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 2025-03-27.

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%	109289
	Night	14% 5% 2 5% 4 4 4 3 5% 7% 47%	109969
Transparency	Day	37% 8% 13% 21% 16% 4	112939
	Night	35% 7% 12% 23% 18% 5%	114458
Seeing	Day	19% 14% 25% 23% 13% 5%	15711
	Night	17% 11% 26% 28% 16% 2	25971

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% 7% 2 5% 3 3 3 2 4 5% 52%	4056
	11th to 20th	Day	10% 6% 2 5% 4 4 3 3 3 5% 55%	4060
	21st to end	Day	15% 6% 3 7% 4 4 4 2 4 5% 46%	4466
February	1st to 10th	Day	11% 7% 2 8% 5% 4 5% 4 6% 6% 42%	1800
	11th to 20th	Day	18% 7% 3 5% 3 3 4 3 6% 7% 41%	1803
	21st to end	Day	17% 8% 2 7% 4 4 3 3 5% 5% 42%	1735
March	1st to 10th	Day	22% 7% 2 5% 3 3 4 4 6% 7% 37%	3545
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 7% 54%	3590
	21st to end	Day	18% 8% 3 5% 4 4 3 3 5% 6% 41%	3881
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% 54%	1900
	21st to end	Day	13% 5% 2 6% 3 3 3 2 4 6% 52%	2090
November	1st to 10th	Day	12% 4 2 4 3 4 2 4 4 5% 55%	1957
	11th to 20th	Day	14% 6% 2 4 3 4 3 3 4 7% 51%	1855
	21st to end	Day	13% 5% 1 5% 4 3 3 4 5% 7% 51%	1800
December	1st to 10th	Day	10% 7% 2 6% 4 5% 4 3 4 5% 51%	2673
	11th to 20th	Day	18% 7% 2 3 3 3 4 4 5% 6% 46%	3373
	21st to end	Day	21% 7% 2 5% 3 4 5% 4 6% 6% 37%	4466
January	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 5% 57%	5889
	11th to 20th	Night	11% 6% 2 5% 3 3 4 2 4 5% 56%	5900
	21st to end	Night	17% 6% 2 4 2 2 3 2 4 5% 50%	6490
February	1st to 10th	Night	14% 7% 2 6% 4 3 4 4 5% 6% 44%	2600
	11th to 20th	Night	20% 6% 2 6% 4 3 4 3 4 6% 42%	2478
	21st to end	Night	13% 7% 2 5% 5% 4 5% 4 6% 5% 44%	1894
March	1st to 10th	Night	23% 7% 3 6% 4 4 4 3 5% 5% 36%	3528
	11th to 20th	Night	10% 3 2 4 3 3 4 3 6% 8% 55%	3590
	21st to end	Night	17% 5% 2 5% 3 4 4 4 5% 6% 46%	3890
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%	2280
	21st to end	Night	14% 4 2 4 3 3 3 2 3 5% 57%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 5% 53%	2341
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 5% 54%	2540
	21st to end	Night	13% 5% 2 5% 3 3 4 3 6% 7% 50%	2600
December	1st to 10th	Night	10% 4 2 5% 4 3 4 3 5% 6% 54%	4428
	11th to 20th	Night	20% 7% 2 7% 4 4 4 3 4 5% 41%	4822
	21st to end	Night	19% 5% 2 4 4 3 4 3 5% 7% 44%	6490

