Port Mouton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Mouton Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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% 9% 4 6% 5% 3 4 4 5% 7% 32%	75043
	Night	21% 9% 3 6% 5% 3 4 4 5% 7% 32%	74700
Transparency	Day	51% 8% 11% 18% 10% 1	74755
	Night	53% 7% 11% 18% 9% 1	74523
Seeing	Day	24% 10% 27% 28% 10% 1	16034
	Night	24% 11% 28% 27% 9% 1	23764

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	19% 14% 6% 7% 6% 3 4 5% 6% 7% 22%	1516
	11th to 20th	Day	22% 13% 4 8% 7% 3 5% 5% 6% 6% 21%	1611
	21st to end	Day	15% 9% 6% 7% 7% 4 5% 6% 8% 8% 27%	1782
February	1st to 10th	Day	20% 11% 5% 7% 7% 4 5% 6% 6% 7% 22%	1620
	11th to 20th	Day	19% 10% 3 8% 8% 4 6% 5% 5% 5% 26%	1573
	21st to end	Day	14% 11% 6% 7% 6% 3 6% 5% 5% 6% 31%	1383
March	1st to 10th	Day	19% 13% 5% 6% 6% 4 5% 4 5% 6% 27%	1975
	11th to 20th	Day	18% 9% 5% 7% 6% 2 5% 4 5% 7% 32%	1980
	21st to end	Day	17% 12% 5% 7% 6% 3 5% 4 4 5% 33%	2178
April	1st to 10th	Day	24% 13% 5% 6% 5% 3 4 4 4 6% 26%	2110
	11th to 20th	Day	21% 9% 3 5% 5% 2 4 4 5% 8% 34%	2226
	21st to end	Day	23% 9% 3 6% 5% 3 4 4 4 7% 33%	2340
May	1st to 10th	Day	26% 10% 3 7% 6% 3 4 4 4 7% 26%	2340
	11th to 20th	Day	23% 8% 3 5% 4 3 4 4 4 6% 35%	2445
	21st to end	Day	20% 8% 4 6% 6% 3 4 4 5% 8% 32%	2724
June	1st to 10th	Day	26% 8% 3 5% 5% 3 5% 3 5% 7% 29%	2520
	11th to 20th	Day	21% 9% 4 6% 5% 2 3 4 4 6% 35%	2604
	21st to end	Day	23% 7% 3 7% 5% 3 4 5% 5% 8% 28%	2700
July	1st to 10th	Day	18% 8% 3 5% 5% 3 4 4 5% 9% 35%	2633
	11th to 20th	Day	17% 9% 3 6% 5% 4 6% 4 6% 9% 33%	2520
	21st to end	Day	19% 9% 4 7% 5% 3 4 4 5% 7% 32%	2558
August	1st to 10th	Day	16% 7% 3 5% 5% 3 6% 6% 6% 8% 37%	2327
	11th to 20th	Day	16% 7% 3 5% 6% 3 5% 5% 6% 7% 36%	2304
	21st to end	Day	15% 7% 3 5% 5% 3 4 6% 5% 7% 42%	2498
September	1st to 10th	Day	14% 6% 3 5% 5% 3 4 4 5% 7% 45%	2160
	11th to 20th	Day	18% 7% 2 4 4 3 3 4 5% 6% 42%	2033
	21st to end	Day	19% 8% 2 5% 4 3 3 3 4 6% 43%	1958
October	1st to 10th	Day	20% 6% 2 5% 4 3 4 5% 4 7% 40%	1800
	11th to 20th	Day	21% 6% 3 5% 5% 3 5% 5% 5% 7% 37%	1800
	21st to end	Day	21% 9% 3 5% 6% 3 6% 5% 5% 6% 31%	1980
November	1st to 10th	Day	20% 9% 4 6% 4 2 4 4 5% 6% 35%	1596
	11th to 20th	Day	21% 9% 4 6% 7% 3 5% 4 6% 7% 29%	1520
	21st to end	Day	21% 9% 4 8% 6% 3 5% 5% 4 7% 29%	1520
December	1st to 10th	Day	25% 11% 3 8% 7% 4 5% 4 5% 7% 21%	1520
	11th to 20th	Day	31% 10% 3 6% 5% 4 5% 5% 5% 7% 20%	1906
	21st to end	Day	23% 10% 6% 7% 5% 3 4 4 5% 7% 27%	2783
January	1st to 10th	Night	19% 11% 5% 9% 7% 4 5% 5% 6% 7% 23%	2649
	11th to 20th	Night	21% 11% 4 8% 6% 4 5% 5% 6% 7% 24%	2410
	21st to end	Night	15% 9% 5% 8% 6% 4 6% 6% 6% 8% 27%	2574
February	1st to 10th	Night	20% 9% 4 7% 7% 4 5% 5% 5% 7% 26%	2340
	11th to 20th	Night	18% 9% 3 8% 7% 4 6% 6% 6% 7% 24%	2273
