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 2026-01-02.

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% 11% 4 8% 5% 4 4 3 5% 6% 30%	128078
	Night	20% 10% 4 8% 5% 4 4 3 5% 6% 30%	123399
Transparency	Day	52% 8% 11% 18% 10% 1	131670
	Night	54% 6% 10% 19% 10% 1	127894
Seeing	Day	23% 10% 27% 28% 10% 1	18761
	Night	24% 11% 28% 27% 9% 1	26394

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% 13% 6% 13% 8% 5% 4 3 4 6% 22%	3624
	11th to 20th	Day	14% 12% 5% 18% 7% 7% 6% 4 6% 5% 15%	3956
	21st to end	Day	16% 11% 5% 10% 6% 5% 4 4 6% 6% 26%	4466
February	1st to 10th	Day	20% 13% 6% 9% 6% 5% 4 4 6% 6% 22%	1800
	11th to 20th	Day	18% 10% 3 10% 7% 6% 5% 4 5% 5% 26%	1753
	21st to end	Day	14% 12% 5% 8% 5% 5% 5% 4 5% 6% 31%	1542
March	1st to 10th	Day	23% 13% 4 9% 7% 5% 4 3 4 4 23%	4929
	11th to 20th	Day	17% 15% 5% 8% 4 3 4 3 5% 6% 30%	4980
	21st to end	Day	26% 13% 4 10% 5% 4 4 3 4 4 22%	5466
April	1st to 10th	Day	24% 15% 4 7% 4 4 4 3 4 6% 27%	2345
	11th to 20th	Day	21% 10% 3 6% 4 4 4 3 5% 7% 34%	2478
	21st to end	Day	22% 9% 3 6% 4 4 4 3 4 7% 35%	2600
May	1st to 10th	Day	26% 13% 3 9% 5% 4 4 3 4 6% 22%	5886
	11th to 20th	Day	24% 13% 3 6% 4 4 4 3 4 5% 29%	6243
	21st to end	Day	18% 10% 3 8% 5% 4 4 3 5% 8% 33%	6948
June	1st to 10th	Day	24% 10% 3 6% 5% 4 4 3 5% 7% 29%	2800
	11th to 20th	Day	19% 10% 3 7% 4 3 4 3 4 6% 37%	2894
	21st to end	Day	24% 8% 3 7% 4 5% 4 4 5% 8% 28%	3000
July	1st to 10th	Day	19% 11% 4 7% 4 4 4 4 5% 7% 30%	6641
	11th to 20th	Day	15% 14% 5% 8% 4 5% 4 3 5% 7% 31%	6360
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 8% 40%	6435
August	1st to 10th	Day	23% 10% 3 7% 4 4 5% 4 5% 6% 29%	4235
	11th to 20th	Day	16% 9% 3 6% 4 4 4 4 6% 7% 37%	4175
	21st to end	Day	9% 6% 2 5% 4 4 4 4 5% 8% 49%	4525
September	1st to 10th	Day	15% 7% 2 6% 4 3 4 3 5% 7% 44%	2400
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 5% 6% 45%	2258
	21st to end	Day	18% 8% 2 6% 4 4 4 3 4 6% 42%	2175
October	1st to 10th	Day	19% 6% 2 6% 4 4 4 3 5% 7% 41%	2000
	11th to 20th	Day	20% 6% 2 6% 4 5% 5% 4 5% 7% 37%	2000
	21st to end	Day	20% 9% 3 6% 5% 5% 6% 4 5% 6% 32%	2200
November	1st to 10th	Day	20% 10% 4 7% 4 3 4 4 5% 6% 34%	1768
	11th to 20th	Day	21% 10% 4 7% 6% 5% 5% 3 5% 7% 27%	1680
	21st to end	Day	20% 10% 4 9% 6% 4 5% 4 4 6% 27%	1680
December	1st to 10th	Day	24% 9% 3 8% 6% 5% 5% 4 5% 7% 23%	2690
	11th to 20th	Day	29% 10% 2 8% 5% 4 5% 3 4 6% 24%	3076
	21st to end	Day	21% 12% 5% 8% 4 3 4 3 5% 6% 30%	4070
January	1st to 10th	Night	12% 14% 6% 16% 7% 6% 6% 4 5% 5% 19%	6364
	11th to 20th	Night	16% 12% 6% 15% 7% 6% 5% 4 5% 6% 19%	5931
	21st to end	Night	14% 12% 4 10% 6% 5% 6% 4 6% 7% 27%	6490
February	1st to 10th	Night	20% 11% 4 9% 6% 5% 5% 4 5% 7% 25%	2600
	11th to 20th	Night	17% 10% 3 10% 7% 6% 5% 5% 6% 7% 25%	2533
