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 2024-03-23.

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% 10% 4 7% 6% 4 5% 5% 5% 7% 33%	78464
	Night	16% 11% 5% 7% 6% 3 4 4 5% 6% 32%	81428
Transparency	Day	50% 7% 12% 19% 10% 1	78170
	Night	52% 7% 12% 19% 10% 1	81243
Seeing	Day	18% 13% 29% 22% 13% 5%	14518
	Night	19% 11% 31% 28% 10% 1	24374

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% 20% 7% 10% 7% 4 4 3 4 5% 20%	2527
	11th to 20th	Day	14% 21% 8% 12% 13% 8% 4 4 3 4 9%	2530
	21st to end	Day	25% 20% 7% 11% 6% 3 4 4 3 4 14%	2952
February	1st to 10th	Day	12% 14% 7% 10% 11% 5% 5% 5% 5% 6% 20%	1710
	11th to 20th	Day	13% 14% 6% 9% 8% 5% 6% 4 4 4 27%	1783
	21st to end	Day	14% 15% 6% 10% 7% 4 6% 3 4 5% 26%	1570
March	1st to 10th	Day	15% 11% 4 8% 8% 4 4 5% 5% 6% 30%	1928
	11th to 20th	Day	13% 12% 5% 8% 7% 3 5% 4 6% 6% 32%	2090
	21st to end	Day	19% 12% 4 8% 6% 3 4 4 4 6% 30%	2200
April	1st to 10th	Day	18% 14% 4 6% 5% 2 4 4 5% 6% 30%	2114
	11th to 20th	Day	18% 9% 3 6% 5% 3 4 4 5% 6% 35%	2283
	21st to end	Day	18% 9% 4 6% 5% 4 4 4 5% 6% 35%	2340
May	1st to 10th	Day	17% 8% 3 6% 6% 3 5% 5% 5% 7% 36%	2340
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 4 8% 37%	2301
	21st to end	Day	13% 6% 3 5% 5% 3 5% 6% 5% 8% 42%	2593
June	1st to 10th	Day	12% 6% 2 5% 6% 3 5% 5% 6% 8% 41%	2675
	11th to 20th	Day	12% 6% 3 5% 5% 3 4 4 6% 8% 43%	2700
	21st to end	Day	11% 5% 2 6% 5% 4 5% 6% 6% 9% 40%	2652
July	1st to 10th	Day	6% 4 2 5% 6% 3 5% 5% 7% 9% 47%	2520
	11th to 20th	Day	10% 4 2 5% 6% 3 5% 5% 7% 8% 45%	2520
	21st to end	Day	9% 4 3 5% 4 3 6% 5% 7% 9% 44%	2650
August	1st to 10th	Day	11% 5% 2 6% 6% 4 6% 6% 7% 9% 40%	2327
	11th to 20th	Day	12% 4 3 5% 6% 4 5% 6% 6% 8% 42%	2303
	21st to end	Day	12% 5% 3 5% 6% 3 4 5% 5% 8% 43%	2556
September	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 5% 6% 9% 37%	2051
	11th to 20th	Day	13% 6% 2 4 4 3 4 5% 6% 8% 45%	1980
	21st to end	Day	15% 6% 2 6% 5% 4 6% 5% 5% 8% 37%	1980
October	1st to 10th	Day	15% 7% 3 6% 6% 3 5% 5% 5% 6% 39%	1931
	11th to 20th	Day	22% 10% 4 7% 5% 4 5% 4 5% 6% 29%	1800
	21st to end	Day	24% 10% 4 7% 5% 3 4 4 5% 7% 28%	1805
November	1st to 10th	Day	14% 10% 3 7% 7% 3 4 5% 5% 8% 35%	1672
	11th to 20th	Day	15% 13% 5% 9% 6% 3 5% 6% 5% 6% 28%	1710
	21st to end	Day	17% 16% 7% 9% 5% 3 4 5% 4 6% 24%	1557
December	1st to 10th	Day	14% 19% 8% 10% 7% 5% 4 4 5% 4 19%	1516
	11th to 20th	Day	16% 15% 5% 9% 8% 4 4 4 5% 6% 23%	1675
	21st to end	Day	26% 21% 7% 11% 5% 2 4 4 4 5% 11%	2623
January	1st to 10th	Night	16% 16% 8% 11% 7% 3 4 3 4 4 23%	4428
	11th to 20th	Night	15% 17% 8% 14% 10% 8% 4 4 3 4 13%	4440
	21st to end	Night	19% 18% 9% 10% 6% 3 4 4 4 5% 17%	4695
February	1st to 10th	Night	15% 14% 6% 9% 7% 5% 6% 5% 5% 6% 23%	2470
	11th to 20th	Night	14% 14% 6% 9% 7% 4 5% 5% 5% 6% 26%	2279
	21st to end	Night	17% 12% 5% 9% 7% 4 5% 5% 4 6% 26%	1884
March	1st to 10th	Night	15% 12% 5% 7% 5% 4 3 3 4 6% 36%	2252
	11th to 20th	Night	13% 9% 4 7% 7% 3 4 4 4 7% 39%	2090
	21st to end	Night	20% 9% 4 7% 6% 3 5% 5% 4 5% 33%	2201
April	1st to 10th	Night	19% 10% 4 6% 5% 2 3 4 5% 7% 34%	1861
