Vassar
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vassar clear sky chart from 2005-11-06 to 2024-11-03.

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	14% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	100996
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 34%	97958
Transparency	Day	48% 7% 13% 20% 11% 1	100791
	Night	49% 7% 12% 20% 11% 1	97834
Seeing	Day	18% 13% 29% 23% 13% 5%	15465
	Night	19% 11% 31% 28% 10% 1	25094

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% 12% 6% 5% 3 3 4 5% 20%	2527
	11th to 20th	Day	14% 22% 7% 14% 11% 9% 4 3 3 3 9%	2530
	21st to end	Day	24% 21% 7% 11% 5% 4 4 3 3 4 14%	2952
February	1st to 10th	Day	12% 16% 6% 13% 9% 6% 5% 4 5% 6% 20%	1710
	11th to 20th	Day	12% 15% 5% 11% 7% 7% 5% 3 4 4 27%	1783
	21st to end	Day	14% 16% 5% 11% 6% 5% 5% 3 4 6% 25%	1570
March	1st to 10th	Day	17% 14% 4 8% 5% 4 4 3 5% 6% 30%	3174
	11th to 20th	Day	12% 15% 5% 10% 6% 5% 5% 3 5% 6% 29%	3480
	21st to end	Day	17% 14% 4 8% 5% 4 5% 3 4 6% 29%	3815
April	1st to 10th	Day	17% 15% 3 7% 5% 4 4 3 5% 6% 30%	2232
	11th to 20th	Day	18% 9% 3 7% 4 4 4 3 6% 6% 36%	2412
	21st to end	Day	17% 10% 3 7% 5% 5% 4 3 5% 6% 35%	2470
May	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 7% 37%	4082
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 5% 9% 37%	4081
	21st to end	Day	11% 6% 2 6% 4 4 4 3 5% 7% 47%	4594
June	1st to 10th	Day	11% 7% 2 6% 6% 4 5% 4 6% 8% 40%	2824
	11th to 20th	Day	12% 7% 2 6% 5% 4 4 3 6% 8% 43%	2850
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	2799
July	1st to 10th	Day	11% 6% 3 8% 6% 5% 6% 4 7% 8% 37%	4440
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 9% 50%	4440
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 6% 9% 43%	4639
August	1st to 10th	Day	12% 5% 2 6% 5% 5% 6% 5% 7% 8% 40%	4104
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 5% 6% 8% 42%	4044
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	4486
September	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	2164
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 6% 9% 46%	2090
	21st to end	Day	16% 7% 3 7% 5% 6% 5% 4 5% 8% 37%	2090
October	1st to 10th	Day	14% 7% 3 7% 4 4 4 4 5% 6% 40%	2038
	11th to 20th	Day	20% 11% 3 7% 4 5% 4 3 5% 7% 31%	1900
	21st to end	Day	22% 11% 4 8% 4 4 4 3 5% 7% 28%	1905
November	1st to 10th	Day	14% 10% 3 8% 6% 4 4 4 5% 8% 34%	1690
	11th to 20th	Day	14% 15% 4 10% 5% 4 5% 4 5% 6% 28%	1710
	21st to end	Day	17% 17% 6% 10% 5% 4 4 4 4 6% 24%	1557
December	1st to 10th	Day	14% 21% 7% 11% 6% 6% 4 3 5% 4 18%	1516
	11th to 20th	Day	16% 17% 4 10% 7% 4 4 4 5% 6% 23%	1675
	21st to end	Day	26% 22% 6% 12% 4 3 4 3 4 5% 11%	2623
January	1st to 10th	Night	16% 16% 8% 13% 6% 5% 4 3 4 4 23%	4428
	11th to 20th	Night	14% 18% 8% 16% 9% 9% 4 3 3 4 13%	4440
	21st to end	Night	19% 19% 8% 11% 5% 4 3 3 5% 5% 17%	4695
February	1st to 10th	Night	14% 16% 5% 10% 7% 6% 6% 3 5% 6% 23%	2470
