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 2026-04-02.

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% 4 8% 5% 5% 5% 4 5% 7% 32%	131605
	Night	15% 11% 4 8% 5% 4 4 3 5% 7% 34%	130216
Transparency	Day	49% 7% 11% 21% 12% 1	135265
	Night	48% 6% 11% 21% 12% 2	134613
Seeing	Day	22% 13% 27% 21% 13% 5%	16709
	Night	20% 10% 26% 28% 14% 2	26875

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	17% 20% 7% 13% 8% 4 3 3 4 5% 16%	5258
	11th to 20th	Day	13% 18% 6% 16% 8% 6% 4 3 3 4 18%	5280
	21st to end	Day	19% 14% 7% 11% 7% 7% 6% 3 5% 5% 15%	5780
February	1st to 10th	Day	21% 13% 5% 9% 7% 4 3 3 5% 5% 25%	1890
	11th to 20th	Day	17% 14% 6% 12% 5% 4 4 3 4 6% 25%	1995
	21st to end	Day	18% 14% 5% 9% 4 4 3 2 5% 5% 31%	1730
March	1st to 10th	Day	18% 13% 6% 10% 4 4 4 3 4 5% 28%	4857
	11th to 20th	Day	13% 8% 3 8% 4 5% 5% 4 5% 6% 39%	5090
	21st to end	Day	19% 9% 3 8% 5% 4 4 3 5% 6% 33%	5586
April	1st to 10th	Day	15% 13% 3 8% 5% 4 4 4 5% 5% 32%	2214
	11th to 20th	Day	16% 9% 3 7% 4 4 4 3 6% 7% 36%	2549
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 3 6% 7% 33%	2600
May	1st to 10th	Day	21% 10% 3 8% 5% 5% 5% 4 6% 7% 27%	5862
	11th to 20th	Day	21% 8% 3 7% 4 4 4 4 5% 8% 31%	5861
	21st to end	Day	16% 6% 3 7% 4 4 5% 5% 7% 9% 33%	6433
June	1st to 10th	Day	13% 6% 2 7% 5% 4 6% 4 7% 10% 37%	2600
	11th to 20th	Day	11% 7% 2 8% 5% 5% 5% 4 7% 10% 36%	2600
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 10% 36%	2600
July	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 3 5% 9% 42%	5900
	11th to 20th	Day	11% 6% 3 7% 5% 6% 6% 5% 7% 10% 34%	5900
	21st to end	Day	12% 7% 2 7% 5% 5% 6% 5% 7% 9% 35%	6424
August	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 9% 38%	4234
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 38%	4174
	21st to end	Day	6% 5% 1 6% 4 5% 6% 4 7% 9% 47%	4609
September	1st to 10th	Day	14% 5% 1 6% 4 3 4 3 5% 7% 48%	2200
	11th to 20th	Day	9% 5% 1 6% 5% 5% 3 4 6% 7% 50%	2200
	21st to end	Day	18% 8% 3 6% 3 4 3 4 5% 6% 40%	2200
October	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 6% 7% 45%	2119
	11th to 20th	Day	14% 6% 2 7% 4 4 4 3 6% 7% 43%	2000
	21st to end	Day	20% 9% 3 7% 5% 3 4 3 4 5% 37%	2180
November	1st to 10th	Day	10% 8% 3 6% 4 4 3 3 5% 6% 48%	1852
	11th to 20th	Day	17% 14% 3 6% 4 4 4 3 4 6% 35%	1890
	21st to end	Day	20% 13% 4 9% 4 3 3 4 4 6% 29%	1890
December	1st to 10th	Day	20% 13% 5% 14% 6% 5% 5% 3 3 4 23%	3020
	11th to 20th	Day	23% 16% 6% 10% 5% 4 4 3 4 5% 21%	3463
	21st to end	Day	21% 21% 6% 12% 5% 4 5% 3 4 4 15%	4565
January	1st to 10th	Night	14% 17% 7% 16% 7% 5% 4 3 4 4 17%	7635
	11th to 20th	Night	12% 16% 8% 15% 8% 7% 6% 3 4 4 17%	7680
	21st to end	Night	17% 13% 6% 10% 6% 5% 5% 2 4 5% 27%	8401
February	1st to 10th	Night	19% 14% 3 9% 5% 5% 3 3 5% 5% 28%	2730
	11th to 20th	Night	19% 13% 3 10% 6% 5% 5% 4 5% 5% 26%	2510
