Somerton Creek
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Somerton Creek 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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	108673
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	110319
Transparency	Day	22% 45% 7% 12% 11% 3	112313
	Night	23% 43% 6% 13% 11% 3	114818
Seeing	Day	17% 12% 26% 25% 15% 5%	15665
	Night	16% 9% 28% 32% 13% 1	25448

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	14% 5% 2 5% 3 3 3 3 3 4 56%	3962
	11th to 20th	Day	13% 7% 2 4 3 3 3 3 4 5% 54%	4060
	21st to end	Day	12% 8% 3 7% 4 4 4 3 4 5% 46%	4466
February	1st to 10th	Day	13% 8% 2 6% 4 4 5% 3 5% 7% 42%	1800
	11th to 20th	Day	22% 6% 2 4 4 4 4 3 4 5% 44%	1891
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 8% 39%	1650
March	1st to 10th	Day	23% 9% 2 5% 3 3 3 4 4 6% 36%	3482
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 3 6% 55%	3491
	21st to end	Day	18% 7% 2 5% 4 4 3 3 5% 6% 45%	3896
April	1st to 10th	Day	13% 5% 1 4 4 3 3 3 5% 7% 52%	2280
	11th to 20th	Day	11% 5% 2 5% 5% 4 4 4 5% 6% 50%	2254
	21st to end	Day	12% 7% 2 6% 6% 4 4 4 5% 6% 45%	2280
May	1st to 10th	Day	12% 6% 3 8% 5% 5% 6% 4 7% 9% 34%	3784
	11th to 20th	Day	20% 7% 2 7% 5% 5% 5% 3 6% 7% 32%	4130
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4823
June	1st to 10th	Day	10% 6% 2 5% 5% 6% 6% 5% 7% 9% 40%	2660
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	2660
	21st to end	Day	9% 5% 2 7% 7% 5% 5% 5% 8% 11% 37%	2660
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 8% 12% 37%	4389
	11th to 20th	Day	11% 8% 3 8% 6% 6% 6% 5% 6% 10% 33%	4120
	21st to end	Day	10% 7% 3 9% 7% 6% 6% 6% 7% 9% 30%	4466
August	1st to 10th	Day	18% 9% 3 9% 6% 6% 6% 5% 7% 9% 21%	4104
	11th to 20th	Day	9% 7% 3 8% 5% 6% 6% 5% 8% 12% 33%	3730
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 7% 11% 41%	4180
September	1st to 10th	Day	14% 6% 2 7% 5% 4 5% 4 7% 8% 38%	2280
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 3 6% 7% 39%	2280
	21st to end	Day	19% 8% 3 7% 5% 4 5% 3 4 6% 35%	2016
October	1st to 10th	Day	17% 7% 3 5% 3 4 4 3 4 7% 44%	1900
	11th to 20th	Day	14% 6% 3 5% 3 3 4 2 4 5% 52%	1900
	21st to end	Day	12% 5% 2 5% 3 4 4 3 4 6% 52%	2090
November	1st to 10th	Day	15% 5% 2 5% 4 4 4 2 5% 5% 49%	1914
	11th to 20th	Day	15% 6% 2 5% 3 3 2 3 4 6% 51%	1800
	21st to end	Day	15% 5% 1 4 3 4 3 3 5% 6% 52%	1725
December	1st to 10th	Day	12% 5% 2 5% 4 4 3 3 5% 5% 53%	2600
	11th to 20th	Day	14% 8% 3 7% 4 3 4 3 4 5% 45%	2990
	21st to end	Day	14% 5% 2 6% 4 3 3 3 5% 7% 48%	3960
January	1st to 10th	Night	13% 7% 3 5% 3 2 3 3 4 5% 53%	5966
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 5% 53%	5900
	21st to end	Night	15% 7% 3 6% 4 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	18% 7% 2 5% 4 4 4 3 5% 7% 43%	2600
	11th to 20th	Night	22% 8% 2 5% 4 3 4 4 4 5% 40%	2393
