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 2025-02-06.

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	18% 12% 4 9% 5% 5% 5% 4 5% 6% 28%	108429
	Night	18% 14% 5% 10% 5% 4 4 3 4 5% 27%	109823
Transparency	Day	55% 6% 12% 19% 7% 1	111933
	Night	58% 5% 11% 17% 8% 1	114483
Seeing	Day	26% 15% 29% 20% 8% 2	14836
	Night	26% 13% 28% 24% 8% 1	23897

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	18% 25% 7% 16% 5% 6% 3 2 3 3 13%	3597
	11th to 20th	Day	11% 17% 8% 20% 9% 6% 5% 3 4 4 12%	3570
	21st to end	Day	12% 17% 7% 13% 6% 6% 6% 4 4 6% 19%	3960
February	1st to 10th	Day	16% 18% 6% 11% 8% 5% 4 3 4 4 20%	1660
	11th to 20th	Day	14% 17% 7% 12% 7% 4 5% 3 4 5% 21%	1807
	21st to end	Day	15% 19% 4 9% 6% 5% 5% 4 4 6% 23%	1570
March	1st to 10th	Day	20% 14% 5% 11% 6% 5% 4 3 5% 5% 23%	3116
	11th to 20th	Day	15% 20% 6% 11% 6% 4 3 3 4 4 24%	3246
	21st to end	Day	17% 13% 5% 9% 4 4 4 2 4 5% 34%	4166
April	1st to 10th	Day	23% 14% 4 8% 5% 4 4 3 4 5% 27%	2280
	11th to 20th	Day	21% 11% 3 7% 4 4 4 3 5% 5% 34%	2259
	21st to end	Day	24% 11% 3 7% 5% 4 4 3 4 5% 30%	2470
May	1st to 10th	Day	23% 10% 3 7% 5% 4 4 4 6% 6% 28%	4261
	11th to 20th	Day	18% 7% 4 8% 5% 5% 5% 5% 6% 8% 29%	4398
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 9% 33%	4837
June	1st to 10th	Day	24% 9% 2 6% 4 4 4 4 5% 7% 29%	2660
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 5% 6% 8% 36%	2660
	21st to end	Day	23% 8% 3 8% 5% 4 4 4 5% 6% 30%	2660
July	1st to 10th	Day	14% 6% 2 6% 6% 6% 6% 4 7% 8% 34%	4440
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 5% 8% 8% 34%	4440
	21st to end	Day	14% 7% 2 6% 4 4 4 4 6% 8% 41%	4814
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 6% 4 7% 9% 29%	4423
	11th to 20th	Day	17% 6% 2 7% 5% 5% 5% 4 6% 8% 34%	4021
	21st to end	Day	18% 6% 3 7% 4 4 4 4 5% 8% 38%	4180
September	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 6% 7% 41%	2280
	11th to 20th	Day	17% 6% 2 4 3 4 4 4 4 7% 45%	2161
	21st to end	Day	18% 8% 2 5% 4 4 4 4 5% 7% 38%	2090
October	1st to 10th	Day	23% 8% 2 7% 5% 4 4 4 6% 7% 29%	2019
	11th to 20th	Day	22% 10% 3 8% 5% 5% 4 4 5% 6% 28%	1816
	21st to end	Day	26% 13% 4 7% 4 4 4 2 4 5% 27%	1881
November	1st to 10th	Day	20% 12% 3 7% 4 4 5% 4 5% 5% 31%	1761
	11th to 20th	Day	18% 16% 4 10% 6% 4 4 3 4 5% 26%	1780
	21st to end	Day	22% 18% 5% 8% 4 4 5% 4 5% 5% 20%	1600
December	1st to 10th	Day	19% 23% 6% 14% 6% 4 4 3 3 3 14%	2600
	11th to 20th	Day	21% 16% 7% 11% 5% 4 4 3 5% 5% 20%	2986
	21st to end	Day	21% 17% 5% 11% 5% 5% 5% 3 5% 6% 17%	3960
January	1st to 10th	Night	16% 24% 9% 14% 5% 4 3 3 3 3 16%	6349
	11th to 20th	Night	10% 20% 8% 19% 8% 6% 5% 3 4 4 13%	6315
	21st to end	Night	13% 18% 6% 13% 8% 5% 4 3 4 4 22%	6996
February	1st to 10th	Night	15% 18% 5% 14% 7% 4 4 3 4 5% 19%	2629
	11th to 20th	Night	15% 16% 5% 12% 7% 5% 5% 3 4 4 24%	2257