	21st to end	Night	19% 14% 6% 10% 5% 4 4 3 3 4 28%	2076
March	1st to 10th	Night	19% 12% 3 7% 4 4 4 3 4 5% 35%	5215
	11th to 20th	Night	12% 8% 3 6% 4 5% 4 4 5% 8% 41%	5090
	21st to end	Night	18% 9% 3 7% 5% 4 4 3 5% 6% 35%	5588
April	1st to 10th	Night	17% 10% 3 6% 4 5% 3 4 6% 6% 37%	2212
	11th to 20th	Night	14% 8% 3 7% 4 5% 4 4 5% 9% 37%	1812
	21st to end	Night	15% 8% 2 7% 4 5% 4 4 6% 8% 37%	1800
May	1st to 10th	Night	24% 11% 3 7% 4 4 4 4 5% 6% 27%	4033
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 6% 10% 36%	4032
	21st to end	Night	13% 7% 3 7% 5% 4 4 4 7% 9% 37%	4421
June	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 5% 8% 47%	1800
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 4 6% 10% 44%	1800
	21st to end	Night	10% 5% 2 5% 3 4 5% 5% 7% 10% 44%	1800
July	1st to 10th	Night	10% 6% 3 6% 4 4 4 4 5% 7% 46%	4058
	11th to 20th	Night	11% 6% 2 6% 5% 6% 6% 5% 7% 10% 38%	4057
	21st to end	Night	8% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	4427
August	1st to 10th	Night	7% 7% 3 8% 6% 5% 5% 5% 8% 10% 38%	2915
	11th to 20th	Night	10% 7% 3 7% 4 4 5% 3 5% 7% 45%	2875
	21st to end	Night	5% 4 1 5% 4 4 5% 4 7% 10% 51%	3223
September	1st to 10th	Night	12% 5% 2 4 3 4 4 3 4 7% 53%	2200
	11th to 20th	Night	9% 3 2 4 3 4 4 3 5% 7% 56%	2200
	21st to end	Night	16% 7% 3 6% 4 3 3 3 5% 6% 46%	2200
October	1st to 10th	Night	10% 5% 2 6% 4 3 4 3 6% 7% 50%	2280
	11th to 20th	Night	14% 7% 3 6% 3 3 4 3 5% 7% 44%	2400
	21st to end	Night	19% 8% 2 6% 4 4 4 3 5% 6% 39%	2655
November	1st to 10th	Night	13% 8% 2 5% 3 3 3 3 5% 5% 49%	2666
	11th to 20th	Night	16% 10% 3 7% 5% 4 4 3 6% 6% 35%	2730
	21st to end	Night	19% 14% 4 8% 3 4 4 4 5% 5% 29%	2730
December	1st to 10th	Night	17% 14% 5% 11% 5% 4 4 3 5% 5% 26%	4380
	11th to 20th	Night	19% 16% 7% 10% 5% 3 4 3 4 5% 23%	4952
	21st to end	Night	21% 20% 6% 10% 5% 4 4 3 4 6% 16%	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	71% 4 12% 10% 3	5259
	11th to 20th	Day	69% 5% 8% 10% 8% 1	5280
	21st to end	Day	70% 5% 9% 10% 5%	5780
February	1st to 10th	Day	61% 5% 11% 15% 8%	1890
	11th to 20th	Day	61% 6% 8% 16% 9%	2005
	21st to end	Day	56% 5% 12% 18% 7% 2	1730
March	1st to 10th	Day	57% 5% 8% 21% 8% 2	4857
	11th to 20th	Day	42% 7% 11% 28% 11% 2	5090
	21st to end	Day	51% 6% 9% 21% 11% 1	5586
April	1st to 10th	Day	52% 7% 9% 19% 11% 2	2214
	11th to 20th	Day	46% 6% 12% 19% 13% 3	2549
	21st to end	Day	48% 7% 10% 21% 11% 2	2600
May	1st to 10th	Day	54% 8% 10% 18% 9% 2	5897
	11th to 20th	Day	50% 7% 11% 20% 11% 1	5865
	21st to end	Day	41% 11% 13% 21% 11% 2	6447
June	1st to 10th	Day	40% 9% 12% 23% 15% 2	2600
	11th to 20th	Day	41% 9% 14% 21% 14% 1	2600
	21st to end	Day	40% 9% 15% 20% 14% 2	2600
July	1st to 10th	Day	39% 6% 12% 27% 15% 2	5835
	11th to 20th	Day	41% 9% 13% 24% 12% 1	5831
	21st to end	Day	42% 8% 12% 18% 17% 2	6240
August	1st to 10th	Day	38% 9% 15% 28% 10%	4221
