Hawthorne
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hawthorne clear sky chart from 2005-11-06 to 2026-08-15.

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	2 3 1 3 3 3 3 3 4 6% 69%	152187
	Night	2 3 1 4 3 3 3 3 5% 7% 67%	147122
Transparency	Day	16% 5% 9% 18% 27% 26%	147200
	Night	17% 5% 10% 22% 27% 19%	143089
Seeing	Day	7% 14% 28% 30% 17% 5%	17362
	Night	4 6% 25% 39% 21% 4	26762

By Month

Percentage Cloud Cover SW (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	1 3 2 5% 3 3 3 3 5% 9% 62%	4722
	11th to 20th	Day	1 3 1 4 3 3 3 2 4 6% 71%	4740
	21st to end	Day	2 6% 2 5% 3 4 4 4 6% 9% 55%	5798
February	1st to 10th	Day	2 4 1 5% 4 3 3 4 6% 7% 60%	2019
	11th to 20th	Day	2 4 2 4 4 5% 4 3 5% 10% 57%	2050
	21st to end	Day	1 2 2 4 3 4 5% 4 7% 9% 60%	1730
March	1st to 10th	Day	2 7% 3 6% 4 4 5% 4 6% 9% 50%	4618
	11th to 20th	Day	2 3 2 4 2 3 4 4 6% 9% 62%	4809
	21st to end	Day	1 5% 2 5% 3 3 4 4 6% 9% 57%	6101
April	1st to 10th	Day	2 4 1 4 3 4 3 5% 6% 8% 61%	2520
	11th to 20th	Day	2 4 1 4 3 3 4 3 6% 7% 62%	2489
	21st to end	Day	3 4 2 4 3 3 4 3 5% 7% 62%	2520
May	1st to 10th	Day	3 3 1 4 3 3 3 3 4 5% 71%	7586
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 5% 77%	7596
	21st to end	Day	3 3 1 4 3 3 3 3 4 5% 68%	8990
June	1st to 10th	Day	3 3 1 4 4 3 4 4 5% 7% 64%	2940
	11th to 20th	Day	2 2 1 2 2 2 3 2 4 5% 76%	2940
	21st to end	Day	2 2 1 2 2 2 3 2 3 4 76%	2940
July	1st to 10th	Day	2 1 2 2 2 2 2 2 3 83%	8219
	11th to 20th	Day	3 2 1 3 3 3 3 3 4 6% 68%	8220
	21st to end	Day	2 2 1 3 3 3 4 4 5% 5% 71%	8922
August	1st to 10th	Day	1 1 1 2 2 3 3 2 3 4 78%	5611
	11th to 20th	Day	2 1 1 2 1 2 2 2 2 3 83%	5413
	21st to end	Day	2 2 1 3 2 3 3 2 3 4 75%	5984
September	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 78%	2400
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 5% 77%	2300
	21st to end	Day	1 2 1 2 2 1 2 2 3 5% 81%	2200
October	1st to 10th	Day	3 2 1 2 2 2 3 2 3 5% 76%	3514
	11th to 20th	Day	1 2 2 2 1 2 2 3 4 81%	3261
	21st to end	Day	2 2 1 2 2 1 3 2 5% 6% 75%	3365
November	1st to 10th	Day	2 2 1 3 2 2 3 3 5% 7% 69%	1854
	11th to 20th	Day	3 2 1 2 2 2 3 3 6% 8% 68%	1890
