Morgan Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morgan Hill clear sky chart from 2005-11-06 to 2025-10-19.

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	6% 4 2 4 3 3 3 3 4 6% 62%	134949
	Night	8% 6% 2 6% 4 4 4 3 4 6% 52%	132193
Transparency	Day	26% 3 7% 19% 26% 20%	129903
	Night	33% 4 7% 18% 22% 16%	128222
Seeing	Day	8% 7% 20% 31% 24% 10%	16693
	Night	10% 4 11% 34% 32% 9%	26055

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	12% 8% 2 5% 3 3 3 3 4 6% 52%	3851
	11th to 20th	Day	11% 5% 2 5% 3 4 3 3 4 4 55%	4100
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 6% 44%	4510
February	1st to 10th	Day	17% 7% 2 5% 4 3 3 2 4 6% 47%	1800
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 6% 45%	1889
	21st to end	Day	14% 5% 2 6% 5% 4 4 3 4 6% 46%	1650
March	1st to 10th	Day	12% 9% 3 8% 6% 6% 7% 5% 6% 6% 32%	4798
	11th to 20th	Day	12% 5% 1 4 4 4 4 4 5% 8% 48%	5000
	21st to end	Day	12% 8% 3 8% 6% 5% 5% 4 7% 9% 34%	5615
April	1st to 10th	Day	11% 7% 2 5% 4 4 4 3 5% 7% 48%	2400
	11th to 20th	Day	9% 5% 1 5% 4 3 4 3 5% 6% 54%	2369
	21st to end	Day	6% 4 2 5% 3 3 3 3 5% 8% 57%	2400
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 6% 73%	5457
	11th to 20th	Day	2 2 2 4 3 3 3 2 3 6% 69%	6010
	21st to end	Day	2 4 2 5% 3 2 2 2 3 5% 70%	6902
June	1st to 10th	Day	3 4 2 5% 3 3 3 3 4 7% 63%	2800
	11th to 20th	Day	1 3 1 2 2 2 2 2 4 6% 75%	2800
	21st to end	Day	1 2 1 3 3 3 2 2 3 4 76%	2800
July	1st to 10th	Day	1 1 2 2 3 2 2 2 3 82%	6319
	11th to 20th	Day	1 1 3 3 3 3 2 3 5% 76%	5936
	21st to end	Day	3 2 5% 3 2 2 2 3 6% 73%	6402
August	1st to 10th	Day	1 1 1 3 2 2 2 2 4 5% 75%	5777
	11th to 20th	Day	1 1 3 2 2 2 2 3 4 79%	5411
	21st to end	Day	1 1 3 3 3 3 2 3 5% 77%	5984
September	1st to 10th	Day	2 2 1 2 2 2 2 1 3 4 80%	2400
	11th to 20th	Day	2 3 1 5% 3 3 3 3 4 8% 66%	2400
	21st to end	Day	3 2 1 2 1 2 2 2 3 5% 76%	2142
October	1st to 10th	Day	2 2 1 3 2 2 2 2 4 6% 73%	3280
	11th to 20th	Day	2 2 1 4 3 2 2 2 3 4 72%	3242
	21st to end	Day	3 3 1 4 3 4 4 4 5% 6% 63%	3448
November	1st to 10th	Day	7% 4 1 4 3 4 4 3 5% 8% 57%	1900
	11th to 20th	Day	8% 4 2 4 4 4 4 3 5% 7% 55%	1800
	21st to end	Day	11% 5% 2 4 2 3 3 3 4 7% 56%	1705
December	1st to 10th	Day	7% 4 1 4 4 3 4 4 5% 8% 56%	2620
	11th to 20th	Day	20% 7% 2 6% 5% 4 4 4 5% 6% 36%	3028
	21st to end	Day	17% 10% 4 8% 5% 4 4 4 6% 6% 31%	4004
January	1st to 10th	Night	9% 6% 2 5% 4 3 3 3 5% 7% 52%	6086
	11th to 20th	Night	13% 5% 2 4 5% 4 4 3 4 5% 51%	5880
	21st to end	Night	15% 9% 3 6% 4 3 4 3 4 6% 45%	6468
February	1st to 10th	Night	17% 6% 2 4 3 2 3 3 4 5% 50%	2600
	11th to 20th	Night	13% 6% 3 5% 4 4 4 4 6% 7% 45%	2397
	21st to end	Night	12% 6% 2 6% 3 2 2 2 4 7% 52%	1980
March	1st to 10th	Night	15% 13% 3 8% 5% 4 4 4 5% 5% 34%	5079
