Port Franks
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Franks clear sky chart from 2005-11-06 to 2024-01-12.

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	14% 10% 4 7% 6% 3 5% 5% 5% 7% 32%	74717
	Night	16% 12% 5% 8% 6% 3 5% 4 5% 6% 29%	74972
Transparency	Day	33% 39% 9% 12% 6% 1	74424
	Night	36% 38% 8% 11% 6% 1	74785
Seeing	Day	20% 10% 27% 24% 13% 5%	14011
	Night	22% 9% 27% 29% 12% 1	23517

By Month

Percentage Cloud Cover SE (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	17% 18% 8% 13% 11% 4 4 3 3 4 14%	1483
	11th to 20th	Day	15% 18% 9% 15% 10% 4 4 4 4 5% 13%	1533
	21st to end	Day	15% 17% 7% 14% 11% 5% 6% 4 4 5% 13%	1782
February	1st to 10th	Day	16% 17% 6% 11% 10% 4 5% 5% 4 5% 16%	1620
	11th to 20th	Day	15% 11% 7% 13% 10% 6% 5% 4 4 4 19%	1601
	21st to end	Day	16% 15% 6% 11% 7% 3 5% 5% 4 5% 24%	1480
March	1st to 10th	Day	14% 12% 5% 10% 8% 3 5% 5% 6% 6% 27%	1898
	11th to 20th	Day	12% 12% 7% 8% 7% 3 4 3 6% 7% 30%	1980
	21st to end	Day	18% 11% 5% 8% 6% 3 5% 4 5% 6% 29%	2178
April	1st to 10th	Day	15% 13% 4 6% 5% 2 4 5% 6% 8% 31%	2046
	11th to 20th	Day	17% 10% 3 5% 5% 2 5% 4 6% 8% 35%	2312
	21st to end	Day	19% 9% 4 7% 5% 3 4 4 4 7% 35%	2340
May	1st to 10th	Day	15% 7% 4 6% 5% 3 4 6% 5% 8% 36%	2340
	11th to 20th	Day	17% 8% 4 5% 5% 3 4 4 5% 8% 39%	2301
	21st to end	Day	11% 5% 3 5% 5% 3 5% 5% 6% 9% 43%	2685
June	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 6% 6% 8% 43%	2598
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 5% 6% 10% 43%	2700
	21st to end	Day	11% 5% 2 6% 4 3 5% 5% 5% 10% 44%	2700
July	1st to 10th	Day	8% 4 2 4 4 3 4 6% 6% 10% 50%	2665
	11th to 20th	Day	10% 5% 3 4 4 3 5% 5% 7% 10% 44%	2477
	21st to end	Day	8% 5% 2 5% 5% 3 5% 5% 7% 10% 44%	2547
August	1st to 10th	Day	9% 4 2 5% 5% 4 5% 6% 7% 11% 42%	2327
	11th to 20th	Day	9% 5% 2 6% 6% 3 6% 7% 6% 10% 40%	2303
	21st to end	Day	12% 6% 2 6% 5% 3 5% 4 6% 8% 44%	2574
September	1st to 10th	Day	10% 7% 3 5% 6% 3 5% 5% 7% 8% 41%	2070
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 6% 9% 44%	1980
	21st to end	Day	16% 6% 3 6% 5% 4 5% 5% 6% 8% 36%	1980
October	1st to 10th	Day	15% 8% 3 6% 6% 4 5% 5% 6% 7% 36%	1864
	11th to 20th	Day	22% 12% 3 7% 7% 3 5% 5% 5% 6% 25%	1800
	21st to end	Day	26% 14% 4 7% 6% 3 4 4 4 6% 22%	1904
November	1st to 10th	Day	16% 13% 4 9% 6% 3 5% 4 5% 7% 28%	1672
	11th to 20th	Day	15% 16% 6% 10% 7% 4 4 4 5% 6% 22%	1596
	21st to end	Day	19% 18% 8% 11% 6% 4 5% 4 4 5% 15%	1520
December	1st to 10th	Day	16% 22% 9% 13% 7% 4 5% 4 4 4 12%	1520
	11th to 20th	Day	19% 19% 10% 12% 6% 4 5% 4 4 5% 12%	1910
	21st to end	Day	26% 24% 8% 10% 7% 3 3 4 4 4 8%	2431
January	1st to 10th	Night	18% 17% 10% 14% 9% 5% 5% 3 3 4 13%	2679
	11th to 20th	Night	19% 19% 8% 13% 8% 3 5% 4 4 4 13%	2446
	21st to end	Night	13% 16% 10% 12% 10% 5% 5% 5% 4 5% 14%	2574
