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 2024-12-24.

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	14% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	101283
	Night	15% 11% 4 8% 5% 4 4 3 5% 7% 34%	98826
Transparency	Day	47% 7% 12% 21% 12% 1	104897
	Night	48% 6% 12% 21% 12% 1	103353
Seeing	Day	18% 12% 29% 22% 14% 6%	15683
	Night	19% 10% 29% 29% 12% 1	25756

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	16% 21% 6% 11% 6% 4 4 4 5% 4 20%	2527
	11th to 20th	Day	16% 20% 5% 13% 9% 7% 5% 3 4 4 12%	2530
	21st to end	Day	28% 22% 6% 10% 5% 4 4 2 3 4 14%	2977
February	1st to 10th	Day	14% 15% 5% 12% 8% 5% 5% 4 5% 5% 22%	1710
	11th to 20th	Day	15% 13% 5% 13% 7% 6% 4 3 5% 5% 24%	1819
	21st to end	Day	15% 17% 5% 11% 6% 4 4 4 4 6% 24%	1570
March	1st to 10th	Day	17% 16% 4 9% 5% 4 4 4 4 6% 28%	3151
	11th to 20th	Day	12% 12% 5% 10% 6% 5% 5% 3 4 6% 32%	3480
	21st to end	Day	18% 14% 4 9% 5% 5% 4 3 4 6% 28%	3815
April	1st to 10th	Day	17% 13% 3 8% 5% 5% 5% 3 5% 6% 30%	2247
	11th to 20th	Day	16% 9% 4 7% 6% 4 4 4 5% 7% 35%	2385
	21st to end	Day	17% 9% 3 8% 5% 4 4 3 4 7% 34%	2470
May	1st to 10th	Day	18% 9% 3 6% 5% 5% 4 3 6% 6% 35%	4082
	11th to 20th	Day	15% 8% 2 6% 5% 5% 5% 4 7% 8% 36%	4081
	21st to end	Day	10% 7% 3 6% 4 4 4 4 5% 8% 45%	4558
June	1st to 10th	Day	10% 6% 3 6% 4 5% 5% 4 6% 8% 43%	2660
	11th to 20th	Day	11% 6% 2 7% 6% 5% 5% 4 7% 8% 40%	2660
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 8% 10% 38%	2660
July	1st to 10th	Day	10% 6% 3 8% 5% 5% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	7% 5% 2 5% 4 4 4 5% 7% 10% 47%	4440
	21st to end	Day	7% 6% 2 7% 6% 6% 5% 5% 7% 9% 41%	4624
August	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 5% 5% 9% 41%	4104
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 4 7% 9% 40%	4044
	21st to end	Day	8% 6% 3 7% 5% 5% 5% 5% 7% 10% 40%	4418
September	1st to 10th	Day	10% 7% 2 8% 5% 4 5% 4 6% 8% 40%	2171
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 3 5% 8% 48%	2090
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 7% 6% 37%	2090
October	1st to 10th	Day	13% 7% 3 8% 5% 4 4 4 5% 7% 41%	2062
	11th to 20th	Day	18% 9% 3 8% 5% 4 5% 4 4 6% 33%	1900
	21st to end	Day	20% 11% 3 8% 5% 4 4 4 5% 7% 29%	1900
November	1st to 10th	Day	12% 10% 2 9% 6% 4 4 3 5% 7% 37%	1762
	11th to 20th	Day	15% 14% 5% 10% 5% 4 4 4 5% 6% 28%	1800
	21st to end	Day	19% 14% 6% 8% 5% 4 3 3 4 6% 26%	1781
December	1st to 10th	Day	14% 17% 7% 12% 7% 5% 5% 4 4 6% 20%	1600
	11th to 20th	Day	15% 18% 6% 11% 5% 5% 5% 5% 6% 6% 20%	1868
	21st to end	Day	24% 23% 7% 12% 5% 4 3 3 4 4 11%	2807
January	1st to 10th	Night	16% 17% 6% 13% 6% 5% 4 3 4 5% 22%	4428
	11th to 20th	Night	15% 17% 7% 15% 6% 7% 6% 4 5% 5% 14%	4440
	21st to end	Night	24% 18% 8% 10% 5% 4 4 2 4 4 16%	4681
February	1st to 10th	Night	17% 16% 5% 10% 7% 5% 4 4 4 5% 22%	2470
