Ferrum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ferrum clear sky chart from 2005-11-06 to 2026-07-31.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	137993
	Night	14% 5% 2 5% 4 4 4 3 5% 7% 46%	134934
Transparency	Day	38% 8% 13% 21% 16% 5%	141599
	Night	36% 7% 12% 22% 18% 5%	139400
Seeing	Day	19% 14% 26% 23% 13% 5%	17564
	Night	17% 11% 27% 28% 15% 2	27717

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	15% 7% 2 4 3 3 4 2 5% 6% 49%	5258
	11th to 20th	Day	9% 6% 2 5% 4 3 3 3 3 5% 57%	5280
	21st to end	Day	15% 8% 3 6% 4 4 4 2 3 4 47%	5780
February	1st to 10th	Day	11% 7% 2 8% 5% 4 5% 4 6% 6% 42%	1890
	11th to 20th	Day	19% 7% 3 5% 3 3 4 3 6% 7% 41%	1896
	21st to end	Day	16% 8% 2 7% 4 4 3 3 5% 5% 42%	1819
March	1st to 10th	Day	17% 7% 2 5% 4 4 4 4 6% 7% 38%	5045
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 6% 7% 53%	5090
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 6% 43%	5586
April	1st to 10th	Day	10% 5% 2 5% 4 3 5% 4 6% 8% 49%	2383
	11th to 20th	Day	12% 6% 2 5% 4 3 3 3 5% 8% 47%	2494
	21st to end	Day	15% 6% 2 7% 5% 4 4 3 4 6% 42%	2646
May	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 10% 36%	7616
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 4 6% 7% 38%	7626
	21st to end	Day	17% 5% 2 6% 5% 5% 5% 4 6% 8% 36%	8391
June	1st to 10th	Day	10% 4 2 5% 5% 5% 6% 5% 8% 9% 40%	2730
	11th to 20th	Day	7% 5% 2 6% 6% 5% 6% 5% 8% 12% 38%	2730
	21st to end	Day	8% 4 2 6% 6% 6% 6% 6% 8% 9% 40%	2730
July	1st to 10th	Day	7% 5% 2 7% 5% 6% 7% 5% 8% 11% 37%	6030
	11th to 20th	Day	7% 5% 2 8% 6% 6% 8% 7% 9% 13% 29%	6030
	21st to end	Day	10% 6% 2 8% 7% 7% 7% 6% 9% 10% 28%	6554
August	1st to 10th	Day	15% 7% 3 7% 4 5% 6% 5% 9% 11% 29%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 6% 6% 5% 9% 11% 36%	4174
	21st to end	Day	8% 5% 2 5% 3 4 4 4 5% 10% 49%	4330
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 6% 7% 47%	2360
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 6% 7% 40%	2200
	21st to end	Day	23% 7% 2 7% 4 4 4 3 5% 6% 35%	2200
October	1st to 10th	Day	17% 4 2 4 3 3 3 2 4 7% 51%	2013
	11th to 20th	Day	14% 4 2 4 3 3 4 3 4 6% 54%	2000
	21st to end	Day	13% 5% 2 6% 3 3 3 2 4 6% 52%	2200
November	1st to 10th	Day	12% 4 2 4 3 4 3 4 4 5% 55%	2057
	11th to 20th	Day	13% 5% 2 4 3 4 3 3 4 7% 52%	1948
	21st to end	Day	12% 5% 1 5% 4 3 4 4 5% 7% 50%	1890
December	1st to 10th	Day	10% 7% 2 6% 4 5% 4 3 4 5% 50%	2755
	11th to 20th	Day	18% 7% 2 3 3 3 4 4 5% 6% 46%	3463
	21st to end	Day	21% 7% 2 5% 3 4 5% 4 6% 6% 37%	4565
January	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	7635
	11th to 20th	Night	10% 6% 2 5% 3 3 3 2 3 4 57%	7680
	21st to end	Night	16% 7% 2 4 3 3 3 3 4 5% 50%	8401
February	1st to 10th	Night	14% 7% 2 6% 4 3 4 4 5% 6% 45%	2730
	11th to 20th	Night	20% 6% 2 6% 4 3 4 3 4 6% 43%	2605
	21st to end	Night	13% 7% 2 5% 5% 4 5% 4 6% 5% 44%	1986
