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 2024-03-23.

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	14% 10% 5% 8% 7% 4 5% 5% 5% 7% 32%	78641
	Night	15% 11% 5% 8% 6% 3 4 4 5% 6% 31%	81295
Transparency	Day	52% 5% 11% 20% 11% 1	78343
	Night	54% 4 11% 19% 10% 1	81114
Seeing	Day	20% 16% 30% 20% 10% 4	14855
	Night	20% 12% 30% 26% 10% 1	24913

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% 22% 7% 9% 9% 5% 5% 4 3 4 16%	2527
	11th to 20th	Day	14% 16% 9% 15% 17% 8% 4 4 4 3 7%	2530
	21st to end	Day	22% 23% 10% 12% 8% 3 4 3 3 4 9%	2783
February	1st to 10th	Day	12% 14% 8% 13% 12% 5% 5% 5% 4 4 19%	1748
	11th to 20th	Day	14% 12% 8% 14% 10% 4 5% 4 3 5% 22%	1848
	21st to end	Day	16% 14% 7% 11% 7% 3 5% 5% 5% 6% 22%	1570
March	1st to 10th	Day	13% 12% 5% 7% 7% 4 6% 5% 4 6% 29%	1900
	11th to 20th	Day	13% 9% 5% 8% 6% 3 4 4 5% 7% 35%	2041
	21st to end	Day	18% 13% 4 7% 5% 3 5% 5% 5% 5% 31%	2262
April	1st to 10th	Day	19% 13% 6% 6% 6% 3 4 4 4 5% 31%	2160
	11th to 20th	Day	16% 11% 3 6% 5% 2 4 4 6% 6% 36%	2195
	21st to end	Day	17% 9% 3 7% 6% 3 5% 5% 5% 7% 34%	2340
May	1st to 10th	Day	17% 8% 3 6% 6% 4 5% 5% 5% 7% 34%	2340
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 5% 7% 37%	2301
	21st to end	Day	12% 6% 3 6% 5% 3 5% 5% 5% 8% 41%	2704
June	1st to 10th	Day	10% 5% 3 5% 5% 3 6% 6% 6% 9% 42%	2520
	11th to 20th	Day	11% 5% 3 5% 5% 4 5% 4 5% 8% 45%	2520
	21st to end	Day	10% 5% 3 6% 5% 3 5% 5% 6% 9% 42%	2520
July	1st to 10th	Day	6% 4 2 4 5% 3 5% 5% 7% 9% 48%	2520
	11th to 20th	Day	10% 5% 2 5% 4 4 6% 5% 5% 9% 45%	2520
	21st to end	Day	7% 4 2 5% 5% 3 5% 6% 7% 10% 46%	2743
August	1st to 10th	Day	9% 5% 2 5% 5% 5% 5% 5% 8% 9% 42%	2337
	11th to 20th	Day	10% 5% 2 6% 6% 3 6% 6% 6% 9% 43%	2303
	21st to end	Day	10% 5% 3 5% 4 3 5% 5% 7% 9% 43%	2425
September	1st to 10th	Day	10% 8% 4 5% 6% 3 5% 5% 6% 8% 42%	2120
	11th to 20th	Day	11% 5% 3 4 4 3 5% 6% 6% 8% 45%	1980
	21st to end	Day	13% 6% 3 7% 6% 3 4 3 5% 8% 41%	1980
October	1st to 10th	Day	15% 7% 3 6% 6% 3 4 4 5% 6% 41%	1980
	11th to 20th	Day	21% 9% 3 7% 7% 4 6% 5% 5% 6% 25%	1757
	21st to end	Day	23% 11% 4 8% 7% 3 5% 4 5% 7% 23%	1782
November	1st to 10th	Day	13% 11% 4 6% 5% 3 4 5% 5% 7% 38%	1672
	11th to 20th	Day	16% 16% 6% 8% 6% 4 5% 4 5% 6% 24%	1710
	21st to end	Day	20% 16% 5% 8% 6% 3 4 4 3 6% 25%	1710
December	1st to 10th	Day	14% 21% 10% 13% 8% 4 4 3 4 5% 15%	1592
	11th to 20th	Day	14% 17% 9% 13% 9% 4 5% 5% 5% 6% 14%	1918
	21st to end	Day	21% 24% 9% 10% 8% 3 4 4 3 3 10%	2783
January	1st to 10th	Night	16% 20% 9% 13% 7% 4 4 4 3 4 16%	4428
	11th to 20th	Night	14% 19% 10% 16% 11% 7% 3 3 3 4 11%	4440
	21st to end	Night	22% 19% 10% 11% 7% 3 4 4 3 4 12%	4884
February	1st to 10th	Night	13% 16% 6% 10% 10% 5% 5% 4 4 5% 20%	2426
	11th to 20th	Night	12% 13% 7% 10% 9% 4 6% 5% 4 5% 25%	2218
	21st to end	Night	17% 12% 5% 8% 7% 6% 5% 4 4 6% 26%	1884
March	1st to 10th	Night	15% 10% 4 8% 6% 2 5% 4 5% 6% 35%	2280
	11th to 20th	Night	13% 10% 3 6% 6% 2 4 4 4 7% 41%	2138
	21st to end	Night	19% 9% 3 7% 5% 3 4 4 5% 6% 34%	2154
April	1st to 10th	Night	16% 10% 4 7% 5% 3 5% 4 5% 5% 36%	1800