	11th to 20th	Night	14% 15% 5% 10% 6% 5% 5% 4 5% 6% 25%	2279
	21st to end	Night	17% 13% 4 10% 6% 5% 5% 3 5% 5% 26%	1884
March	1st to 10th	Night	14% 12% 4 7% 4 4 3 2 4 7% 40%	3679
	11th to 20th	Night	10% 9% 3 7% 5% 4 4 3 5% 8% 41%	3480
	21st to end	Night	17% 10% 5% 8% 5% 5% 4 4 4 5% 33%	3817
April	1st to 10th	Night	18% 12% 4 6% 4 3 3 3 6% 7% 34%	1964
	11th to 20th	Night	18% 8% 3 7% 4 4 3 4 6% 6% 37%	1716
	21st to end	Night	19% 9% 3 6% 4 4 4 4 5% 7% 36%	1710
May	1st to 10th	Night	13% 9% 3 8% 5% 4 4 4 5% 8% 38%	2813
	11th to 20th	Night	15% 8% 2 7% 5% 3 3 3 4 7% 42%	2812
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 6% 49%	2960
June	1st to 10th	Night	13% 5% 2 5% 5% 5% 4 4 5% 8% 45%	1349
	11th to 20th	Night	11% 6% 2 5% 3 5% 4 4 7% 8% 45%	1330
	21st to end	Night	9% 6% 2 5% 4 5% 5% 4 5% 9% 47%	1374
July	1st to 10th	Night	8% 5% 3 7% 4 5% 4 4 6% 9% 46%	2530
	11th to 20th	Night	9% 3 2 3 4 4 4 4 6% 7% 55%	2530
	21st to end	Night	6% 4 1 6% 5% 6% 5% 5% 8% 8% 47%	2903
August	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 5% 8% 46%	2825
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 6% 8% 47%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 5% 8% 47%	3153
September	1st to 10th	Night	9% 6% 2 7% 5% 5% 4 3 5% 7% 47%	1996
	11th to 20th	Night	11% 5% 1 5% 2 3 4 3 5% 7% 54%	2090
	21st to end	Night	16% 7% 2 7% 4 3 4 3 5% 8% 41%	2090
October	1st to 10th	Night	16% 6% 2 5% 4 4 3 2 6% 7% 44%	2141
	11th to 20th	Night	21% 11% 3 8% 4 4 4 3 5% 5% 32%	2280
	21st to end	Night	23% 10% 3 9% 5% 4 3 3 5% 6% 29%	2689
November	1st to 10th	Night	14% 10% 2 6% 4 4 4 3 6% 6% 41%	2433
	11th to 20th	Night	15% 13% 3 8% 5% 5% 6% 6% 6% 7% 27%	2470
	21st to end	Night	19% 15% 4 8% 6% 5% 4 3 4 6% 26%	2621
December	1st to 10th	Night	18% 17% 7% 10% 7% 5% 4 3 4 5% 21%	2660
	11th to 20th	Night	13% 17% 6% 12% 7% 5% 4 3 5% 5% 24%	3522
	21st to end	Night	24% 23% 5% 11% 5% 4 4 3 4 5% 14%	5040

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% 7% 12% 11% 4	2528
	11th to 20th	Day	80% 6% 7% 5% 2	2530
	21st to end	Day	74% 6% 9% 7% 3 1	2952
February	1st to 10th	Day	63% 7% 10% 15% 5%	1710
	11th to 20th	Day	61% 5% 10% 15% 8% 1	1792
	21st to end	Day	59% 6% 12% 15% 6% 1	1570
March	1st to 10th	Day	53% 7% 13% 20% 6% 1	3174
	11th to 20th	Day	54% 6% 13% 18% 8% 1	3480
	21st to end	Day	54% 7% 13% 14% 10% 2	3815
April	1st to 10th	Day	54% 6% 11% 19% 10% 1	2232
	11th to 20th	Day	48% 5% 12% 20% 14% 2	2412
	21st to end	Day	49% 7% 13% 18% 13% 1	2470
May	1st to 10th	Day	44% 7% 12% 20% 14% 2	4120
	11th to 20th	Day	42% 9% 14% 24% 11% 1	4085
	21st to end	Day	35% 7% 11% 19% 23% 5%	4607
June	1st to 10th	Day	38% 8% 13% 25% 14% 2	2824
	11th to 20th	Day	38% 6% 13% 23% 18% 2	2850
	21st to end	Day	39% 8% 13% 24% 15% 1	2799
July	1st to 10th	Day	42% 7% 14% 20% 15% 2	4371
	11th to 20th	Day	29% 7% 14% 33% 16% 1	4365