March	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 5% 5% 45%	5028
	11th to 20th	Night	10% 3 1 4 3 3 3 3 6% 8% 57%	5090
	21st to end	Night	14% 5% 2 5% 4 5% 4 4 6% 7% 44%	5588
April	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 5% 8% 46%	2236
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 6% 7% 50%	2075
	21st to end	Night	15% 4 1 4 4 4 4 3 6% 7% 48%	1990
May	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 6% 9% 41%	5237
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 4 5% 7% 42%	5234
	21st to end	Night	22% 6% 2 5% 3 4 4 3 6% 9% 35%	5761
June	1st to 10th	Night	9% 5% 1 5% 4 5% 6% 5% 8% 11% 42%	1890
	11th to 20th	Night	10% 6% 2 5% 3 4 4 4 7% 11% 45%	1890
	21st to end	Night	7% 3 2 5% 4 4 5% 5% 7% 12% 45%	1890
July	1st to 10th	Night	9% 6% 3 7% 5% 5% 6% 5% 7% 10% 39%	4148
	11th to 20th	Night	8% 6% 3 7% 5% 5% 6% 6% 10% 13% 32%	4147
	21st to end	Night	10% 7% 3 7% 5% 6% 5% 6% 7% 10% 34%	4517
August	1st to 10th	Night	17% 7% 2 7% 4 5% 5% 4 7% 11% 31%	2915
	11th to 20th	Night	8% 4 2 6% 4 4 5% 5% 7% 11% 44%	2875
	21st to end	Night	7% 4 1 5% 4 4 5% 3 5% 9% 52%	3520
September	1st to 10th	Night	11% 6% 3 6% 3 3 4 3 5% 7% 49%	2035
	11th to 20th	Night	16% 5% 2 6% 4 4 4 3 6% 7% 43%	2200
	21st to end	Night	23% 7% 2 6% 5% 4 4 3 4 5% 37%	2200
October	1st to 10th	Night	16% 5% 2 6% 3 3 3 3 4 6% 48%	2385
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 5% 54%	2400
	21st to end	Night	14% 4 2 4 3 3 3 2 3 5% 56%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 53%	2461
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 5% 54%	2667
	21st to end	Night	12% 5% 2 5% 3 3 4 4 5% 7% 50%	2730
December	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 6% 54%	4563
	11th to 20th	Night	19% 7% 2 7% 3 4 4 3 4 5% 41%	4952
	21st to end	Night	19% 5% 2 4 4 3 4 3 5% 7% 44%	6633

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	35% 5% 11% 19% 24% 5%	5259
	11th to 20th	Day	29% 5% 6% 17% 30% 13%	5280
	21st to end	Day	40% 4 6% 25% 19% 6%	5780
February	1st to 10th	Day	40% 6% 12% 21% 18% 3	1890
	11th to 20th	Day	41% 6% 9% 23% 16% 5%	1905
	21st to end	Day	44% 6% 11% 16% 17% 6%	1819
March	1st to 10th	Day	41% 7% 10% 18% 13% 10%	5045
	11th to 20th	Day	29% 6% 8% 22% 23% 11%	5090
	21st to end	Day	40% 7% 10% 21% 17% 5%	5586
April	1st to 10th	Day	31% 8% 13% 24% 18% 7%	2383
	11th to 20th	Day	34% 6% 14% 23% 17% 5%	2494
	21st to end	Day	42% 6% 12% 23% 13% 3	2646
May	1st to 10th	Day	40% 10% 14% 21% 13% 1	7651
	11th to 20th	Day	43% 8% 12% 22% 13% 1	7630
	21st to end	Day	44% 9% 12% 17% 13% 4	8405
June	1st to 10th	Day	36% 11% 16% 23% 13% 1	2730
	11th to 20th	Day	36% 12% 16% 20% 14% 2	2730
	21st to end	Day	35% 12% 17% 21% 13% 2	2730
July	1st to 10th	Day	37% 14% 17% 18% 11% 3	5965
	11th to 20th	Day	40% 15% 22% 17% 5%	5961
	21st to end	Day	45% 13% 15% 19% 7%	6370
August	1st to 10th	Day	44% 12% 18% 20% 5%	4221
