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 2024-11-12.

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% 10% 3 7% 4 4 4 3 5% 7% 31%	102009
	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 31%	97235
Transparency	Day	51% 8% 11% 18% 10% 1	105612
	Night	54% 7% 10% 18% 10% 1	101721
Seeing	Day	23% 10% 27% 28% 10% 1	17228
	Night	24% 11% 28% 28% 9% 1	24797

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% 11% 5% 8% 6% 4 4 3 5% 6% 29%	2526
	11th to 20th	Day	18% 10% 5% 16% 6% 6% 5% 4 6% 6% 19%	2782
	21st to end	Day	21% 10% 5% 9% 5% 4 4 4 6% 6% 26%	3124
February	1st to 10th	Day	20% 13% 5% 8% 6% 5% 4 5% 6% 6% 22%	1710
	11th to 20th	Day	18% 11% 3 9% 7% 6% 5% 4 5% 5% 26%	1663
	21st to end	Day	14% 12% 5% 8% 5% 4 5% 4 5% 6% 31%	1467
March	1st to 10th	Day	27% 13% 4 6% 5% 4 4 3 5% 5% 26%	3429
	11th to 20th	Day	18% 14% 6% 8% 4 4 4 3 5% 6% 28%	3480
	21st to end	Day	30% 12% 5% 9% 5% 3 3 2 3 3 23%	3816
April	1st to 10th	Day	23% 14% 4 7% 4 4 4 3 4 6% 27%	2228
	11th to 20th	Day	21% 10% 3 6% 4 4 3 3 5% 7% 34%	2352
	21st to end	Day	22% 10% 3 7% 4 4 4 3 4 7% 33%	2470
May	1st to 10th	Day	22% 13% 3 8% 5% 4 4 3 4 8% 26%	4106
	11th to 20th	Day	24% 14% 3 6% 4 4 3 2 4 6% 30%	4351
	21st to end	Day	18% 8% 3 7% 4 4 3 4 5% 9% 36%	4836
June	1st to 10th	Day	25% 10% 3 6% 5% 5% 4 3 5% 7% 29%	2660
	11th to 20th	Day	20% 10% 3 7% 5% 3 4 3 4 6% 36%	2749
	21st to end	Day	25% 8% 3 7% 4 5% 4 4 6% 8% 27%	2850
July	1st to 10th	Day	18% 10% 3 7% 5% 4 4 4 6% 7% 32%	4637
	11th to 20th	Day	15% 15% 4 7% 4 4 4 3 5% 7% 31%	4440
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 7% 34%	4477
August	1st to 10th	Day	23% 10% 3 8% 4 4 5% 4 5% 6% 28%	4105
	11th to 20th	Day	16% 9% 3 6% 4 4 4 4 6% 7% 37%	4045
	21st to end	Day	9% 6% 2 5% 4 4 4 4 5% 8% 49%	4386
September	1st to 10th	Day	14% 7% 3 6% 4 3 4 3 5% 7% 44%	2280
	11th to 20th	Day	18% 7% 2 5% 3 3 4 3 5% 6% 44%	2145
	21st to end	Day	19% 8% 2 6% 4 4 4 3 4 6% 41%	2066
October	1st to 10th	Day	20% 6% 2 6% 4 4 5% 3 5% 7% 39%	1900
	11th to 20th	Day	20% 6% 2 6% 4 4 4 4 5% 7% 38%	1900
	21st to end	Day	20% 9% 3 6% 5% 4 5% 3 5% 6% 32%	2090
November	1st to 10th	Day	19% 10% 4 7% 4 3 4 4 5% 6% 34%	1682
	11th to 20th	Day	21% 10% 3 7% 6% 5% 5% 3 6% 7% 29%	1528
	21st to end	Day	21% 10% 3 8% 5% 4 5% 4 4 7% 29%	1520
December	1st to 10th	Day	24% 12% 3 9% 6% 5% 5% 3 5% 7% 21%	1520
	11th to 20th	Day	31% 11% 3 6% 5% 4 5% 4 5% 7% 20%	1906
	21st to end	Day	23% 10% 6% 8% 4 3 4 3 5% 6% 27%	2783
January	1st to 10th	Night	15% 11% 4 13% 6% 5% 5% 4 6% 6% 24%	4429
	11th to 20th	Night	17% 12% 5% 15% 6% 5% 5% 3 5% 5% 21%	4179
	21st to end	Night	19% 12% 4 10% 6% 5% 5% 4 5% 7% 23%	4532