	11th to 20th	Night	19% 7% 4 5% 5% 3 4 4 5% 6% 37%	1712
	21st to end	Night	18% 9% 3 5% 4 3 4 5% 5% 7% 38%	1620
May	1st to 10th	Night	15% 8% 4 7% 6% 3 5% 5% 5% 6% 36%	1620
	11th to 20th	Night	20% 6% 2 5% 4 3 5% 4 5% 7% 40%	1592
	21st to end	Night	10% 7% 3 6% 7% 3 5% 4 6% 8% 41%	1568
June	1st to 10th	Night	11% 5% 2 4 5% 3 5% 5% 6% 8% 47%	1440
	11th to 20th	Night	10% 4 3 6% 4 3 4 6% 6% 10% 45%	1440
	21st to end	Night	9% 4 2 5% 5% 4 5% 6% 6% 10% 45%	1440
July	1st to 10th	Night	7% 4 2 4 5% 2 3 4 5% 8% 55%	1440
	11th to 20th	Night	7% 4 2 4 5% 2 4 5% 6% 9% 52%	1440
	21st to end	Night	6% 4 2 6% 4 2 4 5% 6% 8% 52%	1575
August	1st to 10th	Night	8% 4 2 4 4 3 5% 4 7% 9% 50%	1601
	11th to 20th	Night	9% 4 2 5% 5% 3 4 5% 7% 8% 49%	1598
	21st to end	Night	9% 5% 3 5% 4 3 5% 5% 6% 7% 50%	1928
September	1st to 10th	Night	10% 5% 2 5% 5% 3 4 4 5% 8% 49%	1821
	11th to 20th	Night	11% 5% 2 4 3 2 4 4 5% 7% 52%	1980
	21st to end	Night	12% 6% 4 6% 5% 3 4 4 4 8% 44%	1980
October	1st to 10th	Night	16% 7% 2 5% 4 2 3 4 4 6% 46%	1980
	11th to 20th	Night	19% 10% 4 8% 6% 3 4 4 4 6% 33%	2199
	21st to end	Night	23% 10% 3 6% 5% 3 4 4 5% 6% 31%	2574
November	1st to 10th	Night	14% 9% 3 6% 6% 3 5% 4 5% 6% 39%	2406
	11th to 20th	Night	15% 14% 5% 8% 7% 3 5% 5% 6% 6% 26%	2470
	21st to end	Night	18% 14% 6% 8% 6% 3 4 4 5% 6% 27%	2470
December	1st to 10th	Night	16% 17% 8% 13% 9% 4 5% 4 5% 4 15%	2586
	11th to 20th	Night	16% 16% 8% 11% 8% 4 5% 4 4 6% 17%	3279
	21st to end	Night	27% 20% 7% 9% 6% 3 4 4 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	57% 4 12% 19% 8% 1	1900
	11th to 20th	Day	49% 5% 12% 20% 12% 2	2041
	21st to end	Day	55% 4 12% 20% 8% 2	2262
April	1st to 10th	Day	56% 4 9% 20% 11%	2160
	11th to 20th	Day	48% 5% 10% 21% 14% 2	2195
	21st to end	Day	50% 5% 12% 17% 14% 2	2340
May	1st to 10th	Day	48% 5% 11% 22% 12% 2	2340
	11th to 20th	Day	47% 4 12% 21% 14% 2	2305
	21st to end	Day	41% 6% 13% 25% 14% 1	2704
June	1st to 10th	Day	38% 5% 12% 27% 16% 2	2520
	11th to 20th	Day	38% 5% 12% 24% 20% 2	2520
	21st to end	Day	38% 6% 13% 23% 18% 2	2520
July	1st to 10th	Day	30% 6% 13% 29% 18% 4	2451
	11th to 20th	Day	35% 6% 11% 28% 17% 2	2445
	21st to end	Day	31% 6% 12% 27% 19% 3	2591
August	1st to 10th	Day	35% 6% 14% 29% 13% 2	2323
	11th to 20th	Day	37% 6% 14% 28% 14% 2	2303
	21st to end	Day	35% 5% 13% 27% 16% 3	2425
September	1st to 10th	Day	41% 5% 12% 26% 14% 2	2120
	11th to 20th	Day	35% 5% 13% 31% 15% 2	1980
	21st to end	Day	42% 4 12% 25% 16%	1980
October	1st to 10th	Day	45% 5% 9% 25% 15% 2	1980
	11th to 20th	Day	59% 5% 12% 17% 7%	1757
	21st to end	Day	62% 3 12% 15% 6% 1	1782
November	1st to 10th	Day	46% 5% 12% 23% 13% 1	1672
	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	49% 4 13% 24% 9% 1	2280
	11th to 20th	Night	43% 4 12% 24% 14% 3	2138
	21st to end	Night	51% 4 9% 23% 10% 2	2154
April	1st to 10th	Night	50% 5% 10% 21% 14% 1	1800
	11th to 20th	Night	47% 4 9% 20% 17% 3	1712
	21st to end	Night	45% 5% 13% 19% 15% 3	1620
May	1st to 10th	Night	49% 3 13% 21% 11% 2	1620
	11th to 20th	Night	45% 4 10% 20% 18% 3	1593
	21st to end	Night	41% 5% 13% 24% 15% 2	1569
