James C. Veen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the James C. Veen Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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% 10% 4 8% 5% 5% 4 4 5% 7% 34%	101224
	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 33%	97851
Transparency	Day	48% 5% 11% 22% 13% 1	104796
	Night	50% 4 11% 21% 12% 1	102407
Seeing	Day	20% 16% 30% 20% 10% 4	15811
	Night	20% 12% 30% 26% 10% 1	25655

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	15% 23% 7% 11% 7% 6% 5% 3 3 4 16%	2527
	11th to 20th	Day	14% 17% 8% 18% 14% 9% 3 3 4 3 7%	2530
	21st to end	Day	22% 25% 9% 13% 6% 4 3 2 3 4 8%	2783
February	1st to 10th	Day	12% 16% 6% 15% 10% 6% 5% 4 4 4 19%	1748
	11th to 20th	Day	13% 14% 6% 16% 8% 5% 4 3 3 5% 22%	1848
	21st to end	Day	15% 16% 5% 12% 6% 4 5% 4 5% 6% 22%	1570
March	1st to 10th	Day	16% 15% 4 7% 5% 5% 5% 4 4 6% 30%	3120
	11th to 20th	Day	12% 8% 5% 10% 7% 6% 5% 4 6% 7% 32%	3405
	21st to end	Day	19% 13% 5% 9% 5% 5% 4 3 4 4 28%	3931
April	1st to 10th	Day	18% 15% 4 7% 5% 4 4 3 4 5% 31%	2280
	11th to 20th	Day	16% 12% 2 7% 4 3 4 3 6% 6% 36%	2319
	21st to end	Day	17% 9% 3 7% 5% 4 4 3 5% 7% 35%	2470
May	1st to 10th	Day	17% 8% 3 7% 4 5% 5% 3 5% 7% 36%	4082
	11th to 20th	Day	14% 8% 2 6% 4 4 5% 4 6% 8% 38%	4081
	21st to end	Day	11% 7% 3 6% 4 4 4 4 5% 8% 44%	4789
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 9% 41%	2660
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 3 5% 8% 45%	2660
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	2660
July	1st to 10th	Day	11% 7% 3 8% 5% 5% 5% 5% 7% 8% 36%	4440
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 5% 7% 10% 48%	4440
	21st to end	Day	6% 6% 2 6% 5% 5% 5% 5% 7% 9% 45%	4813
August	1st to 10th	Day	12% 5% 2 6% 4 5% 4 4 7% 9% 41%	4114
	11th to 20th	Day	10% 6% 2 7% 4 4 5% 4 5% 7% 46%	4044
	21st to end	Day	7% 6% 3 7% 5% 5% 5% 4 6% 9% 43%	4257
September	1st to 10th	Day	9% 8% 3 6% 5% 5% 4 4 6% 8% 43%	2237
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 5% 6% 8% 47%	2090
	21st to end	Day	13% 7% 2 8% 5% 4 4 3 6% 8% 40%	2090
October	1st to 10th	Day	14% 7% 3 7% 5% 3 4 3 5% 6% 42%	2090
	11th to 20th	Day	20% 9% 3 8% 6% 6% 5% 4 5% 7% 27%	1853
	21st to end	Day	22% 11% 3 9% 6% 5% 5% 3 5% 7% 24%	1881
November	1st to 10th	Day	12% 12% 4 7% 4 4 4 3 5% 7% 38%	1699
	11th to 20th	Day	15% 17% 5% 9% 6% 5% 5% 3 6% 6% 24%	1710
	21st to end	Day	19% 17% 5% 9% 5% 4 4 3 4 6% 25%	1710
December	1st to 10th	Day	13% 24% 8% 13% 7% 5% 3 3 4 5% 15%	1592
	11th to 20th	Day	13% 18% 7% 15% 7% 5% 5% 4 5% 5% 14%	1918
	21st to end	Day	21% 25% 8% 12% 6% 4 3 3 3 3 10%	2783
January	1st to 10th	Night	16% 21% 9% 14% 6% 5% 4 3 3 4 16%	4428
	11th to 20th	Night	14% 20% 9% 17% 10% 8% 3 3 3 4 11%	4440
	21st to end	Night	22% 20% 9% 13% 5% 4 4 3 3 4 12%	4884
February	1st to 10th	Night	13% 18% 4 12% 10% 6% 4 3 4 5% 20%	2426
