Fred's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fred's Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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	15% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	101509
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 35%	98208
Transparency	Day	47% 7% 11% 21% 12% 1	105213
	Night	46% 7% 11% 21% 13% 2	102630
Seeing	Day	21% 12% 26% 21% 14% 6%	15004
	Night	20% 10% 26% 29% 14% 2	25104

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	18% 21% 6% 11% 4 4 3 4 4 6% 19%	2836
	11th to 20th	Day	15% 21% 7% 13% 7% 8% 4 2 3 4 15%	2840
	21st to end	Day	27% 20% 9% 9% 5% 5% 4 2 3 4 12%	3124
February	1st to 10th	Day	22% 14% 5% 9% 7% 4 3 3 4 5% 25%	1710
	11th to 20th	Day	16% 14% 5% 12% 5% 4 4 3 4 6% 26%	1799
	21st to end	Day	19% 15% 5% 8% 4 4 3 2 5% 5% 30%	1570
March	1st to 10th	Day	22% 14% 5% 9% 4 4 4 3 4 6% 24%	3307
	11th to 20th	Day	14% 9% 3 8% 4 5% 5% 4 5% 6% 37%	3480
	21st to end	Day	20% 9% 3 8% 4 4 4 3 5% 6% 34%	3815
April	1st to 10th	Day	14% 13% 3 9% 5% 5% 4 4 5% 5% 33%	2093
	11th to 20th	Day	17% 9% 3 7% 4 4 4 4 5% 7% 36%	2420
	21st to end	Day	18% 7% 2 7% 5% 5% 6% 3 6% 7% 33%	2470
May	1st to 10th	Day	21% 10% 3 8% 5% 5% 5% 3 6% 7% 28%	4082
	11th to 20th	Day	22% 8% 3 6% 4 4 4 3 5% 8% 33%	4081
	21st to end	Day	10% 6% 3 8% 5% 5% 5% 5% 8% 10% 35%	4476
June	1st to 10th	Day	13% 6% 2 7% 5% 4 6% 4 7% 10% 37%	2470
	11th to 20th	Day	11% 7% 2 7% 5% 5% 5% 5% 7% 10% 37%	2470
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 10% 36%	2470
July	1st to 10th	Day	11% 7% 2 7% 5% 5% 6% 4 6% 10% 38%	4120
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 5% 7% 11% 42%	4120
	21st to end	Day	13% 7% 2 7% 5% 6% 6% 5% 7% 9% 34%	4466
August	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 8% 9% 38%	4104
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	4044
	21st to end	Day	6% 5% 1 6% 4 5% 6% 4 6% 9% 47%	4467
September	1st to 10th	Day	14% 6% 1 6% 4 3 4 3 5% 7% 47%	2090
	11th to 20th	Day	10% 6% 2 6% 5% 4 3 3 6% 7% 48%	2090
	21st to end	Day	18% 8% 3 6% 3 3 3 4 5% 6% 41%	2090
October	1st to 10th	Day	13% 6% 2 7% 4 4 4 3 6% 7% 44%	2013
	11th to 20th	Day	15% 6% 2 7% 4 4 4 3 6% 7% 42%	1900
	21st to end	Day	20% 9% 3 7% 5% 3 4 3 5% 5% 37%	2071
November	1st to 10th	Day	10% 7% 3 6% 4 4 3 3 5% 6% 49%	1762
	11th to 20th	Day	17% 13% 3 6% 5% 4 4 3 5% 6% 34%	1800
	21st to end	Day	20% 13% 3 8% 4 4 4 4 4 6% 29%	1800
December	1st to 10th	Day	20% 17% 5% 11% 6% 4 4 4 4 4 21%	1782
	11th to 20th	Day	19% 16% 5% 11% 6% 3 4 4 5% 5% 22%	2153
	21st to end	Day	21% 23% 6% 11% 5% 4 5% 4 4 5% 13%	3124
January	1st to 10th	Night	16% 19% 7% 12% 6% 5% 4 3 4 4 19%	4109
	11th to 20th	Night	15% 17% 6% 15% 6% 6% 6% 3 4 4 17%	4120
	21st to end	Night	26% 17% 8% 11% 6% 3 3 2 2 4 17%	4532
February	1st to 10th	Night	20% 14% 3 9% 5% 5% 3 3 5% 4 28%	2470
