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 2025-08-05.

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	4 6% 2 4 2 2 3 2 4 5% 64%	129471
	Night	4 6% 2 4 3 2 3 2 4 5% 64%	127468
Transparency	Day	22% 4 7% 19% 27% 21%	129174
	Night	23% 4 7% 20% 27% 19%	127280
Seeing	Day	12% 15% 35% 25% 10% 3	16156
	Night	10% 6% 24% 33% 22% 5%	25574

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	6% 10% 4 6% 2 3 3 2 4 6% 54%	3639
	11th to 20th	Day	7% 9% 2 4 4 3 3 2 3 5% 58%	3847
	21st to end	Day	7% 10% 3 5% 4 3 3 3 5% 7% 49%	4510
February	1st to 10th	Day	9% 12% 4 5% 2 2 3 3 5% 6% 48%	1875
	11th to 20th	Day	6% 11% 4 6% 3 3 5% 3 3 6% 50%	1950
	21st to end	Day	7% 14% 5% 7% 4 3 3 3 3 5% 45%	1650
March	1st to 10th	Day	4 16% 7% 12% 4 3 4 3 5% 7% 36%	4518
	11th to 20th	Day	5% 14% 4 8% 3 3 4 3 4 6% 47%	4885
	21st to end	Day	6% 16% 9% 7% 3 3 3 2 4 6% 41%	5871
April	1st to 10th	Day	7% 11% 4 5% 3 3 3 4 5% 6% 48%	2400
	11th to 20th	Day	7% 13% 3 5% 3 3 3 2 4 6% 50%	2369
	21st to end	Day	6% 12% 2 4 3 3 4 3 6% 6% 52%	2400
May	1st to 10th	Day	6% 6% 1 3 2 2 2 1 3 5% 68%	5649
	11th to 20th	Day	3 6% 2 5% 2 3 3 3 3 5% 67%	5942
	21st to end	Day	3 5% 2 3 2 2 3 2 4 6% 69%	6902
June	1st to 10th	Day	5% 4 2 3 4 3 3 4 5% 7% 61%	2800
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 78%	2800
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 79%	2800
July	1st to 10th	Day	1 1 1 1 1 2 2 90%	6319
	11th to 20th	Day	1 1 2 2 2 2 2 3 5% 81%	6320
	21st to end	Day	1 1 1 2 1 2 2 1 3 6% 81%	6621
August	1st to 10th	Day	2 2 1 2 2 2 2 2 3 3 81%	3973
	11th to 20th	Day	1 1 1 1 1 2 1 3 4 86%	3724
	21st to end	Day	2 1 1 1 1 2 1 2 3 84%	4180
September	1st to 10th	Day	1 2 1 2 1 2 2 2 3 3 81%	2280
	11th to 20th	Day	3 3 1 3 2 2 3 2 4 4 72%	2228
	21st to end	Day	3 2 1 3 2 2 2 3 4 4 75%	2090
October	1st to 10th	Day	3 4 1 3 2 2 2 2 4 5% 73%	3290
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 4 76%	3161
	21st to end	Day	3 4 1 3 3 3 3 3 4 6% 66%	3391
November	1st to 10th	Day	6% 7% 3 5% 2 3 4 3 4 6% 57%	1764
	11th to 20th	Day	7% 5% 2 3 3 3 4 3 4 6% 62%	1800
	21st to end	Day	7% 8% 2 4 3 3 3 2 5% 6% 57%	1800
December	1st to 10th	Day	6% 5% 1 3 2 3 3 3 5% 8% 60%	2691
	11th to 20th	Day	13% 12% 3 7% 3 3 4 3 5% 6% 41%	3028
	21st to end	Day	11% 10% 3 7% 3 3 4 4 6% 8% 40%	4004
January	1st to 10th	Night	4 8% 2 5% 3 3 3 3 5% 7% 58%	6306
	11th to 20th	Night	8% 8% 2 6% 3 3 3 3 4 5% 55%	6119
	21st to end	Night	4 11% 3 5% 3 3 4 3 5% 6% 52%	6468
February	1st to 10th	Night	10% 10% 3 5% 4 3 3 4 4 6% 49%	2523
	11th to 20th	Night	6% 11% 3 5% 3 3 4 3 5% 7% 50%	2338
	21st to end	Night	7% 10% 3 8% 4 4 3 2 3 6% 50%	1980
March	1st to 10th	Night	4 15% 6% 9% 4 3 4 3 5% 6% 42%	5376
	11th to 20th	Night	6% 12% 5% 7% 4 3 3 3 4 5% 49%	5098