	11th to 20th	Night	12% 14% 6% 12% 7% 5% 5% 4 4 5% 25%	2218
	21st to end	Night	17% 13% 4 9% 6% 7% 5% 4 4 6% 26%	1884
March	1st to 10th	Night	15% 10% 4 9% 5% 3 4 3 4 6% 37%	3733
	11th to 20th	Night	10% 8% 2 6% 4 4 4 3 6% 8% 44%	3554
	21st to end	Night	17% 12% 5% 9% 5% 5% 4 4 5% 6% 30%	3729
April	1st to 10th	Night	15% 12% 3 8% 5% 4 4 3 5% 5% 37%	1900
	11th to 20th	Night	20% 8% 3 7% 4 3 4 3 5% 6% 37%	1808
	21st to end	Night	17% 9% 3 5% 4 3 5% 4 5% 7% 38%	1710
May	1st to 10th	Night	13% 9% 4 8% 6% 4 5% 4 5% 7% 36%	2813
	11th to 20th	Night	18% 6% 2 6% 4 3 4 3 5% 6% 42%	2812
	21st to end	Night	13% 8% 3 7% 5% 4 4 3 5% 7% 41%	2773
June	1st to 10th	Night	11% 6% 2 4 5% 5% 4 4 7% 7% 46%	1520
	11th to 20th	Night	10% 5% 2 6% 4 4 4 5% 6% 9% 44%	1520
	21st to end	Night	9% 5% 1 6% 4 5% 5% 5% 6% 10% 44%	1520
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 4 4 7% 9% 43%	2530
	11th to 20th	Night	6% 4 1 4 4 3 5% 3 6% 8% 55%	2530
	21st to end	Night	6% 5% 2 7% 5% 5% 5% 4 6% 8% 47%	2750
August	1st to 10th	Night	9% 6% 2 5% 4 4 5% 3 6% 8% 48%	2815
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 6% 7% 47%	2785
	21st to end	Night	8% 7% 3 7% 5% 4 5% 4 6% 7% 44%	3396
September	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 8% 51%	1923
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 53%	2090
	21st to end	Night	12% 7% 3 7% 4 4 4 3 4 8% 43%	2090
October	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 6% 48%	2090
	11th to 20th	Night	18% 10% 3 9% 4 4 4 2 4 6% 34%	2322
	21st to end	Night	22% 10% 3 7% 5% 5% 4 3 5% 6% 31%	2717
November	1st to 10th	Night	14% 10% 3 7% 5% 4 4 4 5% 6% 39%	2452
	11th to 20th	Night	14% 15% 4 10% 6% 5% 4 4 6% 6% 25%	2470
	21st to end	Night	18% 16% 5% 8% 5% 4 4 3 5% 6% 26%	2470
December	1st to 10th	Night	16% 19% 7% 13% 8% 5% 4 3 5% 4 15%	2586
	11th to 20th	Night	15% 18% 7% 13% 7% 6% 4 4 4 5% 16%	3279
	21st to end	Night	27% 21% 6% 11% 4 4 4 3 4 5% 12%	4884

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	73% 4 11% 10% 2	2528
	11th to 20th	Day	84% 4 6% 5% 2	2530
	21st to end	Day	82% 3 7% 6% 2	2783
February	1st to 10th	Day	69% 4 8% 13% 5%	1748
	11th to 20th	Day	65% 4 8% 15% 7%	1858
	21st to end	Day	63% 4 12% 14% 6% 1	1570
March	1st to 10th	Day	56% 4 11% 20% 8%	3120
	11th to 20th	Day	50% 6% 13% 21% 9% 1	3405
	21st to end	Day	59% 4 10% 18% 8% 1	3931
April	1st to 10th	Day	56% 4 9% 20% 11%	2280
	11th to 20th	Day	48% 5% 10% 22% 14% 2	2319
	21st to end	Day	49% 5% 11% 18% 14% 2	2470
May	1st to 10th	Day	46% 6% 10% 21% 15% 2	4120
	11th to 20th	Day	42% 5% 14% 25% 13% 2	4085
	21st to end	Day	38% 6% 11% 21% 23% 2	4802
June	1st to 10th	Day	37% 6% 12% 27% 16% 2	2660
	11th to 20th	Day	37% 5% 12% 25% 20% 2	2660
	21st to end	Day	38% 6% 13% 23% 18% 2	2660
July	1st to 10th	Day	43% 5% 13% 20% 15% 3	4371
	11th to 20th	Day	29% 6% 14% 31% 18% 2	4365
	21st to end	Day	34% 6% 13% 23% 22% 2	4662