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 47%	5000
	21st to end	Night	15% 11% 3 9% 6% 3 4 3 4 6% 36%	5358
April	1st to 10th	Night	10% 7% 1 5% 4 4 4 4 6% 7% 49%	2000
	11th to 20th	Night	8% 7% 3 6% 4 3 3 3 4 6% 54%	1979
	21st to end	Night	5% 7% 3 4 3 3 3 4 5% 9% 55%	2000
May	1st to 10th	Night	2 4 2 8% 3 4 4 3 4 5% 61%	4524
	11th to 20th	Night	2 4 4 13% 6% 5% 5% 3 4 6% 48%	3891
	21st to end	Night	2 5% 3 7% 5% 4 4 3 4 6% 59%	3972
June	1st to 10th	Night	3 5% 2 7% 4 3 4 3 5% 8% 56%	1600
	11th to 20th	Night	1 4 2 5% 5% 4 4 2 4 6% 65%	1600
	21st to end	Night	1 5% 2 5% 4 4 3 2 3 5% 66%	1600
July	1st to 10th	Night	1 2 5% 5% 6% 4 3 3 4 68%	3640
	11th to 20th	Night	1 2 9% 7% 7% 6% 3 4 5% 55%	4023
	21st to end	Night	1 14% 7% 12% 6% 5% 4 3 4 5% 40%	4472
August	1st to 10th	Night	1 3 4 6% 5% 7% 5% 2 3 5% 60%	4035
	11th to 20th	Night	2 2 7% 4 6% 5% 3 3 5% 62%	4455
	21st to end	Night	1 2 1 5% 5% 5% 5% 3 4 5% 63%	5015
September	1st to 10th	Night	1 3 1 4 3 2 3 2 4 6% 71%	2000
	11th to 20th	Night	3 4 2 8% 3 3 3 2 4 6% 61%	2000
	21st to end	Night	3 4 1 4 3 3 2 1 4 5% 71%	2252
October	1st to 10th	Night	2 3 1 2 2 2 2 2 3 6% 75%	3920
	11th to 20th	Night	3 4 2 9% 4 3 3 2 4 5% 62%	3873
	21st to end	Night	4 4 2 5% 3 3 2 3 4 5% 64%	4121
November	1st to 10th	Night	7% 5% 1 4 3 3 3 3 4 6% 61%	2395
	11th to 20th	Night	8% 5% 1 5% 3 3 3 3 6% 8% 55%	2600
	21st to end	Night	8% 5% 2 4 4 3 4 3 3 6% 57%	2690
December	1st to 10th	Night	9% 3 1 3 2 3 4 3 6% 6% 59%	4560
	11th to 20th	Night	20% 7% 3 6% 5% 4 4 4 7% 9% 32%	5176
	21st to end	Night	20% 10% 3 10% 5% 5% 5% 3 5% 6% 28%	6952

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	35% 2 17% 26% 17% 4	3851
	11th to 20th	Day	35% 2 9% 14% 21% 20%	4100
	21st to end	Day	43% 2 6% 14% 20% 15%	4510
February	1st to 10th	Day	40% 2 6% 22% 18% 11%	1800
	11th to 20th	Day	39% 3 10% 20% 21% 8%	1899
	21st to end	Day	40% 2 8% 23% 20% 6%	1650
March	1st to 10th	Day	50% 4 9% 17% 19% 2	4798
	11th to 20th	Day	34% 3 14% 31% 10% 8%	5000
	21st to end	Day	45% 5% 14% 24% 11% 2	5615
April	1st to 10th	Day	36% 3 11% 24% 20% 4	2400
	11th to 20th	Day	32% 3 12% 26% 20% 7%	2372
	21st to end	Day	28% 3 11% 31% 23% 5%	2400
May	1st to 10th	Day	16% 2 7% 31% 31% 13%	5457
	11th to 20th	Day	20% 3 6% 33% 33% 6%	6013
	21st to end	Day	22% 2 6% 29% 38% 3	6904
June	1st to 10th	Day	24% 3 10% 28% 27% 8%	2800
	11th to 20th	Day	15% 2 6% 23% 37% 17%	2800
	21st to end	Day	16% 2 3 13% 34% 33%	2800
July	1st to 10th	Day	9% 4 1 5% 21% 60%	6251
	11th to 20th	Day	11% 5% 3 5% 16% 60%	5865
	21st to end	Day	15% 4 1 17% 31% 32%	6217
August	1st to 10th	Day	15% 2 1 9% 33% 38%	4206
	11th to 20th	Day	9% 5% 1 9% 41% 35%	3915
	21st to end	Day	13% 3 2 8% 36% 37%	4311
September	1st to 10th	Day	12% 2 1 10% 32% 43%	2400
	11th to 20th	Day	21% 3 3 23% 35% 17%	2400
