Fremont
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fremont clear sky chart from 2005-11-06 to 2025-07-06.

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	18% 9% 3 8% 5% 5% 5% 4 5% 7% 31%	120042
	Night	18% 9% 3 8% 5% 4 4 4 5% 7% 33%	118717
Transparency	Day	32% 40% 7% 12% 7% 1	123577
	Night	32% 40% 8% 12% 8% 1	123343
Seeing	Day	27% 18% 27% 18% 9% 3	15647
	Night	24% 16% 26% 23% 9% 1	24841

By Month

Percentage Cloud Cover NE (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	11% 12% 8% 20% 8% 4 3 3 4 5% 21%	3597
	11th to 20th	Day	14% 13% 7% 17% 7% 5% 5% 3 5% 4 19%	3570
	21st to end	Day	17% 12% 4 8% 5% 5% 5% 3 4 5% 30%	4356
February	1st to 10th	Day	16% 12% 3 10% 7% 6% 5% 4 6% 6% 25%	1800
	11th to 20th	Day	14% 11% 4 10% 6% 5% 5% 3 5% 7% 30%	1873
	21st to end	Day	19% 11% 3 10% 6% 4 4 3 4 6% 30%	1650
March	1st to 10th	Day	21% 12% 4 8% 6% 4 3 3 4 4 31%	4476
	11th to 20th	Day	13% 10% 4 8% 5% 4 4 4 4 7% 37%	4925
	21st to end	Day	19% 10% 3 7% 5% 4 5% 4 6% 6% 33%	5827
April	1st to 10th	Day	19% 12% 3 9% 5% 4 3 3 4 7% 32%	2400
	11th to 20th	Day	17% 10% 3 7% 5% 5% 5% 3 4 6% 35%	2374
	21st to end	Day	18% 9% 4 7% 6% 4 4 3 5% 7% 34%	2400
May	1st to 10th	Day	30% 8% 2 7% 4 4 4 3 4 7% 25%	6108
	11th to 20th	Day	21% 9% 3 8% 6% 5% 5% 4 5% 6% 29%	6318
	21st to end	Day	22% 6% 2 5% 4 5% 6% 5% 7% 8% 29%	6949
June	1st to 10th	Day	17% 6% 3 7% 6% 5% 5% 5% 6% 8% 33%	2800
	11th to 20th	Day	18% 7% 2 7% 5% 5% 5% 4 6% 9% 31%	2800
	21st to end	Day	16% 7% 2 7% 5% 4 4 4 7% 9% 34%	2800
July	1st to 10th	Day	10% 6% 2 7% 6% 5% 6% 5% 7% 10% 35%	4510
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 8% 10% 36%	4440
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 5% 6% 8% 33%	4814
August	1st to 10th	Day	23% 6% 3 7% 5% 5% 5% 4 6% 8% 29%	4184
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 5% 8% 9% 33%	3791
	21st to end	Day	18% 5% 2 4 4 4 4 4 7% 9% 40%	4180
September	1st to 10th	Day	17% 6% 2 6% 5% 4 4 4 5% 8% 39%	2280
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 5% 7% 45%	2243
	21st to end	Day	20% 7% 2 6% 4 5% 5% 4 6% 7% 35%	2090
October	1st to 10th	Day	20% 8% 2 7% 4 4 5% 4 5% 7% 35%	1905
	11th to 20th	Day	17% 9% 3 6% 4 5% 5% 3 6% 7% 36%	1900
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 7% 30%	1971
November	1st to 10th	Day	13% 8% 2 7% 5% 3 4 4 5% 6% 43%	1761
	11th to 20th	Day	17% 12% 2 8% 5% 5% 4 3 5% 5% 35%	1800
	21st to end	Day	19% 10% 4 8% 5% 5% 6% 4 4 6% 29%	1604
December	1st to 10th	Day	18% 12% 7% 13% 5% 5% 5% 4 4 5% 21%	2600
	11th to 20th	Day	23% 12% 5% 7% 5% 3 3 3 4 5% 30%	2986
	21st to end	Day	19% 13% 6% 10% 5% 5% 4 3 5% 6% 25%	3960
January	1st to 10th	Night	13% 10% 5% 18% 6% 4 4 4 5% 6% 24%	6349
	11th to 20th	Night	12% 11% 5% 17% 7% 5% 5% 4 5% 6% 23%	6315
	21st to end	Night	16% 10% 4 9% 7% 5% 5% 4 6% 6% 29%	6584
