Observatoire Vandal
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Vandal clear sky chart from 2005-11-06 to 2024-01-27.

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	19% 10% 4 7% 6% 3 5% 5% 5% 6% 30%	74880
	Night	20% 12% 5% 8% 6% 3 4 4 4 6% 29%	75157
Transparency	Day	55% 6% 12% 18% 8% 1	74585
	Night	57% 6% 11% 17% 8% 1	74986
Seeing	Day	26% 15% 29% 20% 8% 2	13549
	Night	26% 13% 28% 24% 8% 1	22615

By Month

Percentage Cloud Cover NE (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	22% 20% 7% 11% 6% 3 4 3 4 4 14%	1517
	11th to 20th	Day	20% 18% 7% 11% 6% 3 6% 4 4 4 17%	1520
	21st to end	Day	12% 13% 7% 11% 8% 4 7% 5% 4 5% 23%	1632
February	1st to 10th	Day	17% 17% 6% 11% 8% 5% 5% 4 4 4 19%	1534
	11th to 20th	Day	15% 16% 8% 11% 7% 3 5% 4 4 5% 21%	1709
	21st to end	Day	16% 18% 6% 9% 7% 4 5% 5% 4 5% 23%	1480
March	1st to 10th	Day	17% 14% 5% 10% 8% 4 5% 4 4 5% 25%	1797
	11th to 20th	Day	18% 12% 5% 8% 7% 3 4 3 4 5% 31%	1844
	21st to end	Day	19% 13% 4 7% 6% 3 4 4 4 4 32%	2376
April	1st to 10th	Day	23% 13% 5% 7% 5% 3 4 4 4 5% 26%	2160
	11th to 20th	Day	22% 10% 3 5% 5% 3 4 4 5% 5% 35%	2138
	21st to end	Day	25% 11% 3 6% 5% 3 4 4 4 5% 30%	2340
May	1st to 10th	Day	22% 8% 3 6% 5% 3 4 4 5% 6% 33%	2425
	11th to 20th	Day	19% 7% 3 6% 6% 3 4 5% 5% 6% 36%	2478
	21st to end	Day	19% 6% 4 5% 5% 3 5% 6% 6% 8% 32%	2725
June	1st to 10th	Day	25% 8% 3 5% 5% 3 4 5% 5% 7% 29%	2520
	11th to 20th	Day	17% 7% 3 5% 5% 3 4 6% 6% 8% 37%	2520
	21st to end	Day	22% 7% 4 7% 6% 3 4 5% 5% 7% 30%	2520
July	1st to 10th	Day	16% 5% 2 5% 5% 4 6% 5% 6% 8% 38%	2520
	11th to 20th	Day	14% 6% 3 6% 5% 3 5% 7% 8% 8% 35%	2520
	21st to end	Day	15% 7% 3 6% 6% 3 5% 5% 7% 9% 34%	2744
August	1st to 10th	Day	15% 6% 3 7% 6% 4 5% 6% 6% 9% 33%	2506
	11th to 20th	Day	13% 6% 2 5% 6% 3 5% 6% 7% 8% 38%	2295
	21st to end	Day	16% 5% 3 7% 5% 3 4 5% 6% 8% 38%	2376
September	1st to 10th	Day	15% 6% 3 5% 5% 3 4 5% 6% 7% 40%	2160
	11th to 20th	Day	18% 6% 2 4 3 3 4 5% 4 7% 44%	2048
	21st to end	Day	18% 7% 3 4 5% 3 4 5% 5% 7% 38%	1980
October	1st to 10th	Day	24% 8% 3 6% 6% 3 4 5% 6% 7% 28%	1913
	11th to 20th	Day	22% 10% 3 7% 6% 4 5% 6% 5% 6% 27%	1721
	21st to end	Day	27% 13% 4 6% 5% 3 4 3 4 5% 25%	1782
November	1st to 10th	Day	21% 11% 4 6% 5% 3 5% 4 5% 5% 31%	1671
	11th to 20th	Day	19% 15% 5% 9% 6% 3 5% 4 4 5% 25%	1691
	21st to end	Day	22% 16% 5% 8% 5% 3 5% 5% 5% 6% 21%	1520
December	1st to 10th	Day	24% 19% 5% 11% 8% 3 5% 4 3 4 15%	1520
	11th to 20th	Day	23% 15% 6% 8% 6% 3 5% 4 5% 5% 20%	1906
	21st to end	Day	26% 17% 5% 11% 6% 3 4 4 3 4 16%	2772
January	1st to 10th	Night	21% 18% 8% 10% 7% 4 4 4 3 4 17%	2649
	11th to 20th	Night	17% 17% 8% 11% 7% 3 5% 4 4 4 19%	2660
	21st to end	Night	16% 14% 6% 10% 8% 4 4 4 4 5% 23%	2868
February	1st to 10th	Night	16% 16% 6% 14% 8% 3 4 4 4 5% 18%	2424