	21st to end	Night	6% 15% 5% 6% 4 3 3 3 4 7% 44%	5103
April	1st to 10th	Night	6% 10% 3 4 4 3 3 3 4 6% 53%	2000
	11th to 20th	Night	4 9% 2 5% 4 3 3 3 4 5% 58%	1979
	21st to end	Night	4 9% 2 4 3 2 3 3 5% 6% 60%	2000
May	1st to 10th	Night	2 4 1 3 2 2 2 2 3 5% 74%	4340
	11th to 20th	Night	2 4 1 3 2 2 2 3 4 5% 74%	3950
	21st to end	Night	1 2 1 2 1 2 2 2 3 5% 80%	3972
June	1st to 10th	Night	4 3 1 3 3 2 3 3 4 6% 69%	1600
	11th to 20th	Night	1 1 1 2 1 1 2 2 3 5% 82%	1600
	21st to end	Night	2 1 1 1 1 2 2 2 3 85%	1600
July	1st to 10th	Night	1 1 1 1 2 93%	3640
	11th to 20th	Night	1 2 1 2 2 2 1 2 2 3 82%	3640
	21st to end	Night	1 1 1 1 1 1 1 1 3 4 84%	4237
August	1st to 10th	Night	1 1 2 2 2 2 2 3 4 80%	3059
	11th to 20th	Night	1 1 1 1 1 1 2 2 89%	3126
	21st to end	Night	1 1 1 2 1 1 1 1 1 2 88%	3498
September	1st to 10th	Night	1 1 2 1 1 1 1 3 89%	1900
	11th to 20th	Night	1 2 3 2 2 2 1 2 3 81%	1950
	21st to end	Night	2 2 1 2 1 2 2 1 3 4 79%	2090
October	1st to 10th	Night	2 2 1 2 2 2 2 1 2 3 81%	3674
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 5% 79%	3777
	21st to end	Night	3 3 1 3 2 2 3 2 3 5% 73%	4176
November	1st to 10th	Night	6% 7% 2 3 2 2 2 3 5% 7% 61%	2535
	11th to 20th	Night	6% 4 2 4 3 3 3 3 6% 8% 60%	2600
	21st to end	Night	7% 8% 2 5% 3 3 3 3 4 6% 57%	2600
December	1st to 10th	Night	6% 5% 1 3 3 3 3 2 4 6% 64%	4486
	11th to 20th	Night	11% 13% 3 5% 3 3 4 3 7% 9% 39%	5176
	21st to end	Night	8% 9% 3 7% 5% 4 6% 5% 6% 8% 39%	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	29% 7% 14% 21% 20% 10%	3639
	11th to 20th	Day	29% 7% 6% 14% 25% 20%	3847
	21st to end	Day	34% 5% 8% 16% 21% 16%	4510
February	1st to 10th	Day	38% 4 8% 20% 17% 14%	1875
	11th to 20th	Day	35% 5% 8% 23% 22% 7%	1960
	21st to end	Day	43% 5% 7% 20% 18% 7%	1650
March	1st to 10th	Day	46% 6% 7% 13% 21% 7%	4518
	11th to 20th	Day	36% 6% 14% 26% 12% 6%	4885
	21st to end	Day	44% 6% 11% 23% 12% 3	5871
April	1st to 10th	Day	35% 6% 14% 21% 19% 5%	2400
	11th to 20th	Day	35% 5% 11% 26% 18% 5%	2372
	21st to end	Day	30% 6% 13% 31% 17% 2	2400
May	1st to 10th	Day	21% 3 8% 32% 29% 7%	5649
	11th to 20th	Day	20% 3 9% 34% 30% 5%	5945
	21st to end	Day	16% 3 7% 34% 37% 3	6904
June	1st to 10th	Day	21% 6% 12% 28% 27% 7%	2800
	11th to 20th	Day	11% 2 8% 25% 37% 17%	2800
	21st to end	Day	11% 2 3 17% 39% 29%	2800
July	1st to 10th	Day	4 1 1 6% 20% 68%	6251
	11th to 20th	Day	8% 1 4 7% 22% 58%	6245
	21st to end	Day	7% 1 3 15% 35% 38%	6429
August	1st to 10th	Day	10% 1 3 10% 34% 42%	3958
	11th to 20th	Day	5% 1 3 7% 28% 55%	3760
	21st to end	Day	8% 1 2 11% 38% 39%	4179
September	1st to 10th	Day	8% 2 4 13% 36% 37%	2280
	11th to 20th	Day	14% 3 6% 24% 33% 20%	2228
	21st to end	Day	13% 2 5% 22% 36% 21%	2090
October	1st to 10th	Day	16% 2 4 10% 37% 31%	3290