February	1st to 10th	Night	16% 12% 3 8% 6% 5% 5% 4 6% 7% 29%	2600
	11th to 20th	Night	14% 10% 3 11% 5% 5% 5% 5% 7% 8% 28%	2411
	21st to end	Night	18% 10% 3 8% 5% 5% 4 4 6% 7% 29%	1980
March	1st to 10th	Night	20% 10% 3 7% 5% 4 4 4 5% 6% 32%	5364
	11th to 20th	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 37%	5030
	21st to end	Night	17% 11% 3 8% 4 5% 4 3 4 6% 34%	5105
April	1st to 10th	Night	20% 11% 3 7% 6% 5% 4 4 5% 6% 30%	2000
	11th to 20th	Night	19% 10% 3 7% 4 4 3 3 4 7% 35%	1975
	21st to end	Night	19% 8% 3 7% 4 3 4 4 5% 6% 38%	2000
May	1st to 10th	Night	34% 8% 2 5% 3 4 3 3 5% 6% 27%	3859
	11th to 20th	Night	21% 10% 3 6% 4 4 4 4 5% 6% 32%	3575
	21st to end	Night	23% 8% 3 7% 5% 4 4 3 6% 6% 31%	3926
June	1st to 10th	Night	18% 6% 2 6% 4 5% 4 3 6% 7% 40%	1600
	11th to 20th	Night	17% 6% 3 6% 4 3 3 3 6% 8% 42%	1600
	21st to end	Night	12% 5% 2 4 4 4 5% 4 7% 9% 44%	1600
July	1st to 10th	Night	12% 6% 2 6% 5% 5% 4 4 6% 8% 41%	2560
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 7% 9% 39%	2520
	21st to end	Night	17% 6% 2 6% 3 5% 4 4 6% 8% 38%	2739
August	1st to 10th	Night	23% 6% 2 7% 4 4 6% 4 5% 7% 30%	2728
	11th to 20th	Night	18% 7% 2 7% 4 4 4 4 5% 7% 39%	3004
	21st to end	Night	14% 4 1 5% 3 3 4 3 6% 8% 48%	3476
September	1st to 10th	Night	16% 6% 2 5% 4 4 3 4 5% 7% 44%	1900
	11th to 20th	Night	14% 6% 1 4 3 3 4 3 4 6% 52%	1936
	21st to end	Night	19% 6% 2 7% 4 3 4 3 5% 7% 40%	2090
October	1st to 10th	Night	21% 6% 2 6% 3 4 4 4 5% 6% 38%	2270
	11th to 20th	Night	20% 8% 3 4 4 3 4 3 5% 6% 41%	2280
	21st to end	Night	22% 9% 3 5% 4 4 5% 3 5% 7% 32%	2626
November	1st to 10th	Night	15% 7% 3 6% 4 4 3 4 5% 6% 43%	2537
	11th to 20th	Night	20% 9% 2 6% 4 4 3 4 6% 7% 36%	2600
	21st to end	Night	15% 10% 3 7% 5% 4 5% 4 6% 8% 33%	2794
December	1st to 10th	Night	18% 12% 4 10% 4 4 4 4 6% 7% 26%	4580
	11th to 20th	Night	26% 10% 3 7% 4 4 4 3 4 4 32%	5208
	21st to end	Night	22% 14% 6% 8% 5% 4 4 3 5% 6% 25%	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	47% 29% 8% 11% 5% 1	3598
	11th to 20th	Day	51% 28% 6% 9% 6% 1	3600
	21st to end	Day	41% 30% 6% 14% 9%	4356
February	1st to 10th	Day	22% 63% 4 7% 4	1800
	11th to 20th	Day	22% 61% 3 8% 6% 1	1882
	21st to end	Day	19% 63% 5% 10% 4	1650
March	1st to 10th	Day	43% 29% 7% 9% 9% 4	4479
	11th to 20th	Day	30% 33% 13% 17% 4 3	4925
	21st to end	Day	37% 30% 7% 7% 16% 4	5827
April	1st to 10th	Day	24% 62% 3 7% 4 1	2400
	11th to 20th	Day	19% 62% 4 7% 7%	2374
	21st to end	Day	20% 61% 4 8% 6% 1	2400
May	1st to 10th	Day	46% 30% 8% 9% 6% 1	6119
	11th to 20th	Day	41% 32% 7% 13% 7%	6322
	21st to end	Day	37% 32% 9% 14% 8%	6949
June	1st to 10th	Day	18% 63% 5% 8% 6%	2800
	11th to 20th	Day	18% 63% 6% 8% 5%	2800
	21st to end	Day	18% 61% 4 10% 6%	2800
July	1st to 10th	Day	28% 38% 10% 15% 9%	4440
	11th to 20th	Day	29% 37% 11% 15% 8%	4365
	21st to end	Day	34% 36% 9% 13% 8%	4661
