Glacier Point
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Glacier Point clear sky chart from 2005-11-06 to 2024-03-23.

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% 7% 3 4 3 2 3 3 4 6% 59%	78820
	Night	6% 7% 2 4 3 2 3 3 4 6% 59%	81095
Transparency	Day	27% 4 8% 20% 25% 16%	78530
	Night	26% 5% 8% 21% 25% 15%	80907
Seeing	Day	12% 14% 34% 26% 10% 3	14300
	Night	10% 6% 22% 33% 23% 6%	23894

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	7% 13% 6% 6% 3 2 3 3 4 6% 46%	2539
	11th to 20th	Day	11% 12% 3 5% 6% 4 4 5% 4 7% 39%	2677
	21st to end	Day	9% 13% 4 5% 5% 3 3 4 6% 8% 40%	3146
February	1st to 10th	Day	9% 12% 4 4 3 1 3 4 5% 7% 49%	1781
	11th to 20th	Day	6% 10% 5% 6% 3 2 4 4 4 6% 50%	1850
	21st to end	Day	8% 14% 6% 7% 5% 2 3 4 3 4 45%	1570
March	1st to 10th	Day	7% 15% 6% 10% 4 3 5% 4 5% 6% 36%	1900
	11th to 20th	Day	8% 15% 5% 7% 4 2 3 4 5% 6% 41%	2038
	21st to end	Day	8% 14% 6% 5% 4 2 4 3 4 6% 43%	2353
April	1st to 10th	Day	8% 10% 4 5% 4 2 4 5% 5% 7% 46%	2160
	11th to 20th	Day	8% 12% 3 5% 4 2 3 3 4 6% 50%	2129
	21st to end	Day	7% 11% 2 4 3 2 4 4 5% 6% 53%	2160
May	1st to 10th	Day	6% 8% 2 4 3 2 3 3 4 8% 57%	2239
	11th to 20th	Day	6% 8% 2 4 4 2 4 4 4 5% 57%	2337
	21st to end	Day	7% 10% 3 4 4 2 3 4 5% 7% 52%	2723
June	1st to 10th	Day	6% 4 2 3 4 2 3 4 4 7% 61%	2520
	11th to 20th	Day	2 2 1 2 2 1 2 2 3 5% 77%	2520
	21st to end	Day	2 2 1 2 2 1 2 2 3 4 78%	2520
July	1st to 10th	Day	1 1 1 2 1 3 3 3 5% 80%	2520
	11th to 20th	Day	2 1 1 1 2 1 2 2 4 4 79%	2520
	21st to end	Day	2 2 1 3 3 2 3 3 4 6% 70%	2635
August	1st to 10th	Day	2 2 1 2 2 2 2 3 4 3 78%	2235
	11th to 20th	Day	1 1 1 1 2 2 3 3 4 5% 78%	2122
	21st to end	Day	2 1 1 1 2 1 2 2 2 3 82%	2376
September	1st to 10th	Day	1 2 1 2 2 1 2 3 3 3 80%	2160
	11th to 20th	Day	3 2 1 3 2 1 3 2 4 4 73%	2111
	21st to end	Day	3 2 1 2 3 2 2 4 4 4 73%	1980
October	1st to 10th	Day	5% 5% 2 4 2 1 2 2 4 5% 68%	1871
	11th to 20th	Day	4 2 1 2 3 2 3 3 5% 5% 69%	1800
	21st to end	Day	6% 4 2 3 1 2 3 3 5% 70%	1949
November	1st to 10th	Day	7% 7% 3 4 3 2 4 4 4 6% 57%	1674
	11th to 20th	Day	7% 4 2 3 3 2 4 4 4 5% 62%	1710
	21st to end	Day	6% 7% 2 4 3 2 3 3 5% 6% 58%	1710
December	1st to 10th	Day	11% 8% 2 3 5% 3 4 4 4 6% 51%	1563
	11th to 20th	Day	15% 12% 4 7% 5% 3 4 4 4 5% 36%	1928
	21st to end	Day	7% 9% 4 7% 4 3 4 6% 7% 8% 40%	2794
January	1st to 10th	Night	5% 10% 3 6% 4 2 3 4 4 5% 53%	4406
	11th to 20th	Night	11% 11% 4 7% 5% 3 4 5% 5% 7% 37%	4284
	21st to end	Night	6% 14% 4 5% 5% 3 5% 5% 6% 7% 41%	4521
February	1st to 10th	Night	9% 10% 3 4 4 2 4 4 4 6% 50%	2397
	11th to 20th	Night	6% 10% 4 5% 3 2 4 3 5% 8% 51%	2218
	21st to end	Night	8% 9% 4 7% 4 3 3 3 3 6% 49%	1884
March	1st to 10th	Night	7% 13% 5% 8% 5% 2 3 4 4 6% 43%	2280
