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 2024-01-31.

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	16% 10% 4 8% 6% 3 5% 5% 5% 7% 31%	74750
	Night	16% 10% 4 7% 6% 3 5% 4 5% 7% 33%	75371
Transparency	Day	53% 6% 11% 19% 11% 1	74471
	Night	52% 5% 11% 19% 11% 2	75169
Seeing	Day	20% 13% 27% 21% 13% 5%	13557
	Night	21% 10% 27% 28% 12% 2	23592

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	19% 14% 9% 12% 8% 3 5% 5% 4 4 18%	1706
	11th to 20th	Day	20% 19% 8% 11% 8% 4 5% 3 3 5% 15%	1710
	21st to end	Day	18% 18% 9% 12% 8% 4 5% 4 4 5% 13%	1872
February	1st to 10th	Day	19% 17% 6% 12% 9% 4 3 3 3 5% 20%	1620
	11th to 20th	Day	16% 14% 6% 13% 9% 5% 4 5% 4 4 21%	1640
	21st to end	Day	18% 14% 5% 10% 6% 3 5% 4 4 5% 25%	1480
March	1st to 10th	Day	19% 11% 5% 8% 7% 3 3 4 4 6% 28%	1924
	11th to 20th	Day	16% 14% 5% 8% 6% 3 4 5% 4 5% 29%	1980
	21st to end	Day	23% 11% 4 8% 6% 3 4 4 4 7% 26%	2178
April	1st to 10th	Day	14% 11% 4 7% 6% 4 5% 4 5% 7% 31%	2003
	11th to 20th	Day	18% 10% 4 8% 6% 3 5% 4 4 6% 33%	2289
	21st to end	Day	20% 10% 3 7% 5% 4 4 5% 4 6% 32%	2340
May	1st to 10th	Day	18% 9% 4 7% 7% 4 5% 5% 6% 6% 30%	2340
	11th to 20th	Day	19% 7% 3 7% 5% 4 5% 5% 6% 7% 32%	2301
	21st to end	Day	10% 6% 2 6% 7% 4 5% 6% 7% 10% 38%	2531
June	1st to 10th	Day	13% 6% 3 7% 6% 3 5% 6% 6% 9% 36%	2488
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 5% 7% 9% 37%	2520
	21st to end	Day	11% 5% 2 4 6% 4 5% 5% 7% 10% 39%	2441
July	1st to 10th	Day	12% 6% 2 5% 5% 3 5% 5% 6% 9% 41%	2340
	11th to 20th	Day	11% 5% 2 5% 5% 4 6% 6% 7% 9% 40%	2340
	21st to end	Day	11% 5% 3 6% 6% 4 6% 6% 7% 9% 38%	2547
August	1st to 10th	Day	9% 6% 3 6% 6% 4 7% 7% 7% 8% 37%	2327
	11th to 20th	Day	11% 6% 3 6% 6% 4 6% 6% 7% 9% 35%	2303
	21st to end	Day	9% 6% 3 7% 5% 4 6% 6% 7% 9% 38%	2538
September	1st to 10th	Day	14% 6% 2 5% 6% 3 4 6% 5% 8% 42%	2048
	11th to 20th	Day	12% 6% 2 6% 5% 3 4 5% 7% 9% 41%	1980
	21st to end	Day	18% 8% 2 6% 4 3 4 4 5% 6% 39%	1980
October	1st to 10th	Day	15% 7% 3 7% 5% 3 4 4 6% 8% 38%	1869
	11th to 20th	Day	19% 10% 3 7% 5% 3 4 4 5% 6% 35%	1800
	21st to end	Day	22% 11% 4 7% 6% 3 5% 4 5% 5% 29%	1972
November	1st to 10th	Day	11% 11% 4 7% 5% 4 4 5% 5% 6% 39%	1672
	11th to 20th	Day	18% 13% 5% 6% 5% 3 5% 4 5% 5% 31%	1710
	21st to end	Day	18% 15% 6% 8% 7% 3 5% 4 5% 5% 23%	1535
December	1st to 10th	Day	19% 18% 9% 11% 6% 3 4 3 3 4 19%	1520
	11th to 20th	Day	18% 16% 7% 14% 5% 3 4 4 5% 7% 17%	1910
	21st to end	Day	26% 23% 7% 10% 6% 3 4 3 4 4 10%	2996
January	1st to 10th	Night	18% 15% 7% 11% 8% 3 4 4 4 5% 20%	2459
	11th to 20th	Night	19% 19% 6% 11% 8% 3 3 4 4 5% 17%	2470
	21st to end	Night	18% 18% 7% 11% 8% 4 4 3 4 5% 19%	2711
February	1st to 10th	Night	22% 14% 7% 10% 7% 4 4 3 4 5% 20%	2340
	11th to 20th	Night	18% 13% 7% 10% 7% 4 5% 5% 4 6% 22%	2205