	11th to 20th	Night	16% 15% 6% 10% 8% 4 5% 4 4 4 24%	2135
	21st to end	Night	16% 15% 7% 9% 7% 3 5% 3 3 4 28%	1776
March	1st to 10th	Night	17% 16% 6% 8% 7% 3 5% 3 4 5% 26%	2150
	11th to 20th	Night	20% 14% 5% 7% 6% 3 3 4 4 5% 29%	2110
	21st to end	Night	19% 13% 5% 7% 4 2 3 4 4 5% 34%	1980
April	1st to 10th	Night	25% 12% 4 8% 6% 2 4 5% 5% 5% 26%	1800
	11th to 20th	Night	23% 9% 3 6% 6% 2 4 4 4 6% 33%	1769
	21st to end	Night	23% 11% 3 6% 5% 3 3 4 4 6% 32%	1620
May	1st to 10th	Night	23% 9% 4 6% 5% 4 4 4 5% 7% 31%	1550
	11th to 20th	Night	20% 7% 3 7% 4 3 4 5% 4 6% 36%	1415
	21st to end	Night	19% 6% 3 4 4 4 5% 4 6% 8% 37%	1550
June	1st to 10th	Night	25% 7% 3 5% 5% 3 4 5% 5% 8% 30%	1440
	11th to 20th	Night	19% 6% 3 5% 4 2 5% 5% 5% 8% 38%	1440
	21st to end	Night	18% 7% 3 7% 5% 3 5% 5% 6% 8% 33%	1440
July	1st to 10th	Night	14% 6% 2 6% 6% 3 5% 5% 8% 8% 37%	1440
	11th to 20th	Night	12% 5% 2 4 6% 3 7% 7% 6% 9% 38%	1440
	21st to end	Night	13% 6% 3 5% 6% 4 5% 6% 6% 9% 37%	1574
August	1st to 10th	Night	16% 6% 2 6% 6% 2 5% 5% 7% 8% 38%	1432
	11th to 20th	Night	13% 5% 3 8% 6% 3 5% 5% 5% 8% 38%	1584
	21st to end	Night	14% 7% 3 6% 5% 3 5% 5% 6% 8% 39%	1980
September	1st to 10th	Night	16% 7% 3 6% 5% 2 4 4 5% 6% 42%	1800
	11th to 20th	Night	16% 6% 2 5% 4 2 3 4 5% 7% 46%	1912
	21st to end	Night	20% 6% 2 4 5% 3 4 4 4 7% 41%	1980
October	1st to 10th	Night	19% 7% 3 7% 5% 2 4 4 5% 6% 36%	2042
	11th to 20th	Night	23% 9% 4 8% 6% 3 5% 4 4 6% 30%	2237
	21st to end	Night	27% 11% 4 7% 6% 3 4 4 4 5% 26%	2574
November	1st to 10th	Night	21% 12% 4 6% 5% 3 4 5% 4 6% 29%	2407
	11th to 20th	Night	21% 13% 4 8% 6% 3 4 4 4 6% 27%	2487
	21st to end	Night	22% 16% 5% 8% 4 3 4 3 4 5% 25%	2660
December	1st to 10th	Night	25% 19% 7% 9% 6% 3 3 3 4 4 16%	2660
	11th to 20th	Night	24% 17% 6% 10% 7% 3 4 3 3 4 20%	3288
	21st to end	Night	25% 18% 6% 10% 6% 3 4 4 3 3 19%	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	75% 4 10% 9% 2	1518
	11th to 20th	Day	71% 5% 9% 10% 5%	1520
	21st to end	Day	63% 7% 12% 12% 5% 1	1632
February	1st to 10th	Day	69% 6% 9% 12% 4 1	1534
	11th to 20th	Day	67% 5% 13% 10% 5% 1	1718
	21st to end	Day	65% 5% 10% 13% 6% 1	1480
March	1st to 10th	Day	62% 6% 9% 14% 6% 2	1800
	11th to 20th	Day	57% 5% 13% 13% 11% 1	1844
	21st to end	Day	57% 4 10% 18% 9% 2	2376
April	1st to 10th	Day	61% 5% 9% 16% 8% 1	2160
	11th to 20th	Day	52% 5% 9% 21% 13% 1	2138
	21st to end	Day	58% 5% 10% 16% 9% 2	2340
May	1st to 10th	Day	53% 4 9% 19% 13% 2	2425
	11th to 20th	Day	48% 6% 12% 19% 13% 2	2482
	21st to end	Day	50% 8% 12% 17% 12% 1	2725
June	1st to 10th	Day	54% 6% 12% 18% 9% 1	2520
	11th to 20th	Day	44% 7% 13% 23% 11% 2	2520
	21st to end	Day	54% 7% 11% 18% 9%	2520
July	1st to 10th	Day	42% 8% 14% 26% 9%	2450
	11th to 20th	Day	44% 8% 15% 27% 6%	2445
	21st to end	Day	46% 9% 16% 22% 7%	2591
August	1st to 10th	Day	46% 9% 17% 23% 5%	2492