	21st to end	Night	15% 17% 5% 10% 6% 5% 4 2 4 4 28%	1884
March	1st to 10th	Night	22% 12% 4 7% 5% 4 4 3 4 4 31%	3724
	11th to 20th	Night	18% 21% 5% 10% 5% 4 3 2 3 4 25%	3708
	21st to end	Night	14% 16% 5% 10% 4 4 3 3 4 4 33%	3466
April	1st to 10th	Night	24% 12% 4 8% 5% 4 4 4 5% 5% 27%	1900
	11th to 20th	Night	23% 11% 3 6% 5% 3 4 3 4 6% 32%	1868
	21st to end	Night	22% 11% 3 7% 4 4 4 3 4 6% 33%	1710
May	1st to 10th	Night	24% 9% 3 8% 4 5% 4 3 6% 6% 27%	2708
	11th to 20th	Night	22% 9% 3 8% 3 4 5% 4 5% 6% 33%	2495
	21st to end	Night	18% 7% 2 6% 4 4 5% 4 6% 8% 36%	2738
June	1st to 10th	Night	25% 8% 3 6% 4 4 5% 3 6% 7% 30%	1520
	11th to 20th	Night	18% 6% 2 5% 4 4 5% 3 6% 8% 38%	1520
	21st to end	Night	19% 8% 3 7% 4 4 5% 4 6% 7% 32%	1520
July	1st to 10th	Night	14% 8% 3 9% 6% 5% 5% 4 8% 7% 32%	2520
	11th to 20th	Night	16% 7% 2 6% 5% 4 5% 5% 6% 8% 38%	2520
	21st to end	Night	15% 7% 2 6% 4 4 5% 4 7% 7% 38%	2739
August	1st to 10th	Night	21% 9% 3 8% 4 4 4 3 6% 6% 32%	2506
	11th to 20th	Night	16% 7% 3 9% 6% 5% 5% 4 6% 6% 34%	2772
	21st to end	Night	17% 6% 2 6% 4 4 4 3 6% 7% 40%	3476
September	1st to 10th	Night	16% 7% 3 6% 5% 3 4 3 5% 6% 42%	1900
	11th to 20th	Night	15% 6% 2 6% 4 3 3 3 5% 7% 47%	2019
	21st to end	Night	19% 6% 2 5% 5% 5% 3 3 5% 7% 41%	2090
October	1st to 10th	Night	20% 8% 3 7% 4 4 4 3 5% 6% 36%	2156
	11th to 20th	Night	22% 10% 4 9% 4 4 5% 3 4 5% 31%	2362
	21st to end	Night	26% 11% 4 8% 5% 4 4 3 4 5% 27%	2717
November	1st to 10th	Night	21% 13% 3 7% 5% 4 5% 4 4 6% 29%	2537
	11th to 20th	Night	20% 14% 3 9% 5% 4 4 3 4 6% 28%	2618
	21st to end	Night	22% 17% 5% 9% 4 4 3 3 4 4 24%	2800
December	1st to 10th	Night	18% 26% 8% 13% 6% 4 3 2 3 4 12%	4580
	11th to 20th	Night	23% 17% 5% 12% 6% 4 4 2 3 3 21%	5208
	21st to end	Night	22% 18% 5% 10% 5% 4 3 3 4 7% 19%	6996

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	77% 4 9% 8% 1	3598
	11th to 20th	Day	74% 5% 9% 9% 2	3600
	21st to end	Day	66% 7% 9% 11% 7%	3960
February	1st to 10th	Day	68% 6% 9% 12% 4 1	1660
	11th to 20th	Day	67% 5% 13% 10% 5% 1	1816
	21st to end	Day	63% 5% 11% 13% 6% 1	1570
March	1st to 10th	Day	62% 6% 8% 16% 6% 1	3119
	11th to 20th	Day	63% 5% 11% 10% 10% 1	3246
	21st to end	Day	54% 4 10% 18% 14% 1	4166
April	1st to 10th	Day	61% 5% 9% 16% 8% 1	2280
	11th to 20th	Day	53% 5% 9% 20% 12% 1	2259
	21st to end	Day	57% 5% 10% 16% 10% 2	2470
May	1st to 10th	Day	55% 6% 11% 20% 8% 1	4275
	11th to 20th	Day	51% 9% 17% 15% 7% 1	4402
	21st to end	Day	45% 8% 12% 22% 12% 1	4837
June	1st to 10th	Day	54% 6% 12% 19% 9% 1	2660
	11th to 20th	Day	44% 7% 14% 22% 11% 2	2660
	21st to end	Day	55% 7% 11% 18% 9%	2660
July	1st to 10th	Day	45% 7% 14% 27% 7%	4370
	11th to 20th	Day	45% 8% 15% 28% 4	4365
	21st to end	Day	41% 7% 14% 30% 7%	4661
August	1st to 10th	Day	50% 8% 16% 20% 6%	4409