	21st to end	Night	21% 12% 5% 9% 8% 3 4 4 4 5% 25%	1776
March	1st to 10th	Night	18% 9% 5% 9% 6% 3 4 4 4 5% 32%	2034
	11th to 20th	Night	16% 9% 4 7% 5% 2 4 4 6% 7% 35%	1980
	21st to end	Night	19% 10% 4 7% 6% 3 5% 4 5% 5% 31%	2178
April	1st to 10th	Night	15% 11% 4 6% 5% 3 6% 4 5% 7% 33%	1955
	11th to 20th	Night	17% 9% 3 6% 6% 3 4 4 5% 7% 35%	1633
	21st to end	Night	15% 8% 3 6% 6% 3 5% 5% 4 7% 36%	1620
May	1st to 10th	Night	16% 9% 3 8% 7% 4 6% 5% 5% 6% 31%	1620
	11th to 20th	Night	16% 8% 4 5% 3 3 6% 5% 5% 8% 37%	1592
	21st to end	Night	9% 5% 3 5% 5% 3 5% 5% 7% 10% 43%	1744
June	1st to 10th	Night	9% 5% 3 6% 5% 4 5% 5% 6% 7% 44%	1468
	11th to 20th	Night	10% 6% 3 5% 5% 4 5% 5% 6% 10% 42%	1440
	21st to end	Night	12% 4 2 4 5% 3 5% 5% 6% 10% 44%	1511
July	1st to 10th	Night	8% 4 3 5% 6% 4 5% 4 6% 8% 47%	1620
	11th to 20th	Night	7% 4 2 5% 5% 4 5% 5% 6% 11% 46%	1620
	21st to end	Night	8% 5% 3 6% 6% 3 5% 6% 6% 8% 44%	1771
August	1st to 10th	Night	7% 6% 2 5% 5% 3 5% 5% 6% 9% 48%	1611
	11th to 20th	Night	9% 5% 3 6% 6% 4 4 4 6% 9% 45%	1598
	21st to end	Night	9% 6% 2 4 4 2 5% 5% 6% 8% 48%	1798
September	1st to 10th	Night	12% 6% 2 4 5% 3 5% 5% 5% 6% 47%	1908
	11th to 20th	Night	10% 4 2 4 5% 2 4 4 5% 7% 53%	1980
	21st to end	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 44%	1980
October	1st to 10th	Night	14% 6% 2 5% 4 3 4 5% 7% 8% 42%	2089
	11th to 20th	Night	18% 8% 3 7% 5% 4 4 4 5% 5% 36%	2160
	21st to end	Night	23% 10% 3 6% 5% 3 4 5% 4 6% 31%	2381
November	1st to 10th	Night	15% 8% 3 5% 4 2 4 5% 5% 7% 41%	2406
	11th to 20th	Night	17% 13% 5% 7% 5% 3 5% 4 5% 6% 30%	2470
	21st to end	Night	20% 14% 5% 6% 7% 3 5% 5% 4 6% 26%	2642
December	1st to 10th	Night	21% 17% 7% 9% 6% 3 3 4 4 5% 21%	2660
	11th to 20th	Night	17% 15% 7% 11% 6% 3 4 4 5% 6% 23%	3285
	21st to end	Night	26% 19% 8% 8% 6% 3 4 4 3 5% 13%	4656

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 10% 11% 4	1707
	11th to 20th	Day	74% 4 10% 9% 3	1710
	21st to end	Day	75% 4 9% 9% 3 1	1872
February	1st to 10th	Day	70% 3 9% 13% 6%	1620
	11th to 20th	Day	68% 4 9% 13% 6% 1	1649
	21st to end	Day	62% 4 11% 15% 7% 2	1480
March	1st to 10th	Day	58% 3 11% 20% 7%	1924
	11th to 20th	Day	58% 5% 10% 16% 10% 1	1980
	21st to end	Day	59% 5% 8% 18% 10% 1	2178
April	1st to 10th	Day	52% 6% 11% 18% 12% 1	2003
	11th to 20th	Day	53% 6% 10% 16% 13% 2	2289
	21st to end	Day	54% 5% 10% 20% 10% 2	2340
May	1st to 10th	Day	53% 6% 11% 18% 10% 2	2340
	11th to 20th	Day	52% 7% 10% 16% 14% 2	2305
	21st to end	Day	40% 8% 14% 24% 13% 1	2531
June	1st to 10th	Day	44% 7% 10% 24% 13% 1	2488
	11th to 20th	Day	44% 8% 12% 21% 14% 1	2520
	21st to end	Day	39% 8% 13% 23% 16% 2	2441
July	1st to 10th	Day	41% 7% 10% 24% 17% 2	2275
	11th to 20th	Day	40% 7% 14% 23% 15% 2	2271
	21st to end	Day	40% 8% 12% 24% 14% 2	2405
August	1st to 10th	Day	42% 8% 12% 25% 13% 1	2314
