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 2024-10-20.

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 5% 5% 4 6% 8% 41%	100882
	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	97694
Transparency	Day	39% 9% 13% 21% 15% 4	100694
	Night	37% 8% 12% 22% 17% 4	97552
Seeing	Day	19% 14% 25% 23% 14% 5%	15191
	Night	17% 11% 26% 28% 16% 2	25339

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	16% 5% 2 5% 3 4 3 2 4 6% 50%	2836
	11th to 20th	Day	13% 7% 2 6% 5% 5% 4 4 4 6% 47%	2840
	21st to end	Day	19% 8% 3 7% 5% 4 4 2 4 4 40%	3124
February	1st to 10th	Day	11% 7% 2 8% 5% 4 5% 4 5% 6% 42%	1710
	11th to 20th	Day	17% 7% 3 5% 3 3 4 4 6% 7% 41%	1710
	21st to end	Day	17% 8% 2 7% 4 4 3 3 5% 5% 41%	1651
March	1st to 10th	Day	22% 7% 2 5% 3 3 4 4 6% 7% 37%	3435
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 5% 7% 54%	3480
	21st to end	Day	19% 8% 3 5% 4 4 3 3 5% 6% 40%	3815
April	1st to 10th	Day	10% 5% 2 4 4 4 5% 4 6% 8% 49%	2156
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 5% 8% 47%	2254
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 4 6% 42%	2394
May	1st to 10th	Day	15% 6% 3 7% 6% 6% 6% 5% 6% 9% 32%	4082
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 4 7% 7% 32%	4081
	21st to end	Day	9% 4 2 5% 5% 5% 6% 5% 7% 9% 44%	4476
June	1st to 10th	Day	10% 4 2 5% 5% 5% 6% 5% 8% 10% 39%	2470
	11th to 20th	Day	8% 5% 3 6% 6% 5% 6% 5% 8% 11% 38%	2470
	21st to end	Day	9% 4 2 6% 6% 6% 6% 6% 8% 9% 39%	2470
July	1st to 10th	Day	7% 4 2 7% 5% 6% 7% 5% 10% 11% 36%	4120
	11th to 20th	Day	8% 6% 2 7% 5% 5% 7% 6% 8% 12% 33%	4120
	21st to end	Day	12% 7% 3 9% 7% 7% 7% 6% 8% 9% 25%	4466
August	1st to 10th	Day	14% 7% 3 7% 4 5% 7% 5% 9% 11% 29%	4104
	11th to 20th	Day	8% 4 2 6% 5% 6% 6% 5% 8% 12% 36%	4044
	21st to end	Day	8% 5% 2 5% 3 4 4 4 5% 10% 49%	4197
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 6% 7% 47%	2242
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 7% 39%	2090
	21st to end	Day	23% 6% 2 6% 4 4 4 3 5% 5% 36%	2090
October	1st to 10th	Day	17% 4 2 4 3 3 3 2 4 7% 50%	1913
	11th to 20th	Day	15% 4 2 4 2 3 4 3 4 6% 53%	1890
	21st to end	Day	13% 5% 2 6% 3 3 3 2 4 6% 51%	1980
November	1st to 10th	Day	12% 4 2 4 3 4 2 4 4 5% 56%	1857
	11th to 20th	Day	14% 6% 2 4 3 4 3 3 4 7% 51%	1763
	21st to end	Day	13% 5% 1 4 4 3 3 3 5% 7% 50%	1710
December	1st to 10th	Day	14% 8% 2 6% 4 6% 5% 4 5% 6% 41%	1565
	11th to 20th	Day	15% 4 2 3 3 3 4 3 4 6% 55%	2153
	21st to end	Day	20% 7% 2 5% 3 4 5% 4 6% 7% 37%	3124
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 6% 50%	4109
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 4 5% 48%	4120
	21st to end	Night	22% 7% 2 5% 3 3 3 3 4 5% 42%	4532
February	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 5% 6% 45%	2470
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 4 6% 43%	2351
