San Mateo
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Mateo clear sky chart from 2005-11-06 to 2025-04-26.

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% 5% 2 5% 4 4 4 3 5% 8% 54%	117704
	Night	7% 6% 2 6% 4 4 5% 4 6% 8% 48%	119816
Transparency	Day	17% 43% 5% 11% 13% 10%	117465
	Night	21% 42% 6% 11% 12% 8%	119645
Seeing	Day	9% 10% 26% 32% 18% 5%	15998
	Night	11% 5% 16% 34% 28% 6%	25458

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% 5% 2 6% 4 4 4 4 7% 10% 43%	3938
	11th to 20th	Day	9% 7% 3 8% 5% 4 3 3 3 5% 50%	4100
	21st to end	Day	15% 8% 3 7% 5% 4 4 4 6% 6% 38%	4510
February	1st to 10th	Day	13% 8% 2 6% 5% 4 5% 4 4 6% 44%	1800
	11th to 20th	Day	11% 8% 3 7% 4 4 4 3 6% 9% 40%	1836
	21st to end	Day	10% 6% 3 6% 4 4 4 4 5% 7% 46%	1650
March	1st to 10th	Day	9% 7% 3 9% 6% 6% 6% 5% 8% 8% 32%	4904
	11th to 20th	Day	8% 5% 2 6% 5% 5% 4 4 6% 8% 47%	5000
	21st to end	Day	8% 6% 2 7% 5% 6% 6% 5% 8% 11% 36%	5546
April	1st to 10th	Day	10% 6% 2 6% 5% 4 4 3 6% 8% 45%	2400
	11th to 20th	Day	8% 5% 2 6% 3 3 3 4 5% 7% 54%	2369
	21st to end	Day	4 5% 1 4 3 3 4 3 6% 10% 56%	2342
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 70%	3941
	11th to 20th	Day	3 3 1 3 2 3 2 2 5% 8% 68%	4121
	21st to end	Day	3 7% 2 5% 3 3 3 3 5% 8% 58%	4813
June	1st to 10th	Day	5% 6% 2 4 3 3 4 3 6% 9% 57%	2660
	11th to 20th	Day	1 3 1 3 2 2 3 3 5% 8% 70%	2660
	21st to end	Day	2 4 1 3 2 3 4 3 5% 8% 65%	2660
July	1st to 10th	Day	1 3 1 2 2 2 2 2 3 6% 77%	4395
	11th to 20th	Day	1 2 1 3 2 3 3 3 6% 10% 65%	4110
	21st to end	Day	1 3 1 4 3 3 4 4 5% 12% 60%	4455
August	1st to 10th	Day	1 3 1 4 3 4 5% 5% 7% 13% 51%	4091
	11th to 20th	Day	1 3 1 4 2 2 4 3 5% 10% 66%	3843
	21st to end	Day	1 3 2 4 3 3 3 3 5% 10% 63%	4180
September	1st to 10th	Day	2 4 1 3 3 3 2 3 3 8% 69%	2280
	11th to 20th	Day	3 6% 2 5% 3 3 3 3 6% 10% 55%	2280
	21st to end	Day	3 2 1 3 2 2 3 2 3 8% 71%	2007
October	1st to 10th	Day	2 3 1 4 2 2 2 3 4 6% 70%	3180
	11th to 20th	Day	3 3 1 6% 4 4 4 4 5% 7% 59%	3161
	21st to end	Day	4 4 2 5% 3 4 3 3 6% 8% 57%	3448
November	1st to 10th	Day	6% 5% 1 5% 3 4 4 3 5% 9% 54%	1941
	11th to 20th	Day	7% 8% 2 7% 4 4 5% 4 5% 7% 47%	1800
	21st to end	Day	10% 5% 3 5% 4 3 3 3 5% 8% 51%	1631
December	1st to 10th	Day	6% 3 2 6% 3 3 4 3 6% 11% 52%	2620
	11th to 20th	Day	15% 8% 3 9% 6% 7% 6% 4 7% 7% 28%	3028
	21st to end	Day	16% 10% 4 9% 6% 5% 5% 4 6% 8% 28%	4004
January	1st to 10th	Night	9% 7% 3 6% 4 5% 5% 4 6% 10% 42%	6017
	11th to 20th	Night	11% 8% 3 7% 5% 5% 4 3 4 5% 44%	5880
	21st to end	Night	13% 10% 3 8% 5% 4 4 3 5% 6% 39%	6468
February	1st to 10th	Night	13% 7% 3 5% 4 4 4 3 5% 7% 46%	2600
	11th to 20th	Night	10% 7% 2 7% 5% 5% 4 4 5% 8% 42%	2448
	21st to end	Night	10% 6% 2 5% 4 4 4 3 6% 7% 48%	1980
March	1st to 10th	Night	11% 9% 3 8% 6% 5% 6% 5% 7% 8% 33%	4987
