Eastridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eastridge Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	11% 6% 2 5% 4 4 4 3 5% 7% 49%	109516
	Night	10% 6% 2 5% 3 3 3 3 5% 6% 54%	109712
Transparency	Day	34% 5% 10% 24% 21% 5%	113207
	Night	31% 4 9% 24% 24% 8%	114148
Seeing	Day	13% 11% 26% 28% 18% 4	15719
	Night	12% 10% 30% 32% 14% 1	25746

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	12% 10% 4 6% 4 3 3 2 4 5% 46%	4077
	11th to 20th	Day	6% 6% 2 6% 3 4 3 3 4 5% 59%	4080
	21st to end	Day	19% 9% 2 4 2 2 2 2 3 5% 48%	4488
February	1st to 10th	Day	17% 9% 3 7% 5% 4 3 3 4 5% 42%	1800
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 4 6% 43%	1924
	21st to end	Day	18% 9% 2 6% 3 3 3 2 4 5% 45%	1716
March	1st to 10th	Day	15% 6% 2 6% 4 3 3 2 4 5% 50%	3546
	11th to 20th	Day	12% 8% 2 6% 3 4 5% 3 5% 5% 47%	3491
	21st to end	Day	16% 9% 3 7% 5% 4 4 4 5% 6% 38%	3814
April	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 48%	2090
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 5% 6% 46%	2250
	21st to end	Day	15% 7% 2 7% 5% 4 4 3 4 6% 42%	2380
May	1st to 10th	Day	16% 7% 2 7% 5% 4 4 3 4 5% 42%	4082
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 6% 7% 45%	4082
	21st to end	Day	15% 7% 2 7% 5% 5% 4 4 6% 7% 37%	4477
June	1st to 10th	Day	9% 5% 2 5% 4 3 5% 4 7% 9% 48%	2470
	11th to 20th	Day	6% 5% 1 4 4 4 5% 4 6% 10% 50%	2470
	21st to end	Day	4 3 1 4 4 3 4 4 6% 9% 58%	2470
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 5% 6% 9% 49%	4120
	11th to 20th	Day	3 3 2 4 4 4 4 4 6% 8% 58%	4120
	21st to end	Day	6% 3 2 5% 5% 5% 5% 6% 8% 11% 45%	4465
August	1st to 10th	Day	3 3 1 4 3 4 4 4 6% 9% 60%	4104
	11th to 20th	Day	8% 3 1 4 3 4 4 4 7% 8% 54%	4041
	21st to end	Day	4 3 1 4 4 4 4 4 7% 9% 54%	4249
September	1st to 10th	Day	11% 4 2 5% 4 3 4 3 5% 7% 53%	2119
	11th to 20th	Day	11% 4 2 6% 3 5% 5% 3 5% 6% 49%	2090
	21st to end	Day	7% 2 1 4 3 4 4 3 6% 6% 59%	2090
October	1st to 10th	Day	5% 4 2 5% 3 4 4 3 4 6% 59%	1967
	11th to 20th	Day	11% 4 2 5% 2 2 3 2 3 4 62%	1900
	21st to end	Day	16% 5% 2 4 2 2 3 2 4 5% 53%	2090
November	1st to 10th	Day	11% 4 1 5% 3 3 3 3 5% 7% 54%	1958
	11th to 20th	Day	8% 5% 2 4 3 3 3 2 4 5% 60%	1877
	21st to end	Day	14% 5% 1 4 3 3 3 2 4 7% 54%	1800
December	1st to 10th	Day	14% 6% 3 5% 3 3 2 3 4 6% 52%	2940
	11th to 20th	Day	14% 7% 3 5% 4 3 4 3 4 7% 46%	3391
	21st to end	Day	17% 10% 3 7% 3 4 4 3 4 5% 40%	4488
January	1st to 10th	Night	13% 8% 2 8% 4 3 3 3 4 6% 46%	5888
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 5% 62%	5900
	21st to end	Night	20% 9% 2 4 2 2 3 2 4 5% 47%	6490
February	1st to 10th	Night	13% 8% 3 6% 3 3 3 3 5% 5% 48%	2600
	11th to 20th	Night	14% 8% 2 7% 4 4 4 3 4 6% 45%	2378
	21st to end	Night	12% 9% 2 6% 4 3 3 2 4 5% 51%	1912