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 5% 5% 7% 38%	1625
	21st to end	Night	19% 9% 3 5% 5% 3 4 4 5% 7% 37%	1620
May	1st to 10th	Night	15% 9% 3 7% 6% 3 4 5% 5% 8% 35%	1620
	11th to 20th	Night	17% 8% 3 5% 5% 3 4 4 4 7% 40%	1592
	21st to end	Night	12% 4 3 4 5% 3 5% 4 6% 8% 47%	1677
June	1st to 10th	Night	13% 4 2 4 5% 3 5% 5% 5% 8% 46%	1278
	11th to 20th	Night	12% 5% 3 4 4 3 5% 5% 6% 9% 46%	1260
	21st to end	Night	9% 5% 2 4 4 4 5% 5% 5% 9% 48%	1301
July	1st to 10th	Night	8% 3 2 4 4 3 3 5% 5% 7% 55%	1440
	11th to 20th	Night	9% 4 2 3 5% 4 4 5% 6% 8% 51%	1440
	21st to end	Night	7% 4 2 5% 5% 3 4 5% 7% 9% 49%	1665
August	1st to 10th	Night	8% 4 2 4 5% 3 5% 6% 6% 10% 47%	1611
	11th to 20th	Night	8% 3 2 5% 5% 3 5% 6% 6% 8% 49%	1598
	21st to end	Night	10% 5% 2 5% 4 3 4 4 4 9% 50%	1797
September	1st to 10th	Night	9% 5% 3 6% 6% 3 5% 4 5% 8% 45%	1890
	11th to 20th	Night	12% 4 2 4 3 3 3 4 5% 7% 52%	1980
	21st to end	Night	15% 7% 3 6% 5% 2 4 4 5% 8% 42%	1980
October	1st to 10th	Night	17% 6% 3 5% 5% 3 4 3 5% 7% 43%	2028
	11th to 20th	Night	21% 11% 3 7% 5% 3 4 4 5% 5% 31%	2160
	21st to end	Night	24% 10% 3 7% 6% 3 3 4 5% 7% 28%	2547
November	1st to 10th	Night	15% 9% 3 5% 5% 3 4 4 6% 6% 41%	2406
	11th to 20th	Night	15% 12% 4 7% 6% 3 6% 7% 5% 6% 28%	2470
	21st to end	Night	20% 14% 5% 7% 6% 4 4 4 4 6% 26%	2621
December	1st to 10th	Night	18% 15% 7% 9% 8% 4 5% 4 4 5% 21%	2660
	11th to 20th	Night	13% 15% 7% 10% 8% 4 4 4 5% 5% 24%	3522
	21st to end	Night	25% 22% 6% 10% 6% 3 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% 15% 17% 7% 1	1928
	11th to 20th	Day	52% 6% 11% 18% 11% 2	2090
	21st to end	Day	55% 6% 12% 18% 8% 1	2200
April	1st to 10th	Day	54% 6% 11% 19% 9% 1	2114
	11th to 20th	Day	49% 5% 12% 19% 14% 2	2283
	21st to end	Day	49% 7% 13% 17% 12% 1	2340
May	1st to 10th	Day	47% 6% 12% 22% 10% 2	2340
	11th to 20th	Day	46% 6% 11% 19% 15% 2	2305
	21st to end	Day	40% 9% 14% 21% 15% 2	2593
June	1st to 10th	Day	38% 8% 13% 25% 15% 2	2675
	11th to 20th	Day	38% 6% 13% 22% 18% 2	2700
	21st to end	Day	38% 9% 13% 24% 15% 1	2652
July	1st to 10th	Day	31% 8% 12% 27% 18% 3	2451
	11th to 20th	Day	34% 8% 14% 25% 17% 2	2445
	21st to end	Day	34% 9% 14% 23% 17% 3	2503
August	1st to 10th	Day	37% 10% 15% 24% 13% 2	2314
	11th to 20th	Day	36% 9% 15% 24% 13% 2	2303
	21st to end	Day	38% 8% 13% 26% 14% 1	2556
September	1st to 10th	Day	43% 9% 14% 22% 10% 2	2051
	11th to 20th	Day	35% 7% 15% 30% 13%	1980
	21st to end	Day	44% 6% 14% 23% 12%	1980
October	1st to 10th	Day	44% 7% 11% 23% 14% 1	1931
	11th to 20th	Day	54% 7% 13% 18% 7%	1800
	21st to end	Day	56% 6% 15% 17% 6%	1805
November	1st to 10th	Day	47% 8% 13% 21% 10% 1	1672
	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	50% 5% 11% 23% 10% 1	2252
	11th to 20th	Night	44% 6% 12% 21% 14% 2	2090
	21st to end	Night	53% 6% 9% 20% 11% 2	2201
April	1st to 10th	Night	49% 6% 10% 21% 12% 1	1861
	11th to 20th	Night	44% 7% 10% 19% 18% 2	1625
	21st to end	Night	46% 7% 11% 18% 15% 3	1620
May	1st to 10th	Night	48% 5% 12% 20% 12% 2	1620
	11th to 20th	Night	44% 5% 10% 20% 17% 4	1593
	21st to end	Night	35% 9% 12% 24% 16% 4	1678
