Foresthill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foresthill clear sky chart from 2005-11-06 to 2024-01-16.

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	11% 4 1 3 3 2 3 3 4 5% 62%	74773
	Night	13% 4 1 3 3 2 3 3 4 5% 58%	75034
Transparency	Day	26% 4 8% 18% 24% 20%	74486
	Night	29% 4 8% 18% 22% 19%	74842
Seeing	Day	12% 9% 25% 33% 15% 4	14000
	Night	14% 3 14% 35% 28% 5%	23432

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	23% 8% 2 4 5% 2 4 3 3 5% 41%	1519
	11th to 20th	Day	19% 5% 2 3 4 2 3 3 3 5% 51%	1590
	21st to end	Day	20% 9% 2 4 3 2 4 3 5% 7% 42%	1782
February	1st to 10th	Day	21% 7% 2 3 2 1 3 3 4 5% 48%	1620
	11th to 20th	Day	16% 10% 2 4 3 2 3 3 5% 5% 48%	1701
	21st to end	Day	23% 10% 3 3 3 1 2 2 3 5% 44%	1480
March	1st to 10th	Day	22% 11% 3 4 4 3 3 3 5% 6% 35%	1831
	11th to 20th	Day	23% 7% 2 4 4 2 4 3 4 7% 39%	1980
	21st to end	Day	22% 7% 3 4 4 2 4 4 4 8% 40%	2276
April	1st to 10th	Day	17% 7% 3 4 4 2 4 4 5% 7% 43%	2160
	11th to 20th	Day	15% 5% 2 3 3 2 3 5% 4 6% 52%	2129
	21st to end	Day	12% 3 1 3 3 2 3 3 5% 8% 57%	2160
May	1st to 10th	Day	8% 4 2 4 2 2 3 3 4 7% 61%	2216
	11th to 20th	Day	8% 4 1 3 2 2 3 4 5% 7% 61%	2477
	21st to end	Day	9% 4 2 4 3 2 4 4 5% 6% 58%	2723
June	1st to 10th	Day	7% 3 2 3 3 2 3 3 5% 7% 62%	2520
	11th to 20th	Day	2 2 1 2 2 1 2 2 3 5% 79%	2520
	21st to end	Day	3 2 1 1 1 1 1 1 3 3 83%	2520
July	1st to 10th	Day	1 1 1 1 1 1 2 2 88%	2520
	11th to 20th	Day	2 1 1 1 1 1 2 2 2 2 86%	2520
	21st to end	Day	1 1 2 2 1 2 2 3 3 81%	2594
August	1st to 10th	Day	2 1 1 2 1 1 2 3 3 4 81%	2328
	11th to 20th	Day	1 1 1 1 1 2 3 4 85%	2146
	21st to end	Day	1 1 1 1 1 1 1 1 2 91%	2376
September	1st to 10th	Day	2 1 1 1 1 2 2 3 87%	2160
	11th to 20th	Day	4 2 1 2 2 1 2 3 4 4 75%	2160
	21st to end	Day	4 2 1 2 2 1 1 2 3 5% 76%	1953
October	1st to 10th	Day	7% 2 1 3 3 2 3 4 4 5% 68%	1800
	11th to 20th	Day	8% 2 1 2 3 2 3 3 4 5% 68%	1800
	21st to end	Day	8% 3 1 3 3 1 3 3 4 4 65%	1980
November	1st to 10th	Day	15% 5% 2 3 4 2 3 3 4 7% 53%	1688
	11th to 20th	Day	15% 3 1 3 4 2 4 4 5% 7% 52%	1710
	21st to end	Day	13% 5% 2 4 3 3 3 4 5% 6% 54%	1592
December	1st to 10th	Day	18% 6% 2 3 3 2 5% 4 4 7% 47%	1520
	11th to 20th	Day	29% 9% 3 5% 4 2 3 3 4 5% 32%	1928
	21st to end	Day	15% 7% 2 6% 4 3 5% 5% 6% 9% 37%	2794
January	1st to 10th	Night	24% 6% 2 3 3 2 3 4 4 7% 41%	2646
	11th to 20th	Night	17% 6% 2 3 4 3 4 4 5% 7% 46%	2477
	21st to end	Night	20% 6% 2 3 4 2 3 4 4 6% 47%	2574
February	1st to 10th	Night	20% 6% 1 3 4 2 3 2 3 6% 50%	2340
	11th to 20th	Night	17% 7% 1 4 4 2 4 4 4 6% 47%	2145
	21st to end	Night	20% 8% 2 3 2 2 2 4 4 6% 46%	1776
March	1st to 10th	Night	25% 11% 2 4 3 2 4 3 3 4 38%	2124
	11th to 20th	Night	22% 6% 1 4 3 2 3 4 5% 7% 42%	1980
	21st to end	Night	21% 6% 2 3 4 2 3 4 5% 6% 44%	2080
April	1st to 10th	Night	17% 8% 3 5% 4 2 4 3 4 6% 46%	1800
	11th to 20th	Night	16% 4 1 4 4 2 3 4 4 6% 51%	1779
	21st to end	Night	11% 4 2 3 3 2 3 3 4 6% 61%	1800