	21st to end	Night	13% 10% 4 7% 7% 4 6% 4 5% 6% 33%	1870
March	1st to 10th	Night	18% 11% 3 6% 5% 3 5% 5% 4 7% 32%	1971
	11th to 20th	Night	19% 9% 4 6% 6% 4 6% 5% 5% 6% 31%	1979
	21st to end	Night	17% 10% 4 7% 5% 3 4 3 5% 6% 37%	2178
April	1st to 10th	Night	26% 11% 3 6% 5% 2 3 4 5% 6% 30%	1849
	11th to 20th	Night	25% 8% 2 4 4 3 4 4 5% 6% 34%	1680
	21st to end	Night	24% 9% 3 5% 5% 3 3 4 5% 7% 32%	1620
May	1st to 10th	Night	23% 10% 4 6% 6% 3 3 4 5% 6% 31%	1620
	11th to 20th	Night	23% 9% 3 6% 5% 3 4 4 5% 6% 33%	1448
	21st to end	Night	20% 9% 4 8% 4 3 3 4 5% 7% 32%	1550
June	1st to 10th	Night	26% 8% 3 5% 4 2 4 3 7% 7% 30%	1440
	11th to 20th	Night	22% 8% 4 7% 5% 2 4 4 4 6% 34%	1366
	21st to end	Night	27% 8% 3 6% 5% 4 5% 3 5% 7% 27%	1260
July	1st to 10th	Night	18% 10% 3 6% 4 2 4 4 5% 9% 35%	1303
	11th to 20th	Night	18% 10% 3 5% 6% 3 5% 6% 6% 7% 29%	1440
	21st to end	Night	23% 8% 3 5% 6% 3 3 4 5% 7% 33%	1757
August	1st to 10th	Night	14% 8% 4 6% 5% 2 4 3 5% 6% 43%	1612
	11th to 20th	Night	19% 9% 3 6% 5% 3 3 4 5% 7% 37%	1596
	21st to end	Night	16% 8% 3 5% 5% 3 4 4 5% 7% 40%	1858
September	1st to 10th	Night	14% 7% 3 5% 5% 3 4 5% 5% 7% 42%	1800
	11th to 20th	Night	18% 6% 2 4 4 2 3 4 4 6% 44%	1925
	21st to end	Night	21% 7% 3 6% 5% 3 3 3 5% 6% 39%	2002
October	1st to 10th	Night	21% 6% 2 5% 4 2 3 4 4 8% 40%	2160
	11th to 20th	Night	19% 7% 2 4 5% 2 3 5% 5% 7% 41%	2160
	21st to end	Night	24% 7% 2 4 5% 3 3 5% 5% 7% 36%	2376
November	1st to 10th	Night	20% 7% 3 6% 4 3 5% 4 5% 7% 37%	2477
	11th to 20th	Night	19% 8% 4 6% 6% 4 5% 5% 6% 7% 30%	2660
	21st to end	Night	19% 7% 4 7% 5% 4 6% 5% 6% 8% 30%	2660
December	1st to 10th	Night	23% 11% 4 8% 6% 3 5% 4 5% 7% 23%	2660
	11th to 20th	Night	30% 9% 3 5% 5% 3 5% 4 6% 6% 23%	3293
	21st to end	Night	25% 9% 4 7% 5% 3 4 5% 5% 6% 28%	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	62% 7% 12% 13% 6%	1517
	11th to 20th	Day	62% 9% 12% 14% 4	1611
	21st to end	Day	54% 8% 12% 20% 6%	1782
February	1st to 10th	Day	59% 7% 12% 14% 7% 1	1620
	11th to 20th	Day	59% 7% 11% 16% 6% 1	1581
	21st to end	Day	48% 7% 15% 19% 9% 2	1383
March	1st to 10th	Day	56% 4 9% 19% 11% 1	1979
	11th to 20th	Day	46% 6% 15% 18% 13% 2	1980
	21st to end	Day	49% 6% 10% 21% 12% 3	2178
April	1st to 10th	Day	56% 5% 10% 16% 12% 2	2110
	11th to 20th	Day	49% 5% 12% 21% 13% 1	2226
	21st to end	Day	50% 4 12% 20% 12% 2	2340
May	1st to 10th	Day	58% 5% 10% 17% 9% 1	2340
	11th to 20th	Day	49% 6% 10% 19% 14% 2	2449
	21st to end	Day	51% 9% 12% 19% 9% 1	2724
June	1st to 10th	Day	54% 7% 10% 18% 11%	2520
	11th to 20th	Day	51% 7% 10% 18% 12% 2	2604
	21st to end	Day	58% 9% 9% 13% 10% 1	2700
July	1st to 10th	Day	49% 13% 10% 16% 10% 2	2561
	11th to 20th	Day	53% 11% 10% 18% 7% 1	2444
	21st to end	Day	50% 14% 11% 18% 6% 1	2415
August	1st to 10th	Day	45% 11% 11% 19% 12% 1	2314
