Calais
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Calais clear sky chart from 2005-11-06 to 2024-02-25.

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	20% 8% 3 6% 5% 3 4 4 5% 7% 36%	78100
	Night	20% 8% 3 6% 5% 3 4 4 5% 6% 35%	81208
Transparency	Day	48% 4 11% 22% 13% 2	77802
	Night	50% 4 11% 22% 12% 2	81042
Seeing	Day	25% 17% 31% 19% 6% 1	15144
	Night	25% 14% 31% 23% 6%	23373

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	16% 10% 3 6% 6% 3 5% 5% 4 6% 36%	2300
	11th to 20th	Day	17% 10% 3 6% 4 2 3 3 3 4 44%	2520
	21st to end	Day	15% 7% 4 8% 6% 4 4 5% 5% 7% 35%	2963
February	1st to 10th	Day	18% 11% 4 6% 6% 4 5% 3 4 6% 32%	1710
	11th to 20th	Day	15% 11% 3 7% 6% 4 5% 5% 4 6% 34%	1672
	21st to end	Day	12% 12% 4 6% 5% 3 4 5% 5% 7% 36%	1520
March	1st to 10th	Day	18% 11% 4 7% 5% 3 4 5% 4 6% 32%	1863
	11th to 20th	Day	17% 9% 3 6% 5% 3 5% 4 5% 7% 36%	1980
	21st to end	Day	19% 11% 4 7% 6% 3 4 4 4 6% 33%	2178
April	1st to 10th	Day	23% 11% 4 7% 5% 3 4 4 4 7% 28%	2104
	11th to 20th	Day	22% 9% 3 5% 5% 3 4 3 5% 6% 37%	2312
	21st to end	Day	25% 9% 3 5% 4 3 4 4 5% 5% 33%	2340
May	1st to 10th	Day	25% 9% 3 6% 5% 4 4 3 4 6% 30%	2340
	11th to 20th	Day	23% 7% 2 5% 5% 2 4 5% 4 6% 37%	2344
	21st to end	Day	20% 8% 3 5% 4 4 4 5% 6% 8% 33%	2819
June	1st to 10th	Day	24% 7% 4 7% 5% 4 5% 4 5% 6% 29%	2700
	11th to 20th	Day	22% 7% 3 5% 4 2 3 4 5% 7% 38%	2700
	21st to end	Day	27% 7% 3 6% 6% 3 5% 4 5% 7% 28%	2700
July	1st to 10th	Day	18% 5% 2 5% 6% 4 5% 5% 6% 8% 37%	2700
	11th to 20th	Day	17% 7% 2 4 5% 3 5% 5% 7% 9% 35%	2602
	21st to end	Day	17% 5% 3 5% 6% 4 6% 7% 6% 7% 33%	2636
August	1st to 10th	Day	17% 7% 2 6% 5% 3 5% 5% 6% 8% 37%	2327
	11th to 20th	Day	17% 7% 3 6% 6% 4 5% 5% 6% 9% 32%	2304
	21st to end	Day	15% 5% 2 4 4 3 4 5% 5% 8% 44%	2574
September	1st to 10th	Day	13% 4 2 4 6% 3 3 3 4 7% 50%	2111
	11th to 20th	Day	18% 5% 2 4 4 2 3 4 4 5% 49%	1980
	21st to end	Day	20% 6% 2 5% 3 2 3 3 4 6% 47%	1980
October	1st to 10th	Day	18% 7% 2 5% 5% 3 4 3 5% 7% 41%	1868
	11th to 20th	Day	21% 6% 2 4 6% 3 4 4 4 8% 39%	1800
	21st to end	Day	23% 7% 3 5% 7% 3 4 4 4 5% 34%	1839
November	1st to 10th	Day	22% 7% 3 5% 4 3 4 4 5% 4 40%	1671
	11th to 20th	Day	21% 6% 3 5% 4 2 4 4 5% 7% 40%	1532
	21st to end	Day	21% 9% 3 7% 4 3 4 3 4 6% 35%	1520
December	1st to 10th	Day	25% 9% 3 7% 4 3 4 5% 5% 6% 30%	1492
	11th to 20th	Day	25% 10% 4 6% 5% 4 4 4 3 4 30%	1668
	21st to end	Day	25% 8% 3 5% 5% 3 5% 4 4 5% 33%	2431
January	1st to 10th	Night	14% 10% 4 7% 6% 3 4 5% 5% 6% 36%	4650
	11th to 20th	Night	16% 9% 4 8% 6% 4 5% 5% 4 5% 36%	4440
	21st to end	Night	14% 10% 4 7% 7% 3 5% 5% 5% 6% 33%	4692
February	1st to 10th	Night	17% 10% 3 5% 6% 3 5% 6% 5% 7% 32%	2470
	11th to 20th	Night	16% 10% 4 7% 7% 3 5% 5% 5% 6% 32%	2390
	21st to end	Night	16% 11% 4 5% 6% 3 4 4 4 5% 38%	1831