May	1st to 10th	Night	10% 4 1 3 3 2 3 4 4 5% 61%	1738
	11th to 20th	Night	8% 3 1 2 2 1 4 3 4 9% 63%	1416
	21st to end	Night	9% 3 1 2 4 2 3 3 4 6% 62%	1552
June	1st to 10th	Night	7% 4 1 3 3 2 4 3 4 5% 65%	1440
	11th to 20th	Night	2 2 1 2 1 1 2 2 3 5% 79%	1440
	21st to end	Night	3 1 1 2 2 1 2 2 2 3 81%	1440
July	1st to 10th	Night	1 1 1 1 1 1 2 2 3 87%	1440
	11th to 20th	Night	1 1 1 2 1 2 4 86%	1440
	21st to end	Night	1 2 1 3 1 2 2 3 4 4 78%	1720
August	1st to 10th	Night	2 2 1 3 2 1 1 3 3 3 77%	1611
	11th to 20th	Night	1 1 1 1 2 1 1 2 4 4 83%	1755
	21st to end	Night	1 1 1 1 1 1 1 2 2 3 86%	1980
September	1st to 10th	Night	2 1 1 2 1 1 1 2 3 87%	1800
	11th to 20th	Night	4 2 1 1 2 1 1 2 3 4 78%	1800
	21st to end	Night	3 2 2 3 2 2 2 3 4 77%	2002
October	1st to 10th	Night	6% 2 1 3 3 2 3 3 3 3 72%	2160
	11th to 20th	Night	7% 2 1 2 2 1 2 3 3 5% 72%	2160
	21st to end	Night	9% 2 1 2 2 2 2 3 4 6% 68%	2376
November	1st to 10th	Night	16% 5% 2 3 2 1 3 4 4 6% 55%	2391
	11th to 20th	Night	12% 3 2 3 3 2 3 3 5% 7% 56%	2470
	21st to end	Night	13% 5% 2 4 3 2 3 3 4 6% 55%	2584
December	1st to 10th	Night	21% 5% 1 2 2 2 3 4 4 6% 50%	2660
	11th to 20th	Night	33% 7% 2 3 3 2 3 4 4 6% 33%	3276
	21st to end	Night	21% 8% 3 5% 5% 3 4 5% 5% 6% 35%	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	48% 4 8% 21% 15% 4	1519
	11th to 20th	Day	38% 3 9% 18% 17% 14%	1590
	21st to end	Day	43% 4 8% 14% 21% 9%	1782
February	1st to 10th	Day	40% 3 9% 19% 18% 11%	1620
	11th to 20th	Day	39% 4 9% 17% 23% 7%	1711
	21st to end	Day	45% 3 7% 21% 16% 8%	1480
March	1st to 10th	Day	50% 5% 10% 15% 16% 4	1831
	11th to 20th	Day	45% 5% 9% 20% 16% 5%	1980
	21st to end	Day	44% 7% 13% 18% 15% 3	2276
April	1st to 10th	Day	40% 7% 12% 20% 16% 4	2160
	11th to 20th	Day	32% 7% 12% 24% 19% 5%	2132
	21st to end	Day	27% 5% 13% 28% 23% 4	2160
May	1st to 10th	Day	23% 6% 11% 28% 25% 7%	2216
	11th to 20th	Day	23% 6% 10% 30% 24% 8%	2480
	21st to end	Day	26% 7% 13% 29% 20% 5%	2724
June	1st to 10th	Day	22% 6% 9% 24% 28% 10%	2520
	11th to 20th	Day	11% 3 8% 21% 35% 22%	2520
	21st to end	Day	9% 2 3 15% 37% 35%	2520
July	1st to 10th	Day	5% 2 3 10% 27% 54%	2451
	11th to 20th	Day	7% 1 4 12% 26% 49%	2445
	21st to end	Day	10% 2 3 8% 25% 52%	2450
August	1st to 10th	Day	10% 3 2 12% 27% 46%	2312
	11th to 20th	Day	6% 1 3 10% 34% 45%	2146
	21st to end	Day	5% 1 2 10% 38% 44%	2376
September	1st to 10th	Day	5% 2 2 9% 29% 53%	2160
	11th to 20th	Day	14% 3 5% 20% 36% 23%	2160
	21st to end	Day	13% 2 5% 19% 33% 27%	1953
October	1st to 10th	Day	20% 4 5% 19% 28% 24%	1800
	11th to 20th	Day	19% 4 7% 19% 27% 25%	1800
	21st to end	Day	23% 3 7% 19% 25% 23%	1980
November	1st to 10th	Day	33% 4 9% 18% 22% 14%	1688
	11th to 20th	Day	32% 5% 9% 19% 24% 11%	1710
	21st to end	Day	32% 4 9% 18% 25% 12%	1592
December	1st to 10th	Day	38% 5% 7% 19% 19% 11%	1520
	11th to 20th	Day	55% 5% 6% 15% 13% 7%	1928
	21st to end	Day	41% 8% 13% 20% 14% 5%	2794
January	1st to 10th	Night	43% 4 12% 20% 14% 6%	2646
