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 2025-10-19.

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% 11% 4 9% 5% 4 4 4 5% 7% 32%	126474
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 29%	123270
Transparency	Day	49% 8% 13% 19% 11% 1	130043
	Night	52% 7% 12% 17% 10% 1	127758
Seeing	Day	20% 10% 28% 24% 13% 5%	16449
	Night	22% 9% 27% 29% 12% 1	25747

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	13% 18% 8% 29% 7% 4 3 2 3 3 10%	3543
	11th to 20th	Day	12% 20% 9% 26% 9% 7% 4 2 4 3 6%	3822
	21st to end	Day	19% 18% 7% 13% 7% 5% 5% 3 4 4 14%	4466
February	1st to 10th	Day	15% 17% 5% 14% 7% 6% 5% 4 5% 5% 17%	1800
	11th to 20th	Day	15% 13% 7% 15% 9% 7% 5% 3 4 4 20%	1784
	21st to end	Day	15% 16% 5% 12% 7% 4 5% 5% 4 5% 24%	1650
March	1st to 10th	Day	15% 13% 5% 12% 7% 4 3 3 5% 5% 27%	4746
	11th to 20th	Day	11% 13% 5% 10% 6% 4 4 3 6% 8% 30%	4980
	21st to end	Day	17% 13% 6% 12% 5% 4 4 3 5% 5% 28%	5465
April	1st to 10th	Day	15% 14% 4 7% 4 3 4 4 6% 8% 31%	2275
	11th to 20th	Day	17% 10% 3 6% 4 4 5% 3 6% 8% 36%	2572
	21st to end	Day	18% 10% 3 7% 4 4 4 3 5% 7% 36%	2600
May	1st to 10th	Day	16% 8% 3 8% 5% 4 4 4 5% 7% 36%	5862
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 5% 9% 36%	5861
	21st to end	Day	16% 8% 3 6% 4 4 4 3 4 7% 42%	6838
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 8% 42%	2888
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 5% 7% 10% 42%	3000
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 9% 43%	3000
July	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 5% 6% 10% 42%	6729
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 6% 9% 47%	6233
	21st to end	Day	7% 5% 2 6% 4 5% 4 4 7% 10% 47%	6424
August	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 7% 10% 40%	4234
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 5% 7% 11% 38%	4174
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 7% 9% 43%	4661
September	1st to 10th	Day	10% 7% 2 6% 5% 5% 5% 4 6% 9% 43%	2300
	11th to 20th	Day	11% 5% 1 6% 4 4 5% 4 6% 8% 48%	2200
	21st to end	Day	16% 7% 3 7% 4 5% 5% 4 6% 8% 36%	2200
October	1st to 10th	Day	14% 7% 3 7% 5% 5% 4 4 5% 7% 38%	2070
	11th to 20th	Day	21% 11% 3 8% 5% 4 5% 3 5% 6% 28%	1980
	21st to end	Day	25% 15% 4 8% 5% 4 4 3 5% 6% 22%	2009
November	1st to 10th	Day	16% 14% 3 10% 5% 4 4 3 5% 7% 28%	1762
	11th to 20th	Day	16% 17% 5% 11% 5% 5% 4 3 5% 6% 22%	1680
	21st to end	Day	18% 21% 8% 12% 5% 5% 4 3 4 5% 14%	1600
December	1st to 10th	Day	18% 24% 9% 15% 6% 4 4 2 4 4 10%	2600
	11th to 20th	Day	20% 23% 8% 13% 7% 5% 4 2 4 4 10%	2990
	21st to end	Day	25% 23% 7% 13% 6% 4 4 4 4 4 8%	3476
January	1st to 10th	Night	14% 19% 10% 25% 6% 5% 3 2 2 4 11%	6419
	11th to 20th	Night	12% 19% 9% 19% 10% 6% 5% 3 5% 4 8%	6131
	21st to end	Night	17% 17% 7% 13% 6% 6% 5% 4 4 4 16%	6490
February	1st to 10th	Night	16% 17% 5% 14% 8% 6% 5% 4 4 4 17%	2600
