Wadsworth Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wadsworth Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	126159
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	123860
Transparency	Day	52% 5% 11% 20% 11% 1	129791
	Night	52% 5% 10% 19% 11% 1	128297
Seeing	Day	20% 14% 27% 21% 13% 5%	15931
	Night	20% 10% 27% 28% 12% 1	25829

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% 19% 8% 23% 5% 4 3 3 3 4 14%	4056
	11th to 20th	Day	13% 22% 8% 16% 7% 6% 5% 2 3 4 14%	4060
	21st to end	Day	22% 19% 9% 12% 7% 6% 4 3 3 3 12%	4466
February	1st to 10th	Day	18% 17% 5% 13% 7% 5% 3 3 3 4 22%	1800
	11th to 20th	Day	15% 14% 6% 14% 7% 6% 4 4 4 4 22%	1827
	21st to end	Day	17% 14% 4 11% 5% 5% 4 4 4 5% 25%	1650
March	1st to 10th	Day	18% 13% 6% 12% 5% 4 3 3 4 5% 26%	4800
	11th to 20th	Day	11% 11% 4 10% 5% 5% 4 4 4 5% 37%	4980
	21st to end	Day	20% 12% 4 9% 5% 4 4 3 4 6% 28%	5465
April	1st to 10th	Day	15% 13% 4 8% 6% 6% 4 4 5% 7% 30%	2227
	11th to 20th	Day	18% 10% 3 8% 5% 4 4 3 5% 6% 33%	2547
	21st to end	Day	18% 11% 3 8% 5% 5% 4 4 4 6% 33%	2600
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 4 4 5% 6% 28%	5862
	11th to 20th	Day	19% 8% 3 8% 4 4 5% 4 6% 8% 32%	5861
	21st to end	Day	18% 7% 2 6% 4 4 5% 4 6% 8% 34%	6433
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	2765
	11th to 20th	Day	12% 7% 3 8% 5% 4 5% 4 7% 8% 36%	2800
	21st to end	Day	11% 6% 2 5% 6% 5% 5% 5% 7% 10% 40%	2711
July	1st to 10th	Day	9% 6% 2 7% 5% 4 5% 4 6% 9% 42%	5900
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	5900
	21st to end	Day	11% 6% 3 7% 5% 4 5% 4 7% 9% 39%	6424
August	1st to 10th	Day	12% 7% 3 8% 5% 5% 6% 4 6% 9% 35%	4234
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 5% 8% 9% 36%	4174
	21st to end	Day	8% 6% 2 8% 4 6% 6% 4 7% 10% 39%	4609
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 5% 8% 45%	2275
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 3 7% 9% 44%	2200
	21st to end	Day	19% 8% 3 6% 4 4 4 3 5% 6% 38%	2200
October	1st to 10th	Day	14% 7% 2 7% 4 4 4 3 6% 8% 41%	2076
	11th to 20th	Day	17% 10% 3 7% 4 4 4 3 5% 6% 38%	2000
	21st to end	Day	20% 11% 3 7% 5% 4 4 3 5% 6% 31%	2190
November	1st to 10th	Day	12% 11% 3 7% 4 5% 4 4 5% 6% 39%	1780
	11th to 20th	Day	18% 15% 3 7% 5% 4 4 3 5% 5% 30%	1800
	21st to end	Day	17% 17% 6% 10% 6% 5% 4 4 5% 5% 22%	1615
December	1st to 10th	Day	19% 15% 6% 16% 5% 5% 4 3 4 4 20%	2600
	11th to 20th	Day	23% 18% 8% 11% 4 4 3 2 4 5% 17%	2990
	21st to end	Day	25% 22% 7% 13% 5% 4 4 3 4 4 10%	4282
January	1st to 10th	Night	15% 19% 7% 21% 7% 4 3 2 4 4 13%	5889
	11th to 20th	Night	11% 22% 10% 17% 9% 5% 5% 4 3 4 11%	5900
	21st to end	Night	21% 18% 6% 12% 5% 4 4 2 3 4 18%	6490
February	1st to 10th	Night	20% 15% 5% 12% 6% 5% 4 3 4 5% 21%	2600
	11th to 20th	Night	17% 14% 5% 12% 7% 5% 5% 4 4 5% 22%	2457