	11th to 20th	Day	35% 13% 20% 23% 8% 1	4213
	21st to end	Day	30% 9% 18% 29% 13% 2	4330
September	1st to 10th	Day	35% 9% 14% 19% 20% 4	2360
	11th to 20th	Day	41% 8% 12% 21% 17% 2	2200
	21st to end	Day	49% 7% 11% 15% 16% 2	2200
October	1st to 10th	Day	31% 6% 9% 18% 27% 9%	3273
	11th to 20th	Day	23% 3 8% 23% 27% 16%	3240
	21st to end	Day	25% 4 10% 24% 22% 13%	3538
November	1st to 10th	Day	30% 6% 11% 22% 25% 6%	2057
	11th to 20th	Day	34% 4 9% 23% 24% 7%	1948
	21st to end	Day	34% 6% 10% 22% 25% 3	1890
December	1st to 10th	Day	36% 6% 8% 22% 22% 6%	2755
	11th to 20th	Day	38% 6% 12% 19% 21% 5%	3460
	21st to end	Day	43% 8% 12% 21% 14% 1	4565
January	1st to 10th	Night	31% 5% 12% 22% 23% 8%	7636
	11th to 20th	Night	30% 5% 6% 19% 31% 9%	7680
	21st to end	Night	36% 4 7% 27% 20% 6%	8401
February	1st to 10th	Night	39% 6% 11% 23% 19% 3	2730
	11th to 20th	Night	43% 5% 9% 19% 18% 5%	2607
	21st to end	Night	40% 7% 11% 21% 15% 6%	1986
March	1st to 10th	Night	38% 7% 9% 21% 17% 8%	5028
	11th to 20th	Night	26% 6% 8% 21% 26% 13%	5090
	21st to end	Night	38% 7% 10% 23% 18% 5%	5588
April	1st to 10th	Night	37% 6% 13% 19% 20% 4	2236
	11th to 20th	Night	33% 6% 12% 23% 20% 6%	2075
	21st to end	Night	35% 7% 13% 25% 18% 3	1990
May	1st to 10th	Night	39% 9% 15% 21% 13% 3	5264
	11th to 20th	Night	41% 8% 13% 24% 14% 1	5235
	21st to end	Night	45% 8% 15% 20% 9% 3	5771
June	1st to 10th	Night	33% 12% 16% 23% 15% 1	1890
	11th to 20th	Night	31% 12% 19% 22% 14% 2	1890
	21st to end	Night	29% 13% 16% 23% 15% 3	1890
July	1st to 10th	Night	38% 14% 16% 19% 11% 2	4105
	11th to 20th	Night	40% 15% 22% 18% 5% 1	4096
	21st to end	Night	43% 12% 16% 21% 8%	4387
August	1st to 10th	Night	46% 10% 18% 19% 6% 1	2904
	11th to 20th	Night	31% 11% 19% 28% 10% 1	2902
	21st to end	Night	29% 8% 14% 37% 10% 2	3520
September	1st to 10th	Night	35% 9% 13% 22% 16% 6%	2035
	11th to 20th	Night	39% 9% 10% 21% 19% 2	2200
	21st to end	Night	49% 7% 9% 18% 14% 3	2200
October	1st to 10th	Night	36% 5% 9% 17% 25% 8%	3905
	11th to 20th	Night	23% 4 8% 22% 29% 14%	3899
	21st to end	Night	22% 4 7% 27% 26% 13%	4252
November	1st to 10th	Night	34% 5% 10% 21% 24% 7%	2461
	11th to 20th	Night	32% 5% 9% 21% 25% 8%	2667
	21st to end	Night	33% 5% 9% 24% 22% 6%	2730
December	1st to 10th	Night	31% 6% 9% 27% 25% 3	4563
	11th to 20th	Night	44% 6% 9% 18% 15% 7%	4954
	21st to end	Night	37% 7% 15% 23% 16% 2	6633

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	33% 28% 27% 10% 2	330
	11th to 20th	Day	33% 28% 32% 6% 2	330
	21st to end	Day	25% 36% 30% 8% 1	363
February	1st to 10th	Day	24% 30% 32% 11% 3	330
	11th to 20th	Day	44% 27% 23% 4 2	330
	21st to end	Day	30% 27% 28% 12% 2 1	362
March	1st to 10th	Day	25% 21% 31% 17% 5%	540
	11th to 20th	Day	26% 24% 29% 17% 3 1	540
	21st to end	Day	23% 22% 35% 15% 4	594
April	1st to 10th	Day	22% 23% 29% 18% 7% 1	540
	11th to 20th	Day	19% 20% 31% 18% 9% 2	532
