Old Town Pasadena
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Town Pasadena clear sky chart from 2005-11-06 to 2026-03-18.

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	5% 3 1 3 2 2 2 2 4 6% 70%	140053
	Night	6% 3 1 4 2 2 2 2 3 5% 68%	139005
Transparency	Day	18% 5% 6% 16% 28% 28%	135142
	Night	20% 6% 7% 17% 25% 25%	134889
Seeing	Day	7% 9% 27% 36% 16% 5%	17430
	Night	6% 3 15% 37% 32% 7%	27347

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	10% 3 1 3 2 2 2 2 3 5% 66%	4722
	11th to 20th	Day	5% 2 1 2 1 1 2 2 3 6% 74%	5293
	21st to end	Day	10% 4 2 4 3 3 4 3 4 8% 55%	6500
February	1st to 10th	Day	10% 4 2 4 3 2 2 2 4 6% 61%	2100
	11th to 20th	Day	11% 3 1 4 2 3 3 3 5% 7% 58%	2050
	21st to end	Day	9% 5% 1 4 4 2 3 2 5% 7% 57%	1730
March	1st to 10th	Day	12% 7% 2 5% 4 3 3 3 5% 6% 51%	4618
	11th to 20th	Day	12% 4 1 3 2 3 3 3 5% 6% 58%	4780
	21st to end	Day	10% 4 1 4 3 3 4 4 5% 8% 55%	5969
April	1st to 10th	Day	6% 5% 1 3 3 3 3 3 5% 8% 62%	2400
	11th to 20th	Day	6% 4 1 4 3 3 3 2 4 6% 64%	2369
	21st to end	Day	6% 4 1 3 3 2 2 3 3 6% 67%	2400
May	1st to 10th	Day	7% 4 2 4 3 2 2 2 3 5% 66%	5589
	11th to 20th	Day	3 6% 2 5% 3 2 3 2 4 6% 63%	5841
	21st to end	Day	3 5% 3 6% 2 2 2 2 3 5% 66%	6423
June	1st to 10th	Day	4 4 1 4 4 3 3 2 4 8% 64%	2616
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 6% 74%	2800
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 79%	2800
July	1st to 10th	Day	1 1 2 1 1 1 1 2 3 87%	6319
	11th to 20th	Day	1 1 1 4 3 2 2 2 3 4 78%	6242
	21st to end	Day	1 1 1 2 1 1 1 1 3 4 84%	5977
August	1st to 10th	Day	1 1 1 1 1 1 1 2 4 87%	5346
	11th to 20th	Day	1 3 1 1 1 1 1 2 89%	5364
	21st to end	Day	1 1 1 1 1 2 2 2 4 85%	5984
September	1st to 10th	Day	3 2 1 2 1 1 2 2 3 4 80%	2400
	11th to 20th	Day	2 2 1 2 1 1 2 2 3 5% 80%	2253
	21st to end	Day	2 2 1 1 2 2 2 2 4 5% 78%	2200
October	1st to 10th	Day	2 2 1 2 2 1 2 2 2 3 81%	3600
	11th to 20th	Day	3 1 1 4 2 1 2 2 3 6% 75%	3367
	21st to end	Day	3 2 1 4 2 2 3 3 4 6% 70%	3478
November	1st to 10th	Day	3 2 1 3 3 2 2 2 4 5% 73%	1854
	11th to 20th	Day	6% 3 1 3 2 2 2 2 4 6% 68%	1890
	21st to end	Day	5% 3 1 4 4 3 4 3 5% 7% 63%	1890
December	1st to 10th	Day	4 2 1 3 3 3 3 3 4 7% 67%	3040
	11th to 20th	Day	11% 3 1 4 3 3 4 4 5% 9% 52%	3499
	21st to end	Day	11% 5% 2 5% 4 3 3 3 5% 9% 50%	4350
January	1st to 10th	Night	12% 4 1 4 2 2 2 2 3 4 64%	8172
	11th to 20th	Night	4 2 1 4 2 2 2 2 3 7% 71%	7673
	21st to end	Night	11% 5% 2 5% 3 3 3 3 4 6% 55%	7747
February	1st to 10th	Night	10% 5% 1 4 3 2 2 2 3 4 63%	2520
	11th to 20th	Night	10% 4 2 4 3 3 3 2 4 6% 60%	2458
	21st to end	Night	11% 4 1 4 3 3 3 3 3 5% 60%	2076
March	1st to 10th	Night	13% 6% 1 3 3 2 3 3 5% 7% 53%	5496
	11th to 20th	Night	8% 3 1 4 3 3 4 3 5% 6% 60%	5369