March	1st to 10th	Night	17% 11% 4 7% 5% 4 4 4 4 6% 34%	2084
	11th to 20th	Night	19% 10% 3 6% 6% 3 5% 5% 5% 5% 32%	1979
	21st to end	Night	19% 9% 4 6% 6% 3 3 4 4 6% 36%	2178
April	1st to 10th	Night	25% 11% 3 6% 5% 3 4 3 3 5% 32%	1855
	11th to 20th	Night	24% 9% 2 5% 3 3 5% 4 5% 5% 35%	1598
	21st to end	Night	27% 8% 3 5% 5% 2 5% 4 5% 7% 30%	1620
May	1st to 10th	Night	23% 8% 4 5% 4 3 4 5% 5% 6% 33%	1620
	11th to 20th	Night	25% 7% 3 5% 5% 3 5% 5% 5% 6% 31%	1548
	21st to end	Night	19% 9% 3 5% 6% 3 5% 5% 5% 6% 34%	1470
June	1st to 10th	Night	27% 7% 2 5% 5% 3 5% 5% 5% 7% 28%	1260
	11th to 20th	Night	19% 6% 4 6% 5% 3 4 5% 6% 6% 35%	1260
	21st to end	Night	27% 6% 2 6% 6% 3 5% 4 4 7% 30%	1260
July	1st to 10th	Night	17% 7% 3 5% 6% 3 4 6% 6% 9% 34%	1260
	11th to 20th	Night	20% 8% 2 5% 5% 3 5% 5% 7% 9% 33%	1355
	21st to end	Night	20% 7% 3 6% 6% 3 5% 6% 7% 8% 31%	1663
August	1st to 10th	Night	16% 7% 3 5% 5% 3 5% 4 5% 7% 40%	1612
	11th to 20th	Night	17% 8% 3 6% 6% 3 5% 5% 6% 7% 35%	1596
	21st to end	Night	15% 6% 2 5% 5% 2 4 5% 5% 7% 44%	1782
September	1st to 10th	Night	17% 4 2 4 5% 3 3 4 5% 7% 47%	1843
	11th to 20th	Night	18% 6% 2 4 3 3 3 4 5% 5% 48%	1980
	21st to end	Night	20% 6% 2 4 3 2 3 3 5% 7% 46%	1980
October	1st to 10th	Night	20% 6% 2 4 4 3 4 4 5% 7% 41%	2092
	11th to 20th	Night	20% 7% 3 5% 5% 3 3 4 5% 6% 40%	2160
	21st to end	Night	25% 8% 3 6% 5% 3 3 3 4 6% 34%	2515
November	1st to 10th	Night	21% 6% 2 5% 4 3 4 5% 5% 6% 38%	2407
	11th to 20th	Night	20% 7% 3 5% 5% 3 4 4 5% 6% 38%	2645
	21st to end	Night	21% 7% 3 6% 5% 3 4 4 5% 7% 36%	2660
December	1st to 10th	Night	26% 9% 3 6% 5% 3 4 4 4 5% 31%	2686
	11th to 20th	Night	25% 9% 3 5% 5% 3 5% 4 5% 5% 31%	3531
	21st to end	Night	24% 9% 4 6% 5% 3 4 4 4 5% 34%	5236

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	50% 2 17% 23% 8% 1	2301
	11th to 20th	Day	45% 3 9% 28% 13% 2	2520
	21st to end	Day	51% 2 13% 19% 14% 1	2963
February	1st to 10th	Day	54% 3 10% 18% 12% 2	1710
	11th to 20th	Day	52% 3 11% 19% 14% 1	1680
	21st to end	Day	48% 4 10% 23% 11% 5%	1520
March	1st to 10th	Day	52% 4 10% 19% 13% 2	1867
	11th to 20th	Day	48% 4 9% 19% 17% 3	1980
	21st to end	Day	52% 4 7% 20% 13% 4	2178
April	1st to 10th	Day	56% 4 8% 14% 15% 3	2104
	11th to 20th	Day	48% 4 8% 22% 15% 2	2312
	21st to end	Day	52% 4 7% 19% 14% 4	2340
May	1st to 10th	Day	56% 4 7% 18% 13% 2	2340
	11th to 20th	Day	48% 4 7% 24% 13% 3	2348
	21st to end	Day	49% 5% 13% 22% 10% 1	2819
June	1st to 10th	Day	56% 4 10% 19% 11% 1	2700
	11th to 20th	Day	47% 4 11% 21% 16% 2	2700
	21st to end	Day	56% 5% 11% 16% 11% 1	2700
July	1st to 10th	Day	44% 6% 13% 22% 13% 2	2625
	11th to 20th	Day	44% 5% 13% 25% 11% 1	2522
	21st to end	Day	46% 6% 13% 24% 10% 1	2489
August	1st to 10th	Day	44% 5% 14% 26% 10% 1	2314
	11th to 20th	Day	48% 5% 13% 22% 11%	2304