	21st to end	Day	22% 17% 29% 21% 10% 1	540
May	1st to 10th	Day	20% 18% 35% 18% 6% 2	543
	11th to 20th	Day	18% 11% 28% 25% 13% 5%	567
	21st to end	Day	19% 9% 24% 27% 16% 5%	617
June	1st to 10th	Day	12% 7% 29% 31% 15% 6%	570
	11th to 20th	Day	9% 10% 22% 34% 18% 7%	570
	21st to end	Day	9% 6% 25% 32% 22% 5%	570
July	1st to 10th	Day	9% 3 16% 31% 29% 12%	565
	11th to 20th	Day	6% 4 15% 33% 31% 11%	564
	21st to end	Day	12% 3 20% 30% 23% 12%	616
August	1st to 10th	Day	16% 4 21% 26% 20% 12%	526
	11th to 20th	Day	10% 4 19% 28% 25% 14%	519
	21st to end	Day	11% 4 17% 31% 23% 16%	583
September	1st to 10th	Day	15% 4 17% 26% 25% 12%	530
	11th to 20th	Day	18% 5% 15% 24% 29% 10%	530
	21st to end	Day	26% 11% 22% 25% 13% 4	530
October	1st to 10th	Day	20% 7% 21% 29% 17% 5%	530
	11th to 20th	Day	16% 15% 26% 24% 17% 2	530
	21st to end	Day	18% 15% 27% 28% 8% 2	583
November	1st to 10th	Day	18% 14% 30% 25% 10% 2	535
	11th to 20th	Day	20% 24% 28% 20% 6% 1	388
	21st to end	Day	22% 23% 30% 20% 5% 1	330
December	1st to 10th	Day	25% 29% 30% 13% 2	244
	11th to 20th	Day	26% 21% 34% 14% 5%	330
	21st to end	Day	29% 22% 31% 12% 6%	363
January	1st to 10th	Night	19% 22% 35% 20% 3	840
	11th to 20th	Night	19% 22% 35% 23% 2	840
	21st to end	Night	18% 19% 41% 20% 2	924
February	1st to 10th	Night	20% 18% 37% 23% 3	840
	11th to 20th	Night	26% 21% 37% 16% 1	823
	21st to end	Night	21% 19% 38% 19% 2	692
March	1st to 10th	Night	18% 18% 38% 23% 3	840
	11th to 20th	Night	20% 18% 35% 21% 6% 1	840
	21st to end	Night	18% 17% 38% 24% 4	924
April	1st to 10th	Night	17% 21% 34% 22% 6%	840
	11th to 20th	Night	15% 15% 35% 25% 9%	831
	21st to end	Night	18% 8% 36% 29% 10%	730
May	1st to 10th	Night	14% 10% 35% 28% 12% 1	630
	11th to 20th	Night	19% 6% 26% 30% 18% 1	621
	21st to end	Night	18% 4 17% 31% 27% 3	683
June	1st to 10th	Night	10% 2 18% 40% 28% 2	630
	11th to 20th	Night	11% 3 16% 35% 31% 4	630
	21st to end	Night	7% 2 13% 44% 30% 4	630
July	1st to 10th	Night	10% 2 7% 34% 40% 7%	615
	11th to 20th	Night	7% 2 8% 33% 43% 7%	612
	21st to end	Night	9% 2 13% 37% 35% 5%	687
August	1st to 10th	Night	17% 2 11% 36% 31% 4	597
	11th to 20th	Night	13% 2 9% 35% 37% 4	594
	21st to end	Night	10% 2 10% 34% 37% 8%	840
September	1st to 10th	Night	13% 3 12% 33% 33% 7%	800
	11th to 20th	Night	18% 4 13% 30% 30% 5%	800
	21st to end	Night	27% 7% 17% 28% 20% 1	800
October	1st to 10th	Night	18% 7% 20% 26% 24% 5%	800
	11th to 20th	Night	16% 10% 26% 30% 17% 2	800
	21st to end	Night	16% 11% 25% 33% 13% 1	880
November	1st to 10th	Night	17% 9% 27% 32% 13% 2	820
	11th to 20th	Night	16% 15% 29% 31% 9% 1	840
	21st to end	Night	15% 16% 30% 28% 10% 1	840
December	1st to 10th	Night	20% 16% 36% 23% 5%	840
	11th to 20th	Night	17% 14% 40% 25% 4	840
	21st to end	Night	22% 17% 29% 27% 4	924

Page generated at 2026-07-28 03:25:24. Version ac07

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