	11th to 20th	Night	14% 14% 5% 10% 7% 6% 5% 3 5% 5% 26%	2242
	21st to end	Night	17% 12% 5% 11% 6% 4 4 3 5% 6% 28%	1884
March	1st to 10th	Night	15% 12% 4 9% 4 4 3 3 4 7% 34%	3701
	11th to 20th	Night	9% 9% 3 6% 4 4 5% 4 5% 8% 42%	3480
	21st to end	Night	16% 9% 4 9% 5% 5% 4 3 5% 5% 35%	3817
April	1st to 10th	Night	15% 11% 3 7% 4 5% 4 4 5% 8% 35%	1951
	11th to 20th	Night	19% 9% 3 6% 4 5% 4 3 6% 7% 36%	1744
	21st to end	Night	18% 8% 2 5% 5% 4 4 4 5% 6% 38%	1710
May	1st to 10th	Night	15% 10% 3 6% 5% 4 5% 4 6% 7% 35%	2813
	11th to 20th	Night	16% 7% 3 6% 4 3 4 3 5% 7% 42%	2812
	21st to end	Night	10% 6% 3 5% 4 4 4 3 5% 9% 47%	2996
June	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 5% 9% 47%	1520
	11th to 20th	Night	11% 5% 3 6% 4 4 4 3 7% 9% 44%	1520
	21st to end	Night	9% 6% 3 5% 4 3 4 4 7% 9% 47%	1520
July	1st to 10th	Night	8% 3 2 5% 4 5% 5% 5% 7% 10% 46%	2530
	11th to 20th	Night	9% 4 2 4 3 4 4 3 6% 8% 54%	2530
	21st to end	Night	5% 5% 2 7% 5% 4 4 4 7% 9% 47%	2930
August	1st to 10th	Night	10% 6% 2 6% 3 4 4 4 5% 10% 46%	2825
	11th to 20th	Night	10% 6% 2 5% 4 3 4 4 6% 7% 49%	2785
	21st to end	Night	9% 6% 2 7% 4 4 4 3 5% 7% 47%	3220
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 48%	1988
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 5% 6% 55%	2090
	21st to end	Night	14% 8% 2 7% 4 4 4 4 4 8% 42%	2090
October	1st to 10th	Night	14% 6% 3 6% 4 4 3 3 5% 6% 46%	2118
	11th to 20th	Night	18% 10% 3 8% 4 4 4 4 4 5% 34%	2280
	21st to end	Night	21% 10% 3 7% 4 4 4 4 5% 5% 32%	2693
November	1st to 10th	Night	14% 10% 4 7% 4 4 4 3 4 6% 41%	2536
	11th to 20th	Night	15% 12% 4 8% 5% 5% 6% 4 6% 7% 28%	2600
	21st to end	Night	19% 16% 4 8% 5% 4 3 4 5% 6% 27%	2611
December	1st to 10th	Night	17% 17% 5% 12% 7% 5% 4 4 4 5% 20%	2800
	11th to 20th	Night	15% 16% 5% 12% 7% 5% 5% 3 5% 5% 21%	3538
	21st to end	Night	25% 25% 7% 10% 4 4 3 3 3 4 12%	4933

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% 10% 13% 5%	2528
	11th to 20th	Day	72% 8% 11% 7% 3	2530
	21st to end	Day	76% 4 10% 7% 3 1	2977
February	1st to 10th	Day	62% 6% 10% 14% 8%	1710
	11th to 20th	Day	62% 5% 8% 15% 8% 1	1829
	21st to end	Day	60% 5% 11% 16% 7% 2	1570
March	1st to 10th	Day	57% 7% 11% 18% 7%	3151
	11th to 20th	Day	51% 7% 13% 20% 8% 1	3480
	21st to end	Day	57% 5% 11% 15% 9% 2	3815
April	1st to 10th	Day	53% 6% 10% 19% 12%	2247
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2385
	21st to end	Day	49% 6% 13% 17% 14% 1	2470
May	1st to 10th	Day	47% 7% 11% 22% 12% 2	4120
	11th to 20th	Day	43% 9% 14% 24% 9% 1	4085
	21st to end	Day	36% 6% 11% 20% 23% 4	4571
June	1st to 10th	Day	36% 8% 13% 27% 13% 3	2660
	11th to 20th	Day	38% 7% 12% 24% 17% 2	2660
	21st to end	Day	38% 10% 13% 22% 16% 2	2660
July	1st to 10th	Day	40% 9% 15% 20% 15% 2	4371
	11th to 20th	Day	29% 7% 15% 31% 16% 1	4365
	21st to end	Day	35% 9% 14% 20% 21% 1	4478
August	1st to 10th	Day	38% 8% 13% 24% 16% 1	4091