	21st to end	Day	35% 8% 16% 18% 21% 2	4492
August	1st to 10th	Day	39% 8% 13% 22% 16% 1	4091
	11th to 20th	Day	37% 8% 13% 31% 11% 1	4083
	21st to end	Day	37% 7% 15% 27% 13% 1	4485
September	1st to 10th	Day	41% 9% 14% 24% 11% 2	2164
	11th to 20th	Day	34% 7% 14% 29% 15% 1	2090
	21st to end	Day	46% 6% 13% 23% 12%	2090
October	1st to 10th	Day	43% 7% 11% 24% 14% 2	2038
	11th to 20th	Day	52% 7% 13% 19% 8%	1900
	21st to end	Day	56% 6% 14% 18% 6%	1905
November	1st to 10th	Day	47% 8% 13% 21% 10% 1	1690
	11th to 20th	Day	55% 7% 14% 16% 8%	1710
	21st to end	Day	61% 6% 14% 14% 4 1	1557
December	1st to 10th	Day	67% 7% 11% 12% 3	1516
	11th to 20th	Day	61% 7% 13% 15% 4	1671
	21st to end	Day	77% 5% 8% 7% 2	2623
January	1st to 10th	Night	62% 6% 12% 12% 6% 1	4428
	11th to 20th	Night	74% 6% 9% 8% 3	4440
	21st to end	Night	69% 6% 11% 11% 3	4695
February	1st to 10th	Night	60% 7% 10% 15% 6% 1	2470
	11th to 20th	Night	58% 7% 9% 18% 8% 1	2281
	21st to end	Night	59% 5% 11% 17% 8% 1	1884
March	1st to 10th	Night	46% 5% 14% 27% 7% 1	3679
	11th to 20th	Night	39% 7% 17% 22% 14% 1	3480
	21st to end	Night	51% 7% 12% 21% 9% 1	3817
April	1st to 10th	Night	50% 6% 10% 22% 12% 2	1964
	11th to 20th	Night	45% 7% 10% 18% 17% 2	1716
	21st to end	Night	47% 7% 11% 18% 15% 3	1710
May	1st to 10th	Night	42% 7% 13% 20% 14% 4	2840
	11th to 20th	Night	41% 6% 10% 28% 12% 3	2813
	21st to end	Night	34% 8% 10% 21% 21% 6%	2968
June	1st to 10th	Night	37% 6% 11% 26% 19% 2	1349
	11th to 20th	Night	35% 7% 13% 21% 21% 2	1330
	21st to end	Night	33% 7% 14% 26% 18% 2	1374
July	1st to 10th	Night	34% 6% 12% 24% 16% 7%	2490
	11th to 20th	Night	27% 6% 14% 35% 17% 1	2482
	21st to end	Night	31% 10% 18% 20% 20% 1	2810
August	1st to 10th	Night	34% 6% 15% 28% 16% 1	2814
	11th to 20th	Night	34% 7% 15% 29% 13% 2	2812
	21st to end	Night	33% 7% 13% 31% 15% 1	3153
September	1st to 10th	Night	36% 7% 13% 28% 16% 1	1996
	11th to 20th	Night	29% 7% 13% 32% 18% 2	2090
	21st to end	Night	41% 6% 12% 28% 12% 1	2090
October	1st to 10th	Night	39% 7% 12% 25% 15% 1	2141
	11th to 20th	Night	53% 7% 11% 20% 9% 1	2280
	21st to end	Night	55% 6% 12% 19% 8%	2689
November	1st to 10th	Night	42% 7% 14% 24% 12% 1	2433
	11th to 20th	Night	54% 7% 15% 17% 7% 1	2470
	21st to end	Night	59% 5% 15% 15% 4 1	2621
December	1st to 10th	Night	65% 7% 11% 13% 4	2660
	11th to 20th	Night	61% 6% 14% 13% 5% 1	3525
	21st to end	Night	74% 6% 11% 7% 3	5040

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	22% 20% 37% 17% 4 1	270
	11th to 20th	Day	21% 16% 45% 14% 4	270
	21st to end	Day	28% 15% 37% 14% 5% 1	297
February	1st to 10th	Day	23% 27% 33% 14% 2 1	270
	11th to 20th	Day	22% 23% 38% 16% 1	270
	21st to end	Day	28% 32% 29% 9% 3	225
March	1st to 10th	Day	27% 20% 35% 14% 4 1	298
	11th to 20th	Day	21% 19% 29% 23% 8% 1	460
	21st to end	Day	28% 14% 30% 21% 6% 1	506
April	1st to 10th	Day	21% 19% 32% 20% 6% 1	460