	21st to end	Day	37% 4 13% 29% 15% 1	2574
September	1st to 10th	Day	34% 4 12% 31% 18% 1	2111
	11th to 20th	Day	38% 4 9% 27% 19% 4	1980
	21st to end	Day	41% 3 8% 30% 17% 1	1980
October	1st to 10th	Day	42% 4 11% 26% 15% 1	1868
	11th to 20th	Day	45% 4 13% 27% 10%	1800
	21st to end	Day	51% 4 10% 22% 13%	1839
November	1st to 10th	Day	48% 4 11% 22% 15% 1	1671
	11th to 20th	Day	44% 4 12% 22% 16% 2	1532
	21st to end	Day	51% 4 14% 22% 8% 1	1520
December	1st to 10th	Day	56% 4 11% 20% 9%	1492
	11th to 20th	Day	59% 4 8% 19% 10% 1	1668
	21st to end	Day	54% 4 12% 18% 10% 2	2431
January	1st to 10th	Night	49% 3 13% 26% 8% 1	4650
	11th to 20th	Night	50% 4 10% 23% 11% 1	4440
	21st to end	Night	52% 3 11% 22% 11% 1	4692
February	1st to 10th	Night	51% 4 12% 19% 12% 2	2470
	11th to 20th	Night	53% 4 8% 19% 13% 2	2393
	21st to end	Night	50% 2 9% 20% 15% 3	1831
March	1st to 10th	Night	53% 3 8% 19% 15% 2	2093
	11th to 20th	Night	53% 3 10% 16% 14% 3	1980
	21st to end	Night	50% 3 8% 22% 15% 3	2178
April	1st to 10th	Night	56% 4 7% 16% 14% 4	1855
	11th to 20th	Night	51% 3 8% 20% 16% 2	1598
	21st to end	Night	56% 3 8% 16% 14% 2	1620
May	1st to 10th	Night	52% 3 8% 20% 14% 2	1620
	11th to 20th	Night	53% 4 9% 19% 12% 2	1549
	21st to end	Night	50% 5% 13% 20% 11% 1	1471
June	1st to 10th	Night	55% 5% 10% 22% 7% 1	1260
	11th to 20th	Night	48% 4 10% 24% 11% 2	1260
	21st to end	Night	55% 4 10% 19% 8% 3	1260
July	1st to 10th	Night	45% 6% 17% 20% 12%	1225
	11th to 20th	Night	47% 5% 15% 22% 11%	1312
	21st to end	Night	50% 5% 15% 19% 10% 1	1571
August	1st to 10th	Night	45% 5% 14% 26% 10%	1600
	11th to 20th	Night	47% 5% 15% 24% 10%	1596
	21st to end	Night	38% 4 13% 27% 16% 1	1782
September	1st to 10th	Night	38% 4 12% 31% 14% 1	1843
	11th to 20th	Night	38% 3 12% 26% 20% 2	1980
	21st to end	Night	39% 3 9% 31% 16% 2	1980
October	1st to 10th	Night	43% 3 10% 28% 15% 2	2092
	11th to 20th	Night	46% 3 11% 27% 12% 1	2160
	21st to end	Night	53% 3 10% 22% 12% 1	2515
November	1st to 10th	Night	47% 3 13% 23% 13% 1	2407
	11th to 20th	Night	47% 4 11% 23% 15% 1	2645
	21st to end	Night	48% 5% 13% 22% 11% 1	2660
December	1st to 10th	Night	57% 3 10% 20% 9%	2686
	11th to 20th	Night	54% 4 9% 20% 12% 1	3532
	21st to end	Night	54% 3 12% 19% 8% 3	5236

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	29% 22% 32% 14% 2 1	180
	11th to 20th	Day	34% 24% 29% 9% 2 1	180
	21st to end	Day	22% 22% 33% 20% 4	311
February	1st to 10th	Day	26% 23% 30% 16% 3	370
	11th to 20th	Day	24% 28% 34% 11% 3	370
	21st to end	Day	26% 25% 31% 15% 4	362
March	1st to 10th	Day	26% 26% 32% 13% 3	420
	11th to 20th	Day	23% 27% 32% 12% 5% 1	420
	21st to end	Day	29% 24% 27% 12% 5% 2	462
April	1st to 10th	Day	35% 24% 28% 10% 2 2	420
	11th to 20th	Day	31% 21% 30% 14% 3	412
	21st to end	Day	31% 17% 33% 14% 3 1	420