	11th to 20th	Day	45% 12% 13% 19% 10% 1	2304
	21st to end	Day	39% 10% 11% 24% 15% 1	2498
September	1st to 10th	Day	39% 7% 12% 24% 17% 2	2160
	11th to 20th	Day	43% 7% 10% 21% 17% 2	2033
	21st to end	Day	43% 6% 9% 24% 17% 1	1958
October	1st to 10th	Day	42% 6% 12% 26% 12% 2	1800
	11th to 20th	Day	46% 5% 14% 22% 12% 1	1800
	21st to end	Day	52% 7% 12% 19% 10% 1	1980
November	1st to 10th	Day	49% 7% 12% 21% 10% 1	1596
	11th to 20th	Day	55% 7% 11% 18% 8% 1	1520
	21st to end	Day	55% 8% 13% 18% 5% 1	1520
December	1st to 10th	Day	64% 7% 11% 13% 5%	1520
	11th to 20th	Day	65% 8% 9% 11% 6%	1905
	21st to end	Day	59% 9% 13% 13% 6% 1	2783
January	1st to 10th	Night	62% 8% 10% 15% 5%	2649
	11th to 20th	Night	61% 9% 13% 12% 5%	2410
	21st to end	Night	56% 9% 13% 16% 6%	2574
February	1st to 10th	Night	59% 9% 10% 14% 7% 1	2340
	11th to 20th	Night	58% 7% 11% 15% 7% 1	2276
	21st to end	Night	51% 8% 13% 18% 8% 1	1870
March	1st to 10th	Night	54% 6% 10% 18% 11% 2	1980
	11th to 20th	Night	52% 6% 13% 15% 12% 2	1980
	21st to end	Night	49% 5% 10% 20% 14% 2	2178
April	1st to 10th	Night	56% 5% 10% 14% 12% 4	1849
	11th to 20th	Night	51% 4 10% 20% 14% 2	1680
	21st to end	Night	52% 6% 9% 21% 12% 1	1620
May	1st to 10th	Night	56% 4 8% 20% 10% 1	1620
	11th to 20th	Night	52% 5% 8% 21% 12% 1	1449
	21st to end	Night	54% 8% 10% 19% 9% 1	1551
June	1st to 10th	Night	56% 5% 9% 21% 9%	1440
	11th to 20th	Night	52% 7% 11% 16% 13% 2	1366
	21st to end	Night	61% 6% 9% 12% 11% 1	1260
July	1st to 10th	Night	50% 10% 11% 19% 10% 1	1266
	11th to 20th	Night	56% 9% 8% 18% 8% 1	1392
	21st to end	Night	54% 7% 11% 18% 8% 2	1660
August	1st to 10th	Night	46% 9% 9% 23% 12% 1	1600
	11th to 20th	Night	49% 9% 12% 19% 11%	1596
	21st to end	Night	44% 8% 10% 24% 14%	1858
September	1st to 10th	Night	41% 7% 9% 27% 14% 2	1800
	11th to 20th	Night	43% 6% 7% 24% 18% 2	1925
	21st to end	Night	48% 5% 7% 24% 15% 1	2002
October	1st to 10th	Night	45% 5% 11% 23% 13% 2	2160
	11th to 20th	Night	43% 6% 13% 27% 11%	2160
	21st to end	Night	50% 7% 9% 21% 12% 1	2376
November	1st to 10th	Night	49% 8% 12% 20% 10% 1	2477
	11th to 20th	Night	53% 7% 12% 19% 9%	2660
	21st to end	Night	55% 8% 15% 17% 6%	2660
December	1st to 10th	Night	63% 7% 10% 15% 4	2660
	11th to 20th	Night	63% 9% 8% 14% 5% 1	3295
	21st to end	Night	59% 6% 13% 17% 4	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	28% 18% 31% 18% 5%	370
	11th to 20th	Day	27% 15% 33% 20% 4	349
	21st to end	Day	20% 16% 37% 23% 4	374
February	1st to 10th	Day	26% 16% 35% 22% 1	340
	11th to 20th	Day	27% 20% 32% 20% 2	336
	21st to end	Day	16% 18% 39% 22% 4	304
March	1st to 10th	Day	23% 16% 36% 20% 5%	420
	11th to 20th	Day	18% 18% 34% 25% 5%	420
	21st to end	Day	22% 12% 35% 22% 8% 1	462
April	1st to 10th	Day	30% 14% 30% 22% 3	420
	11th to 20th	Day	24% 10% 35% 25% 5%	412