	21st to end	Night	10% 6% 2 5% 3 3 3 2 4 6% 56%	5006
April	1st to 10th	Night	6% 5% 2 4 3 3 3 3 3 6% 63%	2000
	11th to 20th	Night	5% 6% 1 3 2 2 3 3 3 6% 66%	1979
	21st to end	Night	4 6% 1 3 3 3 3 3 4 6% 64%	2000
May	1st to 10th	Night	7% 7% 2 8% 4 1 2 2 3 5% 60%	4404
	11th to 20th	Night	4 6% 2 6% 3 2 2 2 3 5% 64%	4074
	21st to end	Night	2 8% 2 6% 2 1 1 1 2 4 70%	4474
June	1st to 10th	Night	2 4 1 3 4 3 3 3 5% 6% 65%	1777
	11th to 20th	Night	2 3 1 5% 4 2 2 2 3 4 74%	1600
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 76%	1600
July	1st to 10th	Night	1 1 3 2 1 1 1 1 3 87%	3640
	11th to 20th	Night	1 1 5% 5% 3 2 2 3 4 75%	3704
	21st to end	Night	1 1 1 3 1 1 1 2 3 4 82%	4899
August	1st to 10th	Night	1 1 1 1 2 2 2 1 3 4 82%	4486
	11th to 20th	Night	1 1 6% 2 1 1 1 2 3 82%	4506
	21st to end	Night	1 1 2 2 2 2 2 3 4 82%	5015
September	1st to 10th	Night	3 2 1 3 2 2 2 1 3 4 78%	2000
	11th to 20th	Night	2 2 1 3 1 1 2 1 2 3 81%	2145
	21st to end	Night	1 2 2 2 1 1 2 2 5% 81%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 1 2 2 83%	3600
	11th to 20th	Night	3 2 2 10% 2 1 2 2 3 5% 68%	3786
	21st to end	Night	3 2 1 4 2 2 2 1 2 4 77%	4327
November	1st to 10th	Night	4 3 1 2 3 2 2 3 3 5% 73%	2665
	11th to 20th	Night	6% 3 1 3 2 2 2 2 3 6% 70%	2730
	21st to end	Night	6% 3 1 4 3 3 3 4 5% 7% 62%	2730
December	1st to 10th	Night	5% 2 1 4 3 2 3 2 4 6% 69%	4370
	11th to 20th	Night	11% 3 2 4 3 3 4 4 5% 7% 55%	4941
	21st to end	Night	10% 6% 2 5% 3 3 4 4 6% 9% 49%	6836

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	23% 3 7% 15% 27% 24%	4723
	11th to 20th	Day	15% 3 9% 13% 19% 42%	5293
	21st to end	Day	28% 4 4 16% 22% 26%	6500
February	1st to 10th	Day	26% 3 6% 14% 24% 27%	2100
	11th to 20th	Day	24% 5% 7% 18% 25% 22%	2060
	21st to end	Day	25% 3 8% 19% 24% 20%	1730
March	1st to 10th	Day	30% 5% 9% 16% 23% 16%	4618
	11th to 20th	Day	26% 6% 10% 26% 19% 13%	4780
	21st to end	Day	26% 6% 13% 29% 18% 9%	5969
April	1st to 10th	Day	21% 5% 11% 25% 25% 13%	2400
	11th to 20th	Day	20% 4 9% 21% 29% 17%	2372
	21st to end	Day	18% 5% 8% 27% 31% 12%	2400
May	1st to 10th	Day	21% 6% 8% 17% 32% 16%	5589
	11th to 20th	Day	17% 14% 6% 21% 34% 9%	5844
	21st to end	Day	17% 11% 5% 24% 35% 9%	6424
June	1st to 10th	Day	21% 7% 9% 20% 30% 14%	2616
	11th to 20th	Day	15% 4 5% 14% 31% 31%	2800
	21st to end	Day	14% 4 3 8% 30% 41%	2800
July	1st to 10th	Day	7% 2 2 5% 22% 62%	6251
	11th to 20th	Day	14% 3 3 11% 35% 35%	6167
	21st to end	Day	7% 3 4 17% 35% 35%	5805
August	1st to 10th	Day	8% 2 2 13% 40% 36%	3890
	11th to 20th	Day	5% 2 2 11% 32% 48%	3880
	21st to end	Day	6% 2 4 8% 30% 49%	4311
September	1st to 10th	Day	10% 3 4 9% 34% 40%	2400
	11th to 20th	Day	11% 4 2 12% 36% 35%	2253
	21st to end	Day	11% 4 5% 14% 32% 34%	2200