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% 6% 8% 21% 26% 7%	4057
	11th to 20th	Day	31% 5% 7% 19% 29% 9%	4060
	21st to end	Day	40% 5% 7% 23% 21% 4	4466
February	1st to 10th	Day	41% 7% 12% 21% 17% 3	1800
	11th to 20th	Day	41% 7% 9% 23% 16% 5%	1812
	21st to end	Day	44% 6% 12% 15% 17% 6%	1735
March	1st to 10th	Day	43% 7% 12% 18% 13% 7%	3545
	11th to 20th	Day	30% 7% 9% 25% 20% 10%	3590
	21st to end	Day	43% 7% 10% 21% 16% 3	3881
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	31% 6% 10% 18% 27% 9%	3173
	11th to 20th	Day	23% 3 8% 23% 27% 16%	3140
	21st to end	Day	25% 5% 11% 24% 22% 14%	3428
November	1st to 10th	Day	31% 6% 10% 21% 25% 6%	1957
	11th to 20th	Day	34% 4 9% 22% 24% 7%	1855
	21st to end	Day	33% 6% 10% 22% 25% 4	1800
December	1st to 10th	Day	36% 6% 8% 22% 23% 6%	2673
	11th to 20th	Day	38% 6% 12% 19% 20% 5%	3370
	21st to end	Day	43% 8% 12% 21% 14% 1	4466
January	1st to 10th	Night	29% 5% 7% 22% 27% 10%	5889
	11th to 20th	Night	31% 6% 7% 18% 31% 7%	5900
	21st to end	Night	36% 5% 7% 27% 20% 6%	6490
February	1st to 10th	Night	40% 6% 11% 23% 18% 2	2600
	11th to 20th	Night	43% 5% 9% 19% 18% 6%	2480
	21st to end	Night	39% 6% 11% 22% 15% 6%	1894
March	1st to 10th	Night	45% 9% 10% 19% 14% 4	3528
	11th to 20th	Night	27% 7% 9% 21% 23% 12%	3590
	21st to end	Night	37% 7% 10% 23% 18% 5%	3890
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	36% 6% 9% 17% 24% 8%	3786
	11th to 20th	Night	23% 4 8% 22% 29% 14%	3779
	21st to end	Night	22% 4 7% 28% 26% 14%	4120
November	1st to 10th	Night	34% 5% 9% 21% 24% 7%	2341
	11th to 20th	Night	32% 5% 9% 20% 25% 9%	2540
	21st to end	Night	33% 5% 9% 24% 22% 6%	2600
December	1st to 10th	Night	30% 7% 9% 26% 25% 3	4428
	11th to 20th	Night	45% 6% 9% 18% 15% 7%	4824
	21st to end	Night	37% 7% 15% 23% 17% 2	6490

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% 29% 26% 11% 2	300
	11th to 20th	Day	32% 26% 33% 7% 2	300
	21st to end	Day	24% 35% 32% 9% 1	330
February	1st to 10th	Day	24% 30% 31% 11% 3	300
	11th to 20th	Day	43% 28% 23% 4 2	300
	21st to end	Day	30% 27% 28% 13% 1 1	334
March	1st to 10th	Day	26% 22% 31% 17% 4	500
	11th to 20th	Day	26% 24% 30% 17% 3 1	500
	21st to end	Day	25% 22% 34% 15% 4	530
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% 24% 24% 18% 2	490
	21st to end	Day	16% 15% 28% 29% 9% 3	539
November	1st to 10th	Day	19% 14% 28% 26% 11% 2	495
	11th to 20th	Day	20% 23% 28% 21% 7% 1	355
	21st to end	Day	22% 22% 30% 21% 5% 1	300
December	1st to 10th	Day	24% 31% 29% 13% 2	222
	11th to 20th	Day	24% 22% 34% 15% 5%	300
	21st to end	Day	28% 22% 31% 12% 6%	330
January	1st to 10th	Night	20% 23% 35% 20% 3	800
	11th to 20th	Night	19% 22% 34% 23% 2	800
	21st to end	Night	18% 18% 42% 20% 2	880
February	1st to 10th	Night	20% 18% 36% 24% 3	800
	11th to 20th	Night	26% 21% 37% 16% 1	783
	21st to end	Night	20% 19% 39% 20% 2	660
March	1st to 10th	Night	18% 18% 38% 22% 4	800
	11th to 20th	Night	20% 18% 35% 20% 6% 1	800
	21st to end	Night	18% 17% 36% 24% 4	863
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	760
	21st to end	Night	15% 11% 25% 34% 13% 1	836
November	1st to 10th	Night	17% 9% 27% 32% 13% 2	780
	11th to 20th	Night	16% 15% 28% 32% 9% 1	800
	21st to end	Night	16% 15% 30% 28% 11% 1	800
December	1st to 10th	Night	20% 16% 35% 23% 6%	800
	11th to 20th	Night	17% 14% 39% 25% 5%	800
	21st to end	Night	22% 17% 29% 28% 4	880

Page generated at 2025-03-24 09:04:27. Version ac07

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