	21st to end	Night	14% 7% 2 5% 5% 4 5% 4 6% 5% 43%	1803
March	1st to 10th	Night	23% 7% 3 6% 4 4 4 3 5% 5% 36%	3418
	11th to 20th	Night	10% 3 2 4 3 3 4 3 6% 8% 55%	3480
	21st to end	Night	17% 5% 2 5% 3 4 4 4 5% 6% 46%	3817
April	1st to 10th	Night	14% 7% 2 5% 4 4 4 4 4 7% 46%	2024
	11th to 20th	Night	13% 5% 2 5% 3 3 3 4 5% 7% 50%	1875
	21st to end	Night	16% 4 1 4 4 4 4 3 6% 8% 47%	1800
May	1st to 10th	Night	15% 6% 2 7% 5% 5% 5% 3 5% 8% 38%	2813
	11th to 20th	Night	21% 7% 2 6% 4 3 4 3 5% 5% 39%	2812
	21st to end	Night	11% 4 1 4 3 3 5% 4 7% 11% 48%	3079
June	1st to 10th	Night	9% 4 1 5% 4 5% 6% 5% 8% 11% 41%	1710
	11th to 20th	Night	10% 6% 2 4 3 4 4 4 7% 11% 46%	1710
	21st to end	Night	8% 3 2 5% 4 4 5% 5% 7% 12% 45%	1710
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 6% 8% 10% 41%	2840
	11th to 20th	Night	8% 5% 2 6% 5% 4 6% 5% 8% 12% 40%	2840
	21st to end	Night	11% 8% 3 8% 5% 6% 5% 5% 7% 10% 32%	3087
August	1st to 10th	Night	17% 7% 2 7% 4 5% 5% 4 7% 11% 31%	2825
	11th to 20th	Night	8% 4 2 6% 5% 4 5% 5% 7% 11% 44%	2785
	21st to end	Night	7% 4 1 5% 4 4 5% 3 5% 9% 53%	3412
September	1st to 10th	Night	11% 7% 3 6% 3 3 4 3 5% 7% 47%	1933
	11th to 20th	Night	17% 5% 2 6% 4 4 4 3 6% 7% 42%	2090
	21st to end	Night	23% 7% 2 6% 5% 4 3 3 4 5% 38%	2090
October	1st to 10th	Night	16% 5% 2 6% 4 3 3 3 4 7% 47%	2266
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 4 5% 54%	2275
	21st to end	Night	14% 4 2 4 3 3 3 2 4 5% 56%	2376
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 3 4 54%	2221
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 5% 54%	2413
	21st to end	Night	13% 4 2 5% 3 3 4 4 6% 7% 50%	2470
December	1st to 10th	Night	15% 7% 3 5% 5% 4 4 3 5% 6% 44%	2564
	11th to 20th	Night	14% 6% 2 5% 3 4 4 3 4 6% 49%	3042
	21st to end	Night	19% 5% 2 5% 4 4 4 4 6% 7% 39%	4532

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	33% 7% 9% 26% 19% 6%	2837
	11th to 20th	Day	38% 6% 8% 19% 25% 5%	2840
	21st to end	Day	46% 6% 8% 19% 18% 3	3124
February	1st to 10th	Day	41% 6% 12% 21% 18% 3	1710
	11th to 20th	Day	40% 7% 9% 23% 16% 5%	1719
	21st to end	Day	45% 6% 12% 14% 17% 6%	1651
March	1st to 10th	Day	44% 7% 12% 18% 13% 7%	3435
	11th to 20th	Day	30% 7% 9% 25% 20% 10%	3480
	21st to end	Day	43% 7% 10% 21% 16% 3	3815
April	1st to 10th	Day	31% 8% 13% 23% 19% 7%	2156
	11th to 20th	Day	35% 6% 13% 22% 17% 6%	2254
	21st to end	Day	43% 6% 13% 21% 13% 3	2394
May	1st to 10th	Day	45% 11% 15% 17% 10% 2	4120
	11th to 20th	Day	48% 10% 11% 20% 10% 1	4085
	21st to end	Day	31% 11% 17% 21% 15% 4	4489
June	1st to 10th	Day	36% 12% 16% 23% 12% 1	2470
	11th to 20th	Day	36% 12% 16% 19% 14% 2	2470
	21st to end	Day	36% 12% 17% 20% 13% 2	2470
July	1st to 10th	Day	36% 15% 19% 16% 9% 5%	4055
	11th to 20th	Day	39% 12% 20% 22% 7% 1	4051
	21st to end	Day	51% 12% 13% 17% 6%	4324
August	1st to 10th	Day	44% 12% 18% 20% 5%	4091