	11th to 20th	Day	42% 10% 17% 21% 10%	2295
	21st to end	Day	43% 8% 14% 24% 11%	2376
September	1st to 10th	Day	43% 8% 14% 25% 10% 1	2160
	11th to 20th	Day	40% 5% 13% 29% 12%	2048
	21st to end	Day	44% 7% 16% 24% 9% 1	1980
October	1st to 10th	Day	54% 6% 14% 18% 8%	1913
	11th to 20th	Day	55% 8% 15% 17% 5%	1721
	21st to end	Day	64% 5% 10% 14% 5% 1	1782
November	1st to 10th	Day	54% 7% 13% 18% 7%	1671
	11th to 20th	Day	63% 6% 12% 12% 8% 1	1691
	21st to end	Day	66% 7% 14% 10% 3	1520
December	1st to 10th	Day	75% 5% 10% 8% 2	1520
	11th to 20th	Day	68% 4 10% 13% 4	1906
	21st to end	Day	73% 6% 10% 8% 4	2772
January	1st to 10th	Night	72% 5% 10% 9% 3	2649
	11th to 20th	Night	71% 5% 9% 10% 4 1	2660
	21st to end	Night	64% 5% 10% 12% 8%	2868
February	1st to 10th	Night	69% 5% 8% 12% 5% 1	2424
	11th to 20th	Night	64% 5% 10% 14% 6%	2137
	21st to end	Night	63% 4 8% 14% 11% 1	1776
March	1st to 10th	Night	61% 4 9% 16% 8% 1	2160
	11th to 20th	Night	59% 5% 9% 14% 10% 2	2111
	21st to end	Night	54% 4 7% 20% 12% 3	1980
April	1st to 10th	Night	60% 5% 8% 15% 11% 1	1800
	11th to 20th	Night	53% 4 9% 20% 13% 1	1769
	21st to end	Night	54% 5% 9% 17% 12% 2	1620
May	1st to 10th	Night	54% 6% 6% 18% 14% 3	1550
	11th to 20th	Night	49% 6% 10% 17% 16% 3	1416
	21st to end	Night	45% 7% 11% 20% 13% 2	1551
June	1st to 10th	Night	52% 6% 10% 22% 9% 1	1440
	11th to 20th	Night	44% 8% 11% 22% 12% 3	1440
	21st to end	Night	49% 8% 13% 20% 10%	1440
July	1st to 10th	Night	41% 10% 12% 23% 13% 1	1400
	11th to 20th	Night	38% 11% 14% 27% 9%	1392
	21st to end	Night	41% 9% 17% 24% 8%	1486
August	1st to 10th	Night	41% 8% 18% 24% 8%	1422
	11th to 20th	Night	43% 8% 16% 22% 10%	1584
	21st to end	Night	42% 8% 15% 25% 10% 1	1980
September	1st to 10th	Night	43% 6% 13% 25% 13% 1	1800
	11th to 20th	Night	38% 5% 15% 29% 11% 1	1912
	21st to end	Night	43% 6% 12% 25% 13% 1	1980
October	1st to 10th	Night	48% 6% 12% 21% 11%	2042
	11th to 20th	Night	56% 6% 11% 20% 5%	2237
	21st to end	Night	61% 6% 10% 15% 8% 1	2574
November	1st to 10th	Night	57% 5% 13% 18% 6% 1	2407
	11th to 20th	Night	58% 6% 11% 14% 8% 2	2487
	21st to end	Night	63% 5% 13% 15% 5%	2660
December	1st to 10th	Night	74% 4 10% 9% 3	2660
	11th to 20th	Night	71% 4 8% 12% 5%	3288
	21st to end	Night	71% 6% 9% 13% 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	31% 18% 28% 18% 5%	180
	11th to 20th	Day	29% 24% 32% 11% 4	180
	21st to end	Day	23% 20% 33% 18% 5% 1	188
February	1st to 10th	Day	40% 26% 22% 10% 1	202
	11th to 20th	Day	29% 24% 34% 11% 2	377
	21st to end	Day	29% 22% 30% 18% 2	346
March	1st to 10th	Day	29% 17% 32% 19% 4	420
	11th to 20th	Day	27% 21% 33% 15% 4 1	420
	21st to end	Day	30% 19% 25% 18% 6% 1	462
April	1st to 10th	Day	34% 13% 31% 14% 7% 1	420
	11th to 20th	Day	36% 17% 23% 15% 6% 3	412
	21st to end	Day	33% 15% 32% 12% 6% 2	420