	21st to end	Night	16% 7% 2 5% 4 4 3 3 4 6% 47%	1980
March	1st to 10th	Night	25% 8% 2 6% 4 4 4 3 5% 4 34%	3574
	11th to 20th	Night	14% 5% 2 5% 4 3 4 2 5% 5% 52%	3498
	21st to end	Night	18% 6% 2 5% 3 3 3 2 4 4 49%	3734
April	1st to 10th	Night	13% 7% 2 5% 4 3 4 4 6% 6% 46%	1900
	11th to 20th	Night	12% 6% 2 5% 4 4 3 3 5% 6% 51%	1875
	21st to end	Night	13% 5% 2 6% 4 4 4 3 5% 6% 48%	1900
May	1st to 10th	Night	13% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	3130
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 6% 7% 36%	2769
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 6% 9% 44%	2732
June	1st to 10th	Night	11% 4 2 5% 6% 5% 4 5% 7% 12% 40%	1520
	11th to 20th	Night	8% 6% 2 7% 5% 5% 5% 4 7% 11% 40%	1520
	21st to end	Night	8% 5% 1 6% 5% 5% 5% 5% 7% 10% 43%	1520
July	1st to 10th	Night	8% 5% 3 6% 5% 5% 5% 6% 9% 11% 37%	2566
	11th to 20th	Night	10% 8% 3 8% 5% 5% 5% 5% 7% 10% 34%	2840
	21st to end	Night	16% 9% 3 8% 6% 5% 5% 4 6% 8% 29%	3087
August	1st to 10th	Night	20% 8% 3 8% 5% 4 4 4 6% 9% 28%	2825
	11th to 20th	Night	8% 8% 3 9% 5% 5% 5% 4 7% 9% 37%	3067
	21st to end	Night	8% 5% 2 6% 4 4 4 5% 8% 9% 45%	3476
September	1st to 10th	Night	11% 6% 2 6% 3 4 4 3 7% 8% 45%	1900
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 3 5% 6% 44%	1900
	21st to end	Night	18% 8% 3 8% 4 4 4 3 5% 7% 36%	2157
October	1st to 10th	Night	14% 6% 2 6% 3 3 4 3 5% 6% 48%	2280
	11th to 20th	Night	14% 7% 2 5% 4 3 4 2 4 5% 51%	2280
	21st to end	Night	13% 5% 2 5% 4 3 3 3 4 6% 53%	2508
November	1st to 10th	Night	16% 6% 1 5% 3 3 3 3 5% 6% 50%	2381
	11th to 20th	Night	16% 6% 2 4 3 3 3 3 5% 6% 49%	2600
	21st to end	Night	13% 5% 1 6% 4 4 4 3 5% 6% 50%	2670
December	1st to 10th	Night	10% 5% 2 7% 4 4 4 3 4 6% 52%	4580
	11th to 20th	Night	16% 7% 3 8% 5% 5% 5% 3 5% 5% 38%	5205
	21st to end	Night	13% 6% 2 5% 4 4 4 3 5% 7% 49%	6996

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	24% 26% 2 16% 18% 13%	3963
	11th to 20th	Day	24% 26% 3 12% 28% 7%	4060
	21st to end	Day	30% 29% 6% 19% 13% 3	4466
February	1st to 10th	Day	17% 62% 4 10% 6% 1	1800
	11th to 20th	Day	20% 61% 4 7% 6% 3	1900
	21st to end	Day	16% 62% 4 7% 8% 2	1650
March	1st to 10th	Day	37% 41% 5% 7% 6% 4	3482
	11th to 20th	Day	16% 40% 4 12% 19% 9%	3491
	21st to end	Day	23% 40% 9% 11% 13% 3	3896
April	1st to 10th	Day	13% 65% 4 7% 9% 2	2280
	11th to 20th	Day	13% 66% 4 9% 6% 2	2254
	21st to end	Day	14% 66% 5% 8% 5% 2	2280
May	1st to 10th	Day	30% 45% 9% 12% 4 1	3819
	11th to 20th	Day	31% 43% 6% 14% 5%	4134
	21st to end	Day	17% 43% 9% 17% 12% 2	4837
June	1st to 10th	Day	15% 67% 4 8% 6% 1	2660
	11th to 20th	Day	10% 67% 7% 9% 6% 1	2660
	21st to end	Day	13% 65% 6% 11% 6%	2660
July	1st to 10th	Day	23% 43% 16% 11% 7% 1	4320
	11th to 20th	Day	32% 41% 16% 10% 1	4051
	21st to end	Day	34% 40% 8% 8% 9% 1	4324
August	1st to 10th	Day	40% 41% 10% 7% 1	4091