February	1st to 10th	Night	18% 16% 6% 12% 10% 4 6% 5% 4 4 16%	2340
	11th to 20th	Night	16% 14% 6% 10% 8% 4 5% 6% 5% 5% 19%	2241
	21st to end	Night	17% 12% 7% 11% 9% 4 5% 5% 4 5% 22%	1776
March	1st to 10th	Night	15% 11% 5% 9% 8% 4 5% 3 5% 5% 28%	2062
	11th to 20th	Night	13% 10% 6% 9% 6% 3 4 4 5% 7% 32%	1980
	21st to end	Night	19% 10% 5% 8% 7% 3 4 4 4 5% 32%	2178
April	1st to 10th	Night	19% 11% 4 6% 6% 3 4 4 6% 7% 31%	1926
	11th to 20th	Night	17% 8% 4 4 5% 3 5% 5% 5% 7% 38%	1597
	21st to end	Night	17% 11% 3 6% 5% 2 5% 4 5% 7% 34%	1620
May	1st to 10th	Night	17% 8% 3 6% 6% 3 5% 4 6% 8% 34%	1620
	11th to 20th	Night	14% 7% 4 6% 5% 3 4 4 6% 7% 40%	1592
	21st to end	Night	10% 4 2 3 3 3 5% 5% 7% 7% 49%	1589
June	1st to 10th	Night	13% 6% 3 5% 5% 3 5% 4 6% 8% 43%	1358
	11th to 20th	Night	9% 5% 3 5% 5% 3 5% 6% 8% 7% 44%	1260
	21st to end	Night	10% 4 2 5% 6% 3 5% 5% 6% 8% 47%	1260
July	1st to 10th	Night	6% 4 2 4 4 3 3 4 5% 9% 55%	1290
	11th to 20th	Night	10% 4 2 4 4 3 5% 4 6% 9% 48%	1480
	21st to end	Night	7% 5% 2 5% 4 4 6% 5% 8% 8% 47%	1771
August	1st to 10th	Night	8% 5% 2 5% 5% 3 6% 6% 6% 9% 46%	1611
	11th to 20th	Night	9% 4 2 5% 5% 3 4 6% 6% 9% 47%	1598
	21st to end	Night	10% 6% 3 5% 5% 3 4 4 5% 7% 49%	1782
September	1st to 10th	Night	11% 5% 2 5% 6% 3 5% 5% 5% 7% 45%	1868
	11th to 20th	Night	12% 5% 2 4 5% 3 4 4 5% 6% 50%	1980
	21st to end	Night	15% 7% 3 7% 5% 3 4 5% 5% 6% 39%	1980
October	1st to 10th	Night	15% 8% 3 6% 7% 3 4 4 4 7% 39%	2096
	11th to 20th	Night	23% 10% 4 7% 5% 3 4 3 5% 6% 29%	2160
	21st to end	Night	26% 12% 4 8% 7% 2 4 4 4 6% 22%	2448
November	1st to 10th	Night	17% 13% 4 7% 6% 2 4 4 5% 6% 33%	2406
	11th to 20th	Night	17% 17% 6% 10% 7% 3 5% 4 5% 6% 20%	2581
	21st to end	Night	21% 19% 7% 10% 6% 3 4 3 4 5% 19%	2660
December	1st to 10th	Night	19% 22% 10% 11% 7% 3 5% 4 3 4 12%	2660
	11th to 20th	Night	15% 20% 9% 12% 7% 4 4 4 4 5% 16%	3285
	21st to end	Night	25% 22% 8% 11% 7% 3 4 3 3 3 10%	5218

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	45% 42% 7% 5% 1	1484
	11th to 20th	Day	46% 43% 5% 4 2	1533
	21st to end	Day	50% 39% 5% 4 2	1782
February	1st to 10th	Day	46% 38% 8% 6% 2	1620
	11th to 20th	Day	46% 39% 6% 6% 3	1610
	21st to end	Day	44% 39% 6% 7% 4	1480
March	1st to 10th	Day	37% 38% 10% 11% 4	1898
	11th to 20th	Day	35% 38% 8% 10% 7% 2	1980
	21st to end	Day	36% 38% 9% 13% 3 1	2178
April	1st to 10th	Day	31% 38% 9% 14% 7% 1	2046
	11th to 20th	Day	30% 38% 11% 12% 9% 1	2312
	21st to end	Day	31% 38% 10% 12% 7% 1	2340
May	1st to 10th	Day	29% 37% 8% 15% 10% 1	2340
	11th to 20th	Day	28% 38% 7% 12% 12% 3	2305
	21st to end	Day	25% 39% 10% 14% 11% 2	2685
June	1st to 10th	Day	25% 39% 10% 17% 8% 1	2598
	11th to 20th	Day	20% 39% 10% 19% 12% 1	2700
	21st to end	Day	26% 39% 10% 16% 9%	2700
July	1st to 10th	Day	21% 40% 9% 18% 10% 1	2592
	11th to 20th	Day	22% 40% 10% 17% 9% 1	2402