	21st to end	Night	20% 13% 4 11% 7% 4 4 3 4 5% 25%	1980
March	1st to 10th	Night	22% 12% 4 9% 4 3 3 2 3 5% 32%	5051
	11th to 20th	Night	13% 9% 3 7% 4 4 4 4 6% 7% 39%	4980
	21st to end	Night	17% 11% 4 8% 5% 4 4 4 5% 5% 33%	5467
April	1st to 10th	Night	17% 13% 3 7% 5% 5% 4 3 5% 7% 32%	2171
	11th to 20th	Night	16% 10% 3 8% 5% 4 4 4 5% 7% 34%	1817
	21st to end	Night	15% 8% 3 6% 5% 5% 5% 4 5% 7% 37%	1800
May	1st to 10th	Night	24% 10% 3 8% 5% 4 4 4 5% 6% 27%	4033
	11th to 20th	Night	16% 9% 3 6% 3 4 4 3 5% 9% 39%	4032
	21st to end	Night	15% 7% 2 6% 4 4 4 3 7% 8% 39%	4421
June	1st to 10th	Night	10% 6% 2 7% 4 5% 5% 4 6% 7% 44%	1630
	11th to 20th	Night	10% 5% 3 6% 5% 5% 5% 4 7% 9% 41%	1600
	21st to end	Night	11% 5% 2 5% 5% 4 5% 5% 6% 10% 43%	1680
July	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 4 7% 9% 44%	4058
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 4 6% 10% 41%	4057
	21st to end	Night	7% 6% 3 7% 4 4 4 4 6% 8% 46%	4427
August	1st to 10th	Night	11% 8% 3 7% 5% 5% 4 4 6% 8% 40%	2915
	11th to 20th	Night	11% 8% 3 8% 5% 5% 3 3 5% 7% 43%	2875
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 4 6% 9% 45%	3254
September	1st to 10th	Night	11% 6% 2 4 4 4 5% 4 5% 6% 50%	2121
	11th to 20th	Night	9% 4 1 5% 4 3 4 3 5% 7% 55%	2200
	21st to end	Night	16% 7% 3 6% 4 4 5% 3 4 6% 42%	2200
October	1st to 10th	Night	14% 6% 2 5% 3 4 4 4 7% 7% 45%	2322
	11th to 20th	Night	17% 8% 3 8% 4 5% 4 3 5% 5% 38%	2400
	21st to end	Night	22% 9% 3 6% 4 5% 4 4 5% 6% 33%	2647
November	1st to 10th	Night	14% 9% 3 6% 4 4 4 4 5% 7% 41%	2563
	11th to 20th	Night	17% 14% 4 8% 5% 4 4 3 5% 5% 30%	2600
	21st to end	Night	20% 16% 5% 7% 6% 4 4 4 4 7% 25%	2782
December	1st to 10th	Night	17% 15% 6% 12% 6% 5% 4 3 5% 5% 22%	4580
	11th to 20th	Night	20% 18% 7% 12% 5% 4 3 3 4 5% 19%	5205
	21st to end	Night	24% 20% 7% 11% 5% 5% 4 3 3 5% 13%	6656

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	76% 4 8% 8% 4	4057
	11th to 20th	Day	77% 3 7% 8% 6%	4060
	21st to end	Day	78% 3 7% 9% 3	4466
February	1st to 10th	Day	68% 3 9% 13% 7%	1800
	11th to 20th	Day	67% 3 8% 14% 7%	1836
	21st to end	Day	61% 4 12% 15% 7% 2	1650
March	1st to 10th	Day	60% 4 8% 22% 6%	4800
	11th to 20th	Day	49% 4 11% 26% 8% 2	4980
	21st to end	Day	57% 5% 7% 18% 12% 1	5465
April	1st to 10th	Day	54% 6% 11% 17% 12% 1	2227
	11th to 20th	Day	52% 6% 11% 17% 12% 2	2547
	21st to end	Day	52% 5% 10% 21% 10% 2	2600
May	1st to 10th	Day	55% 6% 9% 17% 10% 3	5897
	11th to 20th	Day	50% 7% 11% 21% 11% 1	5865
	21st to end	Day	46% 7% 11% 21% 12% 3	6447
June	1st to 10th	Day	44% 7% 10% 24% 14% 1	2765
	11th to 20th	Day	44% 7% 12% 21% 14% 1	2800
	21st to end	Day	39% 7% 13% 23% 16% 2	2711
July	1st to 10th	Day	37% 5% 12% 28% 16% 1	5835
	11th to 20th	Day	39% 7% 14% 23% 15% 1	5831
	21st to end	Day	39% 6% 11% 22% 20% 2	6240
August	1st to 10th	Day	45% 6% 13% 23% 12%	4221