March	1st to 10th	Night	17% 7% 2 6% 3 3 3 3 5% 5% 46%	3529
	11th to 20th	Night	13% 6% 2 4 3 3 3 2 4 4 56%	3497
	21st to end	Night	12% 7% 2 6% 4 4 4 3 6% 6% 45%	3817
April	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 5% 6% 51%	2090
	11th to 20th	Night	12% 5% 1 4 4 3 4 3 5% 6% 52%	1877
	21st to end	Night	13% 6% 2 5% 4 3 4 3 5% 7% 49%	1796
May	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 9% 42%	2823
	11th to 20th	Night	13% 5% 2 5% 4 4 4 4 5% 7% 46%	2823
	21st to end	Night	12% 4 2 5% 4 5% 4 4 6% 8% 45%	3090
June	1st to 10th	Night	4 3 1 5% 3 3 3 5% 7% 9% 58%	1710
	11th to 20th	Night	4 3 1 3 3 3 4 4 7% 8% 61%	1710
	21st to end	Night	3 2 1 3 3 3 3 3 5% 8% 67%	1710
July	1st to 10th	Night	7% 5% 2 5% 4 4 3 3 5% 9% 54%	2850
	11th to 20th	Night	3 2 1 3 3 2 3 2 5% 9% 67%	2850
	21st to end	Night	4 3 1 4 3 3 4 4 6% 10% 58%	3098
August	1st to 10th	Night	2 1 1 2 3 3 3 4 5% 9% 68%	2836
	11th to 20th	Night	4 3 1 3 2 3 3 3 6% 9% 62%	2798
	21st to end	Night	2 2 1 4 3 3 4 3 5% 8% 65%	3407
September	1st to 10th	Night	8% 5% 2 4 3 3 3 2 4 7% 59%	2057
	11th to 20th	Night	10% 4 1 4 4 5% 3 3 5% 7% 54%	2090
	21st to end	Night	6% 3 1 3 3 3 3 3 4 6% 63%	2090
October	1st to 10th	Night	6% 3 2 4 3 3 4 3 4 5% 63%	2192
	11th to 20th	Night	11% 4 2 3 2 2 3 2 3 4 63%	2280
	21st to end	Night	16% 5% 1 4 3 2 3 2 3 4 58%	2508
November	1st to 10th	Night	10% 5% 2 4 3 3 3 2 4 5% 60%	2340
	11th to 20th	Night	7% 5% 2 4 4 3 3 3 4 5% 60%	2520
	21st to end	Night	12% 6% 2 4 3 3 3 3 4 6% 55%	2600
December	1st to 10th	Night	10% 8% 2 5% 2 3 3 2 4 6% 54%	4250
	11th to 20th	Night	11% 7% 3 8% 4 3 3 2 3 3 52%	4816
	21st to end	Night	16% 9% 3 5% 3 3 3 3 4 5% 47%	6490

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	41% 3 10% 24% 20% 2	4078
	11th to 20th	Day	28% 3 10% 23% 33% 4	4080
	21st to end	Day	41% 3 5% 19% 27% 7%	4488
February	1st to 10th	Day	47% 4 9% 16% 19% 5%	1800
	11th to 20th	Day	44% 5% 8% 21% 18% 5%	1934
	21st to end	Day	43% 4 5% 15% 24% 9%	1716
March	1st to 10th	Day	38% 5% 8% 16% 21% 12%	3546
	11th to 20th	Day	39% 7% 10% 23% 16% 5%	3491
	21st to end	Day	46% 7% 9% 22% 15% 2	3814
April	1st to 10th	Day	37% 6% 12% 21% 18% 7%	2090
	11th to 20th	Day	39% 7% 10% 22% 17% 6%	2250
	21st to end	Day	43% 7% 8% 21% 18% 3	2380
May	1st to 10th	Day	44% 7% 10% 20% 17% 3	4120
	11th to 20th	Day	38% 8% 13% 23% 18% 1	4086
	21st to end	Day	45% 8% 13% 17% 13% 4	4489
June	1st to 10th	Day	31% 9% 17% 28% 13% 2	2470
	11th to 20th	Day	29% 8% 15% 26% 19% 2	2470
	21st to end	Day	23% 7% 15% 30% 21% 4	2470
July	1st to 10th	Day	31% 6% 16% 32% 14% 1	4056
	11th to 20th	Day	23% 4 11% 38% 22% 2	4051
	21st to end	Day	31% 5% 15% 34% 14%	4325
August	1st to 10th	Day	22% 6% 13% 26% 28% 5%	4090
	11th to 20th	Day	28% 4 17% 34% 16% 2	4080
	21st to end	Day	25% 6% 16% 38% 13% 2	4248
September	1st to 10th	Day	32% 6% 9% 22% 22% 10%	2119