	21st to end	Day	22% 41% 11% 15% 10% 1	2405
August	1st to 10th	Day	25% 40% 11% 17% 7%	2314
	11th to 20th	Day	21% 38% 13% 16% 10% 1	2303
	21st to end	Day	24% 38% 9% 19% 9% 1	2574
September	1st to 10th	Day	26% 39% 10% 15% 9% 1	2070
	11th to 20th	Day	23% 39% 11% 20% 7%	1980
	21st to end	Day	31% 39% 9% 14% 6% 1	1980
October	1st to 10th	Day	30% 39% 9% 15% 7% 1	1864
	11th to 20th	Day	39% 39% 9% 9% 4 1	1800
	21st to end	Day	42% 39% 8% 7% 4	1904
November	1st to 10th	Day	31% 42% 8% 13% 5% 1	1672
	11th to 20th	Day	36% 43% 9% 8% 5% 1	1596
	21st to end	Day	43% 42% 6% 7% 1	1520
December	1st to 10th	Day	47% 43% 6% 3 1	1520
	11th to 20th	Day	49% 35% 8% 6% 1	1906
	21st to end	Day	65% 27% 7% 2	2431
January	1st to 10th	Night	48% 41% 4 6% 1	2679
	11th to 20th	Night	45% 44% 6% 4 1	2446
	21st to end	Night	47% 39% 7% 6% 1	2574
February	1st to 10th	Night	45% 39% 6% 8% 2	2340
	11th to 20th	Night	43% 40% 7% 7% 3	2243
	21st to end	Night	42% 37% 8% 9% 3	1776
March	1st to 10th	Night	37% 38% 8% 12% 4	2062
	11th to 20th	Night	32% 38% 9% 13% 7% 1	1980
	21st to end	Night	36% 38% 7% 14% 5% 1	2178
April	1st to 10th	Night	32% 36% 7% 14% 9% 1	1926
	11th to 20th	Night	29% 37% 8% 13% 11% 1	1597
	21st to end	Night	32% 38% 7% 10% 10% 2	1620
May	1st to 10th	Night	31% 38% 7% 13% 9% 2	1620
	11th to 20th	Night	27% 38% 7% 13% 10% 4	1593
	21st to end	Night	21% 38% 11% 15% 13% 3	1590
June	1st to 10th	Night	26% 38% 8% 16% 11% 1	1358
	11th to 20th	Night	22% 38% 10% 15% 14% 1	1260
	21st to end	Night	23% 39% 11% 15% 11% 1	1260
July	1st to 10th	Night	19% 38% 8% 17% 15% 2	1254
	11th to 20th	Night	22% 40% 8% 19% 10% 1	1432
	21st to end	Night	19% 42% 12% 16% 10% 1	1672
August	1st to 10th	Night	26% 39% 9% 18% 7%	1600
	11th to 20th	Night	19% 38% 11% 20% 10% 3	1598
	21st to end	Night	21% 39% 9% 20% 11% 1	1782
September	1st to 10th	Night	26% 39% 9% 15% 10% 1	1868
	11th to 20th	Night	23% 38% 9% 20% 10%	1980
	21st to end	Night	31% 38% 9% 13% 9%	1980
October	1st to 10th	Night	29% 39% 8% 14% 10% 1	2096
	11th to 20th	Night	37% 38% 8% 11% 5% 1	2160
	21st to end	Night	41% 39% 7% 8% 5%	2448
November	1st to 10th	Night	26% 43% 8% 16% 6%	2406
	11th to 20th	Night	36% 43% 9% 8% 3 1	2581
	21st to end	Night	41% 42% 8% 8% 1	2660
December	1st to 10th	Night	49% 41% 6% 3 1	2660
	11th to 20th	Night	49% 36% 9% 6% 1	3288
	21st to end	Night	63% 27% 7% 2	5218

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	24% 22% 27% 20% 6% 1	254
	11th to 20th	Day	26% 15% 40% 14% 4 1	243
	21st to end	Day	27% 14% 34% 20% 3 2	264
February	1st to 10th	Day	27% 20% 43% 9% 1	240
	11th to 20th	Day	26% 20% 40% 13%	240
	21st to end	Day	24% 31% 32% 12% 2	198
March	1st to 10th	Day	27% 16% 35% 16% 6%	338
	11th to 20th	Day	25% 12% 29% 27% 6% 1	420
	21st to end	Day	31% 13% 27% 22% 7% 1	462
April	1st to 10th	Day	24% 14% 31% 23% 8% 1	420
	11th to 20th	Day	28% 10% 29% 19% 10% 4	412