	11th to 20th	Day	37% 8% 11% 31% 12% 1	4083
	21st to end	Day	36% 9% 14% 28% 12% 1	4417
September	1st to 10th	Day	39% 8% 13% 25% 13% 2	2171
	11th to 20th	Day	33% 6% 12% 32% 16% 1	2090
	21st to end	Day	43% 7% 13% 24% 13%	2090
October	1st to 10th	Day	31% 6% 10% 31% 21% 1	3426
	11th to 20th	Day	40% 6% 12% 25% 18%	3140
	21st to end	Day	46% 5% 14% 24% 11% 1	3112
November	1st to 10th	Day	46% 6% 12% 23% 12% 1	1762
	11th to 20th	Day	55% 5% 12% 19% 8% 1	1800
	21st to end	Day	58% 5% 13% 17% 6%	1781
December	1st to 10th	Day	66% 6% 11% 14% 3	1600
	11th to 20th	Day	62% 8% 12% 14% 4	1865
	21st to end	Day	77% 5% 9% 6% 3	2807
January	1st to 10th	Night	64% 6% 11% 14% 5% 1	4428
	11th to 20th	Night	70% 7% 10% 9% 4	4440
	21st to end	Night	72% 5% 10% 10% 3	4681
February	1st to 10th	Night	63% 6% 11% 13% 7% 1	2470
	11th to 20th	Night	58% 6% 10% 17% 8% 1	2243
	21st to end	Night	56% 5% 14% 16% 8% 2	1884
March	1st to 10th	Night	49% 5% 14% 25% 7% 1	3701
	11th to 20th	Night	37% 8% 14% 22% 18% 1	3480
	21st to end	Night	49% 7% 13% 20% 10% 1	3817
April	1st to 10th	Night	47% 6% 11% 20% 13% 2	1951
	11th to 20th	Night	46% 6% 11% 17% 18% 2	1744
	21st to end	Night	46% 6% 13% 18% 15% 3	1710
May	1st to 10th	Night	45% 7% 11% 21% 13% 3	2840
	11th to 20th	Night	41% 7% 10% 27% 13% 2	2813
	21st to end	Night	34% 8% 12% 22% 20% 5%	3004
June	1st to 10th	Night	34% 7% 10% 29% 17% 3	1520
	11th to 20th	Night	34% 8% 14% 24% 17% 2	1520
	21st to end	Night	33% 7% 14% 27% 19% 1	1520
July	1st to 10th	Night	31% 8% 13% 28% 14% 6%	2490
	11th to 20th	Night	28% 5% 13% 32% 21% 2	2482
	21st to end	Night	31% 9% 17% 18% 23% 2	2837
August	1st to 10th	Night	33% 7% 14% 28% 17% 1	2814
	11th to 20th	Night	32% 8% 10% 37% 12% 2	2812
	21st to end	Night	34% 8% 11% 31% 15% 1	3220
September	1st to 10th	Night	33% 8% 12% 27% 18% 2	1988
	11th to 20th	Night	30% 5% 10% 30% 22% 2	2090
	21st to end	Night	40% 6% 13% 26% 14% 1	2090
October	1st to 10th	Night	26% 4 11% 33% 26% 1	3534
	11th to 20th	Night	43% 5% 11% 22% 18%	3779
	21st to end	Night	40% 5% 11% 31% 13% 1	4431
November	1st to 10th	Night	44% 5% 11% 25% 14% 1	2536
	11th to 20th	Night	53% 6% 14% 18% 8% 1	2600
	21st to end	Night	58% 6% 12% 18% 6%	2611
December	1st to 10th	Night	65% 6% 11% 12% 5%	2800
	11th to 20th	Night	62% 7% 12% 13% 6% 1	3540
	21st to end	Night	75% 5% 10% 7% 2	4933

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% 17% 34% 17% 7%	270
	11th to 20th	Day	25% 16% 43% 11% 3 1	270
	21st to end	Day	31% 15% 33% 16% 4 1	297
February	1st to 10th	Day	24% 22% 37% 13% 3 1	270
	11th to 20th	Day	25% 21% 37% 14% 2	270
	21st to end	Day	27% 28% 34% 9% 2	225
March	1st to 10th	Day	26% 21% 36% 15% 2	288
	11th to 20th	Day	22% 13% 35% 22% 7% 1	460
	21st to end	Day	27% 16% 32% 19% 6% 1	506
April	1st to 10th	Day	21% 16% 33% 21% 8% 1	460
	11th to 20th	Day	25% 19% 31% 14% 6% 5%	452