	21st to end	Day	14% 1 3 22% 36% 24%	2142
October	1st to 10th	Day	14% 2 4 12% 28% 40%	3280
	11th to 20th	Day	17% 3 5% 24% 31% 20%	3242
	21st to end	Day	21% 3 4 21% 31% 19%	3448
November	1st to 10th	Day	26% 3 7% 24% 26% 13%	1900
	11th to 20th	Day	29% 3 10% 24% 25% 9%	1800
	21st to end	Day	30% 2 9% 18% 30% 11%	1705
December	1st to 10th	Day	28% 2 8% 21% 34% 7%	2620
	11th to 20th	Day	48% 3 9% 17% 16% 7%	3028
	21st to end	Day	53% 3 12% 19% 10% 3	4004
January	1st to 10th	Night	32% 2 19% 25% 17% 4	6086
	11th to 20th	Night	36% 3 11% 16% 22% 13%	5880
	21st to end	Night	43% 2 7% 14% 17% 16%	6468
February	1st to 10th	Night	39% 1 6% 21% 21% 12%	2600
	11th to 20th	Night	38% 3 10% 21% 21% 8%	2398
	21st to end	Night	33% 2 10% 29% 19% 7%	1980
March	1st to 10th	Night	51% 3 10% 17% 16% 3	5079
	11th to 20th	Night	40% 2 11% 24% 16% 8%	5000
	21st to end	Night	49% 4 9% 23% 12% 3	5358
April	1st to 10th	Night	34% 4 14% 23% 19% 5%	2000
	11th to 20th	Night	34% 3 10% 29% 17% 7%	1979
	21st to end	Night	28% 4 11% 30% 21% 6%	2000
May	1st to 10th	Night	25% 4 8% 23% 26% 13%	4521
	11th to 20th	Night	42% 4 6% 19% 24% 6%	3891
	21st to end	Night	28% 5% 6% 23% 37% 2	3972
June	1st to 10th	Night	30% 4 9% 24% 23% 9%	1600
	11th to 20th	Night	24% 3 6% 14% 34% 21%	1600
	21st to end	Night	27% 3 1 11% 27% 31%	1600
July	1st to 10th	Night	17% 9% 1 4 17% 51%	3600
	11th to 20th	Night	23% 14% 3 4 18% 38%	3971
	21st to end	Night	41% 12% 2 7% 17% 21%	4345
August	1st to 10th	Night	26% 5% 1 7% 28% 33%	2929
	11th to 20th	Night	18% 9% 2 6% 31% 34%	3218
	21st to end	Night	23% 7% 3 7% 34% 26%	3608
September	1st to 10th	Night	18% 4 1 7% 26% 43%	2000
	11th to 20th	Night	29% 3 2 18% 30% 18%	2000
	21st to end	Night	21% 2 3 16% 37% 22%	2252
October	1st to 10th	Night	15% 2 4 11% 23% 45%	3920
	11th to 20th	Night	26% 4 4 23% 24% 19%	3873
	21st to end	Night	23% 3 4 21% 28% 21%	4121
November	1st to 10th	Night	26% 3 7% 24% 24% 16%	2395
	11th to 20th	Night	27% 3 10% 23% 27% 9%	2600
	21st to end	Night	30% 3 9% 20% 24% 15%	2690
December	1st to 10th	Night	26% 2 7% 25% 28% 12%	4560
	11th to 20th	Night	48% 3 9% 16% 19% 5%	5176
	21st to end	Night	56% 4 11% 15% 9% 4	6952

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	25% 7% 21% 25% 18% 4	295
	11th to 20th	Day	15% 8% 19% 29% 22% 8%	400
	21st to end	Day	15% 6% 19% 29% 24% 9%	440
February	1st to 10th	Day	19% 11% 21% 22% 19% 8%	400
	11th to 20th	Day	13% 13% 27% 27% 15% 5%	465
	21st to end	Day	13% 9% 25% 32% 18% 3	414
March	1st to 10th	Day	19% 13% 25% 29% 12% 2	500
	11th to 20th	Day	19% 10% 24% 26% 15% 5%	500
	21st to end	Day	16% 12% 29% 26% 13% 3	550
April	1st to 10th	Day	13% 10% 29% 27% 16% 5%	500
	11th to 20th	Day	11% 12% 28% 27% 15% 7%	492
	21st to end	Day	7% 11% 29% 28% 17% 9%	500