	11th to 20th	Day	37% 8% 12% 30% 13% 1	4213
	21st to end	Day	31% 9% 15% 29% 13% 3	4609
September	1st to 10th	Day	36% 6% 12% 21% 22% 3	2200
	11th to 20th	Day	32% 6% 12% 26% 21% 2	2200
	21st to end	Day	45% 4 10% 22% 19% 1	2200
October	1st to 10th	Day	25% 5% 9% 32% 27% 1	3444
	11th to 20th	Day	33% 4 12% 26% 21% 3	3240
	21st to end	Day	38% 4 11% 27% 18% 2	3506
November	1st to 10th	Day	37% 6% 10% 28% 17% 3	1852
	11th to 20th	Day	50% 6% 9% 21% 12% 2	1890
	21st to end	Day	56% 5% 11% 17% 10% 1	1890
December	1st to 10th	Day	65% 5% 13% 14% 4	3020
	11th to 20th	Day	66% 4 11% 15% 5%	3460
	21st to end	Day	71% 6% 8% 10% 4	4565
January	1st to 10th	Night	69% 5% 9% 13% 3	7636
	11th to 20th	Night	68% 5% 8% 11% 7% 1	7680
	21st to end	Night	60% 4 12% 17% 6% 1	8401
February	1st to 10th	Night	57% 6% 10% 18% 8% 1	2730
	11th to 20th	Night	59% 5% 10% 16% 8% 1	2511
	21st to end	Night	59% 5% 10% 15% 9% 2	2076
March	1st to 10th	Night	51% 5% 7% 25% 10% 2	5215
	11th to 20th	Night	38% 7% 11% 30% 12% 1	5090
	21st to end	Night	49% 6% 8% 22% 13% 2	5588
April	1st to 10th	Night	45% 7% 13% 20% 14% 2	2212
	11th to 20th	Night	43% 6% 12% 18% 18% 4	1812
	21st to end	Night	43% 7% 12% 22% 15% 2	1800
May	1st to 10th	Night	54% 8% 11% 15% 8% 4	4060
	11th to 20th	Night	43% 8% 11% 24% 13% 1	4033
	21st to end	Night	40% 9% 15% 25% 9% 3	4429
June	1st to 10th	Night	34% 9% 12% 23% 21% 2	1800
	11th to 20th	Night	33% 9% 15% 24% 17% 2	1800
	21st to end	Night	33% 10% 14% 24% 17% 2	1800
July	1st to 10th	Night	36% 7% 11% 24% 18% 4	4015
	11th to 20th	Night	38% 10% 15% 25% 11% 1	4006
	21st to end	Night	38% 8% 14% 20% 17% 3	4297
August	1st to 10th	Night	37% 9% 16% 25% 12% 1	2904
	11th to 20th	Night	38% 7% 10% 24% 21% 1	2902
	21st to end	Night	26% 8% 15% 30% 18% 3	3223
September	1st to 10th	Night	31% 5% 12% 22% 23% 6%	2200
	11th to 20th	Night	27% 6% 11% 27% 26% 3	2200
	21st to end	Night	40% 6% 10% 20% 24% 1	2200
October	1st to 10th	Night	27% 5% 7% 28% 30% 3	3722
	11th to 20th	Night	30% 5% 11% 27% 22% 5%	3899
	21st to end	Night	32% 5% 9% 26% 24% 4	4279
November	1st to 10th	Night	37% 5% 11% 25% 19% 3	2666
	11th to 20th	Night	48% 5% 10% 20% 14% 2	2730
	21st to end	Night	56% 4 12% 18% 8% 1	2730
December	1st to 10th	Night	59% 4 12% 19% 5%	4380
	11th to 20th	Night	63% 5% 12% 16% 4	4954
	21st to end	Night	69% 6% 9% 12% 4	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	32% 20% 35% 9% 5%	330
	11th to 20th	Day	33% 24% 34% 7% 2	330
	21st to end	Day	34% 23% 29% 12% 3	363
February	1st to 10th	Day	32% 23% 31% 12% 2	330
	11th to 20th	Day	35% 26% 28% 8% 2 1	330
	21st to end	Day	30% 25% 40% 5% 1	273
March	1st to 10th	Day	28% 23% 30% 15% 4	451
	11th to 20th	Day	32% 15% 34% 14% 5%	540
	21st to end	Day	30% 17% 32% 17% 4	594
April	1st to 10th	Day	27% 17% 30% 17% 7% 2	504
	11th to 20th	Day	27% 16% 29% 16% 9% 3	492
	21st to end	Day	24% 16% 34% 18% 7% 2	500
