Irvington Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Irvington Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	13% 11% 4 9% 5% 5% 5% 4 5% 7% 33%	130647
	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 33%	131648
Transparency	Day	48% 7% 12% 20% 12% 1	134216
	Night	49% 6% 11% 20% 12% 1	136151
Seeing	Day	18% 13% 29% 22% 13% 5%	17424
	Night	19% 10% 30% 28% 11% 1	27576

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	13% 18% 7% 16% 6% 5% 4 3 4 5% 19%	4691
	11th to 20th	Day	12% 17% 8% 18% 11% 10% 4 3 3 3 11%	4710
	21st to end	Day	18% 18% 5% 12% 9% 5% 7% 3 4 4 15%	5529
February	1st to 10th	Day	13% 14% 6% 12% 8% 5% 5% 4 5% 5% 22%	1890
	11th to 20th	Day	14% 13% 6% 13% 8% 6% 4 3 4 5% 23%	2017
	21st to end	Day	14% 16% 6% 12% 6% 4 4 4 5% 6% 24%	1730
March	1st to 10th	Day	15% 12% 4 10% 6% 5% 4 4 4 5% 31%	4626
	11th to 20th	Day	12% 12% 5% 9% 5% 5% 4 3 4 7% 34%	5090
	21st to end	Day	17% 16% 4 9% 5% 4 4 3 4 6% 28%	5586
April	1st to 10th	Day	18% 13% 3 8% 5% 5% 5% 3 5% 6% 30%	2483
	11th to 20th	Day	16% 10% 4 7% 5% 4 4 4 5% 7% 35%	2587
	21st to end	Day	17% 9% 3 7% 5% 4 4 4 4 8% 35%	2600
May	1st to 10th	Day	19% 10% 3 7% 4 5% 4 3 5% 6% 34%	5862
	11th to 20th	Day	15% 9% 2 7% 5% 5% 5% 4 6% 7% 34%	5861
	21st to end	Day	14% 8% 3 6% 4 4 4 4 5% 8% 41%	6557
June	1st to 10th	Day	10% 6% 3 6% 4 5% 5% 4 6% 8% 43%	2800
	11th to 20th	Day	11% 6% 2 7% 6% 5% 5% 4 6% 9% 40%	2800
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 10% 38%	2800
July	1st to 10th	Day	9% 6% 2 7% 4 5% 6% 5% 7% 10% 38%	6360
	11th to 20th	Day	7% 6% 2 6% 4 4 5% 4 7% 9% 46%	6360
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 9% 45%	6652
August	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 5% 6% 9% 41%	4234
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 4 7% 9% 40%	4174
	21st to end	Day	8% 6% 3 7% 5% 5% 5% 5% 7% 10% 40%	4558
September	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 4 6% 9% 41%	2285
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 3 6% 8% 49%	2200
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 7% 6% 37%	2200
October	1st to 10th	Day	13% 7% 2 7% 4 4 4 4 5% 6% 43%	2170
	11th to 20th	Day	18% 9% 3 8% 5% 4 5% 4 4 6% 33%	2000
	21st to end	Day	20% 11% 3 8% 6% 4 4 4 5% 7% 30%	2000
November	1st to 10th	Day	13% 10% 2 8% 6% 4 4 3 5% 7% 37%	1852
	11th to 20th	Day	15% 14% 5% 10% 4 4 4 4 5% 6% 29%	1890
	21st to end	Day	18% 14% 6% 8% 6% 4 4 3 4 6% 26%	1870
December	1st to 10th	Day	13% 16% 8% 13% 8% 5% 5% 4 5% 6% 18%	2680
	11th to 20th	Day	16% 22% 8% 11% 6% 4 4 4 4 4 16%	2873
	21st to end	Day	24% 22% 7% 12% 5% 4 3 3 3 4 13%	4070
January	1st to 10th	Night	14% 16% 6% 15% 6% 5% 4 3 4 5% 22%	8232
	11th to 20th	Night	10% 16% 7% 16% 9% 8% 6% 4 5% 5% 14%	8280
	21st to end	Night	14% 16% 7% 12% 6% 6% 6% 3 5% 5% 20%	8662
February	1st to 10th	Night	16% 15% 5% 11% 7% 5% 5% 3 4 5% 23%	2730