	11th to 20th	Day	14% 1 4 17% 40% 22%	3161
	21st to end	Day	20% 3 6% 20% 37% 14%	3391
November	1st to 10th	Day	28% 4 8% 22% 24% 13%	1764
	11th to 20th	Day	25% 4 8% 22% 29% 13%	1800
	21st to end	Day	30% 4 6% 17% 28% 15%	1800
December	1st to 10th	Day	23% 4 7% 25% 26% 16%	2691
	11th to 20th	Day	44% 3 9% 15% 20% 8%	3028
	21st to end	Day	40% 7% 13% 17% 16% 8%	4004
January	1st to 10th	Night	25% 5% 14% 24% 22% 10%	6306
	11th to 20th	Night	29% 6% 5% 15% 29% 16%	6119
	21st to end	Night	31% 6% 7% 14% 22% 18%	6468
February	1st to 10th	Night	37% 5% 6% 19% 20% 14%	2523
	11th to 20th	Night	33% 4 8% 22% 23% 10%	2339
	21st to end	Night	37% 3 10% 20% 21% 8%	1980
March	1st to 10th	Night	41% 6% 7% 15% 21% 10%	5376
	11th to 20th	Night	37% 5% 9% 28% 15% 7%	5098
	21st to end	Night	38% 8% 16% 22% 14% 3	5103
April	1st to 10th	Night	32% 4 14% 26% 18% 7%	2000
	11th to 20th	Night	28% 6% 12% 25% 21% 8%	1979
	21st to end	Night	25% 5% 13% 35% 21% 2	2000
May	1st to 10th	Night	14% 4 9% 38% 29% 6%	4337
	11th to 20th	Night	13% 3 13% 34% 32% 5%	3951
	21st to end	Night	10% 2 6% 35% 41% 5%	3972
June	1st to 10th	Night	18% 5% 12% 29% 31% 6%	1600
	11th to 20th	Night	8% 3 8% 25% 38% 19%	1600
	21st to end	Night	7% 2 3 16% 40% 32%	1600
July	1st to 10th	Night	2 1 2 5% 20% 71%	3600
	11th to 20th	Night	10% 1 3 6% 27% 53%	3592
	21st to end	Night	7% 1 2 17% 31% 42%	4117
August	1st to 10th	Night	10% 1 3 12% 29% 44%	3050
	11th to 20th	Night	5% 1 3 10% 24% 57%	3156
	21st to end	Night	9% 1 1 10% 41% 38%	3498
September	1st to 10th	Night	6% 1 1 14% 40% 38%	1900
	11th to 20th	Night	12% 2 5% 24% 37% 21%	1950
	21st to end	Night	11% 2 5% 25% 36% 22%	2090
October	1st to 10th	Night	11% 2 4 12% 39% 32%	3674
	11th to 20th	Night	11% 2 4 15% 43% 24%	3777
	21st to end	Night	16% 2 5% 19% 43% 16%	4176
November	1st to 10th	Night	23% 4 8% 22% 27% 16%	2535
	11th to 20th	Night	23% 4 8% 20% 31% 14%	2600
	21st to end	Night	29% 4 7% 18% 28% 15%	2600
December	1st to 10th	Night	23% 3 6% 23% 26% 19%	4486
	11th to 20th	Night	41% 4 8% 17% 22% 7%	5176
	21st to end	Night	40% 8% 10% 17% 19% 7%	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	31% 16% 25% 20% 8% 1	200
	11th to 20th	Day	21% 12% 30% 24% 9% 3	290
	21st to end	Day	15% 16% 32% 21% 13% 3	440
February	1st to 10th	Day	25% 19% 26% 19% 8% 3	437
	11th to 20th	Day	19% 22% 32% 20% 7% 1	495
	21st to end	Day	20% 20% 37% 20% 3 1	414
March	1st to 10th	Day	22% 23% 35% 15% 5% 1	500
	11th to 20th	Day	24% 22% 25% 20% 7% 1	500
	21st to end	Day	22% 25% 31% 17% 4	550
April	1st to 10th	Day	18% 20% 36% 20% 6% 1	500
	11th to 20th	Day	21% 20% 33% 21% 5% 1	492
	21st to end	Day	13% 17% 35% 23% 10% 2	500
May	1st to 10th	Day	18% 20% 31% 21% 8% 2	503
	11th to 20th	Day	20% 14% 30% 22% 12% 3	528