May	1st to 10th	Day	27% 15% 31% 16% 8% 3	503
	11th to 20th	Day	22% 11% 28% 24% 11% 5%	527
	21st to end	Day	15% 10% 24% 24% 20% 6%	573
June	1st to 10th	Day	14% 8% 28% 29% 16% 5%	530
	11th to 20th	Day	16% 8% 23% 29% 18% 6%	530
	21st to end	Day	13% 6% 27% 28% 19% 7%	530
July	1st to 10th	Day	15% 4 16% 24% 23% 17%	525
	11th to 20th	Day	11% 6% 21% 26% 25% 11%	524
	21st to end	Day	15% 5% 20% 27% 20% 13%	576
August	1st to 10th	Day	13% 4 16% 26% 27% 13%	526
	11th to 20th	Day	12% 5% 22% 29% 21% 12%	519
	21st to end	Day	8% 6% 18% 28% 28% 12%	583
September	1st to 10th	Day	14% 6% 15% 27% 25% 13%	530
	11th to 20th	Day	10% 4 13% 25% 33% 15%	530
	21st to end	Day	23% 8% 19% 25% 19% 5%	530
October	1st to 10th	Day	11% 9% 26% 29% 18% 7%	530
	11th to 20th	Day	17% 12% 32% 24% 12% 3	530
	21st to end	Day	26% 14% 31% 19% 9% 1	563
November	1st to 10th	Day	18% 18% 35% 21% 8% 2	330
	11th to 20th	Day	31% 19% 28% 16% 6% 1	330
	21st to end	Day	30% 20% 32% 15% 2	330
December	1st to 10th	Day	30% 19% 35% 12% 4 1	330
	11th to 20th	Day	31% 22% 35% 10% 2 1	330
	21st to end	Day	36% 18% 31% 12% 2	363
January	1st to 10th	Night	26% 19% 31% 20% 3	840
	11th to 20th	Night	27% 20% 31% 18% 3	840
	21st to end	Night	27% 17% 32% 21% 2	924
February	1st to 10th	Night	27% 14% 32% 26% 2	840
	11th to 20th	Night	27% 18% 34% 19% 2	823
	21st to end	Night	27% 13% 38% 19% 2	692
March	1st to 10th	Night	25% 14% 33% 24% 5%	840
	11th to 20th	Night	23% 16% 32% 21% 7% 1	840
	21st to end	Night	25% 13% 31% 24% 6% 1	924
April	1st to 10th	Night	23% 13% 34% 23% 6%	805
	11th to 20th	Night	18% 12% 35% 24% 11% 1	607
	21st to end	Night	20% 7% 37% 29% 7%	600
May	1st to 10th	Night	21% 10% 28% 30% 11% 1	600
	11th to 20th	Night	19% 7% 27% 31% 15% 2	591
	21st to end	Night	10% 5% 21% 37% 24% 3	650
June	1st to 10th	Night	12% 2 23% 40% 22% 1	600
	11th to 20th	Night	12% 4 17% 41% 23% 2	600
	21st to end	Night	12% 2 22% 36% 25% 3	600
July	1st to 10th	Night	11% 2 14% 37% 29% 7%	585
	11th to 20th	Night	7% 3 16% 36% 32% 5%	582
	21st to end	Night	11% 3 12% 36% 35% 3	657
August	1st to 10th	Night	8% 3 12% 39% 35% 4	597
	11th to 20th	Night	10% 3 11% 39% 33% 4	594
	21st to end	Night	9% 2 15% 32% 35% 7%	660
September	1st to 10th	Night	14% 4 14% 35% 28% 5%	800
	11th to 20th	Night	10% 3 11% 36% 35% 5%	800
	21st to end	Night	19% 5% 20% 33% 21% 3	800
October	1st to 10th	Night	13% 5% 23% 34% 22% 3	800
	11th to 20th	Night	19% 10% 28% 30% 12% 1	800
	21st to end	Night	23% 11% 28% 28% 10%	880
November	1st to 10th	Night	18% 11% 27% 29% 14% 1	820
	11th to 20th	Night	22% 15% 28% 27% 7% 1	840
	21st to end	Night	28% 13% 28% 25% 6% 1	840
December	1st to 10th	Night	30% 15% 29% 21% 5%	840
	11th to 20th	Night	25% 16% 30% 24% 5%	840
	21st to end	Night	31% 15% 28% 24% 2	924

Page generated at 2026-03-30 02:18:42. Version ac07

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