February	1st to 10th	Night	20% 11% 4 9% 6% 5% 5% 4 5% 7% 25%	2470
	11th to 20th	Night	18% 10% 3 9% 6% 6% 5% 5% 6% 7% 25%	2403
	21st to end	Night	13% 11% 4 9% 6% 5% 5% 4 5% 6% 33%	1984
March	1st to 10th	Night	24% 12% 3 7% 6% 5% 4 3 3 5% 28%	3410
	11th to 20th	Night	17% 11% 6% 9% 5% 4 5% 4 6% 6% 26%	3479
	21st to end	Night	33% 9% 4 9% 3 3 2 2 3 4 28%	3816
April	1st to 10th	Night	24% 12% 2 6% 5% 3 3 3 5% 5% 31%	1951
	11th to 20th	Night	25% 9% 2 5% 3 4 3 3 4 7% 34%	1774
	21st to end	Night	23% 10% 3 6% 5% 4 3 4 5% 7% 32%	1710
May	1st to 10th	Night	20% 13% 4 9% 5% 4 3 3 5% 6% 29%	2834
	11th to 20th	Night	24% 13% 3 7% 5% 4 4 3 5% 5% 28%	2551
	21st to end	Night	19% 11% 4 8% 4 4 4 2 4 6% 33%	2749
June	1st to 10th	Night	26% 10% 2 6% 4 3 4 2 6% 7% 30%	1520
	11th to 20th	Night	20% 9% 3 7% 4 4 4 3 4 6% 35%	1441
	21st to end	Night	27% 9% 3 7% 4 5% 4 3 5% 6% 26%	1330
July	1st to 10th	Night	20% 12% 4 10% 5% 4 5% 3 5% 7% 26%	2295
	11th to 20th	Night	18% 15% 5% 8% 4 4 5% 3 5% 7% 26%	2530
	21st to end	Night	21% 9% 2 6% 5% 4 4 3 5% 7% 33%	3074
August	1st to 10th	Night	19% 13% 5% 8% 4 3 3 2 4 6% 34%	2825
	11th to 20th	Night	18% 8% 3 8% 4 3 3 4 5% 7% 35%	2782
	21st to end	Night	10% 8% 2 5% 4 3 4 3 5% 7% 51%	3270
September	1st to 10th	Night	14% 8% 3 6% 4 4 4 4 5% 7% 43%	1900
	11th to 20th	Night	18% 7% 2 5% 3 3 3 4 4 6% 45%	2032
	21st to end	Night	21% 7% 3 7% 4 3 3 3 5% 6% 38%	2114
October	1st to 10th	Night	21% 7% 2 6% 3 3 4 3 4 7% 40%	2280
	11th to 20th	Night	19% 7% 2 5% 4 3 3 4 5% 7% 42%	2280
	21st to end	Night	23% 7% 2 5% 4 3 4 4 5% 7% 37%	2508
November	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 7% 38%	2611
	11th to 20th	Night	19% 8% 3 7% 5% 5% 5% 5% 6% 7% 30%	2675
	21st to end	Night	19% 8% 3 7% 5% 5% 5% 4 6% 8% 29%	2660
December	1st to 10th	Night	22% 12% 4 8% 5% 4 5% 4 6% 7% 23%	2660
	11th to 20th	Night	30% 10% 3 6% 4 4 4 4 6% 6% 23%	3293
	21st to end	Night	24% 9% 4 8% 4 4 4 4 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	57% 7% 14% 18% 5%	2527
	11th to 20th	Day	60% 6% 15% 14% 5%	2782
	21st to end	Day	56% 8% 12% 17% 6%	3124
February	1st to 10th	Day	60% 7% 12% 13% 7% 1	1710
	11th to 20th	Day	58% 7% 11% 17% 7% 1	1671
	21st to end	Day	48% 7% 15% 19% 9% 3	1467
March	1st to 10th	Day	60% 4 9% 17% 8% 2	3433
	11th to 20th	Day	48% 10% 15% 17% 9% 1	3480
	21st to end	Day	63% 4 7% 15% 9% 2	3816
April	1st to 10th	Day	55% 5% 10% 16% 12% 2	2228
	11th to 20th	Day	49% 5% 12% 20% 13% 1	2352
	21st to end	Day	48% 5% 12% 20% 13% 2	2470
May	1st to 10th	Day	55% 9% 13% 17% 6%	4120
	11th to 20th	Day	54% 8% 11% 17% 9% 1	4355
	21st to end	Day	46% 14% 13% 15% 12% 1	4836