	11th to 20th	Day	35% 13% 20% 23% 8% 1	4083
	21st to end	Day	29% 9% 18% 29% 13% 2	4197
September	1st to 10th	Day	36% 9% 13% 19% 19% 4	2242
	11th to 20th	Day	42% 8% 11% 21% 16% 2	2090
	21st to end	Day	48% 7% 11% 15% 17% 2	2090
October	1st to 10th	Day	35% 5% 12% 22% 21% 5%	1913
	11th to 20th	Day	32% 5% 12% 24% 21% 7%	1890
	21st to end	Day	34% 5% 8% 25% 21% 7%	1980
November	1st to 10th	Day	30% 6% 10% 20% 27% 6%	1857
	11th to 20th	Day	34% 4 8% 22% 23% 7%	1763
	21st to end	Day	34% 5% 10% 22% 25% 4	1710
December	1st to 10th	Day	43% 7% 10% 18% 17% 4	1565
	11th to 20th	Day	32% 5% 10% 23% 23% 8%	2150
	21st to end	Day	41% 9% 12% 22% 14% 2	3124
January	1st to 10th	Night	33% 6% 10% 23% 22% 5%	4109
	11th to 20th	Night	37% 7% 9% 18% 24% 5%	4120
	21st to end	Night	43% 6% 8% 21% 16% 6%	4532
February	1st to 10th	Night	39% 6% 10% 23% 19% 3	2470
	11th to 20th	Night	42% 5% 9% 20% 18% 6%	2353
	21st to end	Night	41% 6% 12% 22% 14% 5%	1803
March	1st to 10th	Night	45% 9% 11% 18% 13% 4	3418
	11th to 20th	Night	27% 7% 10% 22% 22% 12%	3480
	21st to end	Night	37% 7% 10% 23% 18% 5%	3817
April	1st to 10th	Night	37% 6% 12% 19% 21% 4	2024
	11th to 20th	Night	34% 6% 12% 21% 21% 6%	1875
	21st to end	Night	35% 7% 13% 25% 17% 3	1800
May	1st to 10th	Night	42% 10% 15% 21% 10% 1	2840
	11th to 20th	Night	47% 7% 11% 22% 12% 1	2813
	21st to end	Night	28% 10% 20% 26% 12% 4	3087
June	1st to 10th	Night	33% 13% 16% 24% 13% 1	1710
	11th to 20th	Night	31% 12% 19% 23% 14% 2	1710
	21st to end	Night	29% 14% 17% 23% 15% 4	1710
July	1st to 10th	Night	35% 15% 18% 19% 11% 2	2795
	11th to 20th	Night	34% 13% 22% 24% 7% 1	2786
	21st to end	Night	45% 11% 16% 21% 7%	2988
August	1st to 10th	Night	45% 11% 18% 19% 6% 1	2814
	11th to 20th	Night	31% 11% 18% 28% 11% 1	2812
	21st to end	Night	28% 8% 14% 38% 10% 2	3412
September	1st to 10th	Night	36% 9% 12% 21% 15% 6%	1933
	11th to 20th	Night	40% 9% 10% 20% 19% 2	2090
	21st to end	Night	48% 6% 9% 18% 15% 3	2090
October	1st to 10th	Night	37% 6% 10% 21% 22% 4	2266
	11th to 20th	Night	32% 5% 10% 25% 22% 6%	2275
	21st to end	Night	32% 5% 9% 23% 25% 8%	2376
November	1st to 10th	Night	34% 5% 9% 20% 25% 7%	2221
	11th to 20th	Night	31% 5% 9% 21% 24% 9%	2413
	21st to end	Night	33% 5% 10% 24% 22% 6%	2470
December	1st to 10th	Night	42% 6% 9% 23% 18% 4	2564
	11th to 20th	Night	36% 7% 8% 21% 22% 7%	3044
	21st to end	Night	39% 8% 13% 20% 17% 2	4532

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	30% 28% 27% 12% 2	270
	11th to 20th	Day	33% 25% 34% 7% 1	270
	21st to end	Day	25% 34% 32% 9% 1	297
February	1st to 10th	Day	25% 30% 29% 13% 3	270
	11th to 20th	Day	40% 30% 24% 5% 2	270
	21st to end	Day	31% 27% 27% 12% 2	306
March	1st to 10th	Day	25% 22% 32% 17% 5%	460
	11th to 20th	Day	25% 25% 30% 15% 3 1	460
	21st to end	Day	24% 22% 34% 16% 4	506
April	1st to 10th	Day	22% 25% 29% 17% 7%	460
	11th to 20th	Day	19% 20% 29% 19% 10% 3	452