	11th to 20th	Night	11% 7% 3 6% 4 4 4 3 5% 7% 47%	5000
	21st to end	Night	11% 9% 3 8% 5% 5% 4 4 6% 7% 39%	5429
April	1st to 10th	Night	7% 6% 3 6% 5% 3 4 3 7% 8% 46%	2000
	11th to 20th	Night	8% 6% 2 5% 4 4 3 4 5% 7% 53%	1979
	21st to end	Night	3 4 1 4 3 3 5% 3 5% 9% 58%	1954
May	1st to 10th	Night	4 4 1 4 3 3 2 2 4 6% 67%	3030
	11th to 20th	Night	2 3 2 4 4 4 4 4 6% 9% 57%	2775
	21st to end	Night	3 4 1 5% 3 4 3 3 5% 5% 63%	2762
June	1st to 10th	Night	3 5% 2 6% 4 3 4 3 7% 10% 53%	1520
	11th to 20th	Night	1 2 1 3 3 4 3 3 6% 9% 66%	1520
	21st to end	Night	1 4 2 4 3 3 3 3 5% 8% 63%	1520
July	1st to 10th	Night	1 3 1 3 2 3 3 2 3 7% 73%	2561
	11th to 20th	Night	1 3 2 4 3 4 6% 6% 8% 10% 54%	2860
	21st to end	Night	6% 3 7% 7% 7% 7% 5% 6% 10% 40%	3109
August	1st to 10th	Night	2 4 3 7% 5% 7% 10% 6% 9% 13% 34%	2848
	11th to 20th	Night	4 1 6% 4 4 6% 4 5% 9% 58%	2999
	21st to end	Night	1 3 2 6% 4 4 5% 4 7% 10% 54%	3498
September	1st to 10th	Night	1 2 1 5% 3 3 4 4 6% 10% 62%	1900
	11th to 20th	Night	3 5% 2 5% 4 4 4 3 6% 10% 52%	1900
	21st to end	Night	2 2 1 4 3 3 3 2 3 8% 68%	2166
October	1st to 10th	Night	2 3 1 3 3 3 3 3 5% 6% 67%	3800
	11th to 20th	Night	4 4 2 5% 5% 6% 6% 5% 5% 7% 52%	3777
	21st to end	Night	5% 4 2 4 3 3 4 3 5% 7% 59%	4121
November	1st to 10th	Night	7% 5% 2 4 4 4 4 3 5% 8% 54%	2355
	11th to 20th	Night	8% 7% 3 6% 4 3 4 4 6% 8% 48%	2600
	21st to end	Night	8% 5% 2 5% 4 4 4 4 5% 9% 52%	2765
December	1st to 10th	Night	7% 4 2 5% 4 3 4 4 4 7% 55%	4560
	11th to 20th	Night	17% 9% 3 8% 6% 5% 6% 4 7% 10% 26%	5176
	21st to end	Night	18% 9% 3 9% 5% 5% 6% 4 6% 6% 29%	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	25% 30% 14% 16% 12% 3	3938
	11th to 20th	Day	31% 29% 5% 10% 15% 11%	4100
	21st to end	Day	34% 31% 4 9% 8% 14%	4510
February	1st to 10th	Day	20% 63% 4 6% 6% 2	1800
	11th to 20th	Day	17% 62% 4 7% 7% 2	1845
	21st to end	Day	13% 63% 4 12% 6% 3	1650
March	1st to 10th	Day	33% 34% 4 13% 14% 1	4904
	11th to 20th	Day	20% 31% 12% 22% 8% 7%	5000
	21st to end	Day	27% 36% 13% 18% 6%	5546
April	1st to 10th	Day	15% 63% 5% 9% 7% 1	2400
	11th to 20th	Day	11% 63% 7% 10% 8% 2	2372
	21st to end	Day	9% 65% 7% 9% 9% 1	2342
May	1st to 10th	Day	9% 40% 8% 19% 15% 8%	3941
	11th to 20th	Day	8% 40% 7% 12% 26% 7%	4124
	21st to end	Day	14% 44% 6% 16% 19% 2	4814
June	1st to 10th	Day	8% 65% 4 10% 8% 5%	2660
	11th to 20th	Day	4 65% 3 10% 13% 4	2660
	21st to end	Day	7% 63% 3 4 11% 12%	2660
July	1st to 10th	Day	6% 35% 3 4 5% 48%	4326
	11th to 20th	Day	9% 36% 3 4 9% 39%	4041
	21st to end	Day	13% 34% 2 7% 21% 24%	4315
August	1st to 10th	Day	18% 37% 4 8% 17% 17%	4076
	11th to 20th	Day	10% 36% 3 9% 30% 12%	3882
	21st to end	Day	12% 36% 4 11% 22% 15%	4179
September	1st to 10th	Day	9% 59% 3 6% 13% 11%	2280
	11th to 20th	Day	10% 61% 3 9% 11% 6%	2280