	11th to 20th	Day	35% 6% 11% 20% 21% 7%	2090
	21st to end	Day	24% 6% 11% 22% 28% 9%	2090
October	1st to 10th	Day	18% 3 7% 11% 38% 24%	3266
	11th to 20th	Day	20% 2 4 17% 37% 20%	3140
	21st to end	Day	28% 3 6% 29% 27% 7%	3428
November	1st to 10th	Day	30% 5% 9% 23% 25% 7%	1958
	11th to 20th	Day	28% 4 9% 24% 26% 10%	1877
	21st to end	Day	33% 4 7% 21% 27% 7%	1800
December	1st to 10th	Day	35% 3 8% 34% 17% 3	2940
	11th to 20th	Day	40% 2 5% 21% 26% 6%	3389
	21st to end	Day	46% 4 8% 20% 19% 3	4488
January	1st to 10th	Night	41% 3 9% 25% 21% 3	5888
	11th to 20th	Night	24% 6% 9% 18% 33% 10%	5900
	21st to end	Night	42% 3 6% 15% 27% 7%	6490
February	1st to 10th	Night	38% 5% 10% 21% 20% 6%	2600
	11th to 20th	Night	42% 4 6% 21% 21% 6%	2378
	21st to end	Night	38% 3 6% 17% 24% 11%	1912
March	1st to 10th	Night	41% 6% 8% 16% 19% 12%	3529
	11th to 20th	Night	33% 5% 9% 28% 15% 10%	3497
	21st to end	Night	38% 6% 10% 22% 21% 2	3817
April	1st to 10th	Night	34% 6% 10% 19% 19% 12%	2090
	11th to 20th	Night	32% 7% 10% 23% 18% 11%	1877
	21st to end	Night	36% 7% 10% 24% 21% 3	1796
May	1st to 10th	Night	39% 9% 12% 19% 15% 6%	2850
	11th to 20th	Night	36% 8% 13% 26% 16% 2	2824
	21st to end	Night	35% 9% 16% 21% 14% 6%	3099
June	1st to 10th	Night	21% 8% 16% 32% 19% 3	1710
	11th to 20th	Night	20% 6% 14% 32% 24% 4	1710
	21st to end	Night	16% 5% 16% 29% 27% 7%	1710
July	1st to 10th	Night	28% 5% 17% 29% 17% 4	2805
	11th to 20th	Night	16% 4 9% 43% 26% 4	2796
	21st to end	Night	22% 4 19% 34% 20% 1	2999
August	1st to 10th	Night	14% 5% 12% 26% 35% 8%	2826
	11th to 20th	Night	19% 3 16% 36% 21% 4	2825
	21st to end	Night	19% 4 14% 39% 22% 3	3407
September	1st to 10th	Night	26% 5% 10% 25% 24% 11%	2057
	11th to 20th	Night	30% 5% 10% 21% 24% 10%	2090
	21st to end	Night	22% 4 8% 23% 32% 10%	2090
October	1st to 10th	Night	14% 3 6% 13% 35% 29%	3660
	11th to 20th	Night	18% 2 4 12% 43% 21%	3779
	21st to end	Night	28% 2 4 31% 24% 11%	4120
November	1st to 10th	Night	29% 3 8% 24% 26% 11%	2340
	11th to 20th	Night	28% 4 8% 22% 28% 12%	2520
	21st to end	Night	33% 3 8% 21% 28% 8%	2600
December	1st to 10th	Night	34% 2 7% 30% 22% 5%	4250
	11th to 20th	Night	39% 2 4 18% 25% 11%	4817
	21st to end	Night	41% 3 8% 23% 20% 5%	6490

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	22% 21% 35% 18% 4	300
	11th to 20th	Day	21% 28% 33% 15% 3 1	300
	21st to end	Day	21% 23% 33% 18% 6%	330
February	1st to 10th	Day	25% 18% 41% 13% 3	300
	11th to 20th	Day	26% 28% 32% 12% 2	300
	21st to end	Day	24% 28% 32% 12% 3	249
March	1st to 10th	Day	20% 26% 34% 18% 2	300
	11th to 20th	Day	25% 27% 31% 13% 5%	273
	21st to end	Day	23% 28% 34% 12% 3	297
April	1st to 10th	Day	19% 25% 38% 14% 3 1	270
	11th to 20th	Day	21% 23% 34% 19% 4	448
	21st to end	Day	20% 19% 36% 18% 5% 2	560