	11th to 20th	Day	46% 8% 20% 20% 6%	4061
	21st to end	Day	44% 6% 16% 20% 13% 1	4180
September	1st to 10th	Day	42% 7% 14% 26% 10% 1	2280
	11th to 20th	Day	40% 5% 14% 29% 12%	2161
	21st to end	Day	44% 6% 15% 24% 9%	2090
October	1st to 10th	Day	47% 5% 15% 27% 6%	3357
	11th to 20th	Day	51% 5% 15% 25% 4	2993
	21st to end	Day	51% 5% 17% 20% 6% 1	3081
November	1st to 10th	Day	54% 7% 12% 19% 7%	1761
	11th to 20th	Day	62% 6% 11% 13% 8% 1	1780
	21st to end	Day	67% 6% 13% 10% 3	1600
December	1st to 10th	Day	77% 4 8% 9% 2	2600
	11th to 20th	Day	68% 4 8% 16% 5%	2986
	21st to end	Day	68% 5% 9% 11% 7%	3960
January	1st to 10th	Night	74% 6% 9% 8% 2	6349
	11th to 20th	Night	75% 6% 9% 8% 2	6360
	21st to end	Night	66% 4 9% 10% 11%	6996
February	1st to 10th	Night	67% 5% 8% 13% 6% 1	2629
	11th to 20th	Night	64% 5% 10% 14% 6%	2259
	21st to end	Night	62% 5% 9% 14% 10% 1	1884
March	1st to 10th	Night	56% 6% 9% 19% 8% 3	3734
	11th to 20th	Night	64% 5% 8% 12% 10% 1	3709
	21st to end	Night	56% 4 8% 19% 11% 2	3466
April	1st to 10th	Night	59% 5% 8% 16% 11% 1	1900
	11th to 20th	Night	54% 4 9% 19% 12% 1	1868
	21st to end	Night	53% 5% 10% 17% 13% 2	1710
May	1st to 10th	Night	56% 8% 9% 17% 8% 2	2714
	11th to 20th	Night	49% 8% 13% 19% 9% 2	2496
	21st to end	Night	44% 7% 11% 20% 16% 1	2739
June	1st to 10th	Night	52% 6% 10% 21% 9% 1	1520
	11th to 20th	Night	44% 7% 12% 22% 12% 3	1520
	21st to end	Night	50% 7% 12% 19% 10% 1	1520
July	1st to 10th	Night	48% 7% 14% 20% 11%	2480
	11th to 20th	Night	42% 8% 16% 26% 7%	2472
	21st to end	Night	43% 7% 17% 22% 10%	2651
August	1st to 10th	Night	51% 6% 14% 23% 6%	2496
	11th to 20th	Night	49% 7% 17% 20% 7%	2798
	21st to end	Night	43% 6% 13% 23% 14% 1	3476
September	1st to 10th	Night	43% 6% 13% 25% 13% 1	1900
	11th to 20th	Night	37% 5% 16% 30% 12% 1	2019
	21st to end	Night	43% 6% 12% 25% 13% 1	2090
October	1st to 10th	Night	51% 5% 12% 23% 9%	3598
	11th to 20th	Night	50% 5% 11% 27% 7%	3924
	21st to end	Night	53% 4 13% 18% 10% 2	4467
November	1st to 10th	Night	57% 5% 13% 18% 6% 1	2537
	11th to 20th	Night	58% 6% 11% 15% 9% 2	2618
	21st to end	Night	64% 5% 13% 14% 4	2800
December	1st to 10th	Night	78% 3 9% 7% 2	4580
	11th to 20th	Night	71% 4 6% 12% 7%	5208
	21st to end	Night	66% 5% 8% 13% 8%	6996

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	32% 20% 28% 17% 5%	200
	11th to 20th	Day	27% 26% 34% 10% 4	200
	21st to end	Day	25% 20% 32% 18% 5%	220
February	1st to 10th	Day	38% 25% 22% 13% 1	238
	11th to 20th	Day	29% 24% 33% 11% 2	415
	21st to end	Day	28% 23% 31% 17% 2	382
March	1st to 10th	Day	30% 17% 30% 19% 3	460
	11th to 20th	Day	27% 21% 33% 14% 4 1	460
	21st to end	Day	30% 21% 26% 17% 6% 1	506
April	1st to 10th	Day	34% 14% 30% 15% 7% 1	460
	11th to 20th	Day	35% 17% 25% 14% 6% 3	452
	21st to end	Day	33% 15% 32% 12% 6% 2	460