August	1st to 10th	Day	38% 5% 12% 28% 16% 1	4100
	11th to 20th	Day	37% 5% 10% 29% 18% 1	4083
	21st to end	Day	36% 6% 13% 29% 16% 2	4256
September	1st to 10th	Day	39% 5% 12% 27% 15% 2	2237
	11th to 20th	Day	33% 5% 12% 30% 17% 2	2090
	21st to end	Day	43% 4 12% 25% 16%	2090
October	1st to 10th	Day	31% 3 8% 36% 20% 2	3480
	11th to 20th	Day	43% 4 12% 27% 15%	3043
	21st to end	Day	51% 3 12% 24% 9% 1	3081
November	1st to 10th	Day	46% 5% 12% 23% 13% 1	1699
	11th to 20th	Day	61% 4 13% 17% 5% 1	1710
	21st to end	Day	62% 3 15% 14% 5%	1710
December	1st to 10th	Day	72% 4 9% 12% 2 1	1592
	11th to 20th	Day	70% 5% 12% 11% 2	1915
	21st to end	Day	79% 4 7% 8% 2	2783
January	1st to 10th	Night	73% 4 9% 9% 5% 1	4428
	11th to 20th	Night	79% 3 7% 6% 4	4440
	21st to end	Night	77% 4 9% 8% 2	4884
February	1st to 10th	Night	67% 4 9% 15% 6% 1	2426
	11th to 20th	Night	62% 5% 8% 17% 8%	2219
	21st to end	Night	59% 5% 11% 16% 7% 1	1884
March	1st to 10th	Night	48% 5% 12% 27% 7% 1	3733
	11th to 20th	Night	36% 6% 14% 25% 17% 2	3554
	21st to end	Night	55% 4 11% 20% 9% 1	3729
April	1st to 10th	Night	49% 5% 10% 21% 14% 1	1900
	11th to 20th	Night	48% 4 9% 20% 17% 3	1808
	21st to end	Night	45% 5% 12% 20% 15% 3	1710
May	1st to 10th	Night	46% 5% 13% 19% 15% 2	2840
	11th to 20th	Night	43% 4 8% 22% 19% 3	2813
	21st to end	Night	43% 5% 10% 20% 20% 3	2781
June	1st to 10th	Night	35% 6% 10% 30% 17% 2	1520
	11th to 20th	Night	35% 6% 14% 25% 19% 1	1520
	21st to end	Night	34% 6% 15% 26% 16% 2	1520
July	1st to 10th	Night	38% 5% 12% 26% 13% 7%	2490
	11th to 20th	Night	27% 5% 14% 29% 24% 1	2482
	21st to end	Night	34% 6% 15% 19% 24% 3	2662
August	1st to 10th	Night	33% 4 14% 30% 18% 1	2805
	11th to 20th	Night	34% 5% 10% 34% 16% 1	2812
	21st to end	Night	37% 5% 10% 29% 17% 2	3396
September	1st to 10th	Night	32% 6% 12% 29% 19% 2	1923
	11th to 20th	Night	30% 4 10% 34% 20% 1	2090
	21st to end	Night	41% 4 12% 28% 14% 1	2090
October	1st to 10th	Night	28% 3 9% 37% 22% 1	3480
	11th to 20th	Night	42% 4 11% 24% 19%	3858
	21st to end	Night	43% 4 10% 29% 14%	4467
November	1st to 10th	Night	46% 4 11% 24% 14% 1	2452
	11th to 20th	Night	57% 4 13% 18% 7% 1	2470
	21st to end	Night	58% 5% 14% 18% 4	2470
December	1st to 10th	Night	71% 5% 9% 11% 4	2586
	11th to 20th	Night	69% 5% 11% 11% 4	3281
	21st to end	Night	76% 4 10% 7% 2	4884

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% 21% 36% 15% 5% 1	270
	11th to 20th	Day	23% 24% 38% 13% 1 1	270
	21st to end	Day	29% 22% 31% 16% 2	297
February	1st to 10th	Day	26% 27% 33% 11% 2 1	270
	11th to 20th	Day	25% 35% 27% 12% 1	270
	21st to end	Day	32% 34% 27% 7% 1	225
March	1st to 10th	Day	26% 33% 33% 6% 2	270
	11th to 20th	Day	27% 18% 34% 15% 6% 1	411
	21st to end	Day	32% 17% 30% 15% 4 2	506
April	1st to 10th	Day	29% 19% 29% 18% 5% 1	460
	11th to 20th	Day	31% 22% 27% 11% 6% 3	452