	21st to end	Day	7% 60% 3 9% 15% 6%	2007
October	1st to 10th	Day	8% 36% 1 6% 18% 32%	3180
	11th to 20th	Day	16% 38% 5% 15% 17% 10%	3161
	21st to end	Day	15% 37% 6% 17% 17% 8%	3448
November	1st to 10th	Day	7% 56% 4 14% 13% 5%	1941
	11th to 20th	Day	16% 58% 4 12% 7% 2	1800
	21st to end	Day	16% 58% 5% 7% 11% 3	1631
December	1st to 10th	Day	16% 37% 3 22% 19% 3	2620
	11th to 20th	Day	37% 36% 6% 8% 11% 2	3028
	21st to end	Day	40% 29% 8% 14% 8% 1	4004
January	1st to 10th	Night	24% 32% 17% 16% 10% 3	6017
	11th to 20th	Night	33% 30% 7% 10% 13% 7%	5880
	21st to end	Night	35% 30% 3 8% 14% 10%	6468
February	1st to 10th	Night	19% 62% 4 6% 6% 3	2600
	11th to 20th	Night	17% 62% 5% 7% 6% 3	2450
	21st to end	Night	11% 62% 5% 13% 6% 2	1980
March	1st to 10th	Night	35% 34% 5% 14% 11% 1	4987
	11th to 20th	Night	26% 30% 10% 17% 11% 6%	5000
	21st to end	Night	32% 34% 11% 16% 7%	5429
April	1st to 10th	Night	13% 63% 5% 10% 7% 1	2000
	11th to 20th	Night	12% 63% 4 11% 7% 2	1979
	21st to end	Night	9% 66% 6% 10% 7% 1	1954
May	1st to 10th	Night	13% 41% 7% 15% 14% 9%	3030
	11th to 20th	Night	15% 43% 10% 6% 17% 10%	2776
	21st to end	Night	14% 43% 5% 18% 19% 1	2762
June	1st to 10th	Night	10% 66% 3 8% 8% 5%	1520
	11th to 20th	Night	6% 66% 5% 10% 11% 3	1520
	21st to end	Night	8% 63% 2 4 10% 13%	1520
July	1st to 10th	Night	8% 35% 3 3 7% 45%	2521
	11th to 20th	Night	14% 35% 4 4 11% 33%	2806
	21st to end	Night	22% 35% 2 5% 15% 20%	3011
August	1st to 10th	Night	28% 37% 3 11% 15% 6%	2839
	11th to 20th	Night	14% 37% 3 5% 32% 9%	3026
	21st to end	Night	15% 38% 5% 10% 20% 12%	3498
September	1st to 10th	Night	9% 61% 4 5% 12% 9%	1900
	11th to 20th	Night	10% 60% 3 12% 10% 5%	1900
	21st to end	Night	8% 60% 4 9% 11% 8%	2166
October	1st to 10th	Night	8% 36% 1 4 17% 34%	3800
	11th to 20th	Night	18% 38% 8% 12% 13% 11%	3777
	21st to end	Night	15% 37% 5% 15% 17% 11%	4121
November	1st to 10th	Night	10% 56% 4 13% 11% 6%	2355
	11th to 20th	Night	15% 58% 6% 12% 7% 3	2600
	21st to end	Night	16% 58% 5% 8% 9% 4	2765
December	1st to 10th	Night	17% 38% 4 20% 16% 5%	4560
	11th to 20th	Night	36% 36% 6% 10% 10% 2	5176
	21st to end	Night	39% 31% 10% 13% 5% 2	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% 9% 25% 22% 16% 2	330
	11th to 20th	Day	15% 11% 23% 31% 16% 5%	400
	21st to end	Day	14% 11% 25% 28% 18% 4	440
February	1st to 10th	Day	16% 15% 24% 25% 18% 3	400
	11th to 20th	Day	16% 16% 29% 26% 12% 1	436
	21st to end	Day	12% 15% 28% 28% 16% 2	414
March	1st to 10th	Day	18% 18% 31% 21% 10% 2	500
	11th to 20th	Day	17% 15% 28% 22% 15% 3	500
	21st to end	Day	13% 18% 32% 26% 8% 3	550
April	1st to 10th	Day	13% 16% 31% 24% 13% 3	500
	11th to 20th	Day	11% 18% 32% 25% 10% 4	492
	21st to end	Day	6% 20% 31% 28% 11% 6%	481
May	1st to 10th	Day	8% 17% 34% 28% 11% 2	463