	11th to 20th	Day	43% 9% 13% 24% 10% 2	2303
	21st to end	Day	40% 8% 12% 21% 14% 4	2538
September	1st to 10th	Day	41% 5% 14% 20% 16% 4	2048
	11th to 20th	Day	38% 6% 14% 24% 15% 1	1980
	21st to end	Day	46% 5% 9% 22% 17% 1	1980
October	1st to 10th	Day	44% 6% 10% 24% 15% 1	1869
	11th to 20th	Day	51% 5% 11% 20% 12% 1	1800
	21st to end	Day	57% 4 13% 16% 9% 1	1972
November	1st to 10th	Day	46% 5% 11% 22% 15% 1	1672
	11th to 20th	Day	55% 4 10% 20% 9% 2	1710
	21st to end	Day	63% 5% 11% 14% 6% 1	1535
December	1st to 10th	Day	71% 4 8% 12% 4	1520
	11th to 20th	Day	69% 4 9% 14% 4	1906
	21st to end	Day	78% 4 10% 6% 2	2996
January	1st to 10th	Night	67% 4 10% 12% 6%	2459
	11th to 20th	Night	71% 4 10% 12% 3	2470
	21st to end	Night	70% 3 9% 13% 4 1	2711
February	1st to 10th	Night	68% 4 9% 13% 5%	2340
	11th to 20th	Night	64% 4 10% 14% 7%	2207
	21st to end	Night	64% 4 8% 14% 9% 2	1776
March	1st to 10th	Night	55% 4 9% 22% 10% 1	2034
	11th to 20th	Night	49% 5% 12% 20% 11% 3	1980
	21st to end	Night	55% 4 8% 19% 11% 2	2178
April	1st to 10th	Night	51% 5% 10% 18% 14% 1	1955
	11th to 20th	Night	48% 5% 12% 16% 16% 3	1633
	21st to end	Night	47% 5% 12% 19% 15% 3	1620
May	1st to 10th	Night	53% 6% 9% 17% 12% 3	1620
	11th to 20th	Night	45% 7% 11% 18% 15% 4	1593
	21st to end	Night	36% 7% 15% 26% 15% 1	1745
June	1st to 10th	Night	38% 7% 10% 25% 16% 3	1468
	11th to 20th	Night	39% 6% 13% 21% 18% 2	1440
	21st to end	Night	36% 8% 11% 26% 16% 2	1511
July	1st to 10th	Night	36% 7% 12% 23% 19% 4	1575
	11th to 20th	Night	32% 7% 13% 27% 18% 2	1566
	21st to end	Night	36% 8% 13% 24% 17% 3	1672
August	1st to 10th	Night	32% 7% 13% 28% 19% 1	1600
	11th to 20th	Night	36% 7% 15% 27% 13% 2	1598
	21st to end	Night	33% 7% 13% 26% 18% 4	1798
September	1st to 10th	Night	36% 7% 11% 25% 17% 4	1908
	11th to 20th	Night	31% 6% 13% 26% 23% 2	1980
	21st to end	Night	41% 6% 11% 22% 19% 1	1980
October	1st to 10th	Night	38% 7% 12% 22% 20% 1	2089
	11th to 20th	Night	49% 5% 10% 22% 13% 1	2160
	21st to end	Night	54% 5% 9% 20% 10% 2	2381
November	1st to 10th	Night	43% 5% 11% 22% 16% 3	2406
	11th to 20th	Night	55% 3 10% 18% 11% 2	2470
	21st to end	Night	59% 5% 13% 17% 6%	2642
December	1st to 10th	Night	67% 5% 10% 12% 6%	2660
	11th to 20th	Night	63% 5% 12% 16% 3	3288
	21st to end	Night	75% 5% 10% 8% 2	4656

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	28% 23% 33% 11% 4 1	270
	11th to 20th	Day	30% 23% 34% 9% 3	270
	21st to end	Day	29% 24% 32% 13% 2	294
February	1st to 10th	Day	30% 23% 37% 10%	240
	11th to 20th	Day	28% 31% 33% 8% 1	240
	21st to end	Day	27% 31% 34% 7% 1	198
March	1st to 10th	Day	24% 21% 32% 18% 4	364
	11th to 20th	Day	27% 16% 31% 18% 6%	420
	21st to end	Day	31% 17% 27% 19% 4 1	462
April	1st to 10th	Day	22% 19% 30% 19% 8% 2	420
	11th to 20th	Day	26% 18% 26% 17% 9% 5%	412