	21st to end	Day	25% 18% 31% 18% 7% 1	460
May	1st to 10th	Day	25% 21% 28% 18% 6% 2	463
	11th to 20th	Day	21% 15% 30% 22% 7% 4	487
	21st to end	Day	15% 12% 31% 26% 13% 3	712
June	1st to 10th	Day	13% 14% 36% 24% 9% 4	680
	11th to 20th	Day	16% 12% 34% 24% 11% 3	680
	21st to end	Day	15% 12% 37% 21% 11% 3	680
July	1st to 10th	Day	9% 8% 28% 27% 19% 8%	671
	11th to 20th	Day	12% 10% 29% 28% 14% 8%	668
	21st to end	Day	9% 7% 33% 28% 17% 6%	739
August	1st to 10th	Day	11% 6% 26% 26% 20% 11%	496
	11th to 20th	Day	13% 8% 21% 27% 23% 8%	479
	21st to end	Day	13% 6% 26% 27% 19% 9%	539
September	1st to 10th	Day	14% 9% 26% 26% 18% 7%	490
	11th to 20th	Day	11% 6% 20% 30% 24% 9%	490
	21st to end	Day	20% 13% 23% 23% 14% 6%	490
October	1st to 10th	Day	20% 14% 27% 22% 13% 3	490
	11th to 20th	Day	25% 19% 32% 19% 5% 1	410
	21st to end	Day	32% 20% 35% 10% 3	330
November	1st to 10th	Day	24% 17% 38% 14% 6% 1	279
	11th to 20th	Day	26% 26% 36% 9% 4	270
	21st to end	Day	32% 21% 34% 10% 2	270
December	1st to 10th	Day	30% 29% 25% 10% 5%	270
	11th to 20th	Day	26% 31% 30% 11% 3	270
	21st to end	Day	28% 28% 35% 6% 2	297
January	1st to 10th	Night	23% 21% 31% 19% 6% 1	950
	11th to 20th	Night	25% 19% 30% 21% 4 1	950
	21st to end	Night	24% 12% 33% 27% 3	1045
February	1st to 10th	Night	24% 18% 32% 23% 3	768
	11th to 20th	Night	20% 18% 36% 21% 4	743
	21st to end	Night	24% 21% 36% 16% 3	628
March	1st to 10th	Night	20% 19% 31% 23% 7%	760
	11th to 20th	Night	19% 17% 35% 22% 6% 1	760
	21st to end	Night	25% 13% 33% 23% 7%	786
April	1st to 10th	Night	21% 15% 35% 23% 5% 1	570
	11th to 20th	Night	26% 13% 34% 21% 6% 1	563
	21st to end	Night	23% 12% 34% 25% 6%	570
May	1st to 10th	Night	19% 11% 34% 28% 8% 1	570
	11th to 20th	Night	20% 10% 30% 28% 11%	561
	21st to end	Night	15% 6% 27% 33% 18% 1	617
June	1st to 10th	Night	13% 6% 31% 34% 15% 1	570
	11th to 20th	Night	13% 6% 32% 34% 15% 1	570
	21st to end	Night	11% 5% 28% 36% 17% 2	570
July	1st to 10th	Night	10% 3 22% 40% 22% 3	555
	11th to 20th	Night	8% 2 27% 38% 22% 3	552
	21st to end	Night	8% 2 25% 38% 26% 2	624
August	1st to 10th	Night	10% 3 20% 39% 25% 3	567
	11th to 20th	Night	11% 2 19% 40% 23% 5%	564
	21st to end	Night	12% 3 21% 36% 25% 3	627
September	1st to 10th	Night	10% 5% 24% 35% 21% 5%	593
	11th to 20th	Night	11% 5% 22% 36% 23% 3	760
	21st to end	Night	18% 9% 22% 30% 18% 3	760
October	1st to 10th	Night	18% 7% 28% 30% 15% 1	760
	11th to 20th	Night	25% 15% 29% 23% 8%	760
	21st to end	Night	27% 13% 32% 21% 6%	836
November	1st to 10th	Night	21% 14% 31% 25% 9%	755
	11th to 20th	Night	23% 17% 34% 20% 6% 1	760
	21st to end	Night	26% 15% 33% 23% 4	760
December	1st to 10th	Night	26% 17% 30% 21% 5%	876
	11th to 20th	Night	26% 17% 35% 18% 4 1	950
	21st to end	Night	32% 15% 35% 17% 1	1045

Page generated at 2024-11-01 22:47:51. Version ac07

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