June	1st to 10th	Night	37% 6% 11% 26% 19% 2	1278
	11th to 20th	Night	34% 7% 14% 22% 21% 2	1260
	21st to end	Night	33% 8% 14% 26% 18% 2	1301
July	1st to 10th	Night	27% 7% 11% 29% 22% 5%	1400
	11th to 20th	Night	29% 7% 15% 27% 20% 2	1392
	21st to end	Night	29% 10% 16% 23% 19% 2	1572
August	1st to 10th	Night	30% 8% 18% 29% 14% 1	1600
	11th to 20th	Night	30% 8% 17% 28% 14% 3	1598
	21st to end	Night	32% 7% 14% 29% 17% 1	1797
September	1st to 10th	Night	37% 7% 13% 26% 16% 1	1890
	11th to 20th	Night	30% 7% 13% 32% 17% 1	1980
	21st to end	Night	41% 6% 12% 28% 12% 1	1980
October	1st to 10th	Night	41% 7% 11% 25% 15% 1	2028
	11th to 20th	Night	53% 7% 11% 19% 9% 1	2160
	21st to end	Night	56% 6% 12% 18% 8%	2547
November	1st to 10th	Night	42% 7% 14% 24% 12% 1	2406
	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	20% 19% 29% 23% 8% 1	460
	21st to end	Day	29% 13% 29% 21% 7% 1	470
April	1st to 10th	Day	22% 18% 32% 21% 6% 1	420
	11th to 20th	Day	26% 18% 28% 15% 8% 4	412
	21st to end	Day	23% 15% 29% 21% 10% 1	420
May	1st to 10th	Day	23% 14% 31% 20% 9% 2	423
	11th to 20th	Day	21% 9% 30% 23% 11% 6%	447
	21st to end	Day	15% 8% 25% 28% 18% 6%	547
June	1st to 10th	Day	12% 12% 29% 31% 12% 4	630
	11th to 20th	Day	16% 9% 30% 26% 16% 4	630
	21st to end	Day	11% 9% 30% 30% 15% 5%	630
July	1st to 10th	Day	8% 5% 25% 29% 22% 11%	621
	11th to 20th	Day	11% 8% 27% 30% 16% 8%	618
	21st to end	Day	9% 4 30% 31% 20% 7%	591
August	1st to 10th	Day	11% 2 25% 25% 24% 14%	446
	11th to 20th	Day	11% 5% 18% 29% 25% 12%	439
	21st to end	Day	15% 6% 21% 25% 24% 9%	495
September	1st to 10th	Day	14% 7% 26% 25% 20% 8%	450
	11th to 20th	Day	12% 7% 20% 25% 24% 13%	450
	21st to end	Day	16% 14% 20% 25% 16% 9%	450
October	1st to 10th	Day	17% 14% 28% 18% 18% 5%	450
	11th to 20th	Day	23% 18% 30% 23% 5% 1	450
	21st to end	Day	31% 16% 35% 13% 4 1	320
November	1st to 10th	Day	20% 21% 32% 19% 7% 1	270
	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% 11% 31% 27% 6% 1	801
April	1st to 10th	Night	22% 15% 36% 22% 5%	601
	11th to 20th	Night	21% 12% 34% 24% 7% 2	533
	21st to end	Night	21% 10% 33% 28% 7% 1	540
May	1st to 10th	Night	18% 7% 34% 33% 8% 1	540
	11th to 20th	Night	21% 7% 29% 32% 11% 1	531
	21st to end	Night	13% 3 27% 37% 18% 2	584
June	1st to 10th	Night	12% 6% 31% 41% 10%	378
	11th to 20th	Night	12% 5% 33% 38% 13%	360
	21st to end	Night	9% 5% 26% 44% 15% 1	401
July	1st to 10th	Night	10% 2 18% 43% 26% 2	525
	11th to 20th	Night	10% 3 22% 40% 23% 3	522
	21st to end	Night	8% 3 22% 45% 21% 1	591
August	1st to 10th	Night	9% 2 17% 44% 25% 2	537
	11th to 20th	Night	9% 1 20% 37% 28% 4	534
	21st to end	Night	12% 4 22% 32% 27% 3	594
September	1st to 10th	Night	11% 4 25% 32% 22% 5%	630
	11th to 20th	Night	13% 4 19% 36% 24% 4	720
	21st to end	Night	16% 8% 21% 32% 19% 4	720
October	1st to 10th	Night	18% 10% 25% 30% 15% 1	720
	11th to 20th	Night	25% 14% 28% 24% 8%	720
	21st to end	Night	29% 11% 34% 22% 4	792
November	1st to 10th	Night	20% 14% 30% 28% 9%	740
	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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