May	1st to 10th	Day	6% 10% 30% 30% 17% 6%	503
	11th to 20th	Day	8% 8% 29% 31% 18% 7%	528
	21st to end	Day	6% 10% 24% 32% 20% 8%	573
June	1st to 10th	Day	5% 6% 25% 32% 20% 12%	530
	11th to 20th	Day	2 6% 24% 38% 24% 6%	530
	21st to end	Day	2 4 19% 34% 29% 12%	530
July	1st to 10th	Day	1 1 16% 38% 31% 14%	526
	11th to 20th	Day	1 1 9% 37% 35% 18%	524
	21st to end	Day	1 1 10% 35% 36% 17%	575
August	1st to 10th	Day	3 1 16% 35% 31% 14%	527
	11th to 20th	Day	1 1 14% 36% 30% 17%	517
	21st to end	Day	2 1 9% 36% 34% 18%	583
September	1st to 10th	Day	3 3 10% 35% 32% 17%	530
	11th to 20th	Day	3 4 19% 34% 26% 13%	530
	21st to end	Day	4 5% 13% 34% 32% 13%	530
October	1st to 10th	Day	4 5% 19% 29% 28% 15%	530
	11th to 20th	Day	5% 6% 18% 27% 30% 14%	523
	21st to end	Day	8% 4 19% 29% 29% 11%	539
November	1st to 10th	Day	7% 8% 15% 28% 29% 13%	436
	11th to 20th	Day	8% 6% 12% 32% 30% 12%	300
	21st to end	Day	10% 3 17% 32% 31% 7%	253
December	1st to 10th	Day	11% 8% 17% 28% 24% 15%	200
	11th to 20th	Day	16% 9% 17% 22% 27% 11%	200
	21st to end	Day	19% 10% 15% 30% 21% 5%	220
January	1st to 10th	Night	17% 4 14% 37% 23% 6%	800
	11th to 20th	Night	14% 3 13% 38% 25% 8%	800
	21st to end	Night	18% 3 10% 37% 26% 6%	880
February	1st to 10th	Night	20% 4 13% 33% 23% 6%	800
	11th to 20th	Night	15% 4 18% 34% 22% 6%	784
	21st to end	Night	16% 4 16% 42% 20% 3	660
March	1st to 10th	Night	18% 7% 14% 36% 23% 4	800
	11th to 20th	Night	18% 4 15% 33% 25% 5%	800
	21st to end	Night	15% 7% 17% 37% 21% 3	880
April	1st to 10th	Night	12% 7% 15% 39% 24% 4	800
	11th to 20th	Night	11% 7% 17% 35% 24% 6%	791
	21st to end	Night	9% 5% 17% 35% 26% 8%	800
May	1st to 10th	Night	7% 2 17% 40% 28% 6%	790
	11th to 20th	Night	5% 3 11% 36% 35% 10%	513
	21st to end	Night	7% 7% 10% 36% 33% 6%	433
June	1st to 10th	Night	4 5% 9% 38% 39% 6%	400
	11th to 20th	Night	2 5% 10% 45% 36% 4	400
	21st to end	Night	2 3 5% 34% 49% 8%	400
July	1st to 10th	Night	2 2 4 26% 58% 9%	390
	11th to 20th	Night	2 1 2 17% 59% 20%	546
	21st to end	Night	2 5% 3 18% 54% 18%	657
August	1st to 10th	Night	4 5% 4 24% 48% 15%	597
	11th to 20th	Night	2 4 5% 33% 45% 11%	766
	21st to end	Night	2 1 5% 27% 50% 15%	880
September	1st to 10th	Night	2 2 5% 29% 48% 14%	800
	11th to 20th	Night	5% 3 7% 32% 42% 11%	800
	21st to end	Night	4 3 8% 31% 41% 13%	800
October	1st to 10th	Night	6% 4 6% 36% 35% 14%	800
	11th to 20th	Night	7% 4 9% 31% 37% 12%	792
	21st to end	Night	7% 2 10% 35% 29% 17%	836
November	1st to 10th	Night	8% 3 13% 33% 32% 11%	780
	11th to 20th	Night	10% 3 10% 39% 31% 7%	800
	21st to end	Night	10% 3 11% 43% 27% 8%	800
December	1st to 10th	Night	17% 2 11% 39% 25% 7%	800
	11th to 20th	Night	21% 4 11% 37% 23% 4	800
	21st to end	Night	20% 5% 17% 36% 19% 4	880

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