October	1st to 10th	Day	10% 3 3 11% 18% 55%	3600
	11th to 20th	Day	13% 3 4 15% 34% 31%	3367
	21st to end	Day	16% 4 3 10% 27% 41%	3478
November	1st to 10th	Day	14% 4 5% 17% 26% 35%	1854
	11th to 20th	Day	19% 3 6% 15% 20% 37%	1890
	21st to end	Day	21% 5% 8% 12% 23% 30%	1890
December	1st to 10th	Day	19% 4 5% 17% 22% 33%	3040
	11th to 20th	Day	29% 3 7% 15% 27% 19%	3498
	21st to end	Day	32% 5% 10% 25% 20% 9%	4350
January	1st to 10th	Night	27% 3 6% 14% 28% 23%	8172
	11th to 20th	Night	16% 4 7% 15% 15% 43%	7673
	21st to end	Night	30% 6% 4 15% 20% 25%	7747
February	1st to 10th	Night	28% 3 6% 15% 22% 27%	2520
	11th to 20th	Night	26% 5% 7% 18% 22% 22%	2459
	21st to end	Night	27% 5% 7% 17% 21% 23%	2076
March	1st to 10th	Night	29% 6% 10% 19% 24% 13%	5496
	11th to 20th	Night	24% 7% 12% 27% 17% 13%	5369
	21st to end	Night	27% 9% 15% 24% 20% 6%	5006
April	1st to 10th	Night	25% 6% 10% 24% 21% 14%	2000
	11th to 20th	Night	20% 7% 9% 22% 26% 16%	1979
	21st to end	Night	22% 8% 10% 25% 27% 9%	2000
May	1st to 10th	Night	27% 9% 7% 20% 31% 7%	4401
	11th to 20th	Night	18% 17% 7% 25% 28% 6%	4075
	21st to end	Night	14% 12% 6% 34% 26% 8%	4474
June	1st to 10th	Night	20% 10% 13% 20% 25% 12%	1777
	11th to 20th	Night	15% 7% 6% 14% 34% 23%	1600
	21st to end	Night	17% 5% 6% 7% 31% 34%	1600
July	1st to 10th	Night	8% 3 2 8% 22% 57%	3600
	11th to 20th	Night	15% 5% 4 12% 34% 30%	3656
	21st to end	Night	9% 4 4 24% 34% 25%	4759
August	1st to 10th	Night	10% 3 3 15% 41% 28%	3256
	11th to 20th	Night	7% 4 3 15% 35% 36%	3256
	21st to end	Night	8% 5% 3 9% 39% 36%	3608
September	1st to 10th	Night	15% 5% 4 8% 31% 37%	2000
	11th to 20th	Night	11% 6% 3 11% 35% 33%	2145
	21st to end	Night	12% 5% 5% 14% 35% 30%	2200
October	1st to 10th	Night	11% 3 2 11% 20% 52%	3600
	11th to 20th	Night	21% 5% 3 14% 28% 30%	3786
	21st to end	Night	14% 3 3 9% 27% 43%	4327
November	1st to 10th	Night	15% 4 6% 16% 25% 33%	2665
	11th to 20th	Night	19% 3 7% 13% 21% 37%	2730
	21st to end	Night	23% 4 7% 17% 18% 30%	2730
December	1st to 10th	Night	19% 4 6% 14% 23% 33%	4370
	11th to 20th	Night	29% 3 9% 14% 23% 21%	4941
	21st to end	Night	29% 8% 13% 25% 16% 9%	6836

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	15% 5% 29% 35% 14% 2	220
	11th to 20th	Day	10% 11% 32% 30% 14% 3	397
	21st to end	Day	9% 14% 29% 35% 12% 1	594
February	1st to 10th	Day	14% 10% 29% 32% 12% 3	540
	11th to 20th	Day	13% 15% 32% 30% 7% 1	535
	21st to end	Day	9% 17% 33% 30% 9% 2	446
March	1st to 10th	Day	13% 19% 33% 25% 10% 1	540
	11th to 20th	Day	13% 15% 34% 26% 10% 3	529
	21st to end	Day	12% 19% 33% 26% 9% 1	550
April	1st to 10th	Day	9% 14% 34% 26% 13% 3	500
	11th to 20th	Day	9% 18% 37% 24% 9% 3	492
	21st to end	Day	5% 15% 32% 29% 14% 4	500
May	1st to 10th	Day	12% 13% 34% 27% 11% 3	503