	11th to 20th	Night	19% 13% 3 10% 6% 5% 4 4 5% 5% 27%	2266
	21st to end	Night	20% 14% 6% 10% 5% 4 4 3 4 4 27%	1884
March	1st to 10th	Night	21% 13% 3 6% 4 3 4 3 4 6% 33%	3545
	11th to 20th	Night	13% 9% 3 6% 4 5% 5% 4 5% 7% 39%	3480
	21st to end	Night	19% 9% 3 6% 4 4 4 3 5% 6% 39%	3817
April	1st to 10th	Night	16% 9% 3 6% 4 5% 4 4 6% 6% 38%	2090
	11th to 20th	Night	14% 8% 3 7% 4 4 4 4 6% 9% 37%	1721
	21st to end	Night	15% 8% 2 7% 4 5% 4 4 6% 7% 37%	1710
May	1st to 10th	Night	25% 12% 3 7% 4 4 4 4 5% 6% 27%	2813
	11th to 20th	Night	18% 9% 3 6% 4 3 3 3 4 7% 40%	2812
	21st to end	Night	9% 6% 3 6% 4 4 4 4 8% 10% 42%	3079
June	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 5% 9% 46%	1710
	11th to 20th	Night	9% 4 2 5% 5% 4 5% 4 6% 10% 45%	1710
	21st to end	Night	11% 6% 2 5% 3 4 5% 5% 7% 10% 43%	1710
July	1st to 10th	Night	11% 6% 3 6% 4 3 4 4 5% 7% 47%	2840
	11th to 20th	Night	9% 5% 2 5% 4 5% 5% 4 7% 9% 45%	2840
	21st to end	Night	8% 6% 3 8% 6% 5% 5% 5% 7% 9% 38%	3087
August	1st to 10th	Night	7% 7% 3 8% 6% 5% 5% 5% 8% 10% 37%	2825
	11th to 20th	Night	10% 7% 3 7% 4 4 5% 3 5% 7% 44%	2785
	21st to end	Night	5% 3 1 5% 4 4 5% 4 7% 10% 51%	3124
September	1st to 10th	Night	13% 5% 2 4 4 4 4 3 4 7% 52%	2090
	11th to 20th	Night	9% 3 2 4 3 4 4 3 5% 7% 56%	2090
	21st to end	Night	15% 7% 3 6% 4 3 3 3 5% 6% 46%	2090
October	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 7% 49%	2166
	11th to 20th	Night	15% 7% 3 6% 3 3 4 3 5% 7% 43%	2280
	21st to end	Night	20% 8% 2 6% 3 4 4 3 5% 6% 39%	2522
November	1st to 10th	Night	14% 8% 2 5% 3 3 3 3 5% 5% 49%	2536
	11th to 20th	Night	16% 11% 3 7% 5% 4 4 3 6% 6% 35%	2600
	21st to end	Night	19% 14% 3 8% 4 5% 4 4 5% 5% 29%	2600
December	1st to 10th	Night	21% 15% 5% 10% 5% 4 4 3 5% 5% 23%	2581
	11th to 20th	Night	18% 14% 7% 8% 5% 3 4 3 5% 6% 27%	3042
	21st to end	Night	21% 21% 6% 10% 5% 5% 4 3 5% 6% 16%	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	66% 6% 12% 12% 5%	2837
	11th to 20th	Day	72% 6% 9% 10% 3	2840
	21st to end	Day	76% 5% 7% 8% 3 1	3124
February	1st to 10th	Day	62% 4 12% 13% 8%	1710
	11th to 20th	Day	60% 6% 9% 16% 9%	1809
	21st to end	Day	58% 5% 11% 17% 8% 2	1570
March	1st to 10th	Day	60% 7% 10% 18% 5% 1	3307
	11th to 20th	Day	44% 9% 11% 26% 10% 1	3480
	21st to end	Day	49% 7% 10% 23% 11%	3815
April	1st to 10th	Day	51% 7% 10% 19% 11% 2	2093
	11th to 20th	Day	46% 7% 12% 19% 14% 3	2420
	21st to end	Day	49% 7% 10% 21% 11% 2	2470
May	1st to 10th	Day	53% 7% 10% 21% 7% 1	4120
	11th to 20th	Day	48% 8% 12% 19% 13% 1	4085
	21st to end	Day	37% 14% 15% 18% 12% 4	4489
June	1st to 10th	Day	40% 9% 12% 24% 14% 2	2470
	11th to 20th	Day	40% 9% 14% 21% 14% 1	2470
	21st to end	Day	40% 9% 16% 20% 14% 2	2470
July	1st to 10th	Day	41% 7% 13% 21% 15% 2	4055
	11th to 20th	Day	33% 8% 13% 30% 16% 1	4051
	21st to end	Day	44% 8% 12% 16% 17% 3	4324
August	1st to 10th	Day	38% 10% 14% 27% 10% 1	4091