	11th to 20th	Night	14% 14% 5% 11% 7% 5% 5% 3 4 5% 27%	2484
	21st to end	Night	16% 12% 5% 10% 6% 4 4 4 5% 6% 28%	2076
March	1st to 10th	Night	15% 11% 4 9% 5% 4 4 3 5% 7% 35%	5446
	11th to 20th	Night	10% 9% 4 6% 4 4 5% 4 5% 8% 41%	5090
	21st to end	Night	16% 10% 4 8% 5% 4 4 3 5% 5% 35%	5588
April	1st to 10th	Night	16% 11% 3 7% 4 5% 4 4 5% 7% 34%	2156
	11th to 20th	Night	19% 9% 3 7% 4 5% 4 3 6% 6% 35%	1894
	21st to end	Night	17% 8% 2 5% 6% 4 4 4 6% 7% 38%	1800
May	1st to 10th	Night	19% 11% 3 8% 5% 4 4 3 5% 6% 32%	4033
	11th to 20th	Night	17% 9% 3 6% 4 3 4 3 5% 6% 40%	4032
	21st to end	Night	13% 6% 3 5% 4 4 5% 3 5% 8% 44%	4299
June	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 5% 9% 47%	1600
	11th to 20th	Night	11% 6% 3 6% 4 4 4 3 7% 9% 44%	1600
	21st to end	Night	9% 5% 3 5% 4 3 4 4 7% 9% 47%	1600
July	1st to 10th	Night	7% 4 2 5% 4 5% 5% 5% 7% 10% 46%	3618
	11th to 20th	Night	8% 5% 2 5% 3 4 5% 3 6% 9% 50%	3617
	21st to end	Night	8% 7% 2 6% 4 4 4 4 5% 8% 48%	4205
August	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 5% 10% 47%	2915
	11th to 20th	Night	9% 6% 2 5% 4 3 4 4 6% 7% 49%	2875
	21st to end	Night	8% 6% 2 7% 4 4 4 3 5% 7% 48%	3322
September	1st to 10th	Night	10% 6% 2 6% 4 3 4 4 5% 7% 49%	2093
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 5% 6% 56%	2200
	21st to end	Night	13% 8% 2 7% 4 4 4 4 4 8% 42%	2200
October	1st to 10th	Night	14% 6% 3 6% 4 4 3 3 4 6% 47%	2230
	11th to 20th	Night	18% 10% 3 8% 4 4 4 4 4 5% 35%	2400
	21st to end	Night	20% 10% 3 7% 4 4 4 4 5% 5% 34%	2835
November	1st to 10th	Night	13% 10% 4 7% 4 4 4 3 4 6% 41%	2666
	11th to 20th	Night	15% 12% 4 8% 5% 5% 6% 4 6% 7% 29%	2730
	21st to end	Night	19% 16% 4 9% 5% 4 3 4 5% 6% 26%	2742
December	1st to 10th	Night	12% 15% 6% 16% 6% 6% 5% 3 4 5% 21%	4720
	11th to 20th	Night	15% 20% 6% 13% 7% 5% 5% 3 5% 5% 18%	5528
	21st to end	Night	23% 24% 7% 11% 4 4 3 2 3 4 14%	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	66% 6% 11% 13% 4	4692
	11th to 20th	Day	76% 6% 8% 5% 4	4710
	21st to end	Day	71% 5% 10% 9% 4	5529
February	1st to 10th	Day	62% 6% 9% 14% 7%	1890
	11th to 20th	Day	63% 5% 9% 15% 8% 1	2027
	21st to end	Day	60% 5% 11% 16% 7% 1	1730
March	1st to 10th	Day	53% 6% 12% 17% 9% 2	4626
	11th to 20th	Day	48% 6% 15% 21% 8% 2	5090
	21st to end	Day	58% 5% 10% 15% 11% 1	5586
April	1st to 10th	Day	54% 6% 11% 18% 11%	2483
	11th to 20th	Day	49% 6% 11% 21% 12% 2	2587
	21st to end	Day	48% 6% 14% 18% 13% 1	2600
May	1st to 10th	Day	49% 6% 10% 19% 13% 2	5897
	11th to 20th	Day	46% 8% 12% 21% 12% 1	5865
	21st to end	Day	41% 5% 11% 22% 19% 3	6571
June	1st to 10th	Day	36% 8% 13% 27% 14% 2	2800
	11th to 20th	Day	39% 8% 12% 24% 16% 2	2800
	21st to end	Day	37% 10% 13% 22% 16% 2	2800
July	1st to 10th	Day	36% 9% 15% 24% 14% 2	6291
	11th to 20th	Day	31% 7% 15% 28% 18% 1	6285
	21st to end	Day	33% 8% 14% 24% 20% 1	6463