	21st to end	Day	25% 11% 31% 24% 7% 2	420
May	1st to 10th	Day	23% 9% 29% 24% 11% 4	423
	11th to 20th	Day	18% 9% 24% 27% 16% 6%	447
	21st to end	Day	15% 6% 21% 29% 21% 8%	485
June	1st to 10th	Day	13% 5% 30% 32% 14% 6%	528
	11th to 20th	Day	13% 8% 28% 31% 15% 5%	630
	21st to end	Day	14% 6% 26% 35% 15% 4	630
July	1st to 10th	Day	11% 3 24% 28% 24% 10%	621
	11th to 20th	Day	15% 4 26% 29% 19% 8%	575
	21st to end	Day	9% 3 21% 34% 23% 10%	488
August	1st to 10th	Day	11% 2 16% 29% 28% 13%	446
	11th to 20th	Day	12% 4 18% 28% 22% 15%	439
	21st to end	Day	16% 4 16% 31% 23% 10%	495
September	1st to 10th	Day	14% 5% 22% 27% 20% 12%	450
	11th to 20th	Day	11% 5% 19% 26% 26% 13%	450
	21st to end	Day	18% 9% 23% 23% 18% 9%	450
October	1st to 10th	Day	20% 8% 24% 25% 17% 6%	450
	11th to 20th	Day	27% 13% 30% 21% 8%	450
	21st to end	Day	34% 10% 33% 18% 4 1	419
November	1st to 10th	Day	23% 16% 31% 20% 9% 2	270
	11th to 20th	Day	28% 15% 33% 17% 5% 2	216
	21st to end	Day	28% 18% 38% 12% 3	180
December	1st to 10th	Day	30% 17% 32% 14% 5% 1	180
	11th to 20th	Day	28% 22% 32% 15% 3	180
	21st to end	Day	33% 19% 34% 12% 2	198
January	1st to 10th	Night	27% 15% 35% 17% 5% 1	950
	11th to 20th	Night	30% 13% 30% 20% 6%	817
	21st to end	Night	22% 10% 34% 26% 7%	792
February	1st to 10th	Night	26% 13% 37% 22% 2 1	720
	11th to 20th	Night	25% 14% 29% 25% 6%	703
	21st to end	Night	27% 15% 35% 19% 4	592
March	1st to 10th	Night	20% 13% 33% 27% 7%	720
	11th to 20th	Night	20% 12% 35% 25% 7% 1	720
	21st to end	Night	24% 9% 27% 29% 10% 1	792
April	1st to 10th	Night	25% 13% 29% 27% 6%	707
	11th to 20th	Night	23% 9% 29% 27% 11% 1	533
	21st to end	Night	22% 7% 27% 34% 9% 1	540
May	1st to 10th	Night	20% 5% 29% 33% 11% 2	540
	11th to 20th	Night	16% 5% 25% 40% 12% 2	531
	21st to end	Night	13% 3 24% 41% 18% 1	429
June	1st to 10th	Night	15% 3 26% 39% 17% 1	360
	11th to 20th	Night	11% 4 24% 46% 15%	360
	21st to end	Night	11% 3 21% 43% 20% 1	360
July	1st to 10th	Night	8% 1 15% 50% 24% 3	379
	11th to 20th	Night	12% 2 17% 40% 25% 4	522
	21st to end	Night	10% 2 16% 48% 23% 2	591
August	1st to 10th	Night	10% 2 11% 42% 32% 3	537
	11th to 20th	Night	12% 2 12% 39% 31% 4	534
	21st to end	Night	13% 2 16% 37% 29% 3	594
September	1st to 10th	Night	13% 4 18% 35% 25% 5%	702
	11th to 20th	Night	14% 4 16% 36% 25% 5%	720
	21st to end	Night	18% 5% 21% 33% 18% 4	720
October	1st to 10th	Night	20% 6% 25% 30% 17% 2	720
	11th to 20th	Night	32% 9% 24% 26% 9% 1	720
	21st to end	Night	34% 10% 27% 24% 4	792
November	1st to 10th	Night	26% 11% 26% 27% 10% 1	740
	11th to 20th	Night	28% 14% 31% 19% 6% 1	760
	21st to end	Night	33% 10% 29% 22% 5%	760
December	1st to 10th	Night	33% 16% 30% 17% 4	760
	11th to 20th	Night	26% 19% 33% 19% 3	760
	21st to end	Night	34% 13% 31% 20% 2	1040

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