	11th to 20th	Night	15% 16% 6% 11% 7% 5% 5% 5% 5% 5% 20%	2498
	21st to end	Night	16% 13% 6% 13% 7% 5% 5% 4 5% 5% 22%	1980
March	1st to 10th	Night	15% 13% 3 10% 6% 4 4 3 5% 6% 31%	5107
	11th to 20th	Night	9% 10% 5% 10% 5% 5% 4 4 6% 8% 34%	4980
	21st to end	Night	16% 11% 4 11% 6% 4 4 3 4 4 33%	5467
April	1st to 10th	Night	18% 13% 4 7% 5% 4 4 3 6% 7% 30%	2138
	11th to 20th	Night	17% 9% 3 5% 4 5% 5% 4 4 7% 37%	1777
	21st to end	Night	16% 11% 3 7% 5% 4 5% 3 5% 7% 34%	1800
May	1st to 10th	Night	17% 9% 3 8% 5% 4 4 3 5% 7% 35%	4033
	11th to 20th	Night	13% 9% 3 8% 5% 5% 4 4 5% 6% 37%	4032
	21st to end	Night	18% 7% 2 4 3 3 4 3 5% 7% 42%	4015
June	1st to 10th	Night	13% 7% 3 5% 4 4 4 4 5% 8% 43%	1508
	11th to 20th	Night	9% 5% 3 6% 5% 5% 4 5% 8% 8% 43%	1400
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 6% 8% 47%	1400
July	1st to 10th	Night	8% 6% 3 7% 4 5% 4 3 6% 9% 46%	3244
	11th to 20th	Night	10% 4 2 6% 3 4 4 3 5% 9% 49%	3722
	21st to end	Night	8% 5% 2 5% 4 5% 5% 4 6% 8% 49%	4427
August	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 6% 7% 45%	2915
	11th to 20th	Night	10% 7% 2 7% 4 4 3 4 6% 7% 46%	2875
	21st to end	Night	9% 6% 2 6% 5% 4 5% 3 5% 8% 47%	3223
September	1st to 10th	Night	11% 5% 2 6% 5% 4 5% 4 5% 6% 46%	2076
	11th to 20th	Night	11% 5% 1 4 4 3 4 4 5% 6% 53%	2200
	21st to end	Night	16% 8% 3 8% 4 5% 4 4 5% 6% 39%	2200
October	1st to 10th	Night	14% 8% 3 7% 5% 4 3 3 5% 7% 42%	2330
	11th to 20th	Night	22% 10% 3 8% 4 4 4 3 5% 6% 32%	2377
	21st to end	Night	24% 12% 4 9% 5% 3 5% 4 5% 6% 23%	2584
November	1st to 10th	Night	16% 14% 3 8% 5% 4 3 3 5% 6% 33%	2536
	11th to 20th	Night	17% 18% 5% 10% 6% 4 5% 3 5% 6% 20%	2717
	21st to end	Night	21% 20% 6% 11% 5% 4 4 3 4 4 18%	2800
December	1st to 10th	Night	13% 24% 12% 15% 6% 5% 5% 4 4 4 9%	4580
	11th to 20th	Night	15% 24% 8% 14% 7% 4 4 2 3 4 14%	5205
	21st to end	Night	22% 21% 7% 14% 6% 5% 4 3 3 4 11%	7484

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	76% 8% 9% 6% 1	3544
	11th to 20th	Day	81% 8% 7% 3 1	3822
	21st to end	Day	69% 8% 11% 7% 4 1	4466
February	1st to 10th	Day	66% 8% 10% 11% 4	1800
	11th to 20th	Day	65% 6% 10% 13% 5%	1793
	21st to end	Day	59% 8% 13% 12% 6% 1	1650
March	1st to 10th	Day	58% 6% 14% 16% 5%	4746
	11th to 20th	Day	49% 7% 16% 20% 6% 2	4980
	21st to end	Day	56% 8% 12% 15% 8% 1	5465
April	1st to 10th	Day	49% 7% 13% 20% 9% 1	2275
	11th to 20th	Day	46% 7% 14% 18% 14% 1	2572
	21st to end	Day	48% 7% 14% 17% 12% 2	2600
May	1st to 10th	Day	46% 7% 12% 17% 13% 5%	5897
	11th to 20th	Day	44% 9% 14% 19% 12% 2	5865
	21st to end	Day	42% 7% 10% 20% 17% 5%	6853
June	1st to 10th	Day	37% 9% 13% 26% 13% 1	2888
	11th to 20th	Day	36% 9% 13% 24% 16% 2	3000
	21st to end	Day	37% 8% 14% 25% 16% 1	3000
July	1st to 10th	Day	36% 7% 13% 24% 18% 1	6656
	11th to 20th	Day	32% 7% 13% 30% 16% 1	6158
	21st to end	Day	31% 8% 16% 25% 20% 1	6240