	21st to end	Night	13% 11% 4 8% 6% 5% 5% 4 5% 6% 34%	2085
March	1st to 10th	Night	24% 12% 4 9% 7% 5% 4 3 3 5% 24%	4910
	11th to 20th	Night	19% 12% 6% 9% 4 4 4 4 6% 6% 27%	4979
	21st to end	Night	30% 10% 4 8% 3 3 3 2 4 5% 28%	5466
April	1st to 10th	Night	25% 12% 2 6% 5% 3 3 3 5% 6% 30%	2053
	11th to 20th	Night	25% 9% 2 5% 4 4 3 3 4 7% 34%	1868
	21st to end	Night	22% 10% 3 6% 5% 4 3 3 5% 7% 33%	1800
May	1st to 10th	Night	26% 14% 4 8% 5% 4 3 3 4 4 25%	4054
	11th to 20th	Night	24% 13% 3 6% 4 3 3 3 4 5% 30%	3667
	21st to end	Night	18% 13% 4 8% 5% 4 4 2 4 6% 33%	3948
June	1st to 10th	Night	25% 10% 2 6% 4 3 4 2 6% 7% 31%	1600
	11th to 20th	Night	20% 9% 3 7% 4 3 4 3 4 6% 36%	1517
	21st to end	Night	26% 9% 3 7% 4 6% 4 3 5% 6% 27%	1400
July	1st to 10th	Night	20% 13% 4 9% 5% 4 4 3 5% 7% 26%	3287
	11th to 20th	Night	19% 12% 4 8% 5% 4 5% 3 5% 6% 28%	3620
	21st to end	Night	18% 8% 2 5% 5% 4 4 3 5% 8% 39%	4415
August	1st to 10th	Night	19% 13% 4 8% 4 3 3 2 4 6% 35%	2915
	11th to 20th	Night	18% 8% 3 8% 4 3 4 4 5% 7% 36%	2872
	21st to end	Night	10% 8% 2 5% 3 3 4 3 5% 7% 51%	3373
September	1st to 10th	Night	15% 8% 3 6% 4 4 4 4 5% 7% 42%	2000
	11th to 20th	Night	17% 6% 2 5% 3 3 3 4 4 6% 46%	2139
	21st to end	Night	21% 7% 3 7% 4 3 3 3 5% 7% 38%	2225
October	1st to 10th	Night	20% 6% 2 6% 3 3 4 3 4 7% 42%	2400
	11th to 20th	Night	19% 7% 2 5% 4 4 3 4 5% 6% 41%	2400
	21st to end	Night	23% 8% 2 5% 4 3 4 3 5% 7% 36%	2640
November	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 7% 38%	2745
	11th to 20th	Night	19% 10% 4 7% 6% 5% 5% 4 6% 7% 29%	2940
	21st to end	Night	20% 8% 3 7% 5% 5% 5% 4 6% 7% 29%	2940
December	1st to 10th	Night	16% 9% 4 9% 6% 5% 5% 4 6% 9% 27%	4720
	11th to 20th	Night	25% 10% 3 6% 4 4 3 3 5% 6% 30%	5353
	21st to end	Night	23% 10% 4 8% 5% 3 3 3 4 5% 30%	7150

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% 6% 12% 15% 4	3625
	11th to 20th	Day	63% 8% 13% 13% 4	3989
	21st to end	Day	54% 7% 10% 15% 12% 2	4466
February	1st to 10th	Day	60% 7% 12% 13% 8% 1	1800
	11th to 20th	Day	58% 7% 11% 16% 6% 1	1761
	21st to end	Day	49% 7% 14% 19% 10% 3	1542
March	1st to 10th	Day	60% 6% 9% 14% 10% 3	4933
	11th to 20th	Day	50% 8% 16% 17% 8% 1	4980
	21st to end	Day	60% 4 6% 17% 11% 1	5466
April	1st to 10th	Day	55% 5% 10% 16% 12% 2	2345
	11th to 20th	Day	49% 5% 12% 20% 13% 1	2478
	21st to end	Day	47% 5% 12% 20% 13% 2	2600
May	1st to 10th	Day	63% 8% 10% 14% 5%	5897
	11th to 20th	Day	55% 7% 9% 19% 9% 2	6247
	21st to end	Day	49% 12% 11% 17% 11% 1	6948
June	1st to 10th	Day	54% 7% 10% 18% 11%	2800
	11th to 20th	Day	49% 7% 10% 19% 13% 2	2894
	21st to end	Day	58% 8% 9% 14% 9% 1	3000
July	1st to 10th	Day	57% 10% 9% 13% 10% 1	6569
	11th to 20th	Day	56% 10% 8% 19% 7%	6284
	21st to end	Day	42% 11% 12% 23% 11%	6251
August	1st to 10th	Day	56% 9% 8% 16% 10% 1	4222