	21st to end	Day	23% 15% 31% 20% 10% 2	460
May	1st to 10th	Day	25% 14% 29% 19% 11% 2	463
	11th to 20th	Day	19% 9% 29% 22% 13% 8%	487
	21st to end	Day	13% 7% 24% 27% 20% 8%	572
June	1st to 10th	Day	12% 8% 29% 29% 16% 5%	680
	11th to 20th	Day	13% 8% 31% 27% 18% 4	680
	21st to end	Day	12% 10% 29% 29% 16% 5%	680
July	1st to 10th	Day	8% 6% 23% 28% 25% 11%	671
	11th to 20th	Day	11% 6% 27% 27% 19% 9%	668
	21st to end	Day	8% 5% 24% 31% 22% 10%	625
August	1st to 10th	Day	9% 4 21% 25% 25% 15%	486
	11th to 20th	Day	14% 5% 16% 26% 26% 13%	479
	21st to end	Day	12% 4 20% 27% 25% 12%	539
September	1st to 10th	Day	12% 8% 22% 23% 22% 12%	490
	11th to 20th	Day	10% 5% 18% 24% 28% 16%	490
	21st to end	Day	19% 10% 22% 23% 18% 9%	490
October	1st to 10th	Day	16% 10% 28% 23% 16% 7%	490
	11th to 20th	Day	19% 18% 27% 27% 9% 1	490
	21st to end	Day	30% 17% 33% 15% 5%	349
November	1st to 10th	Day	16% 20% 35% 19% 8% 1	300
	11th to 20th	Day	22% 23% 33% 16% 6%	300
	21st to end	Day	27% 19% 39% 12% 3	290
December	1st to 10th	Day	25% 19% 33% 16% 8% 1	200
	11th to 20th	Day	24% 25% 37% 10% 4	230
	21st to end	Day	27% 21% 37% 14% 1	306
January	1st to 10th	Night	22% 16% 36% 21% 5% 1	950
	11th to 20th	Night	25% 15% 34% 21% 4 1	950
	21st to end	Night	21% 12% 35% 26% 5%	920
February	1st to 10th	Night	26% 13% 36% 22% 3	760
	11th to 20th	Night	22% 15% 36% 24% 4	743
	21st to end	Night	24% 16% 38% 20% 2	628
March	1st to 10th	Night	21% 13% 36% 24% 6% 1	760
	11th to 20th	Night	18% 16% 36% 24% 5% 1	760
	21st to end	Night	24% 11% 31% 27% 6% 1	836
April	1st to 10th	Night	20% 14% 36% 25% 4 1	621
	11th to 20th	Night	23% 10% 35% 23% 7% 1	563
	21st to end	Night	22% 7% 34% 29% 8%	570
May	1st to 10th	Night	21% 6% 32% 32% 8% 1	570
	11th to 20th	Night	22% 5% 28% 32% 12% 1	561
	21st to end	Night	12% 5% 24% 38% 19% 2	617
June	1st to 10th	Night	14% 4 24% 43% 15% 1	570
	11th to 20th	Night	13% 4 23% 43% 18%	570
	21st to end	Night	11% 4 21% 45% 18% 1	570
July	1st to 10th	Night	9% 2 17% 40% 28% 4	555
	11th to 20th	Night	11% 3 20% 39% 26% 3	552
	21st to end	Night	5% 3 20% 47% 24% 2	624
August	1st to 10th	Night	9% 1 16% 43% 28% 3	567
	11th to 20th	Night	10% 2 19% 38% 27% 4	564
	21st to end	Night	11% 4 16% 37% 29% 4	627
September	1st to 10th	Night	12% 4 21% 34% 26% 3	658
	11th to 20th	Night	12% 4 17% 33% 30% 4	760
	21st to end	Night	16% 8% 20% 32% 20% 4	760
October	1st to 10th	Night	17% 6% 28% 30% 17% 2	760
	11th to 20th	Night	24% 13% 28% 26% 8% 1	760
	21st to end	Night	26% 11% 33% 24% 6%	836
November	1st to 10th	Night	19% 12% 31% 27% 11% 1	780
	11th to 20th	Night	22% 16% 35% 20% 7% 1	800
	21st to end	Night	26% 14% 33% 22% 5%	800
December	1st to 10th	Night	22% 17% 35% 20% 5%	800
	11th to 20th	Night	24% 14% 38% 21% 3	971
	21st to end	Night	33% 12% 34% 19% 2	1063

Page generated at 2024-12-21 04:13:49. Version ac07

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