May	1st to 10th	Day	14% 14% 43% 24% 4	653
	11th to 20th	Day	15% 11% 33% 27% 12% 2	675
	21st to end	Day	12% 9% 34% 25% 16% 4	734
June	1st to 10th	Day	9% 5% 20% 37% 23% 6%	680
	11th to 20th	Day	5% 3 24% 43% 19% 5%	680
	21st to end	Day	3 3 24% 38% 28% 6%	680
July	1st to 10th	Day	5% 1 16% 37% 31% 10%	671
	11th to 20th	Day	4 1 12% 40% 36% 7%	668
	21st to end	Day	5% 1 9% 38% 37% 12%	739
August	1st to 10th	Day	4 2 11% 44% 34% 5%	676
	11th to 20th	Day	9% 2 12% 41% 32% 5%	665
	21st to end	Day	5% 1 9% 32% 39% 14%	582
September	1st to 10th	Day	9% 1 11% 29% 38% 12%	329
	11th to 20th	Day	5% 4 14% 41% 29% 7%	300
	21st to end	Day	11% 4 20% 34% 26% 6%	300
October	1st to 10th	Day	4 6% 34% 31% 19% 5%	300
	11th to 20th	Day	12% 12% 34% 29% 12% 1	300
	21st to end	Day	28% 11% 28% 25% 8%	330
November	1st to 10th	Day	13% 13% 42% 22% 8% 2	300
	11th to 20th	Day	13% 18% 37% 25% 5% 2	300
	21st to end	Day	18% 23% 36% 17% 5%	300
December	1st to 10th	Day	17% 21% 32% 22% 7%	300
	11th to 20th	Day	23% 17% 40% 17% 4	300
	21st to end	Day	22% 23% 35% 15% 4	330
January	1st to 10th	Night	16% 15% 38% 27% 6%	1000
	11th to 20th	Night	15% 19% 37% 24% 4	1000
	21st to end	Night	15% 14% 37% 29% 5%	1100
February	1st to 10th	Night	18% 18% 37% 23% 4	1000
	11th to 20th	Night	20% 20% 37% 21% 2	824
	21st to end	Night	19% 19% 36% 24% 2	592
March	1st to 10th	Night	20% 17% 31% 28% 4	600
	11th to 20th	Night	20% 13% 39% 24% 4	575
	21st to end	Night	16% 15% 40% 23% 5%	627
April	1st to 10th	Night	13% 18% 39% 24% 5%	570
	11th to 20th	Night	14% 15% 39% 26% 6%	563
	21st to end	Night	15% 13% 39% 24% 8% 1	570
May	1st to 10th	Night	10% 10% 39% 32% 8% 1	570
	11th to 20th	Night	12% 7% 33% 32% 15% 1	561
	21st to end	Night	9% 5% 33% 33% 19% 2	617
June	1st to 10th	Night	4 4 20% 42% 27% 3	570
	11th to 20th	Night	5% 3 24% 48% 19% 1	570
	21st to end	Night	2 1 20% 45% 28% 4	570
July	1st to 10th	Night	5% 1 10% 46% 34% 5%	555
	11th to 20th	Night	3 1 13% 45% 36% 4	552
	21st to end	Night	3 1 8% 44% 39% 5%	624
August	1st to 10th	Night	2 1 12% 47% 36% 2	567
	11th to 20th	Night	5% 2 9% 44% 37% 4	564
	21st to end	Night	3 2 7% 42% 42% 5%	627
September	1st to 10th	Night	9% 1 10% 42% 36% 2	570
	11th to 20th	Night	12% 1 14% 38% 32% 3	570
	21st to end	Night	7% 2 21% 36% 30% 3	570
October	1st to 10th	Night	7% 4 25% 34% 26% 3	672
	11th to 20th	Night	13% 7% 32% 34% 13% 1	760
	21st to end	Night	19% 9% 30% 28% 12% 1	836
November	1st to 10th	Night	12% 9% 35% 33% 11% 1	780
	11th to 20th	Night	10% 15% 35% 30% 10% 1	920
	21st to end	Night	17% 14% 36% 24% 8%	1000
December	1st to 10th	Night	15% 16% 35% 29% 5%	1000
	11th to 20th	Night	18% 14% 38% 25% 5%	1000
	21st to end	Night	17% 17% 37% 25% 5%	1100

Page generated at 2025-03-09 07:08:45. Version ac07

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