June	1st to 10th	Day	54% 7% 10% 18% 11%	2660
	11th to 20th	Day	50% 7% 11% 19% 12% 2	2749
	21st to end	Day	59% 8% 9% 13% 9% 1	2850
July	1st to 10th	Day	54% 12% 8% 13% 12% 1	4565
	11th to 20th	Day	55% 10% 9% 17% 8% 1	4364
	21st to end	Day	50% 12% 10% 19% 8% 1	4334
August	1st to 10th	Day	58% 9% 8% 16% 9% 1	4092
	11th to 20th	Day	46% 13% 13% 21% 7%	4084
	21st to end	Day	31% 9% 10% 28% 20% 1	4386
September	1st to 10th	Day	40% 7% 11% 24% 17% 2	2280
	11th to 20th	Day	42% 7% 11% 22% 17% 2	2145
	21st to end	Day	44% 6% 9% 24% 16% 1	2066
October	1st to 10th	Day	42% 7% 12% 27% 11% 1	3160
	11th to 20th	Day	35% 5% 11% 24% 24% 1	3140
	21st to end	Day	43% 6% 13% 23% 11% 4	3428
November	1st to 10th	Day	49% 7% 12% 21% 10% 1	1682
	11th to 20th	Day	55% 7% 11% 18% 8% 1	1528
	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	60% 8% 11% 16% 4 1	4429
	11th to 20th	Night	60% 9% 13% 13% 6%	4179
	21st to end	Night	62% 9% 12% 13% 5%	4532
February	1st to 10th	Night	60% 9% 10% 14% 7% 1	2470
	11th to 20th	Night	57% 7% 11% 16% 8% 1	2406
	21st to end	Night	51% 8% 13% 18% 9% 1	1984
March	1st to 10th	Night	61% 4 8% 14% 12% 1	3419
	11th to 20th	Night	54% 8% 15% 14% 8% 1	3480
	21st to end	Night	61% 3 8% 16% 10% 2	3816
April	1st to 10th	Night	55% 5% 9% 15% 13% 4	1951
	11th to 20th	Night	50% 4 10% 21% 13% 1	1774
	21st to end	Night	51% 5% 9% 20% 12% 2	1710
May	1st to 10th	Night	58% 6% 10% 19% 7% 1	2840
	11th to 20th	Night	57% 7% 9% 19% 7% 1	2552
	21st to end	Night	54% 10% 9% 20% 8%	2750
June	1st to 10th	Night	55% 5% 10% 21% 9%	1520
	11th to 20th	Night	50% 7% 11% 18% 12% 2	1441
	21st to end	Night	61% 6% 9% 12% 11% 1	1330
July	1st to 10th	Night	62% 8% 7% 13% 10% 1	2258
	11th to 20th	Night	64% 7% 6% 13% 9% 2	2482
	21st to end	Night	55% 7% 9% 18% 10% 1	2977
August	1st to 10th	Night	56% 7% 7% 20% 9%	2813
	11th to 20th	Night	50% 8% 13% 21% 7%	2809
	21st to end	Night	34% 7% 11% 28% 20%	3269
September	1st to 10th	Night	41% 7% 8% 26% 16% 2	1900
	11th to 20th	Night	42% 5% 8% 25% 18% 2	2032
	21st to end	Night	48% 5% 7% 23% 15% 1	2114
October	1st to 10th	Night	45% 6% 11% 24% 12% 1	3800
	11th to 20th	Night	32% 5% 12% 28% 23% 1	3779
	21st to end	Night	40% 5% 9% 27% 17% 1	4120
November	1st to 10th	Night	48% 8% 12% 21% 10% 1	2611
	11th to 20th	Night	53% 7% 12% 19% 9%	2675
	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	26% 18% 33% 19% 4	370
	21st to end	Day	20% 16% 36% 23% 5%	407
February	1st to 10th	Day	26% 15% 33% 22% 3	370
	11th to 20th	Day	25% 20% 33% 20% 2	366
	21st to end	Day	17% 19% 37% 23% 4	334
March	1st to 10th	Day	24% 16% 35% 20% 4	460
	11th to 20th	Day	20% 18% 33% 24% 5%	460
	21st to end	Day	24% 14% 34% 20% 7% 1	506