August	1st to 10th	Day	37% 8% 13% 25% 16% 1	4221
	11th to 20th	Day	37% 8% 11% 31% 12% 1	4213
	21st to end	Day	36% 8% 14% 29% 12% 1	4557
September	1st to 10th	Day	38% 8% 13% 26% 13% 2	2285
	11th to 20th	Day	32% 7% 12% 32% 16% 1	2200
	21st to end	Day	43% 7% 12% 23% 14%	2200
October	1st to 10th	Day	31% 6% 10% 31% 21% 1	3534
	11th to 20th	Day	40% 6% 12% 25% 18%	3240
	21st to end	Day	46% 5% 14% 23% 12% 1	3212
November	1st to 10th	Day	46% 6% 12% 24% 12% 1	1852
	11th to 20th	Day	55% 5% 12% 19% 7% 2	1890
	21st to end	Day	59% 5% 12% 17% 6%	1870
December	1st to 10th	Day	65% 8% 14% 10% 3	2680
	11th to 20th	Day	70% 6% 9% 12% 3	2870
	21st to end	Day	75% 4 8% 8% 4	4070
January	1st to 10th	Night	63% 6% 10% 15% 6% 1	8233
	11th to 20th	Night	69% 6% 10% 9% 5%	8280
	21st to end	Night	64% 6% 10% 14% 6%	8662
February	1st to 10th	Night	62% 6% 10% 14% 7% 1	2730
	11th to 20th	Night	58% 6% 10% 18% 8% 1	2485
	21st to end	Night	56% 5% 14% 16% 8% 2	2076
March	1st to 10th	Night	48% 5% 14% 22% 10% 1	5446
	11th to 20th	Night	39% 6% 14% 25% 15% 1	5090
	21st to end	Night	49% 6% 12% 18% 13% 1	5588
April	1st to 10th	Night	48% 6% 11% 19% 13% 2	2156
	11th to 20th	Night	47% 6% 11% 17% 17% 2	1894
	21st to end	Night	45% 6% 13% 18% 15% 3	1800
May	1st to 10th	Night	52% 6% 10% 16% 13% 4	4060
	11th to 20th	Night	45% 7% 9% 25% 11% 3	4033
	21st to end	Night	37% 7% 11% 24% 17% 4	4307
June	1st to 10th	Night	34% 7% 10% 28% 19% 3	1600
	11th to 20th	Night	35% 8% 14% 25% 16% 2	1600
	21st to end	Night	32% 7% 14% 27% 19% 1	1600
July	1st to 10th	Night	30% 8% 14% 28% 15% 5%	3580
	11th to 20th	Night	30% 7% 14% 29% 19% 2	3572
	21st to end	Night	33% 7% 13% 21% 24% 2	4082
August	1st to 10th	Night	32% 7% 15% 28% 18% 1	2904
	11th to 20th	Night	32% 8% 10% 37% 11% 2	2902
	21st to end	Night	34% 8% 11% 31% 15% 1	3322
September	1st to 10th	Night	33% 8% 12% 28% 17% 2	2093
	11th to 20th	Night	29% 5% 11% 31% 22% 2	2200
	21st to end	Night	40% 6% 14% 26% 15% 1	2200
October	1st to 10th	Night	26% 4 10% 33% 26% 1	3646
	11th to 20th	Night	43% 5% 11% 22% 18%	3899
	21st to end	Night	40% 5% 11% 31% 13% 1	4573
November	1st to 10th	Night	44% 6% 11% 25% 14% 1	2666
	11th to 20th	Night	52% 6% 14% 19% 8% 1	2730
	21st to end	Night	59% 6% 11% 17% 7%	2742
December	1st to 10th	Night	63% 7% 13% 13% 3	4720
	11th to 20th	Night	67% 6% 10% 12% 5%	5530
	21st to end	Night	74% 4 8% 7% 5%	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	25% 18% 36% 15% 6%	330
	11th to 20th	Day	26% 21% 40% 10% 2 1	330
	21st to end	Day	30% 17% 34% 14% 4 1	363
February	1st to 10th	Day	24% 23% 36% 12% 3 1	330
	11th to 20th	Day	26% 22% 36% 13% 3	330
	21st to end	Day	27% 29% 33% 10% 1	273
March	1st to 10th	Day	27% 21% 36% 14% 2	350
	11th to 20th	Day	24% 16% 34% 20% 6% 1	540
	21st to end	Day	27% 17% 32% 18% 5% 1	594
April	1st to 10th	Day	23% 17% 34% 19% 7% 1	540
	11th to 20th	Day	25% 19% 31% 14% 7% 4	516