	11th to 20th	Night	6% 11% 6% 6% 4 3 3 4 5% 7% 46%	2138
	21st to end	Night	7% 11% 4 4 3 3 3 3 4 6% 51%	2047
April	1st to 10th	Night	6% 10% 4 4 4 3 4 3 4 6% 52%	1800
	11th to 20th	Night	5% 8% 3 4 4 2 3 4 3 6% 58%	1779
	21st to end	Night	5% 7% 2 3 3 2 3 3 4 7% 61%	1800
May	1st to 10th	Night	4 4 1 3 3 2 3 4 5% 7% 62%	1720
	11th to 20th	Night	4 6% 1 2 2 2 2 3 3 7% 67%	1554
	21st to end	Night	3 5% 2 3 3 2 2 4 4 7% 65%	1552
June	1st to 10th	Night	4 3 1 3 3 2 3 3 4 6% 68%	1440
	11th to 20th	Night	1 1 1 1 1 1 2 4 3 5% 81%	1440
	21st to end	Night	2 1 1 1 1 1 2 2 2 4 84%	1440
July	1st to 10th	Night	1 1 1 2 2 3 4 85%	1440
	11th to 20th	Night	2 2 1 2 3 2 1 2 3 4 79%	1440
	21st to end	Night	3 2 2 2 3 2 3 3 5% 5% 70%	1678
August	1st to 10th	Night	2 2 1 2 2 2 2 3 3 3 79%	1702
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 81%	1780
	21st to end	Night	2 1 1 2 1 2 1 2 3 85%	1980
September	1st to 10th	Night	1 1 2 2 1 1 1 1 3 89%	1800
	11th to 20th	Night	1 2 1 2 2 2 2 1 2 3 82%	1847
	21st to end	Night	3 2 1 2 2 1 2 2 3 4 78%	1980
October	1st to 10th	Night	4 4 1 3 3 1 2 2 2 4 73%	2086
	11th to 20th	Night	5% 2 1 2 2 2 2 3 4 5% 74%	2160
	21st to end	Night	4 2 1 2 2 1 2 2 3 5% 77%	2407
November	1st to 10th	Night	6% 7% 2 3 3 1 2 3 4 8% 60%	2405
	11th to 20th	Night	6% 4 2 3 3 1 3 3 5% 8% 60%	2470
	21st to end	Night	6% 7% 2 4 3 2 3 4 5% 6% 58%	2470
December	1st to 10th	Night	11% 8% 2 4 3 2 3 3 4 6% 53%	2612
	11th to 20th	Night	13% 15% 4 5% 3 3 4 4 5% 6% 38%	3276
	21st to end	Night	7% 9% 4 7% 6% 3 5% 5% 5% 8% 40%	4862

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	34% 9% 10% 21% 20% 6%	2539
	11th to 20th	Day	42% 9% 8% 19% 13% 9%	2677
	21st to end	Day	41% 7% 10% 22% 14% 6%	3146
February	1st to 10th	Day	37% 4 8% 20% 17% 13%	1781
	11th to 20th	Day	35% 5% 8% 23% 22% 7%	1860
	21st to end	Day	44% 5% 7% 20% 19% 5%	1570
March	1st to 10th	Day	49% 6% 9% 15% 15% 6%	1900
	11th to 20th	Day	43% 5% 11% 19% 16% 6%	2038
	21st to end	Day	43% 5% 14% 21% 14% 4	2353
April	1st to 10th	Day	37% 6% 14% 20% 18% 5%	2160
	11th to 20th	Day	36% 5% 11% 25% 17% 5%	2132
	21st to end	Day	31% 6% 14% 30% 17% 3	2160
May	1st to 10th	Day	28% 4 12% 32% 21% 3	2239
	11th to 20th	Day	29% 5% 10% 29% 22% 5%	2340
	21st to end	Day	31% 5% 13% 30% 18% 3	2724
June	1st to 10th	Day	23% 5% 12% 27% 26% 7%	2520
	11th to 20th	Day	12% 3 8% 26% 37% 14%	2520
	21st to end	Day	12% 2 3 18% 39% 27%	2520
July	1st to 10th	Day	8% 3 3 13% 31% 43%	2451
	11th to 20th	Day	10% 2 6% 15% 31% 36%	2445
	21st to end	Day	14% 3 7% 15% 27% 33%	2488
August	1st to 10th	Day	11% 2 4 14% 28% 40%	2219
	11th to 20th	Day	9% 2 6% 13% 39% 32%	2122
	21st to end	Day	9% 2 3 14% 39% 34%	2376
September	1st to 10th	Day	9% 2 4 14% 34% 38%	2160
	11th to 20th	Day	13% 3 6% 25% 33% 20%	2111