	21st to end	Day	1 4 2 4 3 3 3 3 5% 8% 66%	1890
December	1st to 10th	Day	1 2 1 2 2 2 3 3 5% 7% 74%	2836
	11th to 20th	Day	1 3 1 4 4 4 5% 4 7% 10% 57%	3108
	21st to end	Day	1 6% 2 5% 4 4 5% 4 6% 10% 53%	4092
January	1st to 10th	Night	1 5% 2 5% 3 3 3 3 5% 8% 63%	8172
	11th to 20th	Night	1 3 2 4 2 3 3 3 5% 7% 67%	8220
	21st to end	Night	2 6% 2 5% 3 4 4 4 6% 8% 56%	8411
February	1st to 10th	Night	1 6% 2 5% 3 3 4 3 5% 7% 60%	2599
	11th to 20th	Night	2 5% 2 5% 4 3 4 3 6% 9% 58%	2458
	21st to end	Night	3 2 5% 3 3 4 3 6% 8% 63%	2076
March	1st to 10th	Night	2 6% 3 6% 4 4 4 3 5% 7% 56%	5496
	11th to 20th	Night	1 4 2 4 3 3 4 3 5% 7% 63%	5394
	21st to end	Night	2 5% 2 4 3 3 4 4 5% 8% 60%	5116
April	1st to 10th	Night	2 4 2 4 3 3 3 3 5% 7% 63%	2100
	11th to 20th	Night	2 5% 1 4 3 3 3 3 4 6% 66%	2079
	21st to end	Night	4 6% 1 3 3 2 3 3 3 6% 65%	2100
May	1st to 10th	Night	5% 3 1 3 2 2 3 2 3 6% 69%	5363
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 5% 75%	5281
	21st to end	Night	4 3 1 4 3 2 3 2 4 6% 67%	5181
June	1st to 10th	Night	3 3 1 3 3 3 4 4 5% 6% 66%	1680
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 5% 79%	1680
	21st to end	Night	1 2 1 2 3 3 2 2 4 4 77%	1680
July	1st to 10th	Night	1 1 2 2 2 2 2 3 4 82%	4740
	11th to 20th	Night	3 3 2 4 4 3 4 3 5% 7% 62%	4740
	21st to end	Night	1 2 1 3 2 2 3 3 4 6% 72%	5153
August	1st to 10th	Night	2 2 1 3 2 2 3 2 3 5% 77%	4417
	11th to 20th	Night	2 1 1 2 2 2 2 1 3 5% 79%	4546
	21st to end	Night	4 2 1 3 2 2 2 2 4 4 74%	5015
September	1st to 10th	Night	2 1 1 2 2 2 3 2 4 4 77%	2000
	11th to 20th	Night	3 1 1 3 2 2 3 3 4 5% 73%	2099
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 77%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 76%	3681
	11th to 20th	Night	2 2 1 2 1 1 2 1 3 5% 80%	3897
	21st to end	Night	1 3 1 2 1 1 2 2 3 5% 77%	4438
November	1st to 10th	Night	2 3 1 3 2 2 2 3 3 7% 72%	2665
	11th to 20th	Night	1 3 1 3 2 2 4 3 5% 5% 71%	2730
	21st to end	Night	2 3 1 4 3 2 4 3 5% 6% 67%	2730
December	1st to 10th	Night	1 2 1 3 2 2 3 3 4 7% 72%	4563
	11th to 20th	Night	1 4 2 4 3 4 4 4 7% 13% 55%	5316
	21st to end	Night	1 5% 2 6% 5% 6% 7% 6% 8% 11% 43%	7106