	21st to end	Day	23% 14% 30% 20% 10% 2	500
May	1st to 10th	Day	24% 15% 30% 19% 10% 2	503
	11th to 20th	Day	20% 10% 29% 22% 12% 7%	527
	21st to end	Day	15% 9% 24% 26% 18% 8%	619
June	1st to 10th	Day	13% 8% 30% 29% 15% 4	730
	11th to 20th	Day	13% 8% 31% 27% 18% 4	730
	21st to end	Day	12% 9% 28% 30% 16% 5%	730
July	1st to 10th	Day	8% 6% 24% 28% 24% 10%	721
	11th to 20th	Day	11% 7% 28% 27% 19% 9%	718
	21st to end	Day	9% 5% 23% 31% 23% 10%	674
August	1st to 10th	Day	9% 4 21% 26% 25% 16%	526
	11th to 20th	Day	13% 6% 16% 26% 27% 12%	519
	21st to end	Day	11% 5% 20% 28% 24% 12%	583
September	1st to 10th	Day	12% 8% 22% 23% 21% 13%	530
	11th to 20th	Day	9% 4 16% 24% 28% 18%	530
	21st to end	Day	18% 10% 21% 22% 19% 9%	530
October	1st to 10th	Day	15% 9% 28% 24% 17% 7%	530
	11th to 20th	Day	19% 17% 28% 27% 9% 1	530
	21st to end	Day	28% 19% 32% 16% 4	383
November	1st to 10th	Day	18% 19% 35% 19% 7% 1	330
	11th to 20th	Day	23% 24% 32% 15% 6%	330
	21st to end	Day	27% 20% 38% 12% 3	319
December	1st to 10th	Day	25% 19% 33% 15% 8%	220
	11th to 20th	Day	25% 26% 35% 9% 4	253
	21st to end	Day	29% 21% 35% 12% 1	363
January	1st to 10th	Night	21% 17% 36% 21% 4 1	1050
	11th to 20th	Night	25% 16% 35% 20% 4	1050
	21st to end	Night	21% 14% 35% 25% 5%	1016
February	1st to 10th	Night	25% 13% 36% 23% 3	840
	11th to 20th	Night	22% 16% 35% 23% 3	823
	21st to end	Night	24% 16% 38% 19% 2	692
March	1st to 10th	Night	21% 14% 36% 23% 5%	840
	11th to 20th	Night	19% 17% 36% 23% 5% 1	840
	21st to end	Night	23% 13% 32% 26% 6% 1	924
April	1st to 10th	Night	22% 15% 36% 23% 3 1	686
	11th to 20th	Night	23% 12% 34% 23% 7% 1	611
	21st to end	Night	21% 8% 34% 29% 8%	600
May	1st to 10th	Night	21% 7% 32% 31% 8% 1	600
	11th to 20th	Night	22% 6% 28% 31% 12% 1	591
	21st to end	Night	13% 5% 24% 38% 18% 2	650
June	1st to 10th	Night	14% 4 25% 42% 15% 1	600
	11th to 20th	Night	13% 4 23% 43% 18%	600
	21st to end	Night	11% 4 22% 44% 18% 1	600
July	1st to 10th	Night	9% 3 17% 41% 28% 4	585
	11th to 20th	Night	11% 3 19% 39% 25% 3	582
	21st to end	Night	6% 2 19% 48% 23% 2	657
August	1st to 10th	Night	9% 1 16% 44% 28% 3	597
	11th to 20th	Night	9% 3 19% 38% 27% 4	594
	21st to end	Night	10% 3 17% 38% 29% 3	660
September	1st to 10th	Night	11% 4 21% 33% 27% 3	693
	11th to 20th	Night	11% 4 16% 33% 32% 4	800
	21st to end	Night	15% 8% 20% 32% 20% 5%	800
October	1st to 10th	Night	17% 6% 26% 31% 18% 2	800
	11th to 20th	Night	24% 12% 28% 27% 8% 1	800
	21st to end	Night	25% 11% 33% 24% 5%	880
November	1st to 10th	Night	19% 13% 32% 26% 10% 1	820
	11th to 20th	Night	21% 16% 35% 20% 6% 1	840
	21st to end	Night	26% 14% 34% 21% 5%	840
December	1st to 10th	Night	22% 17% 35% 20% 5%	840
	11th to 20th	Night	24% 15% 38% 20% 3	1020
	21st to end	Night	33% 13% 34% 18% 2	1155

Page generated at 2026-04-14 02:44:27. Version ac07

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