	11th to 20th	Day	37% 8% 12% 29% 13% 1	4083
	21st to end	Day	31% 9% 15% 29% 13% 3	4467
September	1st to 10th	Day	36% 6% 12% 21% 22% 3	2090
	11th to 20th	Day	33% 6% 12% 25% 21% 3	2090
	21st to end	Day	45% 4 9% 22% 19% 1	2090
October	1st to 10th	Day	26% 5% 10% 32% 27% 1	3338
	11th to 20th	Day	34% 4 12% 26% 21% 3	3140
	21st to end	Day	38% 4 11% 27% 18% 2	3397
November	1st to 10th	Day	36% 6% 10% 28% 17% 3	1762
	11th to 20th	Day	50% 6% 9% 20% 12% 3	1800
	21st to end	Day	55% 6% 11% 17% 10% 1	1800
December	1st to 10th	Day	66% 5% 11% 14% 4	1782
	11th to 20th	Day	62% 5% 10% 16% 7%	2150
	21st to end	Day	72% 7% 9% 8% 3	3124
January	1st to 10th	Night	67% 6% 9% 13% 4	4109
	11th to 20th	Night	69% 7% 10% 11% 3	4120
	21st to end	Night	73% 4 6% 12% 3 1	4532
February	1st to 10th	Night	58% 6% 10% 18% 8% 1	2470
	11th to 20th	Night	58% 5% 10% 17% 9% 1	2267
	21st to end	Night	60% 5% 9% 15% 9% 2	1884
March	1st to 10th	Night	52% 6% 8% 23% 10% 1	3545
	11th to 20th	Night	41% 9% 12% 28% 10% 1	3480
	21st to end	Night	45% 7% 8% 27% 12% 1	3817
April	1st to 10th	Night	44% 7% 12% 20% 14% 2	2090
	11th to 20th	Night	42% 6% 12% 17% 18% 4	1721
	21st to end	Night	43% 7% 12% 21% 15% 2	1710
May	1st to 10th	Night	56% 8% 10% 17% 8% 2	2840
	11th to 20th	Night	43% 8% 10% 26% 11% 2	2813
	21st to end	Night	32% 10% 16% 26% 13% 4	3087
June	1st to 10th	Night	34% 9% 12% 23% 20% 2	1710
	11th to 20th	Night	32% 9% 16% 24% 17% 3	1710
	21st to end	Night	33% 10% 14% 25% 16% 3	1710
July	1st to 10th	Night	36% 7% 12% 22% 18% 6%	2795
	11th to 20th	Night	32% 9% 13% 29% 15% 2	2786
	21st to end	Night	39% 9% 14% 20% 14% 4	2988
August	1st to 10th	Night	38% 9% 16% 25% 12% 1	2814
	11th to 20th	Night	38% 7% 10% 23% 21% 1	2812
	21st to end	Night	26% 8% 15% 30% 18% 3	3124
September	1st to 10th	Night	32% 5% 13% 22% 22% 6%	2090
	11th to 20th	Night	27% 6% 12% 26% 25% 3	2090
	21st to end	Night	39% 6% 10% 20% 24% 1	2090
October	1st to 10th	Night	28% 5% 7% 28% 29% 3	3608
	11th to 20th	Night	30% 5% 11% 27% 22% 5%	3779
	21st to end	Night	32% 5% 9% 26% 25% 4	4146
November	1st to 10th	Night	37% 5% 11% 24% 19% 3	2536
	11th to 20th	Night	49% 5% 10% 19% 15% 3	2600
	21st to end	Night	56% 4 12% 18% 8% 1	2600
December	1st to 10th	Night	62% 5% 10% 17% 6%	2581
	11th to 20th	Night	57% 6% 12% 19% 6%	3044
	21st to end	Night	69% 7% 9% 12% 3	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	29% 21% 33% 11% 6%	270
	11th to 20th	Day	30% 23% 37% 9% 2	270
	21st to end	Day	33% 22% 29% 13% 3	297
February	1st to 10th	Day	34% 21% 30% 13% 2	270
	11th to 20th	Day	31% 28% 29% 9% 2 1	270
	21st to end	Day	33% 24% 37% 5% 1	225
March	1st to 10th	Day	27% 22% 30% 16% 5%	379
	11th to 20th	Day	31% 14% 34% 15% 6%	460
	21st to end	Day	30% 15% 32% 18% 4	506
April	1st to 10th	Day	25% 16% 32% 18% 7% 3	460
	11th to 20th	Day	27% 16% 28% 17% 9% 3	452
	21st to end	Day	24% 17% 33% 17% 7% 2	460