May	1st to 10th	Day	30% 20% 31% 15% 3	423
	11th to 20th	Day	24% 22% 32% 18% 3 1	490
	21st to end	Day	26% 17% 31% 17% 8% 1	680
June	1st to 10th	Day	27% 17% 31% 19% 5% 1	630
	11th to 20th	Day	24% 13% 32% 22% 7% 2	630
	21st to end	Day	32% 7% 31% 20% 8% 1	630
July	1st to 10th	Day	22% 8% 34% 25% 10% 1	620
	11th to 20th	Day	22% 8% 33% 26% 10% 2	521
	21st to end	Day	17% 9% 36% 30% 7% 1	488
August	1st to 10th	Day	17% 9% 35% 28% 10% 1	446
	11th to 20th	Day	20% 8% 33% 27% 10% 2	439
	21st to end	Day	20% 11% 29% 27% 10% 3	495
September	1st to 10th	Day	14% 11% 36% 30% 8% 2	450
	11th to 20th	Day	25% 13% 28% 19% 11% 4	450
	21st to end	Day	19% 16% 31% 21% 8% 4	450
October	1st to 10th	Day	18% 16% 29% 23% 10% 3	394
	11th to 20th	Day	23% 16% 29% 19% 10% 3	360
	21st to end	Day	27% 17% 33% 17% 6%	396
November	1st to 10th	Day	27% 18% 27% 21% 5% 2	365
	11th to 20th	Day	26% 24% 28% 16% 6% 1	370
	21st to end	Day	30% 21% 26% 19% 3 1	370
December	1st to 10th	Day	31% 19% 31% 14% 5%	342
	11th to 20th	Day	37% 28% 24% 9% 2	180
	21st to end	Day	31% 31% 24% 10% 4 1	198
January	1st to 10th	Night	25% 21% 29% 21% 3	821
	11th to 20th	Night	28% 20% 31% 19% 3	760
	21st to end	Night	17% 24% 37% 17% 4	836
February	1st to 10th	Night	25% 19% 31% 20% 4	760
	11th to 20th	Night	24% 26% 30% 16% 2 1	743
	21st to end	Night	24% 21% 32% 19% 4	611
March	1st to 10th	Night	24% 21% 34% 19% 2	720
	11th to 20th	Night	24% 17% 33% 21% 5%	720
	21st to end	Night	25% 16% 33% 20% 6% 1	792
April	1st to 10th	Night	33% 17% 33% 15% 3	676
	11th to 20th	Night	28% 17% 32% 19% 4	534
	21st to end	Night	34% 11% 32% 18% 6%	540
May	1st to 10th	Night	26% 12% 32% 25% 5%	540
	11th to 20th	Night	29% 12% 32% 22% 5%	532
	21st to end	Night	22% 11% 36% 25% 5% 1	503
June	1st to 10th	Night	29% 6% 31% 28% 5%	360
	11th to 20th	Night	23% 7% 33% 28% 9% 1	360
	21st to end	Night	29% 2 30% 33% 7%	360
July	1st to 10th	Night	19% 4 40% 28% 9%	350
	11th to 20th	Night	22% 6% 24% 36% 12%	348
	21st to end	Night	21% 3 31% 34% 11%	483
August	1st to 10th	Night	18% 5% 31% 36% 10%	538
	11th to 20th	Night	21% 3 28% 36% 12%	532
	21st to end	Night	17% 7% 28% 35% 13% 1	594
September	1st to 10th	Night	18% 4 36% 29% 12% 1	620
	11th to 20th	Night	21% 8% 28% 27% 13% 2	720
	21st to end	Night	21% 10% 30% 26% 12% 1	720
October	1st to 10th	Night	23% 8% 29% 28% 12%	720
	11th to 20th	Night	23% 11% 33% 22% 11% 1	720
	21st to end	Night	27% 17% 30% 20% 6%	792
November	1st to 10th	Night	27% 13% 29% 23% 6% 1	740
	11th to 20th	Night	26% 18% 27% 22% 7% 1	843
	21st to end	Night	28% 19% 28% 20% 5%	850
December	1st to 10th	Night	33% 14% 31% 18% 4	850
	11th to 20th	Night	29% 23% 29% 16% 2	850
	21st to end	Night	30% 22% 28% 15% 4	935

Page generated at 2024-02-22 06:05:47. Version ac07

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