	11th to 20th	Day	46% 12% 13% 22% 7% 1	4214
	21st to end	Day	32% 9% 10% 28% 20% 1	4525
September	1st to 10th	Day	40% 7% 11% 24% 17% 2	2400
	11th to 20th	Day	40% 6% 11% 22% 18% 2	2258
	21st to end	Day	43% 6% 9% 23% 17% 2	2175
October	1st to 10th	Day	41% 7% 12% 28% 12% 1	3260
	11th to 20th	Day	36% 5% 11% 24% 23% 1	3240
	21st to end	Day	43% 6% 13% 23% 11% 4	3538
November	1st to 10th	Day	49% 7% 12% 22% 10% 1	1768
	11th to 20th	Day	55% 7% 11% 17% 8% 1	1680
	21st to end	Day	56% 7% 13% 18% 5% 1	1680
December	1st to 10th	Day	57% 7% 12% 20% 4	2690
	11th to 20th	Day	59% 7% 11% 13% 9%	3075
	21st to end	Day	55% 7% 12% 17% 8% 1	4070
January	1st to 10th	Night	63% 8% 10% 15% 4	6364
	11th to 20th	Night	59% 9% 12% 15% 5%	5973
	21st to end	Night	55% 7% 12% 14% 11% 1	6490
February	1st to 10th	Night	59% 9% 10% 14% 8% 1	2600
	11th to 20th	Night	57% 7% 11% 16% 8% 1	2536
	21st to end	Night	51% 7% 13% 19% 9% 1	2085
March	1st to 10th	Night	62% 4 7% 12% 14% 1	4919
	11th to 20th	Night	55% 6% 14% 17% 7% 1	4980
	21st to end	Night	59% 3 7% 18% 11% 1	5466
April	1st to 10th	Night	55% 5% 9% 15% 13% 3	2053
	11th to 20th	Night	50% 4 10% 21% 13% 1	1868
	21st to end	Night	50% 5% 9% 21% 12% 2	1800
May	1st to 10th	Night	64% 5% 8% 15% 8% 1	4060
	11th to 20th	Night	54% 6% 8% 21% 8% 3	3668
	21st to end	Night	54% 8% 8% 19% 10% 1	3949
June	1st to 10th	Night	54% 5% 10% 22% 10%	1600
	11th to 20th	Night	50% 7% 10% 19% 13% 1	1517
	21st to end	Night	61% 6% 9% 12% 11% 1	1400
July	1st to 10th	Night	62% 7% 8% 14% 9% 1	3250
	11th to 20th	Night	61% 8% 5% 15% 10% 2	3572
	21st to end	Night	50% 7% 9% 20% 13% 1	4285
August	1st to 10th	Night	55% 7% 7% 20% 10% 1	2903
	11th to 20th	Night	49% 8% 13% 22% 7%	2899
	21st to end	Night	34% 7% 11% 28% 20%	3372
September	1st to 10th	Night	41% 7% 8% 25% 16% 3	2000
	11th to 20th	Night	41% 5% 8% 25% 19% 2	2139
	21st to end	Night	48% 5% 7% 24% 15% 1	2225
October	1st to 10th	Night	44% 6% 11% 24% 12% 2	3920
	11th to 20th	Night	33% 5% 12% 27% 23% 1	3899
	21st to end	Night	41% 5% 9% 27% 17% 1	4252
November	1st to 10th	Night	48% 7% 12% 21% 11% 1	2745
	11th to 20th	Night	54% 7% 12% 18% 9%	2940
	21st to end	Night	55% 8% 15% 16% 6%	2940
December	1st to 10th	Night	53% 7% 12% 23% 5%	4720
	11th to 20th	Night	55% 7% 11% 15% 11% 1	5355
	21st to end	Night	58% 5% 11% 17% 9% 1	7150

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	27% 19% 31% 18% 5%	403
	11th to 20th	Day	24% 17% 33% 21% 5% 1	400
	21st to end	Day	20% 16% 37% 23% 5%	440
February	1st to 10th	Day	26% 14% 34% 22% 3	400
	11th to 20th	Day	24% 21% 34% 20% 2	396
	21st to end	Day	17% 18% 36% 23% 6%	361
March	1st to 10th	Day	23% 16% 36% 21% 4	500
	11th to 20th	Day	20% 16% 32% 26% 5%	500
	21st to end	Day	24% 13% 34% 21% 7% 1	550
April	1st to 10th	Day	29% 13% 32% 22% 3	500
	11th to 20th	Day	23% 11% 35% 26% 5%	492