	11th to 20th	Day	8% 15% 35% 29% 11% 2	488
	21st to end	Day	8% 15% 32% 29% 12% 3	529
June	1st to 10th	Day	6% 12% 34% 32% 13% 3	490
	11th to 20th	Day	3 10% 31% 38% 13% 4	490
	21st to end	Day	3 7% 29% 38% 19% 5%	490
July	1st to 10th	Day	5% 4 23% 46% 17% 5%	486
	11th to 20th	Day	1 1 20% 50% 24% 4	484
	21st to end	Day	3 1 23% 46% 20% 7%	531
August	1st to 10th	Day	5% 4 24% 43% 18% 6%	487
	11th to 20th	Day	3 2 24% 45% 21% 5%	477
	21st to end	Day	3 3 20% 44% 23% 6%	539
September	1st to 10th	Day	3 4 16% 44% 24% 9%	490
	11th to 20th	Day	5% 10% 25% 36% 18% 7%	490
	21st to end	Day	3 8% 21% 32% 27% 9%	490
October	1st to 10th	Day	3 7% 26% 30% 22% 11%	490
	11th to 20th	Day	6% 9% 21% 29% 26% 9%	490
	21st to end	Day	9% 7% 22% 30% 23% 8%	539
November	1st to 10th	Day	8% 10% 21% 27% 26% 8%	477
	11th to 20th	Day	9% 8% 17% 34% 25% 7%	300
	21st to end	Day	12% 7% 22% 32% 27% 1	215
December	1st to 10th	Day	12% 15% 18% 23% 23% 10%	200
	11th to 20th	Day	14% 13% 21% 28% 21% 5%	200
	21st to end	Day	20% 9% 28% 28% 14% 1	220
January	1st to 10th	Night	19% 6% 16% 33% 22% 5%	800
	11th to 20th	Night	15% 6% 15% 33% 25% 6%	800
	21st to end	Night	18% 5% 14% 35% 24% 4	880
February	1st to 10th	Night	18% 7% 17% 33% 23% 4	800
	11th to 20th	Night	13% 9% 18% 34% 21% 4	784
	21st to end	Night	12% 10% 20% 34% 20% 3	660
March	1st to 10th	Night	17% 9% 23% 28% 21% 3	800
	11th to 20th	Night	17% 9% 18% 31% 22% 4	800
	21st to end	Night	15% 10% 22% 32% 18% 2	880
April	1st to 10th	Night	11% 8% 24% 34% 20% 3	800
	11th to 20th	Night	11% 11% 22% 31% 20% 5%	791
	21st to end	Night	7% 8% 25% 32% 22% 5%	782
May	1st to 10th	Night	6% 6% 23% 38% 23% 5%	675
	11th to 20th	Night	8% 4 17% 36% 30% 4	529
	21st to end	Night	8% 9% 22% 37% 21% 4	411
June	1st to 10th	Night	6% 5% 20% 41% 26% 3	380
	11th to 20th	Night	1 3 18% 47% 28% 2	380
	21st to end	Night	4 3 14% 37% 38% 4	380
July	1st to 10th	Night	4 2 11% 33% 45% 5%	378
	11th to 20th	Night	3 1 7% 38% 41% 10%	552
	21st to end	Night	5% 2 9% 34% 41% 9%	624
August	1st to 10th	Night	8% 4 12% 33% 36% 8%	567
	11th to 20th	Night	5% 2 11% 40% 35% 7%	673
	21st to end	Night	3 1 12% 36% 38% 9%	836
September	1st to 10th	Night	3 1 11% 33% 41% 11%	760
	11th to 20th	Night	8% 3 14% 35% 33% 7%	760
	21st to end	Night	3 3 13% 30% 40% 11%	760
October	1st to 10th	Night	7% 2 13% 33% 35% 10%	760
	11th to 20th	Night	7% 4 12% 29% 38% 10%	760
	21st to end	Night	8% 4 12% 30% 34% 12%	836
November	1st to 10th	Night	9% 3 16% 28% 35% 9%	780
	11th to 20th	Night	11% 4 12% 37% 30% 7%	800
	21st to end	Night	11% 5% 13% 39% 28% 5%	800
December	1st to 10th	Night	15% 6% 14% 34% 25% 5%	800
	11th to 20th	Night	23% 6% 16% 31% 21% 3	800
	21st to end	Night	20% 8% 21% 30% 19% 2	880

Page generated at 2025-04-23 06:25:06. Version ac07

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