	11th to 20th	Day	27% 18% 29% 15% 8% 4	452
	21st to end	Day	23% 15% 30% 21% 10% 1	460
May	1st to 10th	Day	22% 15% 31% 21% 9% 2	463
	11th to 20th	Day	20% 9% 30% 24% 11% 6%	487
	21st to end	Day	14% 9% 25% 28% 18% 6%	595
June	1st to 10th	Day	12% 14% 29% 30% 12% 4	680
	11th to 20th	Day	16% 10% 31% 25% 15% 3	680
	21st to end	Day	13% 10% 30% 29% 15% 4	680
July	1st to 10th	Day	8% 6% 25% 29% 21% 10%	671
	11th to 20th	Day	11% 8% 27% 30% 16% 8%	668
	21st to end	Day	9% 4 28% 31% 21% 7%	640
August	1st to 10th	Day	11% 3 25% 25% 23% 13%	486
	11th to 20th	Day	11% 5% 19% 29% 24% 12%	479
	21st to end	Day	14% 6% 22% 26% 23% 9%	539
September	1st to 10th	Day	13% 9% 25% 25% 20% 8%	490
	11th to 20th	Day	11% 6% 20% 25% 26% 13%	490
	21st to end	Day	18% 13% 20% 25% 16% 8%	490
October	1st to 10th	Day	17% 15% 28% 19% 17% 4	490
	11th to 20th	Day	22% 17% 32% 22% 6% 1	490
	21st to end	Day	30% 17% 35% 14% 4 1	354
November	1st to 10th	Day	20% 21% 32% 19% 7% 1	276
	11th to 20th	Day	23% 22% 38% 11% 5% 1	270
	21st to end	Day	25% 19% 37% 13% 5%	197
December	1st to 10th	Day	26% 20% 33% 16% 5%	180
	11th to 20th	Day	24% 25% 37% 11% 3 1	180
	21st to end	Day	29% 24% 32% 13% 2	252
January	1st to 10th	Night	22% 15% 36% 21% 6% 1	950
	11th to 20th	Night	25% 15% 33% 22% 4 1	950
	21st to end	Night	20% 11% 36% 27% 5%	934
February	1st to 10th	Night	23% 14% 38% 22% 4	760
	11th to 20th	Night	22% 15% 36% 23% 4	743
	21st to end	Night	23% 18% 36% 19% 2	628
March	1st to 10th	Night	20% 13% 36% 25% 6%	760
	11th to 20th	Night	16% 15% 37% 26% 6% 1	760
	21st to end	Night	23% 12% 31% 27% 6% 1	836
April	1st to 10th	Night	22% 15% 37% 22% 4	634
	11th to 20th	Night	22% 13% 34% 23% 7% 2	563
	21st to end	Night	21% 11% 33% 28% 7% 1	570
May	1st to 10th	Night	17% 8% 33% 34% 8% 1	570
	11th to 20th	Night	20% 7% 30% 32% 10% 1	561
	21st to end	Night	13% 4 27% 36% 18% 2	617
June	1st to 10th	Night	13% 6% 32% 40% 10%	399
	11th to 20th	Night	12% 6% 33% 37% 12%	380
	21st to end	Night	9% 6% 27% 43% 14% 1	424
July	1st to 10th	Night	10% 2 19% 43% 25% 2	555
	11th to 20th	Night	9% 3 23% 39% 23% 3	552
	21st to end	Night	8% 3 21% 45% 21% 1	624
August	1st to 10th	Night	8% 3 17% 44% 25% 2	567
	11th to 20th	Night	10% 2 21% 37% 27% 4	564
	21st to end	Night	12% 4 22% 33% 26% 3	627
September	1st to 10th	Night	11% 5% 25% 31% 22% 5%	666
	11th to 20th	Night	13% 4 19% 36% 25% 4	760
	21st to end	Night	17% 8% 21% 33% 18% 4	760
October	1st to 10th	Night	18% 11% 26% 30% 15% 1	760
	11th to 20th	Night	24% 14% 29% 24% 8%	760
	21st to end	Night	28% 11% 35% 22% 4	836
November	1st to 10th	Night	20% 14% 30% 28% 9%	749
	11th to 20th	Night	20% 15% 37% 20% 7%	760
	21st to end	Night	26% 14% 38% 19% 3	760
December	1st to 10th	Night	26% 15% 34% 20% 4 1	760
	11th to 20th	Night	23% 16% 38% 20% 3 1	950
	21st to end	Night	32% 12% 35% 19% 1	1045

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