	21st to end	Day	14% 3 6% 22% 36% 19%	1980
October	1st to 10th	Day	20% 3 6% 17% 32% 22%	1871
	11th to 20th	Day	17% 3 7% 23% 29% 21%	1800
	21st to end	Day	18% 4 5% 21% 33% 19%	1949
November	1st to 10th	Day	29% 4 8% 21% 24% 13%	1674
	11th to 20th	Day	24% 5% 8% 21% 29% 13%	1710
	21st to end	Day	29% 4 7% 17% 29% 15%	1710
December	1st to 10th	Day	34% 4 8% 20% 20% 13%	1563
	11th to 20th	Day	52% 4 6% 14% 14% 10%	1928
	21st to end	Day	38% 8% 10% 18% 17% 9%	2794
January	1st to 10th	Night	31% 6% 10% 26% 20% 7%	4406
	11th to 20th	Night	41% 9% 7% 18% 17% 9%	4284
	21st to end	Night	39% 9% 9% 19% 16% 7%	4521
February	1st to 10th	Night	36% 5% 6% 20% 21% 13%	2397
	11th to 20th	Night	32% 5% 8% 22% 23% 11%	2219
	21st to end	Night	38% 3 10% 20% 22% 6%	1884
March	1st to 10th	Night	42% 5% 9% 19% 18% 7%	2280
	11th to 20th	Night	38% 5% 9% 22% 19% 7%	2138
	21st to end	Night	34% 6% 14% 26% 15% 5%	2047
April	1st to 10th	Night	33% 5% 14% 25% 17% 7%	1800
	11th to 20th	Night	28% 6% 12% 24% 22% 8%	1779
	21st to end	Night	24% 5% 14% 35% 20% 2	1800
May	1st to 10th	Night	22% 5% 11% 33% 24% 5%	1720
	11th to 20th	Night	19% 4 13% 34% 26% 4	1555
	21st to end	Night	20% 5% 14% 33% 23% 5%	1552
June	1st to 10th	Night	18% 5% 12% 27% 32% 7%	1440
	11th to 20th	Night	8% 3 9% 26% 38% 16%	1440
	21st to end	Night	8% 3 3 17% 39% 30%	1440
July	1st to 10th	Night	4 2 4 12% 35% 44%	1400
	11th to 20th	Night	12% 2 5% 13% 31% 38%	1392
	21st to end	Night	16% 3 5% 15% 29% 32%	1585
August	1st to 10th	Night	11% 2 4 16% 26% 41%	1693
	11th to 20th	Night	8% 3 4 17% 36% 31%	1780
	21st to end	Night	9% 1 2 14% 41% 33%	1980
September	1st to 10th	Night	6% 1 1 15% 38% 39%	1800
	11th to 20th	Night	11% 2 5% 24% 37% 20%	1847
	21st to end	Night	11% 2 5% 25% 36% 20%	1980
October	1st to 10th	Night	17% 3 6% 20% 32% 22%	2086
	11th to 20th	Night	14% 4 6% 20% 33% 23%	2160
	21st to end	Night	13% 2 6% 22% 37% 19%	2407
November	1st to 10th	Night	24% 4 8% 22% 27% 16%	2405
	11th to 20th	Night	23% 4 8% 20% 31% 14%	2470
	21st to end	Night	28% 4 7% 18% 28% 15%	2470
December	1st to 10th	Night	32% 5% 7% 19% 22% 14%	2612
	11th to 20th	Night	46% 4 8% 14% 18% 9%	3276
	21st to end	Night	38% 8% 8% 17% 20% 9%	4862

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	32% 14% 24% 20% 8% 1	180
	11th to 20th	Day	22% 12% 27% 26% 9% 4	265
	21st to end	Day	16% 15% 31% 21% 13% 3	407
February	1st to 10th	Day	22% 18% 27% 20% 9% 3	403
	11th to 20th	Day	18% 21% 31% 21% 7% 1	455
	21st to end	Day	21% 20% 34% 21% 3 1	382
March	1st to 10th	Day	23% 22% 34% 15% 5%	460
	11th to 20th	Day	22% 22% 25% 22% 8% 1	460
	21st to end	Day	21% 25% 33% 17% 4	471
April	1st to 10th	Day	19% 19% 35% 20% 7%	420
	11th to 20th	Day	23% 21% 31% 20% 4 1	412
	21st to end	Day	14% 17% 34% 23% 10% 2	420
May	1st to 10th	Day	18% 19% 30% 22% 8% 2	423