	11th to 20th	Day	40% 8% 11% 29% 11% 1	4213
	21st to end	Day	38% 7% 13% 29% 10% 2	4609
September	1st to 10th	Day	38% 5% 13% 23% 17% 4	2275
	11th to 20th	Day	36% 6% 15% 25% 16% 2	2200
	21st to end	Day	48% 5% 9% 20% 17% 1	2200
October	1st to 10th	Day	31% 4 13% 31% 20% 1	3375
	11th to 20th	Day	46% 4 10% 20% 16% 4	3240
	21st to end	Day	42% 3 13% 25% 16% 1	3516
November	1st to 10th	Day	45% 5% 11% 23% 15% 1	1780
	11th to 20th	Day	56% 4 10% 19% 9% 2	1800
	21st to end	Day	65% 5% 10% 14% 6% 1	1615
December	1st to 10th	Day	69% 4 12% 13% 3	2600
	11th to 20th	Day	71% 4 9% 13% 3	2986
	21st to end	Day	79% 4 8% 7% 2	4282
January	1st to 10th	Night	75% 3 8% 10% 4	5889
	11th to 20th	Night	78% 3 8% 8% 3	5900
	21st to end	Night	71% 3 9% 11% 5% 1	6490
February	1st to 10th	Night	67% 4 9% 14% 6%	2600
	11th to 20th	Night	64% 4 10% 15% 7%	2459
	21st to end	Night	64% 4 8% 14% 9% 2	1980
March	1st to 10th	Night	57% 3 11% 20% 9%	5051
	11th to 20th	Night	43% 5% 14% 26% 10% 2	4980
	21st to end	Night	52% 5% 8% 20% 14% 1	5467
April	1st to 10th	Night	53% 5% 10% 17% 13% 2	2171
	11th to 20th	Night	49% 5% 12% 17% 15% 2	1817
	21st to end	Night	46% 5% 11% 20% 16% 2	1800
May	1st to 10th	Night	58% 5% 9% 15% 8% 5%	4060
	11th to 20th	Night	44% 6% 10% 23% 15% 2	4033
	21st to end	Night	41% 7% 13% 24% 13% 3	4429
June	1st to 10th	Night	39% 7% 9% 25% 17% 3	1630
	11th to 20th	Night	39% 7% 13% 21% 17% 2	1600
	21st to end	Night	36% 8% 11% 26% 16% 2	1680
July	1st to 10th	Night	35% 6% 13% 25% 17% 3	4015
	11th to 20th	Night	37% 7% 14% 27% 13% 1	4006
	21st to end	Night	34% 7% 12% 21% 25% 2	4297
August	1st to 10th	Night	42% 6% 12% 24% 15% 1	2904
	11th to 20th	Night	40% 6% 10% 28% 14% 1	2902
	21st to end	Night	35% 7% 14% 29% 14% 2	3254
September	1st to 10th	Night	34% 7% 10% 26% 18% 4	2121
	11th to 20th	Night	29% 5% 13% 27% 24% 2	2200
	21st to end	Night	43% 6% 10% 20% 19% 1	2200
October	1st to 10th	Night	30% 5% 8% 30% 26% 1	3803
	11th to 20th	Night	43% 5% 10% 21% 19% 3	3899
	21st to end	Night	39% 4 10% 28% 17% 2	4271
November	1st to 10th	Night	42% 5% 12% 22% 16% 3	2563
	11th to 20th	Night	56% 3 10% 17% 11% 3	2600
	21st to end	Night	60% 5% 13% 16% 6% 1	2782
December	1st to 10th	Night	64% 5% 11% 16% 4	4580
	11th to 20th	Night	68% 4 11% 15% 2	5208
	21st to end	Night	74% 5% 10% 10% 1	6656

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	27% 23% 35% 10% 4 1	300
	11th to 20th	Day	30% 25% 33% 8% 3	300
	21st to end	Day	28% 26% 32% 12% 2	330
February	1st to 10th	Day	27% 24% 37% 10% 2 1	300
	11th to 20th	Day	28% 30% 32% 9% 2	300
	21st to end	Day	27% 29% 35% 8%	249
March	1st to 10th	Day	24% 20% 33% 18% 4	438
	11th to 20th	Day	27% 17% 31% 18% 6%	500
	21st to end	Day	31% 18% 28% 19% 4 1	550
April	1st to 10th	Day	23% 21% 31% 18% 7% 1	500
	11th to 20th	Day	26% 20% 25% 17% 8% 4	492