	21st to end	Day	22% 9% 28% 31% 10% 1	500
May	1st to 10th	Day	25% 11% 31% 25% 7% 1	503
	11th to 20th	Day	22% 6% 28% 32% 10% 1	688
	21st to end	Day	26% 6% 25% 32% 10% 1	790
June	1st to 10th	Day	27% 6% 22% 34% 10% 1	730
	11th to 20th	Day	21% 7% 22% 35% 14% 1	730
	21st to end	Day	30% 5% 20% 33% 11% 2	730
July	1st to 10th	Day	23% 6% 19% 36% 14% 1	720
	11th to 20th	Day	23% 5% 17% 36% 16% 2	718
	21st to end	Day	22% 5% 18% 36% 18% 1	586
August	1st to 10th	Day	17% 5% 18% 35% 21% 2	526
	11th to 20th	Day	20% 6% 20% 30% 20% 4	519
	21st to end	Day	17% 5% 20% 35% 19% 4	583
September	1st to 10th	Day	18% 3 21% 37% 19% 2	530
	11th to 20th	Day	21% 5% 20% 30% 19% 4	530
	21st to end	Day	19% 6% 24% 33% 15% 4	516
October	1st to 10th	Day	18% 4 29% 28% 18% 3	420
	11th to 20th	Day	18% 12% 27% 28% 12% 3	420
	21st to end	Day	23% 11% 30% 26% 10%	462
November	1st to 10th	Day	21% 13% 28% 27% 10%	425
	11th to 20th	Day	23% 21% 27% 21% 7%	430
	21st to end	Day	27% 15% 30% 23% 6%	430
December	1st to 10th	Day	30% 15% 29% 23% 3	430
	11th to 20th	Day	30% 21% 30% 15% 3 1	430
	21st to end	Day	23% 18% 33% 20% 5% 1	473
January	1st to 10th	Night	24% 18% 35% 20% 4	906
	11th to 20th	Night	26% 14% 32% 22% 6%	816
	21st to end	Night	19% 16% 39% 21% 4	880
February	1st to 10th	Night	27% 16% 33% 22% 3	800
	11th to 20th	Night	22% 19% 37% 20% 2	783
	21st to end	Night	17% 11% 41% 24% 6% 1	660
March	1st to 10th	Night	24% 14% 36% 24% 3	800
	11th to 20th	Night	22% 11% 31% 29% 8%	800
	21st to end	Night	25% 12% 31% 25% 6% 1	880
April	1st to 10th	Night	31% 7% 31% 26% 5%	653
	11th to 20th	Night	27% 6% 34% 27% 6%	594
	21st to end	Night	27% 7% 25% 30% 11% 2	600
May	1st to 10th	Night	28% 7% 25% 32% 7% 2	600
	11th to 20th	Night	27% 4 26% 31% 10% 1	592
	21st to end	Night	25% 3 21% 37% 12% 1	649
June	1st to 10th	Night	30% 5% 21% 31% 14%	600
	11th to 20th	Night	25% 5% 20% 39% 11%	517
	21st to end	Night	30% 5% 18% 32% 15% 1	400
July	1st to 10th	Night	26% 5% 15% 39% 15%	437
	11th to 20th	Night	21% 7% 16% 39% 15% 2	582
	21st to end	Night	25% 5% 11% 43% 16%	656
August	1st to 10th	Night	17% 5% 16% 39% 21% 2	598
	11th to 20th	Night	20% 3 16% 37% 22% 2	592
	21st to end	Night	17% 5% 18% 35% 22% 4	660
September	1st to 10th	Night	18% 6% 18% 36% 19% 3	600
	11th to 20th	Night	21% 4 21% 31% 19% 3	739
	21st to end	Night	23% 6% 22% 31% 14% 3	800
October	1st to 10th	Night	24% 6% 25% 31% 13% 2	800
	11th to 20th	Night	21% 10% 29% 26% 12% 2	800
	21st to end	Night	27% 11% 29% 26% 7%	880
November	1st to 10th	Night	22% 12% 29% 28% 8% 1	875
	11th to 20th	Night	23% 17% 30% 22% 8%	950
	21st to end	Night	24% 15% 34% 19% 7% 1	950
December	1st to 10th	Night	28% 15% 32% 21% 4 1	950
	11th to 20th	Night	27% 20% 33% 16% 3	950
	21st to end	Night	25% 17% 34% 19% 4 1	1045

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