	21st to end	Day	15% 14% 27% 28% 11% 4	573
June	1st to 10th	Day	7% 14% 36% 25% 12% 5%	530
	11th to 20th	Day	8% 16% 36% 24% 12% 5%	530
	21st to end	Day	3 12% 39% 29% 10% 7%	530
July	1st to 10th	Day	3 7% 42% 31% 12% 4	526
	11th to 20th	Day	2 5% 37% 36% 13% 7%	524
	21st to end	Day	2 7% 41% 32% 10% 7%	575
August	1st to 10th	Day	2 11% 48% 25% 10% 5%	504
	11th to 20th	Day	3 9% 44% 28% 11% 5%	477
	21st to end	Day	4 7% 40% 31% 14% 3	539
September	1st to 10th	Day	3 9% 44% 30% 13% 2	490
	11th to 20th	Day	8% 14% 43% 25% 9% 1	490
	21st to end	Day	8% 12% 41% 30% 8% 1	490
October	1st to 10th	Day	8% 19% 32% 29% 11%	490
	11th to 20th	Day	7% 13% 37% 33% 9% 1	490
	21st to end	Day	10% 14% 33% 31% 10% 2	506
November	1st to 10th	Day	13% 12% 33% 22% 17% 2	300
	11th to 20th	Day	13% 11% 28% 28% 13% 6%	300
	21st to end	Day	15% 11% 30% 28% 12% 3	300
December	1st to 10th	Day	17% 9% 29% 24% 17% 3	223
	11th to 20th	Day	15% 16% 33% 21% 13% 4	200
	21st to end	Day	24% 13% 32% 18% 10% 4	220
January	1st to 10th	Night	16% 10% 30% 22% 14% 7%	800
	11th to 20th	Night	13% 11% 30% 30% 12% 5%	800
	21st to end	Night	14% 11% 32% 24% 15% 4	880
February	1st to 10th	Night	18% 10% 27% 29% 13% 4	800
	11th to 20th	Night	14% 12% 27% 25% 16% 6%	784
	21st to end	Night	16% 8% 32% 31% 11% 2	660
March	1st to 10th	Night	18% 11% 28% 28% 12% 3	800
	11th to 20th	Night	16% 10% 23% 27% 18% 6%	800
	21st to end	Night	15% 10% 30% 29% 14% 2	880
April	1st to 10th	Night	12% 8% 28% 33% 15% 4	800
	11th to 20th	Night	9% 9% 30% 30% 16% 5%	791
	21st to end	Night	10% 6% 27% 33% 19% 5%	800
May	1st to 10th	Night	6% 8% 27% 34% 20% 5%	710
	11th to 20th	Night	8% 5% 23% 39% 21% 3	546
	21st to end	Night	5% 3 24% 34% 27% 6%	433
June	1st to 10th	Night	6% 4 16% 41% 31% 4	400
	11th to 20th	Night	2 2 22% 41% 30% 4	400
	21st to end	Night	2 23% 39% 33% 4	400
July	1st to 10th	Night	1 17% 39% 41% 3	390
	11th to 20th	Night	3 1 13% 42% 38% 4	388
	21st to end	Night	3 1 14% 34% 38% 11%	552
August	1st to 10th	Night	2 3 17% 38% 34% 7%	676
	11th to 20th	Night	2 14% 44% 33% 7%	752
	21st to end	Night	2 1 9% 41% 41% 6%	836
September	1st to 10th	Night	2 1 10% 44% 35% 8%	760
	11th to 20th	Night	4 1 17% 39% 31% 8%	760
	21st to end	Night	3 2 18% 38% 29% 9%	760
October	1st to 10th	Night	5% 3 24% 37% 25% 6%	760
	11th to 20th	Night	5% 4 23% 36% 26% 7%	760
	21st to end	Night	6% 5% 23% 32% 24% 11%	836
November	1st to 10th	Night	10% 6% 17% 36% 23% 7%	780
	11th to 20th	Night	9% 7% 27% 35% 17% 5%	800
	21st to end	Night	11% 6% 30% 32% 16% 5%	800
December	1st to 10th	Night	17% 7% 27% 33% 12% 4	800
	11th to 20th	Night	20% 10% 28% 26% 14% 3	800
	21st to end	Night	18% 13% 30% 25% 11% 3	880

Page generated at 2025-08-01 22:32:25. Version ac07

Copyright © 2025 Attilla Danko.