	21st to end	Day	25% 17% 29% 20% 8% 1	420
May	1st to 10th	Day	26% 14% 29% 19% 9% 3	423
	11th to 20th	Day	20% 11% 33% 20% 11% 5%	447
	21st to end	Day	16% 6% 24% 28% 19% 7%	485
June	1st to 10th	Day	14% 8% 28% 32% 15% 3	450
	11th to 20th	Day	16% 11% 21% 33% 15% 5%	450
	21st to end	Day	14% 9% 27% 30% 15% 6%	450
July	1st to 10th	Day	13% 3 19% 29% 23% 14%	445
	11th to 20th	Day	11% 6% 20% 29% 23% 10%	444
	21st to end	Day	12% 3 23% 26% 25% 10%	488
August	1st to 10th	Day	12% 4 18% 28% 26% 12%	446
	11th to 20th	Day	15% 5% 21% 26% 21% 12%	439
	21st to end	Day	9% 5% 20% 29% 22% 15%	495
September	1st to 10th	Day	16% 5% 21% 24% 22% 12%	450
	11th to 20th	Day	11% 4 18% 24% 28% 15%	450
	21st to end	Day	22% 10% 19% 23% 17% 8%	450
October	1st to 10th	Day	12% 10% 26% 24% 22% 5%	450
	11th to 20th	Day	21% 15% 35% 19% 9% 1	450
	21st to end	Day	29% 16% 29% 21% 4 1	487
November	1st to 10th	Day	16% 22% 32% 18% 11% 1	270
	11th to 20th	Day	29% 18% 32% 14% 6% 1	270
	21st to end	Day	25% 21% 41% 11% 2	187
December	1st to 10th	Day	27% 20% 36% 12% 4 2	180
	11th to 20th	Day	24% 22% 38% 14% 2	180
	21st to end	Day	33% 22% 31% 12% 2	261
January	1st to 10th	Night	25% 21% 33% 17% 4	760
	11th to 20th	Night	31% 16% 33% 16% 3	760
	21st to end	Night	28% 17% 30% 23% 3	835
February	1st to 10th	Night	29% 15% 33% 20% 3	720
	11th to 20th	Night	27% 15% 35% 21% 2	703
	21st to end	Night	29% 16% 36% 17% 2	592
March	1st to 10th	Night	23% 14% 35% 23% 4	720
	11th to 20th	Night	22% 14% 33% 24% 7% 1	720
	21st to end	Night	24% 11% 32% 25% 8% 1	792
April	1st to 10th	Night	22% 12% 34% 25% 6% 1	720
	11th to 20th	Night	22% 11% 35% 21% 10% 1	569
	21st to end	Night	18% 9% 34% 31% 7% 1	540
May	1st to 10th	Night	20% 8% 31% 30% 10% 1	540
	11th to 20th	Night	20% 7% 25% 36% 11% 1	531
	21st to end	Night	12% 2 21% 38% 24% 3	584
June	1st to 10th	Night	10% 3 27% 41% 19% 1	388
	11th to 20th	Night	11% 4 25% 44% 16% 1	360
	21st to end	Night	13% 4 22% 41% 19% 1	431
July	1st to 10th	Night	9% 2 13% 42% 30% 4	525
	11th to 20th	Night	9% 2 19% 40% 27% 4	522
	21st to end	Night	9% 2 16% 43% 29% 3	591
August	1st to 10th	Night	9% 2 15% 42% 29% 3	537
	11th to 20th	Night	11% 2 11% 42% 29% 4	534
	21st to end	Night	10% 3 18% 33% 31% 5%	610
September	1st to 10th	Night	14% 4 16% 33% 28% 5%	720
	11th to 20th	Night	11% 2 15% 35% 33% 5%	720
	21st to end	Night	17% 5% 23% 31% 21% 4	720
October	1st to 10th	Night	17% 7% 21% 33% 19% 4	720
	11th to 20th	Night	23% 11% 28% 28% 9% 1	720
	21st to end	Night	27% 12% 28% 27% 6%	792
November	1st to 10th	Night	21% 10% 29% 29% 12% 1	740
	11th to 20th	Night	24% 15% 31% 22% 7% 1	760
	21st to end	Night	29% 15% 31% 19% 6%	760
December	1st to 10th	Night	31% 15% 32% 18% 4	760
	11th to 20th	Night	26% 16% 33% 21% 3 1	760
	21st to end	Night	29% 15% 33% 21% 2	836

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