	21st to end	Day	24% 17% 30% 20% 8% 1	500
May	1st to 10th	Day	26% 16% 29% 18% 9% 2	503
	11th to 20th	Day	20% 11% 34% 19% 12% 5%	527
	21st to end	Day	18% 8% 24% 26% 18% 6%	573
June	1st to 10th	Day	15% 9% 29% 30% 14% 3	530
	11th to 20th	Day	15% 11% 22% 34% 15% 5%	530
	21st to end	Day	14% 9% 27% 28% 16% 6%	530
July	1st to 10th	Day	12% 4 20% 29% 23% 12%	525
	11th to 20th	Day	11% 7% 22% 29% 22% 10%	524
	21st to end	Day	12% 4 22% 26% 25% 11%	576
August	1st to 10th	Day	12% 5% 20% 28% 25% 10%	526
	11th to 20th	Day	14% 5% 21% 26% 21% 12%	519
	21st to end	Day	9% 5% 20% 30% 22% 14%	583
September	1st to 10th	Day	15% 6% 21% 23% 22% 13%	530
	11th to 20th	Day	9% 3 16% 26% 30% 16%	530
	21st to end	Day	24% 10% 20% 23% 16% 7%	530
October	1st to 10th	Day	12% 9% 26% 25% 23% 5%	530
	11th to 20th	Day	19% 15% 35% 19% 9% 3	530
	21st to end	Day	27% 18% 29% 20% 5% 1	573
November	1st to 10th	Day	16% 22% 33% 18% 10% 1	306
	11th to 20th	Day	29% 18% 33% 14% 5% 1	300
	21st to end	Day	27% 21% 39% 12% 1	207
December	1st to 10th	Day	28% 19% 36% 12% 4 2	200
	11th to 20th	Day	26% 21% 36% 15% 3	200
	21st to end	Day	32% 22% 31% 13% 2	290
January	1st to 10th	Night	25% 20% 33% 17% 4	800
	11th to 20th	Night	30% 17% 34% 15% 3	800
	21st to end	Night	28% 18% 30% 22% 3	880
February	1st to 10th	Night	28% 16% 32% 21% 3	800
	11th to 20th	Night	26% 16% 35% 21% 2	783
	21st to end	Night	28% 16% 37% 17% 2	660
March	1st to 10th	Night	24% 14% 34% 23% 5%	800
	11th to 20th	Night	22% 14% 33% 24% 6% 1	800
	21st to end	Night	23% 12% 33% 24% 7% 1	880
April	1st to 10th	Night	23% 13% 34% 24% 6% 1	800
	11th to 20th	Night	23% 11% 35% 21% 9% 1	633
	21st to end	Night	19% 9% 34% 31% 7% 1	600
May	1st to 10th	Night	20% 9% 30% 31% 9% 1	600
	11th to 20th	Night	20% 7% 24% 36% 11% 2	591
	21st to end	Night	13% 2 20% 39% 22% 3	650
June	1st to 10th	Night	10% 3 27% 40% 18% 1	430
	11th to 20th	Night	11% 4 25% 45% 15% 1	400
	21st to end	Night	13% 5% 22% 40% 19% 1	480
July	1st to 10th	Night	9% 2 14% 42% 29% 4	585
	11th to 20th	Night	9% 2 19% 40% 27% 3	582
	21st to end	Night	8% 2 15% 42% 30% 3	657
August	1st to 10th	Night	9% 2 15% 42% 28% 3	597
	11th to 20th	Night	11% 3 11% 41% 30% 4	594
	21st to end	Night	10% 3 17% 34% 31% 5%	678
September	1st to 10th	Night	13% 4 17% 34% 27% 5%	800
	11th to 20th	Night	10% 2 15% 36% 34% 5%	800
	21st to end	Night	19% 5% 22% 31% 21% 4	800
October	1st to 10th	Night	16% 7% 20% 33% 20% 3	800
	11th to 20th	Night	22% 11% 28% 28% 10% 2	800
	21st to end	Night	26% 11% 29% 27% 7%	880
November	1st to 10th	Night	20% 10% 30% 29% 11% 1	789
	11th to 20th	Night	24% 16% 31% 22% 6% 1	800
	21st to end	Night	29% 15% 31% 19% 6%	800
December	1st to 10th	Night	30% 15% 32% 18% 4	800
	11th to 20th	Night	27% 17% 32% 21% 3 1	800
	21st to end	Night	28% 15% 33% 21% 3	880

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