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	18% 7% 23% 36% 15% 1	4723
	11th to 20th	Day	17% 5% 8% 21% 38% 11%	4740
	21st to end	Day	25% 6% 12% 26% 27% 5%	5798
February	1st to 10th	Day	21% 7% 13% 25% 22% 11%	2019
	11th to 20th	Day	23% 6% 12% 24% 27% 8%	2060
	21st to end	Day	20% 7% 17% 26% 23% 7%	1730
March	1st to 10th	Day	27% 8% 10% 25% 23% 7%	4618
	11th to 20th	Day	15% 7% 13% 34% 25% 6%	4809
	21st to end	Day	21% 8% 15% 23% 26% 8%	6101
April	1st to 10th	Day	23% 7% 11% 25% 23% 10%	2520
	11th to 20th	Day	21% 6% 11% 23% 27% 13%	2492
	21st to end	Day	21% 7% 11% 26% 27% 8%	2520
May	1st to 10th	Day	16% 5% 6% 21% 35% 17%	7586
	11th to 20th	Day	11% 4 6% 17% 38% 25%	7599
	21st to end	Day	18% 4 7% 18% 35% 19%	8992
June	1st to 10th	Day	20% 6% 9% 17% 30% 19%	2940
	11th to 20th	Day	11% 4 7% 14% 30% 35%	2940
	21st to end	Day	13% 3 6% 8% 26% 44%	2940
July	1st to 10th	Day	9% 2 3 5% 13% 68%	8151
	11th to 20th	Day	16% 3 4 9% 14% 54%	8145
	21st to end	Day	14% 4 5% 11% 21% 45%	8723
August	1st to 10th	Day	15% 3 5% 7% 23% 46%	4104
	11th to 20th	Day	10% 2 4 5% 15% 66%	3929
	21st to end	Day	10% 2 3 6% 25% 55%	4311
September	1st to 10th	Day	12% 3 5% 7% 25% 48%	2400
	11th to 20th	Day	12% 3 7% 13% 33% 32%	2300
	21st to end	Day	10% 3 6% 13% 34% 35%	2200
October	1st to 10th	Day	12% 3 6% 11% 32% 36%	3514
	11th to 20th	Day	9% 3 4 15% 44% 26%	3261
	21st to end	Day	11% 4 6% 23% 42% 15%	3365
November	1st to 10th	Day	16% 5% 12% 23% 32% 12%	1854
	11th to 20th	Day	15% 7% 11% 25% 33% 9%	1890
	21st to end	Day	19% 5% 11% 27% 30% 9%	1890
December	1st to 10th	Day	12% 5% 13% 31% 32% 6%	2836
	11th to 20th	Day	21% 10% 17% 30% 18% 5%	3108
	21st to end	Day	23% 10% 26% 23% 16% 2	4092
January	1st to 10th	Night	20% 7% 20% 35% 16% 2	8172
	11th to 20th	Night	17% 5% 10% 26% 36% 6%	8220
	21st to end	Night	24% 7% 10% 30% 26% 3	8411
February	1st to 10th	Night	24% 6% 11% 26% 23% 9%	2599
	11th to 20th	Night	22% 7% 12% 26% 26% 7%	2459
	21st to end	Night	20% 6% 16% 29% 21% 8%	2076
March	1st to 10th	Night	25% 6% 9% 26% 24% 9%	5496
	11th to 20th	Night	17% 7% 11% 34% 25% 6%	5394
	21st to end	Night	19% 7% 16% 30% 24% 4	5116
April	1st to 10th	Night	22% 6% 11% 30% 21% 10%	2100
	11th to 20th	Night	20% 6% 9% 28% 26% 11%	2079
	21st to end	Night	22% 6% 10% 29% 27% 5%	2100
May	1st to 10th	Night	17% 4 7% 21% 37% 14%	5361
	11th to 20th	Night	12% 4 7% 15% 42% 20%	5282
	21st to end	Night	17% 5% 7% 17% 40% 13%	5182
June	1st to 10th	Night	19% 6% 9% 21% 28% 17%	1680
	11th to 20th	Night	10% 4 5% 15% 34% 32%	1680
	21st to end	Night	12% 3 5% 11% 27% 42%	1680
July	1st to 10th	Night	8% 3 3 7% 15% 64%	4700
	11th to 20th	Night	19% 4 5% 7% 18% 47%	4692
	21st to end	Night	12% 4 5% 15% 21% 43%	5040
August	1st to 10th	Night	15% 3 5% 12% 21% 44%	3241
	11th to 20th	Night	9% 2 3 7% 17% 62%	3296
	21st to end	Night	11% 2 3 7% 27% 51%	3608
September	1st to 10th	Night	11% 3 6% 9% 27% 45%	2000
	11th to 20th	Night	14% 4 7% 14% 29% 32%	2099
	21st to end	Night	13% 3 4 13% 33% 35%	2200
October	1st to 10th	Night	12% 3 6% 11% 31% 36%	3681
	11th to 20th	Night	9% 3 5% 15% 46% 21%	3897
	21st to end	Night	11% 3 5% 22% 41% 18%	4438
November	1st to 10th	Night	14% 6% 10% 29% 29% 13%	2665
	11th to 20th	Night	16% 6% 11% 28% 32% 8%	2730
	21st to end	Night	19% 5% 12% 27% 30% 8%	2730
December	1st to 10th	Night	14% 4 14% 31% 27% 10%	4563
	11th to 20th	Night	20% 8% 19% 32% 17% 4	5316
	21st to end	Night	29% 11% 20% 20% 16% 2	7106