	21st to end	Day	23% 18% 29% 19% 10% 2	460
May	1st to 10th	Day	21% 17% 33% 19% 7% 3	463
	11th to 20th	Day	18% 11% 27% 25% 13% 6%	487
	21st to end	Day	16% 7% 23% 29% 19% 6%	529
June	1st to 10th	Day	13% 7% 29% 30% 16% 6%	490
	11th to 20th	Day	9% 10% 21% 34% 19% 8%	490
	21st to end	Day	10% 6% 27% 31% 20% 5%	490
July	1st to 10th	Day	10% 3 16% 32% 28% 10%	485
	11th to 20th	Day	6% 3 16% 34% 31% 10%	484
	21st to end	Day	10% 3 20% 32% 23% 12%	532
August	1st to 10th	Day	14% 3 22% 26% 21% 13%	486
	11th to 20th	Day	10% 4 19% 28% 24% 15%	479
	21st to end	Day	11% 3 15% 30% 24% 17%	539
September	1st to 10th	Day	16% 4 16% 26% 26% 13%	490
	11th to 20th	Day	18% 5% 15% 24% 29% 9%	490
	21st to end	Day	26% 10% 21% 25% 13% 4	490
October	1st to 10th	Day	21% 7% 21% 29% 17% 4	490
	11th to 20th	Day	16% 15% 25% 23% 18% 2	486
	21st to end	Day	17% 15% 28% 29% 8% 2	495
November	1st to 10th	Day	19% 14% 28% 25% 12% 3	455
	11th to 20th	Day	21% 24% 26% 21% 7% 1	323
	21st to end	Day	20% 20% 31% 23% 5% 1	270
December	1st to 10th	Day	23% 32% 29% 14% 3 1	200
	11th to 20th	Day	23% 23% 34% 15% 5%	270
	21st to end	Day	29% 21% 33% 11% 6%	297
January	1st to 10th	Night	19% 23% 34% 21% 3	760
	11th to 20th	Night	19% 22% 33% 24% 2	760
	21st to end	Night	19% 19% 40% 21% 2	836
February	1st to 10th	Night	19% 18% 36% 24% 3	760
	11th to 20th	Night	26% 21% 37% 16% 1	743
	21st to end	Night	21% 19% 39% 19% 2	628
March	1st to 10th	Night	18% 18% 38% 22% 4	760
	11th to 20th	Night	20% 18% 35% 20% 6% 1	760
	21st to end	Night	19% 17% 36% 25% 4	836
April	1st to 10th	Night	17% 21% 34% 21% 6%	760
	11th to 20th	Night	16% 15% 34% 24% 10% 1	751
	21st to end	Night	19% 8% 36% 28% 9%	660
May	1st to 10th	Night	15% 10% 34% 28% 13% 1	570
	11th to 20th	Night	20% 6% 26% 28% 19% 1	561
	21st to end	Night	16% 3 17% 32% 30% 3	617
June	1st to 10th	Night	10% 2 17% 40% 29% 2	570
	11th to 20th	Night	11% 3 15% 35% 32% 5%	570
	21st to end	Night	8% 2 13% 44% 28% 4	570
July	1st to 10th	Night	10% 3 8% 35% 39% 6%	555
	11th to 20th	Night	6% 2 9% 34% 43% 7%	552
	21st to end	Night	8% 2 12% 39% 35% 5%	624
August	1st to 10th	Night	16% 2 11% 36% 31% 4	567
	11th to 20th	Night	13% 2 9% 36% 36% 4	564
	21st to end	Night	11% 1 9% 32% 38% 8%	798
September	1st to 10th	Night	13% 3 12% 31% 33% 7%	760
	11th to 20th	Night	18% 4 13% 29% 30% 5%	760
	21st to end	Night	27% 6% 17% 28% 21% 1	760
October	1st to 10th	Night	18% 8% 20% 26% 23% 5%	760
	11th to 20th	Night	16% 10% 26% 30% 17% 2	759
	21st to end	Night	16% 12% 25% 33% 12% 1	792
November	1st to 10th	Night	18% 9% 26% 31% 14% 2	740
	11th to 20th	Night	16% 15% 28% 31% 9% 1	760
	21st to end	Night	16% 15% 30% 28% 11% 1	760
December	1st to 10th	Night	20% 17% 34% 23% 6%	760
	11th to 20th	Night	16% 14% 40% 25% 5%	760
	21st to end	Night	22% 17% 29% 27% 4	836

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