April	1st to 10th	Day	29% 14% 32% 22% 3	460
	11th to 20th	Day	24% 10% 35% 25% 5%	452
	21st to end	Day	22% 9% 28% 30% 10% 1	460
May	1st to 10th	Day	25% 11% 32% 25% 7% 1	463
	11th to 20th	Day	22% 7% 29% 32% 10% 1	640
	21st to end	Day	26% 6% 26% 32% 9% 1	735
June	1st to 10th	Day	28% 6% 23% 35% 9% 1	680
	11th to 20th	Day	21% 8% 22% 35% 14% 1	680
	21st to end	Day	31% 5% 21% 32% 10% 2	680
July	1st to 10th	Day	24% 6% 19% 36% 14% 1	670
	11th to 20th	Day	24% 5% 17% 37% 15% 1	668
	21st to end	Day	23% 5% 19% 35% 17% 1	542
August	1st to 10th	Day	18% 6% 20% 35% 20% 2	486
	11th to 20th	Day	21% 6% 20% 30% 20% 3	479
	21st to end	Day	18% 5% 20% 34% 19% 4	539
September	1st to 10th	Day	17% 3 22% 37% 19% 2	490
	11th to 20th	Day	22% 6% 21% 30% 17% 4	490
	21st to end	Day	20% 5% 25% 33% 14% 3	477
October	1st to 10th	Day	20% 4 28% 28% 17% 3	390
	11th to 20th	Day	17% 12% 29% 27% 12% 3	390
	21st to end	Day	22% 12% 31% 26% 10%	429
November	1st to 10th	Day	20% 13% 28% 27% 11%	395
	11th to 20th	Day	25% 21% 25% 22% 7% 1	373
	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% 15% 33% 22% 5%	775
	21st to end	Night	20% 16% 37% 22% 5%	836
February	1st to 10th	Night	28% 15% 32% 22% 3	760
	11th to 20th	Night	22% 19% 37% 20% 2	743
	21st to end	Night	17% 11% 41% 24% 6% 1	628
March	1st to 10th	Night	24% 13% 36% 23% 3	760
	11th to 20th	Night	22% 11% 31% 29% 7%	760
	21st to end	Night	25% 12% 31% 25% 6% 1	836
April	1st to 10th	Night	31% 7% 30% 27% 5%	621
	11th to 20th	Night	27% 6% 34% 27% 6%	564
	21st to end	Night	27% 7% 26% 28% 11% 2	570
May	1st to 10th	Night	27% 7% 24% 32% 8% 2	570
	11th to 20th	Night	28% 4 25% 31% 10% 1	562
	21st to end	Night	25% 4 21% 37% 11% 1	616
June	1st to 10th	Night	31% 5% 21% 31% 12%	570
	11th to 20th	Night	25% 5% 20% 39% 11%	491
	21st to end	Night	31% 5% 17% 31% 15% 1	380
July	1st to 10th	Night	26% 5% 15% 38% 15%	414
	11th to 20th	Night	22% 7% 16% 39% 15% 1	552
	21st to end	Night	26% 5% 11% 43% 15%	623
August	1st to 10th	Night	17% 5% 17% 40% 20% 2	568
	11th to 20th	Night	21% 3 17% 36% 22% 1	562
	21st to end	Night	17% 5% 17% 35% 22% 4	627
September	1st to 10th	Night	17% 6% 19% 36% 19% 3	570
	11th to 20th	Night	21% 5% 21% 31% 18% 3	702
	21st to end	Night	23% 7% 22% 31% 14% 3	760
October	1st to 10th	Night	25% 6% 25% 31% 12% 2	760
	11th to 20th	Night	20% 10% 30% 26% 12% 2	760
	21st to end	Night	27% 11% 29% 26% 6%	836
November	1st to 10th	Night	22% 12% 28% 29% 8% 1	830
	11th to 20th	Night	23% 16% 30% 23% 8%	856
	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

Page generated at 2024-11-08 23:45:35. Version ac07

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