	21st to end	Day	24% 9% 27% 29% 11% 1	420
May	1st to 10th	Day	25% 12% 32% 22% 7% 1	423
	11th to 20th	Day	21% 6% 30% 31% 10% 1	591
	21st to end	Day	27% 6% 26% 31% 10% 1	680
June	1st to 10th	Day	29% 5% 22% 35% 9% 1	630
	11th to 20th	Day	22% 8% 23% 33% 13% 1	630
	21st to end	Day	29% 5% 22% 33% 10% 2	630
July	1st to 10th	Day	24% 6% 20% 36% 13% 1	620
	11th to 20th	Day	24% 5% 17% 37% 16% 1	618
	21st to end	Day	22% 5% 19% 36% 16% 1	498
August	1st to 10th	Day	17% 5% 21% 37% 19% 2	446
	11th to 20th	Day	23% 5% 21% 30% 18% 2	439
	21st to end	Day	19% 5% 21% 32% 20% 3	495
September	1st to 10th	Day	16% 4 23% 37% 19% 1	450
	11th to 20th	Day	22% 6% 23% 31% 15% 3	450
	21st to end	Day	20% 5% 26% 33% 13% 4	439
October	1st to 10th	Day	21% 4 28% 29% 15% 3	360
	11th to 20th	Day	18% 11% 28% 28% 12% 3	360
	21st to end	Day	23% 12% 31% 25% 9% 1	396
November	1st to 10th	Day	21% 14% 26% 26% 12%	365
	11th to 20th	Day	24% 21% 25% 22% 7% 1	370
	21st to end	Day	26% 16% 31% 22% 4	370
December	1st to 10th	Day	31% 16% 29% 21% 4	370
	11th to 20th	Day	31% 22% 29% 15% 2	370
	21st to end	Day	24% 17% 35% 19% 4 1	407
January	1st to 10th	Night	24% 17% 35% 20% 4	850
	11th to 20th	Night	26% 14% 33% 22% 5%	747
	21st to end	Night	20% 16% 37% 22% 5%	792
February	1st to 10th	Night	27% 16% 33% 22% 2	720
	11th to 20th	Night	23% 19% 36% 19% 2	703
	21st to end	Night	17% 12% 42% 23% 6%	592
March	1st to 10th	Night	24% 13% 37% 23% 3	720
	11th to 20th	Night	22% 11% 31% 28% 8%	720
	21st to end	Night	23% 12% 32% 26% 6% 1	792
April	1st to 10th	Night	32% 7% 31% 26% 5%	589
	11th to 20th	Night	27% 6% 35% 27% 5%	534
	21st to end	Night	28% 7% 25% 27% 11% 2	540
May	1st to 10th	Night	28% 6% 24% 31% 8% 2	540
	11th to 20th	Night	28% 4 26% 30% 10% 2	532
	21st to end	Night	26% 4 22% 36% 11% 2	583
June	1st to 10th	Night	31% 4 22% 30% 12%	540
	11th to 20th	Night	26% 5% 20% 38% 10%	466
	21st to end	Night	31% 5% 17% 31% 16% 1	360
July	1st to 10th	Night	25% 5% 14% 40% 16%	391
	11th to 20th	Night	22% 7% 16% 39% 15% 1	522
	21st to end	Night	26% 5% 11% 43% 14%	590
August	1st to 10th	Night	17% 4 16% 41% 20% 2	538
	11th to 20th	Night	21% 4 17% 36% 20% 2	532
	21st to end	Night	18% 5% 17% 35% 21% 4	594
September	1st to 10th	Night	17% 6% 19% 36% 19% 3	540
	11th to 20th	Night	21% 5% 23% 31% 17% 3	665
	21st to end	Night	23% 7% 22% 31% 14% 4	720
October	1st to 10th	Night	25% 6% 24% 31% 12% 2	720
	11th to 20th	Night	21% 9% 30% 26% 12% 2	720
	21st to end	Night	27% 11% 29% 26% 6%	792
November	1st to 10th	Night	23% 12% 27% 29% 9% 1	785
	11th to 20th	Night	23% 16% 30% 23% 8%	850
	21st to end	Night	24% 16% 33% 20% 7% 1	850
December	1st to 10th	Night	28% 15% 32% 20% 4 1	850
	11th to 20th	Night	28% 21% 32% 16% 3	850
	21st to end	Night	26% 17% 33% 18% 4 1	935

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