May	1st to 10th	Day	32% 14% 28% 18% 5% 2	423
	11th to 20th	Day	23% 15% 28% 22% 9% 1	447
	21st to end	Day	22% 17% 32% 20% 8% 2	485
June	1st to 10th	Day	30% 12% 32% 18% 6% 2	450
	11th to 20th	Day	20% 14% 32% 23% 11%	450
	21st to end	Day	24% 10% 34% 20% 8% 3	450
July	1st to 10th	Day	19% 9% 31% 26% 14% 2	445
	11th to 20th	Day	17% 10% 32% 29% 9% 3	444
	21st to end	Day	21% 8% 31% 29% 10% 1	488
August	1st to 10th	Day	17% 9% 29% 28% 14% 2	446
	11th to 20th	Day	18% 7% 28% 27% 17% 4	439
	21st to end	Day	19% 12% 24% 24% 14% 7%	495
September	1st to 10th	Day	16% 12% 32% 23% 11% 6%	450
	11th to 20th	Day	19% 11% 23% 23% 16% 7%	450
	21st to end	Day	19% 13% 26% 24% 13% 5%	450
October	1st to 10th	Day	24% 15% 28% 18% 11% 4	450
	11th to 20th	Day	30% 16% 24% 19% 10% 2	410
	21st to end	Day	38% 16% 23% 18% 4	396
November	1st to 10th	Day	33% 16% 28% 18% 5%	365
	11th to 20th	Day	30% 21% 26% 14% 8% 1	351
	21st to end	Day	32% 22% 31% 13% 3	180
December	1st to 10th	Day	35% 18% 24% 17% 5% 2	180
	11th to 20th	Day	33% 24% 29% 10% 3 1	180
	21st to end	Day	36% 23% 25% 11% 4 1	198
January	1st to 10th	Night	32% 14% 32% 18% 4	760
	11th to 20th	Night	27% 20% 31% 17% 4	760
	21st to end	Night	24% 14% 31% 25% 5%	820
February	1st to 10th	Night	26% 18% 35% 18% 3	720
	11th to 20th	Night	25% 17% 33% 20% 4	703
	21st to end	Night	25% 17% 32% 24% 2	592
March	1st to 10th	Night	27% 16% 29% 25% 3	720
	11th to 20th	Night	28% 16% 28% 23% 6%	720
	21st to end	Night	27% 13% 28% 24% 7% 1	792
April	1st to 10th	Night	33% 13% 28% 23% 4	720
	11th to 20th	Night	28% 15% 26% 24% 7%	705
	21st to end	Night	30% 15% 27% 21% 7%	540
May	1st to 10th	Night	28% 9% 32% 21% 8% 1	470
	11th to 20th	Night	23% 11% 30% 27% 8% 1	354
	21st to end	Night	23% 6% 32% 31% 7% 1	389
June	1st to 10th	Night	30% 6% 28% 27% 8% 1	360
	11th to 20th	Night	20% 8% 27% 32% 12%	360
	21st to end	Night	17% 8% 29% 34% 12% 1	360
July	1st to 10th	Night	15% 6% 29% 38% 12% 1	350
	11th to 20th	Night	14% 5% 24% 39% 17% 1	348
	21st to end	Night	15% 6% 25% 40% 14%	394
August	1st to 10th	Night	19% 4 20% 37% 18% 1	358
	11th to 20th	Night	16% 6% 22% 34% 21% 2	520
	21st to end	Night	17% 6% 23% 34% 17% 2	792
September	1st to 10th	Night	19% 7% 25% 34% 14% 1	720
	11th to 20th	Night	19% 8% 23% 29% 20% 2	720
	21st to end	Night	22% 7% 28% 26% 14% 2	720
October	1st to 10th	Night	23% 11% 28% 25% 12% 1	720
	11th to 20th	Night	29% 12% 25% 25% 8% 1	720
	21st to end	Night	35% 11% 29% 20% 5%	792
November	1st to 10th	Night	30% 14% 31% 20% 6% 1	740
	11th to 20th	Night	27% 19% 27% 17% 10% 1	760
	21st to end	Night	32% 18% 25% 19% 5% 1	760
December	1st to 10th	Night	37% 16% 27% 16% 4	760
	11th to 20th	Night	30% 18% 29% 17% 5% 1	760
	21st to end	Night	35% 18% 27% 17% 4	836

Page generated at 2024-01-24 09:37:30. Version ac07

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