May	1st to 10th	Day	32% 15% 28% 18% 6% 2	463
	11th to 20th	Day	23% 15% 28% 23% 10% 1	487
	21st to end	Day	22% 17% 31% 21% 7% 2	529
June	1st to 10th	Day	29% 12% 32% 19% 7% 1	490
	11th to 20th	Day	19% 15% 32% 23% 11%	490
	21st to end	Day	27% 11% 34% 19% 7% 3	490
July	1st to 10th	Day	18% 10% 32% 26% 13% 2	485
	11th to 20th	Day	19% 11% 32% 27% 8% 2	484
	21st to end	Day	20% 8% 31% 29% 9% 2	532
August	1st to 10th	Day	18% 9% 30% 29% 13% 2	486
	11th to 20th	Day	19% 7% 28% 27% 16% 3	479
	21st to end	Day	19% 12% 25% 25% 14% 6%	539
September	1st to 10th	Day	16% 13% 32% 24% 10% 6%	490
	11th to 20th	Day	18% 10% 22% 25% 18% 8%	490
	21st to end	Day	19% 12% 26% 25% 13% 5%	490
October	1st to 10th	Day	24% 14% 29% 19% 11% 3	490
	11th to 20th	Day	30% 16% 24% 19% 9% 2	445
	21st to end	Day	37% 17% 23% 18% 4	429
November	1st to 10th	Day	33% 16% 28% 17% 5%	395
	11th to 20th	Day	31% 20% 26% 14% 8% 1	380
	21st to end	Day	34% 22% 31% 12% 3	200
December	1st to 10th	Day	34% 19% 26% 16% 5% 2	200
	11th to 20th	Day	34% 24% 29% 10% 3 1	200
	21st to end	Day	34% 20% 26% 13% 5% 1	220
January	1st to 10th	Night	31% 15% 33% 18% 3	800
	11th to 20th	Night	27% 20% 31% 18% 5%	800
	21st to end	Night	23% 14% 33% 24% 5%	880
February	1st to 10th	Night	25% 18% 35% 19% 3	781
	11th to 20th	Night	24% 18% 33% 20% 3	743
	21st to end	Night	25% 17% 33% 24% 2	628
March	1st to 10th	Night	28% 16% 29% 24% 4	760
	11th to 20th	Night	28% 16% 28% 23% 6%	760
	21st to end	Night	26% 14% 28% 24% 6% 1	836
April	1st to 10th	Night	32% 13% 28% 23% 4	760
	11th to 20th	Night	28% 16% 26% 22% 7%	744
	21st to end	Night	30% 16% 27% 20% 7%	570
May	1st to 10th	Night	28% 9% 33% 22% 8% 1	496
	11th to 20th	Night	22% 12% 30% 26% 8% 1	374
	21st to end	Night	23% 7% 32% 31% 6%	411
June	1st to 10th	Night	30% 6% 27% 28% 8% 1	380
	11th to 20th	Night	20% 8% 28% 32% 12%	380
	21st to end	Night	19% 7% 29% 33% 12% 1	380
July	1st to 10th	Night	15% 6% 28% 39% 11% 1	370
	11th to 20th	Night	15% 5% 23% 40% 16% 1	368
	21st to end	Night	16% 6% 24% 40% 14%	416
August	1st to 10th	Night	20% 4 20% 37% 18% 1	378
	11th to 20th	Night	16% 6% 22% 33% 21% 2	550
	21st to end	Night	17% 6% 23% 34% 17% 2	836
September	1st to 10th	Night	19% 7% 26% 33% 14% 1	760
	11th to 20th	Night	18% 7% 21% 29% 22% 2	760
	21st to end	Night	21% 7% 28% 27% 15% 2	760
October	1st to 10th	Night	24% 11% 28% 25% 12% 1	760
	11th to 20th	Night	29% 12% 25% 25% 8% 1	760
	21st to end	Night	34% 12% 28% 20% 5%	836
November	1st to 10th	Night	30% 14% 30% 19% 6% 1	780
	11th to 20th	Night	27% 18% 27% 17% 10% 1	800
	21st to end	Night	32% 19% 25% 18% 5% 1	800
December	1st to 10th	Night	36% 16% 27% 17% 4	800
	11th to 20th	Night	30% 18% 29% 16% 5% 1	800
	21st to end	Night	34% 17% 27% 17% 4	880

Page generated at 2025-02-03 03:05:52. Version ac07

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