	11th to 20th	Day	21% 14% 31% 21% 11% 3	448
	21st to end	Day	18% 15% 24% 28% 11% 4	485
June	1st to 10th	Day	9% 16% 34% 25% 12% 5%	450
	11th to 20th	Day	7% 15% 35% 25% 13% 5%	450
	21st to end	Day	3 12% 38% 32% 10% 6%	450
July	1st to 10th	Day	2 4 43% 35% 12% 4	446
	11th to 20th	Day	2 4 34% 41% 12% 7%	444
	21st to end	Day	2 6% 39% 36% 10% 7%	487
August	1st to 10th	Day	2 10% 48% 25% 10% 5%	447
	11th to 20th	Day	3 7% 44% 29% 12% 5%	437
	21st to end	Day	3 6% 41% 33% 14% 4	495
September	1st to 10th	Day	3 8% 44% 30% 14% 2	450
	11th to 20th	Day	7% 13% 43% 26% 10% 1	450
	21st to end	Day	7% 12% 40% 31% 9% 1	450
October	1st to 10th	Day	8% 19% 32% 30% 11%	450
	11th to 20th	Day	6% 12% 36% 34% 9% 2	450
	21st to end	Day	10% 13% 32% 32% 11% 2	464
November	1st to 10th	Day	13% 11% 31% 23% 19% 3	270
	11th to 20th	Day	11% 10% 27% 30% 15% 6%	270
	21st to end	Day	13% 11% 30% 30% 13% 3	270
December	1st to 10th	Day	19% 10% 28% 22% 16% 3	201
	11th to 20th	Day	14% 16% 33% 20% 14% 3	180
	21st to end	Day	23% 13% 30% 19% 11% 4	198
January	1st to 10th	Night	17% 9% 28% 23% 15% 8%	760
	11th to 20th	Night	13% 10% 28% 31% 13% 5%	760
	21st to end	Night	15% 10% 31% 24% 15% 5%	836
February	1st to 10th	Night	17% 8% 27% 30% 14% 4	760
	11th to 20th	Night	13% 12% 26% 26% 17% 7%	744
	21st to end	Night	16% 8% 29% 32% 12% 2	628
March	1st to 10th	Night	18% 10% 27% 29% 13% 4	760
	11th to 20th	Night	16% 9% 22% 28% 19% 6%	760
	21st to end	Night	14% 9% 29% 29% 15% 2	800
April	1st to 10th	Night	12% 7% 27% 34% 16% 4	720
	11th to 20th	Night	10% 9% 29% 29% 18% 6%	711
	21st to end	Night	11% 5% 26% 33% 20% 6%	720
May	1st to 10th	Night	6% 6% 26% 35% 21% 5%	640
	11th to 20th	Night	9% 5% 21% 40% 22% 3	492
	21st to end	Night	6% 3 20% 34% 30% 7%	389
June	1st to 10th	Night	6% 4 13% 38% 35% 4	360
	11th to 20th	Night	2 2 19% 41% 33% 4	360
	21st to end	Night	2 18% 39% 36% 4	360
July	1st to 10th	Night	1 13% 37% 45% 4	350
	11th to 20th	Night	3 1 11% 39% 41% 5%	348
	21st to end	Night	3 1 11% 31% 42% 12%	496
August	1st to 10th	Night	2 3 14% 37% 37% 7%	628
	11th to 20th	Night	2 11% 45% 35% 7%	712
	21st to end	Night	2 8% 41% 42% 6%	792
September	1st to 10th	Night	2 1 9% 43% 37% 9%	720
	11th to 20th	Night	4 1 15% 39% 33% 8%	720
	21st to end	Night	3 2 17% 37% 31% 10%	720
October	1st to 10th	Night	5% 3 24% 36% 25% 7%	720
	11th to 20th	Night	5% 3 21% 35% 27% 8%	720
	21st to end	Night	6% 4 20% 32% 25% 11%	792
November	1st to 10th	Night	10% 5% 17% 36% 24% 8%	740
	11th to 20th	Night	9% 6% 25% 37% 18% 6%	760
	21st to end	Night	11% 5% 29% 34% 17% 5%	760
December	1st to 10th	Night	18% 7% 26% 34% 12% 4	760
	11th to 20th	Night	20% 9% 26% 27% 15% 3	760
	21st to end	Night	18% 11% 30% 26% 12% 3	836

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