	21st to end	Night	16% 5% 26% 38% 15% 1	433
June	1st to 10th	Night	20% 6% 21% 39% 13% 1	400
	11th to 20th	Night	16% 8% 20% 41% 15% 1	400
	21st to end	Night	15% 4 22% 43% 16%	400
July	1st to 10th	Night	8% 2 20% 47% 19% 3	390
	11th to 20th	Night	13% 2 16% 46% 21% 1	388
	21st to end	Night	15% 3 16% 42% 22% 2	462
August	1st to 10th	Night	13% 2 17% 40% 25% 3	597
	11th to 20th	Night	13% 4 15% 36% 29% 4	634
	21st to end	Night	17% 4 18% 37% 23% 2	880
September	1st to 10th	Night	18% 6% 22% 32% 21% 2	800
	11th to 20th	Night	17% 5% 15% 30% 28% 5%	800
	21st to end	Night	20% 8% 25% 27% 19% 2	800
October	1st to 10th	Night	23% 9% 24% 26% 18% 1	800
	11th to 20th	Night	27% 11% 26% 27% 8% 1	800
	21st to end	Night	35% 13% 25% 22% 5%	880
November	1st to 10th	Night	25% 13% 27% 26% 9%	789
	11th to 20th	Night	30% 15% 28% 19% 6% 1	800
	21st to end	Night	29% 17% 27% 22% 4 1	800
December	1st to 10th	Night	35% 19% 25% 17% 4	800
	11th to 20th	Night	31% 20% 27% 20% 3	800
	21st to end	Night	34% 19% 27% 18% 2	880

Page generated at 2025-10-31 02:02:37. Version ac07

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