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	5% 18% 34% 31% 10% 1	220
	11th to 20th	Day	4 13% 28% 31% 17% 7%	220
	21st to end	Day	6% 10% 26% 33% 20% 4	454
February	1st to 10th	Day	5% 18% 32% 25% 13% 6%	499
	11th to 20th	Day	8% 15% 35% 29% 11% 1	535
	21st to end	Day	10% 16% 35% 31% 8%	446
March	1st to 10th	Day	11% 16% 35% 28% 8% 2	540
	11th to 20th	Day	8% 19% 35% 22% 12% 3	540
	21st to end	Day	11% 21% 34% 25% 8% 2	594
April	1st to 10th	Day	9% 21% 30% 24% 13% 3	540
	11th to 20th	Day	12% 22% 31% 22% 10% 3	532
	21st to end	Day	13% 18% 28% 26% 11% 4	540
May	1st to 10th	Day	10% 24% 29% 22% 12% 3	543
	11th to 20th	Day	9% 20% 29% 25% 14% 4	568
	21st to end	Day	12% 14% 25% 25% 18% 6%	617
June	1st to 10th	Day	8% 14% 27% 27% 18% 6%	570
	11th to 20th	Day	7% 17% 30% 24% 18% 5%	570
	21st to end	Day	3 14% 29% 31% 18% 5%	570
July	1st to 10th	Day	4 10% 24% 34% 23% 6%	566
	11th to 20th	Day	4 7% 20% 34% 27% 8%	564
	21st to end	Day	4 6% 24% 34% 23% 9%	619
August	1st to 10th	Day	5% 8% 27% 31% 20% 8%	567
	11th to 20th	Day	5% 11% 25% 28% 24% 7%	534
	21st to end	Day	3 7% 26% 36% 21% 8%	583
September	1st to 10th	Day	3 8% 25% 36% 22% 6%	530
	11th to 20th	Day	4 11% 28% 36% 17% 4	530
	21st to end	Day	4 10% 24% 39% 20% 3	530
October	1st to 10th	Day	5% 15% 27% 31% 19% 3	530
	11th to 20th	Day	5% 10% 28% 33% 20% 3	530
	21st to end	Day	6% 11% 24% 31% 23% 5%	465
November	1st to 10th	Day	5% 9% 24% 32% 25% 6%	330
	11th to 20th	Day	5% 13% 25% 34% 17% 6%	330
	21st to end	Day	5% 7% 28% 36% 21% 2	330
December	1st to 10th	Day	8% 8% 21% 33% 20% 10%	264
	11th to 20th	Day	5% 10% 36% 25% 17% 7%	220
	21st to end	Day	5% 21% 26% 36% 10% 2	242
January	1st to 10th	Night	4 5% 36% 31% 15% 8%	840
	11th to 20th	Night	2 6% 28% 39% 20% 6%	840
	21st to end	Night	4 6% 27% 39% 18% 6%	924
February	1st to 10th	Night	5% 8% 30% 36% 17% 4	840
	11th to 20th	Night	4 9% 31% 36% 16% 4	824
	21st to end	Night	3 7% 38% 38% 11% 3	692
March	1st to 10th	Night	5% 11% 32% 35% 15% 3	840
	11th to 20th	Night	3 9% 30% 34% 19% 5%	840
	21st to end	Night	5% 11% 34% 34% 15% 2	924
April	1st to 10th	Night	6% 10% 28% 38% 14% 3	840
	11th to 20th	Night	5% 13% 29% 34% 15% 4	831
	21st to end	Night	6% 11% 29% 35% 17% 3	840
May	1st to 10th	Night	7% 12% 29% 35% 15% 2	646
	11th to 20th	Night	5% 9% 30% 37% 18% 2	619
	21st to end	Night	7% 4 26% 40% 20% 2	455
June	1st to 10th	Night	2 4 21% 47% 24% 2	420
	11th to 20th	Night	2 4 21% 49% 21% 2	420
	21st to end	Night	1 3 19% 53% 23% 1	420
July	1st to 10th	Night	2 1 15% 53% 28% 2	410
	11th to 20th	Night	3 10% 58% 27% 1	408
	21st to end	Night	2 1 10% 55% 31% 1	460
August	1st to 10th	Night	4 2 18% 46% 27% 3	757
	11th to 20th	Night	3 2 15% 48% 28% 3	808
	21st to end	Night	4 2 15% 41% 36% 3	880
September	1st to 10th	Night	3 3 15% 42% 31% 6%	800
	11th to 20th	Night	3 4 16% 43% 28% 6%	800
	21st to end	Night	3 4 16% 39% 32% 6%	800
October	1st to 10th	Night	4 4 17% 47% 22% 7%	800
	11th to 20th	Night	3 5% 19% 41% 26% 6%	800
	21st to end	Night	3 4 24% 33% 27% 10%	880
November	1st to 10th	Night	4 5% 21% 37% 26% 8%	820
	11th to 20th	Night	3 4 29% 40% 19% 5%	840
	21st to end	Night	3 5% 28% 37% 22% 5%	840
December	1st to 10th	Night	4 5% 28% 37% 19% 6%	840
	11th to 20th	Night	5% 6% 31% 37% 17% 5%	840
	21st to end	Night	4 11% 35% 34% 15% 2	924

Page generated at 2026-08-12 00:37:19. Version ac07

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