August	1st to 10th	Day	37% 38% 9% 12% 4	4170
	11th to 20th	Day	29% 38% 11% 18% 4	3829
	21st to end	Day	24% 37% 8% 25% 6%	4180
September	1st to 10th	Day	15% 61% 5% 11% 7%	2280
	11th to 20th	Day	16% 61% 4 9% 10% 1	2243
	21st to end	Day	20% 61% 5% 8% 6% 1	2090
October	1st to 10th	Day	31% 36% 3 14% 15% 2	3165
	11th to 20th	Day	25% 36% 4 16% 12% 6%	3140
	21st to end	Day	17% 37% 13% 21% 10% 2	3222
November	1st to 10th	Day	21% 57% 7% 10% 5%	1761
	11th to 20th	Day	18% 57% 5% 11% 9% 1	1800
	21st to end	Day	22% 57% 8% 9% 3 1	1604
December	1st to 10th	Day	45% 36% 6% 11% 2	2600
	11th to 20th	Day	43% 31% 7% 11% 8%	2986
	21st to end	Day	46% 27% 5% 10% 11% 1	3960
January	1st to 10th	Night	45% 28% 8% 10% 8% 1	6349
	11th to 20th	Night	46% 28% 10% 8% 7% 1	6360
	21st to end	Night	41% 30% 7% 7% 13% 2	6584
February	1st to 10th	Night	20% 63% 6% 8% 3	2600
	11th to 20th	Night	21% 61% 4 8% 5%	2413
	21st to end	Night	18% 62% 5% 9% 5% 1	1980
March	1st to 10th	Night	38% 29% 9% 10% 12% 1	5374
	11th to 20th	Night	31% 32% 16% 13% 5% 3	5031
	21st to end	Night	38% 31% 4 12% 14% 1	5105
April	1st to 10th	Night	23% 62% 3 7% 4	2000
	11th to 20th	Night	18% 62% 5% 7% 8%	1975
	21st to end	Night	19% 62% 4 8% 7% 1	2000
May	1st to 10th	Night	44% 30% 9% 10% 5% 2	3865
	11th to 20th	Night	40% 32% 10% 12% 6%	3576
	21st to end	Night	41% 32% 7% 13% 6%	3927
June	1st to 10th	Night	16% 62% 4 10% 7%	1600
	11th to 20th	Night	17% 62% 7% 9% 5%	1600
	21st to end	Night	16% 61% 6% 9% 7% 1	1600
July	1st to 10th	Night	28% 36% 12% 11% 12% 1	2520
	11th to 20th	Night	29% 36% 9% 18% 8%	2472
	21st to end	Night	32% 36% 7% 15% 9% 1	2651
August	1st to 10th	Night	39% 38% 8% 11% 3	2718
	11th to 20th	Night	28% 37% 10% 19% 5%	3032
	21st to end	Night	20% 37% 8% 28% 7%	3476
September	1st to 10th	Night	15% 61% 6% 11% 7% 1	1900
	11th to 20th	Night	13% 60% 5% 9% 12% 1	1936
	21st to end	Night	20% 59% 4 10% 7%	2090
October	1st to 10th	Night	31% 37% 5% 15% 11% 1	3790
	11th to 20th	Night	21% 37% 7% 14% 15% 6%	3779
	21st to end	Night	21% 36% 12% 21% 10% 1	4325
November	1st to 10th	Night	19% 57% 6% 10% 7%	2537
	11th to 20th	Night	18% 58% 5% 10% 8% 1	2600
	21st to end	Night	21% 58% 8% 10% 3	2794
December	1st to 10th	Night	38% 36% 7% 17% 1	4580
	11th to 20th	Night	41% 32% 8% 11% 9%	5208
	21st to end	Night	46% 28% 6% 8% 11% 2	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	37% 25% 25% 12% 1 1	200
	11th to 20th	Day	44% 25% 21% 9% 2	200
	21st to end	Day	31% 29% 28% 9% 3	307
February	1st to 10th	Day	35% 32% 27% 5% 1	300
	11th to 20th	Day	39% 28% 26% 7% 1	421
	21st to end	Day	36% 23% 29% 11% 1	414
March	1st to 10th	Day	33% 22% 28% 15% 2	500
	11th to 20th	Day	32% 26% 27% 11% 3 1	500
	21st to end	Day	33% 28% 25% 8% 5% 1	550
April	1st to 10th	Day	37% 23% 24% 12% 4	500
	11th to 20th	Day	35% 22% 23% 11% 6% 3	492
	21st to end	Day	31% 20% 32% 9% 5% 2	500