August	1st to 10th	Day	37% 9% 15% 23% 16% 1	4221
	11th to 20th	Day	37% 9% 16% 30% 8% 1	4213
	21st to end	Day	36% 7% 13% 30% 13% 1	4661
September	1st to 10th	Day	37% 9% 16% 25% 12% 1	2300
	11th to 20th	Day	32% 8% 15% 29% 15% 1	2200
	21st to end	Day	44% 8% 14% 23% 10% 1	2200
October	1st to 10th	Day	35% 8% 13% 28% 15% 2	3369
	11th to 20th	Day	46% 6% 11% 21% 16%	3220
	21st to end	Day	51% 7% 16% 19% 7%	3285
November	1st to 10th	Day	53% 7% 13% 19% 7% 1	1762
	11th to 20th	Day	58% 7% 15% 12% 8% 1	1680
	21st to end	Day	69% 8% 11% 11% 2	1600
December	1st to 10th	Day	72% 10% 11% 6% 1	2600
	11th to 20th	Day	72% 9% 11% 7% 1	2986
	21st to end	Day	79% 7% 8% 5% 2	3476
January	1st to 10th	Night	73% 8% 10% 8% 2	6419
	11th to 20th	Night	73% 9% 10% 6% 2	6131
	21st to end	Night	65% 8% 11% 10% 6%	6490
February	1st to 10th	Night	66% 8% 10% 12% 4	2600
	11th to 20th	Night	61% 7% 13% 14% 6%	2500
	21st to end	Night	61% 7% 12% 14% 6%	1980
March	1st to 10th	Night	52% 6% 13% 20% 8% 1	5107
	11th to 20th	Night	42% 7% 18% 22% 9% 1	4980
	21st to end	Night	50% 7% 12% 17% 12% 1	5467
April	1st to 10th	Night	52% 6% 11% 19% 11% 1	2138
	11th to 20th	Night	46% 7% 11% 18% 17% 2	1777
	21st to end	Night	47% 7% 11% 17% 15% 4	1800
May	1st to 10th	Night	48% 7% 11% 14% 13% 7%	4060
	11th to 20th	Night	44% 9% 11% 22% 11% 3	4033
	21st to end	Night	40% 8% 12% 21% 17% 4	4022
June	1st to 10th	Night	37% 6% 13% 25% 17% 2	1508
	11th to 20th	Night	34% 8% 14% 24% 18% 2	1400
	21st to end	Night	33% 7% 14% 25% 19% 1	1400
July	1st to 10th	Night	34% 6% 14% 24% 18% 4	3210
	11th to 20th	Night	32% 6% 13% 28% 20% 2	3677
	21st to end	Night	31% 8% 13% 22% 24% 2	4297
August	1st to 10th	Night	35% 8% 14% 25% 17% 1	2904
	11th to 20th	Night	35% 7% 14% 29% 13% 2	2902
	21st to end	Night	33% 8% 13% 30% 15% 1	3223
September	1st to 10th	Night	36% 8% 14% 27% 14% 2	2076
	11th to 20th	Night	31% 6% 15% 29% 19% 1	2200
	21st to end	Night	44% 7% 13% 22% 13% 1	2200
October	1st to 10th	Night	29% 7% 11% 27% 25% 1	3811
	11th to 20th	Night	48% 5% 12% 19% 15%	3876
	21st to end	Night	48% 7% 14% 21% 10%	4245
November	1st to 10th	Night	48% 6% 13% 22% 10% 1	2536
	11th to 20th	Night	59% 8% 14% 13% 6% 1	2717
	21st to end	Night	66% 7% 14% 11% 2	2800
December	1st to 10th	Night	73% 8% 11% 6% 1	4580
	11th to 20th	Night	69% 9% 10% 9% 4 1	5208
	21st to end	Night	75% 7% 10% 7% 1	7484

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	23% 22% 31% 18% 5% 1	283
	11th to 20th	Day	25% 16% 41% 14% 3 1	300
	21st to end	Day	29% 13% 34% 20% 3 1	330
February	1st to 10th	Day	25% 20% 40% 11% 3	300
	11th to 20th	Day	26% 20% 39% 15%	300
	21st to end	Day	24% 28% 33% 14% 1	249
March	1st to 10th	Day	28% 15% 35% 17% 5%	410
	11th to 20th	Day	25% 13% 30% 26% 5% 1	500
	21st to end	Day	31% 13% 28% 21% 6% 1	550
April	1st to 10th	Day	25% 15% 32% 21% 7% 1	500
	11th to 20th	Day	27% 12% 29% 19% 10% 3	492