May	1st to 10th	Day	26% 15% 31% 16% 8% 3	463
	11th to 20th	Day	22% 10% 27% 24% 12% 5%	487
	21st to end	Day	12% 9% 24% 26% 22% 7%	529
June	1st to 10th	Day	15% 7% 27% 29% 17% 6%	490
	11th to 20th	Day	15% 8% 22% 29% 18% 7%	490
	21st to end	Day	14% 6% 28% 27% 18% 7%	490
July	1st to 10th	Day	15% 5% 16% 24% 22% 18%	485
	11th to 20th	Day	11% 6% 20% 26% 25% 11%	484
	21st to end	Day	15% 5% 20% 27% 20% 13%	532
August	1st to 10th	Day	14% 4 15% 26% 27% 14%	486
	11th to 20th	Day	13% 5% 23% 30% 19% 11%	479
	21st to end	Day	8% 5% 17% 28% 29% 13%	539
September	1st to 10th	Day	14% 5% 16% 27% 26% 12%	490
	11th to 20th	Day	11% 4 14% 26% 32% 14%	490
	21st to end	Day	24% 8% 19% 25% 18% 6%	490
October	1st to 10th	Day	11% 9% 27% 29% 17% 7%	490
	11th to 20th	Day	17% 12% 31% 24% 12% 3	490
	21st to end	Day	26% 14% 29% 20% 9% 1	520
November	1st to 10th	Day	16% 19% 34% 22% 9% 2	300
	11th to 20th	Day	29% 19% 28% 18% 6% 1	300
	21st to end	Day	28% 21% 33% 16% 3	300
December	1st to 10th	Day	30% 19% 33% 13% 4 1	294
	11th to 20th	Day	27% 23% 36% 11% 2 1	270
	21st to end	Day	35% 16% 32% 13% 3	297
January	1st to 10th	Night	25% 19% 31% 20% 4	760
	11th to 20th	Night	27% 18% 32% 19% 3	760
	21st to end	Night	28% 15% 32% 22% 2	836
February	1st to 10th	Night	28% 14% 31% 25% 2	760
	11th to 20th	Night	27% 18% 34% 20% 2	743
	21st to end	Night	28% 13% 37% 20% 2	628
March	1st to 10th	Night	24% 14% 32% 25% 5%	760
	11th to 20th	Night	22% 16% 32% 22% 7% 1	760
	21st to end	Night	24% 11% 31% 25% 7% 1	836
April	1st to 10th	Night	22% 13% 34% 24% 7%	760
	11th to 20th	Night	18% 11% 35% 24% 11% 1	577
	21st to end	Night	19% 7% 38% 29% 7%	570
May	1st to 10th	Night	21% 9% 28% 29% 12% 1	570
	11th to 20th	Night	18% 6% 28% 31% 15% 2	561
	21st to end	Night	9% 5% 21% 37% 25% 3	617
June	1st to 10th	Night	12% 2 23% 40% 22% 1	570
	11th to 20th	Night	11% 4 18% 42% 24% 2	570
	21st to end	Night	13% 2 22% 36% 24% 2	570
July	1st to 10th	Night	12% 2 14% 37% 28% 7%	555
	11th to 20th	Night	7% 3 16% 36% 32% 5%	552
	21st to end	Night	11% 3 13% 36% 34% 3	624
August	1st to 10th	Night	8% 2 12% 40% 35% 4	567
	11th to 20th	Night	10% 3 11% 40% 31% 5%	564
	21st to end	Night	9% 2 15% 31% 36% 7%	627
September	1st to 10th	Night	14% 4 14% 35% 28% 5%	760
	11th to 20th	Night	11% 3 12% 37% 33% 5%	760
	21st to end	Night	19% 5% 21% 32% 21% 3	760
October	1st to 10th	Night	12% 5% 24% 34% 21% 3	760
	11th to 20th	Night	19% 10% 28% 29% 12% 1	760
	21st to end	Night	23% 11% 27% 28% 10%	836
November	1st to 10th	Night	19% 10% 26% 29% 15% 1	780
	11th to 20th	Night	22% 15% 27% 28% 8% 1	800
	21st to end	Night	27% 13% 28% 25% 6% 1	800
December	1st to 10th	Night	30% 15% 29% 21% 5%	795
	11th to 20th	Night	25% 15% 30% 25% 5%	760
	21st to end	Night	31% 14% 28% 24% 2	836

Page generated at 2024-12-06 03:54:52. Version ac07

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