	11th to 20th	Day	25% 42% 10% 16% 7%	3766
	21st to end	Day	18% 41% 8% 21% 10% 1	4180
September	1st to 10th	Day	18% 62% 7% 6% 5% 1	2280
	11th to 20th	Day	19% 61% 6% 6% 5% 2	2280
	21st to end	Day	24% 61% 4 9% 3	2016
October	1st to 10th	Day	18% 39% 8% 15% 18% 2	3160
	11th to 20th	Day	11% 36% 3 11% 30% 8%	3140
	21st to end	Day	8% 37% 4 21% 20% 10%	3428
November	1st to 10th	Day	18% 57% 3 11% 8% 2	1914
	11th to 20th	Day	17% 57% 3 8% 12% 3	1800
	21st to end	Day	15% 57% 4 11% 11% 2	1725
December	1st to 10th	Day	21% 36% 4 19% 16% 4	2600
	11th to 20th	Day	31% 33% 8% 12% 11% 5%	2986
	21st to end	Day	26% 28% 9% 21% 16%	3960
January	1st to 10th	Night	25% 26% 4 14% 22% 9%	5966
	11th to 20th	Night	25% 27% 3 14% 26% 5%	5900
	21st to end	Night	29% 29% 5% 17% 16% 4	6490
February	1st to 10th	Night	15% 63% 6% 10% 5% 1	2600
	11th to 20th	Night	21% 61% 3 7% 7% 2	2395
	21st to end	Night	16% 62% 2 10% 7% 4	1980
March	1st to 10th	Night	40% 41% 4 8% 5% 2	3574
	11th to 20th	Night	18% 40% 4 10% 17% 10%	3498
	21st to end	Night	23% 40% 4 19% 10% 3	3734
April	1st to 10th	Night	14% 67% 5% 5% 9% 1	1900
	11th to 20th	Night	14% 65% 4 9% 6% 2	1875
	21st to end	Night	13% 66% 5% 9% 5% 1	1900
May	1st to 10th	Night	30% 44% 10% 12% 4 1	3160
	11th to 20th	Night	29% 44% 6% 17% 3	2770
	21st to end	Night	21% 44% 11% 15% 8% 1	2739
June	1st to 10th	Night	14% 67% 6% 7% 5% 1	1520
	11th to 20th	Night	10% 68% 7% 9% 6%	1520
	21st to end	Night	11% 66% 5% 11% 7% 1	1520
July	1st to 10th	Night	25% 45% 12% 12% 4 1	2525
	11th to 20th	Night	31% 41% 15% 11% 1	2786
	21st to end	Night	39% 38% 5% 8% 10%	2988
August	1st to 10th	Night	39% 40% 11% 9% 1	2814
	11th to 20th	Night	25% 41% 12% 15% 8%	3097
	21st to end	Night	18% 39% 8% 19% 15% 1	3476
September	1st to 10th	Night	15% 62% 9% 7% 5% 1	1900
	11th to 20th	Night	18% 61% 4 7% 8% 2	1900
	21st to end	Night	23% 61% 6% 7% 3	2157
October	1st to 10th	Night	23% 39% 7% 16% 14% 1	3800
	11th to 20th	Night	12% 37% 3 12% 25% 12%	3779
	21st to end	Night	8% 36% 5% 22% 16% 12%	4120
November	1st to 10th	Night	20% 56% 4 10% 8% 2	2381
	11th to 20th	Night	17% 57% 3 8% 9% 6%	2600
	21st to end	Night	14% 57% 3 12% 11% 2	2670
December	1st to 10th	Night	20% 37% 4 17% 17% 6%	4580
	11th to 20th	Night	36% 33% 6% 10% 10% 5%	5208
	21st to end	Night	26% 28% 11% 21% 13% 1	6996

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	24% 26% 34% 12% 5%	280
	11th to 20th	Day	21% 30% 34% 14% 1	300
	21st to end	Day	22% 25% 39% 12% 2 1	330
February	1st to 10th	Day	17% 27% 39% 13% 4	300
	11th to 20th	Day	30% 24% 30% 13% 3	439
	21st to end	Day	24% 20% 34% 17% 4	414
March	1st to 10th	Day	22% 22% 34% 17% 5%	500
	11th to 20th	Day	22% 20% 35% 18% 5% 1	464
	21st to end	Day	22% 19% 36% 17% 5% 2	506
April	1st to 10th	Day	20% 18% 37% 17% 6% 1	460
	11th to 20th	Day	14% 18% 33% 21% 12% 2	452
	21st to end	Day	17% 15% 31% 27% 9% 2	460