May	1st to 10th	Day	37% 17% 27% 11% 6% 2	503
	11th to 20th	Day	22% 21% 31% 17% 6% 3	527
	21st to end	Day	23% 12% 29% 22% 11% 3	573
June	1st to 10th	Day	22% 14% 33% 20% 10% 3	530
	11th to 20th	Day	22% 16% 23% 25% 12% 2	530
	21st to end	Day	19% 14% 34% 22% 9% 2	530
July	1st to 10th	Day	14% 10% 29% 28% 15% 4	505
	11th to 20th	Day	12% 7% 31% 32% 16% 3	484
	21st to end	Day	18% 9% 36% 24% 11% 3	532
August	1st to 10th	Day	17% 11% 26% 24% 16% 6%	486
	11th to 20th	Day	16% 10% 25% 27% 16% 6%	479
	21st to end	Day	17% 10% 28% 24% 14% 7%	539
September	1st to 10th	Day	24% 10% 23% 19% 16% 8%	490
	11th to 20th	Day	15% 10% 25% 23% 15% 12%	490
	21st to end	Day	26% 10% 27% 21% 13% 4	490
October	1st to 10th	Day	29% 12% 23% 19% 14% 3	490
	11th to 20th	Day	25% 18% 26% 21% 9% 1	490
	21st to end	Day	32% 17% 25% 19% 7% 1	476
November	1st to 10th	Day	26% 18% 23% 21% 10% 2	395
	11th to 20th	Day	28% 25% 26% 16% 5% 1	400
	21st to end	Day	33% 23% 30% 12% 2	204
December	1st to 10th	Day	33% 26% 25% 13% 4 1	200
	11th to 20th	Day	40% 23% 27% 9% 2	200
	21st to end	Day	30% 26% 28% 13% 2	220
January	1st to 10th	Night	27% 27% 31% 14% 2	800
	11th to 20th	Night	29% 21% 28% 19% 2	800
	21st to end	Night	22% 25% 35% 15% 3	880
February	1st to 10th	Night	28% 21% 35% 15% 1	800
	11th to 20th	Night	29% 26% 31% 13% 1	783
	21st to end	Night	33% 22% 28% 14% 3	660
March	1st to 10th	Night	29% 22% 32% 15% 3	800
	11th to 20th	Night	26% 26% 26% 19% 3	800
	21st to end	Night	25% 23% 31% 16% 5%	880
April	1st to 10th	Night	30% 20% 30% 17% 3	800
	11th to 20th	Night	26% 19% 30% 19% 6%	791
	21st to end	Night	26% 19% 27% 21% 7%	800
May	1st to 10th	Night	32% 13% 23% 24% 6% 1	522
	11th to 20th	Night	22% 11% 30% 29% 8%	394
	21st to end	Night	20% 8% 24% 33% 14% 1	433
June	1st to 10th	Night	19% 8% 27% 32% 15%	400
	11th to 20th	Night	20% 8% 22% 35% 16%	400
	21st to end	Night	15% 4 26% 42% 14%	400
July	1st to 10th	Night	16% 6% 18% 41% 18% 2	380
	11th to 20th	Night	15% 4 22% 40% 19% 1	368
	21st to end	Night	16% 5% 19% 41% 18% 1	416
August	1st to 10th	Night	15% 3 22% 36% 22% 2	509
	11th to 20th	Night	17% 5% 15% 40% 19% 4	693
	21st to end	Night	18% 5% 18% 33% 24% 2	836
September	1st to 10th	Night	19% 6% 26% 29% 19% 2	760
	11th to 20th	Night	17% 8% 20% 30% 20% 6%	760
	21st to end	Night	22% 9% 22% 30% 14% 3	760
October	1st to 10th	Night	26% 12% 22% 22% 17% 1	760
	11th to 20th	Night	25% 12% 27% 28% 8%	760
	21st to end	Night	27% 18% 28% 22% 5%	836
November	1st to 10th	Night	19% 19% 28% 25% 9% 1	780
	11th to 20th	Night	28% 20% 23% 21% 7% 1	800
	21st to end	Night	23% 21% 30% 22% 5% 1	800
December	1st to 10th	Night	31% 24% 25% 16% 4	800
	11th to 20th	Night	32% 21% 27% 17% 3	800
	21st to end	Night	30% 23% 28% 17% 3	880

Page generated at 2025-07-03 01:30:17. Version ac07

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