	11th to 20th	Day	7% 12% 33% 32% 13% 3	528
	21st to end	Day	8% 10% 35% 32% 13% 3	573
June	1st to 10th	Day	4 6% 29% 39% 17% 5%	530
	11th to 20th	Day	5% 5% 28% 41% 15% 6%	530
	21st to end	Day	2 3 27% 44% 16% 8%	530
July	1st to 10th	Day	1 1 17% 48% 24% 8%	526
	11th to 20th	Day	1 1 9% 50% 26% 12%	524
	21st to end	Day	2 1 13% 49% 24% 11%	575
August	1st to 10th	Day	2 1 18% 50% 20% 9%	527
	11th to 20th	Day	2 2 15% 47% 25% 10%	517
	21st to end	Day	1 2 20% 48% 23% 6%	583
September	1st to 10th	Day	2 2 25% 44% 19% 8%	530
	11th to 20th	Day	2 6% 30% 40% 17% 4	530
	21st to end	Day	2 6% 32% 34% 20% 6%	530
October	1st to 10th	Day	4 8% 31% 33% 19% 5%	530
	11th to 20th	Day	4 10% 28% 37% 18% 4	530
	21st to end	Day	5% 8% 26% 35% 20% 6%	539
November	1st to 10th	Day	4 6% 21% 37% 25% 8%	330
	11th to 20th	Day	9% 7% 21% 38% 22% 4	330
	21st to end	Day	5% 6% 25% 39% 19% 5%	330
December	1st to 10th	Day	8% 4 21% 41% 20% 6%	330
	11th to 20th	Day	5% 5% 24% 35% 25% 5%	330
	21st to end	Day	18% 8% 27% 33% 13% 1	302
January	1st to 10th	Night	11% 3 21% 42% 19% 3	840
	11th to 20th	Night	8% 3 21% 43% 23% 2	840
	21st to end	Night	13% 4 20% 42% 19% 2	924
February	1st to 10th	Night	11% 4 18% 41% 22% 4	840
	11th to 20th	Night	10% 5% 22% 38% 21% 4	824
	21st to end	Night	11% 4 22% 42% 19% 2	692
March	1st to 10th	Night	9% 5% 25% 42% 18% 2	840
	11th to 20th	Night	8% 5% 18% 40% 24% 5%	828
	21st to end	Night	8% 5% 21% 42% 21% 3	880
April	1st to 10th	Night	7% 4 17% 45% 24% 3	800
	11th to 20th	Night	5% 7% 19% 40% 26% 3	791
	21st to end	Night	4 6% 20% 37% 27% 6%	800
May	1st to 10th	Night	6% 5% 20% 40% 26% 4	735
	11th to 20th	Night	5% 4 17% 39% 30% 4	591
	21st to end	Night	4 3 21% 38% 30% 3	650
June	1st to 10th	Night	2 2 17% 38% 39% 2	577
	11th to 20th	Night	3 1 6% 34% 52% 5%	400
	21st to end	Night	2 2 4 36% 50% 7%	400
July	1st to 10th	Night	2 1 4 16% 65% 12%	390
	11th to 20th	Night	1 3 15% 66% 14%	413
	21st to end	Night	2 1 3 28% 51% 15%	840
August	1st to 10th	Night	2 1 5% 28% 51% 13%	796
	11th to 20th	Night	2 1 5% 21% 54% 18%	792
	21st to end	Night	2 1 4 25% 51% 17%	880
September	1st to 10th	Night	3 1 7% 28% 47% 14%	800
	11th to 20th	Night	3 2 8% 34% 42% 11%	800
	21st to end	Night	2 2 8% 35% 45% 8%	800
October	1st to 10th	Night	4 2 9% 42% 39% 4	800
	11th to 20th	Night	6% 2 13% 38% 35% 7%	800
	21st to end	Night	4 2 14% 36% 34% 10%	880
November	1st to 10th	Night	5% 2 13% 41% 32% 8%	820
	11th to 20th	Night	5% 3 13% 44% 28% 7%	840
	21st to end	Night	7% 2 17% 45% 25% 4	840
December	1st to 10th	Night	7% 4 19% 43% 23% 5%	840
	11th to 20th	Night	11% 5% 17% 44% 22% 2	840
	21st to end	Night	11% 6% 23% 39% 17% 3	924

Page generated at 2026-03-14 23:23:57. Version ac07

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