	21st to end	Day	24% 11% 32% 23% 7% 2	500
May	1st to 10th	Day	23% 11% 29% 24% 10% 3	503
	11th to 20th	Day	17% 9% 24% 27% 17% 6%	527
	21st to end	Day	17% 6% 23% 27% 19% 7%	573
June	1st to 10th	Day	13% 7% 32% 29% 13% 6%	618
	11th to 20th	Day	13% 8% 29% 31% 14% 5%	730
	21st to end	Day	15% 6% 26% 35% 14% 4	730
July	1st to 10th	Day	12% 4 25% 28% 22% 9%	721
	11th to 20th	Day	15% 4 26% 29% 19% 8%	669
	21st to end	Day	9% 3 22% 32% 24% 10%	576
August	1st to 10th	Day	11% 3 16% 30% 27% 12%	526
	11th to 20th	Day	13% 4 18% 28% 22% 14%	519
	21st to end	Day	15% 4 16% 32% 24% 9%	583
September	1st to 10th	Day	13% 6% 23% 25% 21% 13%	530
	11th to 20th	Day	9% 4 16% 25% 30% 15%	530
	21st to end	Day	18% 7% 23% 24% 18% 9%	530
October	1st to 10th	Day	18% 8% 24% 25% 19% 6%	530
	11th to 20th	Day	24% 12% 30% 22% 10% 1	522
	21st to end	Day	32% 12% 33% 18% 4 1	458
November	1st to 10th	Day	24% 16% 31% 19% 8% 2	300
	11th to 20th	Day	29% 16% 32% 18% 4 2	240
	21st to end	Day	29% 18% 38% 13% 3	200
December	1st to 10th	Day	30% 17% 32% 17% 5% 1	200
	11th to 20th	Day	29% 21% 32% 15% 4	200
	21st to end	Day	33% 19% 33% 12% 4	220
January	1st to 10th	Night	26% 16% 35% 17% 5% 1	1000
	11th to 20th	Night	30% 14% 31% 20% 5%	901
	21st to end	Night	23% 11% 35% 25% 6%	880
February	1st to 10th	Night	25% 13% 38% 23% 2 1	800
	11th to 20th	Night	25% 14% 30% 25% 5%	783
	21st to end	Night	24% 15% 37% 20% 3	660
March	1st to 10th	Night	22% 13% 33% 26% 7%	800
	11th to 20th	Night	20% 13% 36% 25% 7% 1	800
	21st to end	Night	25% 10% 28% 28% 9% 1	880
April	1st to 10th	Night	26% 13% 30% 26% 5%	785
	11th to 20th	Night	23% 9% 29% 26% 11% 1	593
	21st to end	Night	22% 9% 27% 33% 8% 1	600
May	1st to 10th	Night	21% 5% 29% 34% 10% 2	600
	11th to 20th	Night	16% 5% 26% 39% 12% 2	591
	21st to end	Night	14% 4 24% 39% 17% 1	477
June	1st to 10th	Night	15% 3 27% 39% 16% 1	400
	11th to 20th	Night	11% 4 24% 47% 14%	400
	21st to end	Night	12% 3 23% 43% 20% 1	400
July	1st to 10th	Night	8% 2 15% 50% 23% 2	423
	11th to 20th	Night	11% 2 18% 40% 25% 3	582
	21st to end	Night	10% 2 15% 48% 24% 2	657
August	1st to 10th	Night	9% 2 12% 41% 33% 3	597
	11th to 20th	Night	12% 2 12% 39% 30% 4	594
	21st to end	Night	13% 2 15% 39% 29% 2	660
September	1st to 10th	Night	12% 5% 19% 34% 26% 5%	780
	11th to 20th	Night	13% 4 14% 35% 30% 5%	800
	21st to end	Night	19% 5% 20% 35% 18% 4	800
October	1st to 10th	Night	19% 6% 25% 31% 17% 2	800
	11th to 20th	Night	30% 8% 25% 27% 9% 1	793
	21st to end	Night	33% 11% 27% 25% 4	836
November	1st to 10th	Night	26% 11% 27% 27% 9% 1	780
	11th to 20th	Night	29% 14% 31% 19% 6% 1	800
	21st to end	Night	33% 11% 29% 22% 5%	800
December	1st to 10th	Night	32% 16% 31% 17% 4	800
	11th to 20th	Night	26% 19% 33% 19% 3	800
	21st to end	Night	33% 12% 31% 21% 2	1095

Page generated at 2025-10-16 01:50:23. Version ac07

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