June	1st to 10th	Night	35% 6% 10% 30% 17% 2	1440
	11th to 20th	Night	34% 6% 15% 26% 19% 1	1440
	21st to end	Night	33% 7% 15% 27% 16% 2	1440
July	1st to 10th	Night	27% 5% 14% 30% 20% 5%	1400
	11th to 20th	Night	29% 5% 15% 30% 20% 2	1392
	21st to end	Night	28% 6% 14% 27% 20% 4	1487
August	1st to 10th	Night	30% 5% 16% 31% 15% 2	1591
	11th to 20th	Night	30% 7% 13% 32% 16% 2	1598
	21st to end	Night	33% 5% 10% 28% 21% 3	1928
September	1st to 10th	Night	33% 6% 12% 29% 18% 2	1821
	11th to 20th	Night	32% 4 11% 35% 17% 1	1980
	21st to end	Night	39% 4 13% 28% 14% 1	1980
October	1st to 10th	Night	40% 4 10% 28% 17% 2	1980
	11th to 20th	Night	52% 4 12% 21% 10% 1	2199
	21st to end	Night	55% 4 12% 19% 9% 1	2574
November	1st to 10th	Night	46% 4 10% 24% 14% 1	2406
	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	33% 16% 30% 16% 4 2	470
April	1st to 10th	Day	27% 19% 30% 18% 5% 1	420
	11th to 20th	Day	30% 23% 25% 12% 7% 3	412
	21st to end	Day	25% 18% 31% 18% 7% 1	420
May	1st to 10th	Day	26% 20% 28% 18% 6% 3	423
	11th to 20th	Day	22% 17% 29% 21% 8% 4	447
	21st to end	Day	16% 11% 31% 26% 13% 4	658
June	1st to 10th	Day	12% 13% 38% 25% 9% 4	630
	11th to 20th	Day	17% 11% 33% 25% 11% 3	630
	21st to end	Day	14% 11% 38% 22% 12% 3	630
July	1st to 10th	Day	8% 7% 27% 28% 20% 8%	621
	11th to 20th	Day	12% 10% 29% 27% 14% 8%	618
	21st to end	Day	10% 7% 34% 28% 16% 6%	684
August	1st to 10th	Day	10% 5% 26% 26% 20% 12%	456
	11th to 20th	Day	13% 7% 21% 28% 24% 9%	439
	21st to end	Day	14% 7% 25% 27% 18% 9%	495
September	1st to 10th	Day	15% 8% 25% 26% 18% 7%	450
	11th to 20th	Day	12% 6% 21% 30% 22% 9%	450
	21st to end	Day	20% 13% 22% 23% 15% 6%	450
October	1st to 10th	Day	21% 14% 26% 22% 14% 4	450
	11th to 20th	Day	26% 19% 32% 18% 4 1	375
	21st to end	Day	34% 19% 35% 9% 4	297
November	1st to 10th	Day	25% 17% 37% 14% 6% 1	270
	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% 36% 22% 6% 1	760
	21st to end	Night	25% 12% 33% 23% 7%	754
April	1st to 10th	Night	21% 15% 35% 24% 5% 1	540
	11th to 20th	Night	24% 12% 35% 22% 6% 1	533
	21st to end	Night	24% 12% 33% 26% 6%	540
May	1st to 10th	Night	19% 10% 34% 28% 8% 1	540
	11th to 20th	Night	21% 10% 31% 27% 11%	531
	21st to end	Night	15% 5% 27% 33% 19% 1	584
June	1st to 10th	Night	13% 6% 30% 35% 15% 1	540
	11th to 20th	Night	13% 5% 31% 35% 15% 1	540
	21st to end	Night	11% 5% 28% 37% 18% 2	540
July	1st to 10th	Night	9% 3 21% 40% 23% 3	525
	11th to 20th	Night	8% 2 26% 39% 22% 3	522
	21st to end	Night	7% 2 26% 39% 25% 2	591
August	1st to 10th	Night	10% 2 19% 39% 26% 3	537
	11th to 20th	Night	11% 3 19% 39% 24% 5%	534
	21st to end	Night	12% 2 21% 37% 25% 3	594
September	1st to 10th	Night	10% 4 24% 36% 20% 5%	561
	11th to 20th	Night	12% 5% 23% 35% 22% 3	720
	21st to end	Night	18% 9% 22% 30% 19% 3	720
October	1st to 10th	Night	18% 7% 28% 30% 16% 1	720
	11th to 20th	Night	26% 15% 29% 22% 8%	720
	21st to end	Night	29% 13% 31% 21% 6% 1	792
November	1st to 10th	Night	21% 14% 31% 25% 9%	740
	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 2024-03-19 23:31:02. Version ac07

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