May	1st to 10th	Day	16% 15% 34% 24% 9% 2	463
	11th to 20th	Day	20% 7% 28% 27% 15% 3	487
	21st to end	Day	18% 4 22% 29% 21% 6%	529
June	1st to 10th	Day	12% 6% 23% 33% 21% 5%	490
	11th to 20th	Day	11% 6% 19% 32% 23% 8%	490
	21st to end	Day	10% 4 20% 37% 23% 6%	490
July	1st to 10th	Day	10% 2 12% 35% 31% 9%	485
	11th to 20th	Day	7% 2 12% 36% 32% 10%	484
	21st to end	Day	9% 2 18% 38% 25% 8%	532
August	1st to 10th	Day	14% 3 19% 31% 24% 9%	486
	11th to 20th	Day	12% 3 16% 27% 30% 12%	479
	21st to end	Day	9% 3 13% 32% 28% 14%	539
September	1st to 10th	Day	14% 4 18% 27% 22% 15%	490
	11th to 20th	Day	16% 5% 17% 27% 23% 12%	490
	21st to end	Day	26% 8% 19% 24% 16% 7%	490
October	1st to 10th	Day	20% 6% 19% 25% 22% 9%	490
	11th to 20th	Day	14% 10% 27% 27% 18% 3	490
	21st to end	Day	13% 10% 29% 26% 16% 5%	539
November	1st to 10th	Day	17% 8% 32% 26% 13% 3	472
	11th to 20th	Day	18% 15% 32% 23% 10% 2	400
	21st to end	Day	19% 18% 25% 23% 13% 2	325
December	1st to 10th	Day	22% 16% 41% 14% 6% 1	200
	11th to 20th	Day	20% 18% 36% 20% 7% 1	200
	21st to end	Day	22% 19% 29% 20% 9% 1	220
January	1st to 10th	Night	18% 17% 36% 25% 3	800
	11th to 20th	Night	20% 14% 35% 25% 6%	800
	21st to end	Night	19% 13% 41% 26% 2	880
February	1st to 10th	Night	21% 13% 39% 24% 4	800
	11th to 20th	Night	27% 15% 40% 16% 2	783
	21st to end	Night	20% 16% 38% 23% 3	660
March	1st to 10th	Night	18% 15% 37% 27% 4	800
	11th to 20th	Night	20% 14% 39% 21% 5%	767
	21st to end	Night	20% 12% 38% 25% 5%	836
April	1st to 10th	Night	17% 14% 43% 23% 4	760
	11th to 20th	Night	15% 12% 39% 25% 9%	751
	21st to end	Night	16% 8% 37% 34% 6%	760
May	1st to 10th	Night	16% 11% 35% 31% 7%	760
	11th to 20th	Night	17% 7% 26% 39% 10% 1	544
	21st to end	Night	12% 2 17% 50% 19% 1	411
June	1st to 10th	Night	11% 3 21% 47% 18% 1	380
	11th to 20th	Night	6% 3 21% 47% 21% 1	380
	21st to end	Night	8% 2 19% 50% 19% 1	380
July	1st to 10th	Night	11% 2 13% 48% 25% 2	389
	11th to 20th	Night	7% 1 10% 44% 35% 3	552
	21st to end	Night	8% 2 17% 45% 26% 2	624
August	1st to 10th	Night	10% 2 12% 44% 28% 4	567
	11th to 20th	Night	12% 2 12% 44% 26% 3	732
	21st to end	Night	12% 2 12% 34% 36% 4	836
September	1st to 10th	Night	14% 3 13% 38% 28% 5%	760
	11th to 20th	Night	16% 4 14% 33% 29% 4	760
	21st to end	Night	22% 6% 18% 33% 20% 1	760
October	1st to 10th	Night	17% 3 18% 33% 26% 3	760
	11th to 20th	Night	19% 5% 27% 31% 17% 1	760
	21st to end	Night	15% 7% 27% 32% 17% 2	836
November	1st to 10th	Night	17% 7% 26% 33% 14% 3	780
	11th to 20th	Night	19% 9% 31% 32% 9% 1	800
	21st to end	Night	15% 9% 30% 34% 11% 1	800
December	1st to 10th	Night	20% 12% 34% 28% 7%	800
	11th to 20th	Night	18% 9% 35% 30% 8%	800
	21st to end	Night	17% 14% 33% 30% 6%	880

Page generated at 2025-03-09 08:08:47. Version ac07

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