	11th to 20th	Night	39% 5% 10% 15% 17% 15%	2477
	21st to end	Night	41% 5% 7% 15% 21% 12%	2574
February	1st to 10th	Night	39% 3 6% 19% 20% 14%	2340
	11th to 20th	Night	38% 5% 7% 20% 21% 9%	2146
	21st to end	Night	40% 4 9% 23% 18% 7%	1776
March	1st to 10th	Night	50% 5% 8% 17% 15% 5%	2124
	11th to 20th	Night	43% 4 7% 21% 18% 7%	1980
	21st to end	Night	42% 8% 10% 20% 14% 5%	2080
April	1st to 10th	Night	41% 5% 12% 18% 18% 6%	1800
	11th to 20th	Night	34% 6% 13% 23% 17% 8%	1779
	21st to end	Night	25% 5% 12% 33% 20% 5%	1800
May	1st to 10th	Night	25% 6% 10% 23% 27% 8%	1738
	11th to 20th	Night	21% 3 13% 30% 23% 9%	1416
	21st to end	Night	23% 6% 16% 29% 21% 5%	1552
June	1st to 10th	Night	21% 7% 11% 26% 24% 12%	1440
	11th to 20th	Night	10% 3 7% 23% 33% 23%	1440
	21st to end	Night	10% 3 5% 13% 33% 37%	1440
July	1st to 10th	Night	5% 2 3 10% 28% 52%	1400
	11th to 20th	Night	6% 2 4 12% 23% 53%	1392
	21st to end	Night	10% 3 4 8% 22% 53%	1624
August	1st to 10th	Night	13% 2 3 14% 25% 43%	1602
	11th to 20th	Night	7% 2 4 12% 29% 46%	1755
	21st to end	Night	8% 1 2 12% 28% 50%	1980
September	1st to 10th	Night	6% 2 3 8% 31% 51%	1800
	11th to 20th	Night	11% 3 6% 18% 40% 23%	1800
	21st to end	Night	13% 2 4 20% 32% 29%	2002
October	1st to 10th	Night	19% 3 6% 17% 29% 26%	2160
	11th to 20th	Night	17% 3 5% 17% 30% 27%	2160
	21st to end	Night	19% 3 6% 19% 28% 24%	2376
November	1st to 10th	Night	30% 5% 9% 19% 20% 17%	2391
	11th to 20th	Night	29% 4 10% 20% 26% 11%	2470
	21st to end	Night	32% 3 7% 19% 23% 15%	2584
December	1st to 10th	Night	37% 5% 8% 18% 18% 15%	2660
	11th to 20th	Night	52% 4 7% 13% 17% 7%	3276
	21st to end	Night	47% 8% 12% 15% 13% 5%	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	42% 7% 22% 18% 8% 2	180
	11th to 20th	Day	26% 6% 20% 33% 11% 3	280
	21st to end	Day	25% 7% 21% 28% 14% 4	374
February	1st to 10th	Day	26% 11% 22% 26% 12% 2	340
	11th to 20th	Day	23% 15% 28% 23% 10% 1	393
	21st to end	Day	25% 15% 27% 24% 8% 1	346
March	1st to 10th	Day	31% 17% 27% 18% 6% 1	420
	11th to 20th	Day	27% 17% 24% 18% 10% 3	420
	21st to end	Day	25% 16% 31% 20% 8%	462
April	1st to 10th	Day	22% 15% 30% 24% 8% 1	420
	11th to 20th	Day	19% 18% 29% 25% 7% 1	412
	21st to end	Day	12% 14% 28% 29% 14% 3	420
May	1st to 10th	Day	10% 16% 32% 27% 11% 4	423
	11th to 20th	Day	11% 15% 25% 30% 14% 5%	448
	21st to end	Day	11% 11% 24% 31% 17% 6%	485
June	1st to 10th	Day	7% 11% 25% 37% 12% 8%	450
	11th to 20th	Day	5% 13% 28% 36% 14% 4	450
	21st to end	Day	3 5% 28% 41% 16% 8%	450
July	1st to 10th	Day	2 26% 48% 17% 6%	446
	11th to 20th	Day	2 23% 48% 21% 7%	444
	21st to end	Day	1 1 26% 46% 20% 7%	487
August	1st to 10th	Day	4 3 23% 45% 19% 6%	447
	11th to 20th	Day	2 2 27% 43% 19% 7%	437
	21st to end	Day	1 3 19% 53% 21% 3	495
September	1st to 10th	Day	2 3 26% 45% 20% 4	450
	11th to 20th	Day	7% 4 28% 38% 19% 3	450
	21st to end	Day	4 8% 28% 42% 17% 2	450
October	1st to 10th	Day	5% 11% 29% 30% 20% 5%	450
	11th to 20th	Day	8% 8% 23% 34% 21% 6%	450
	21st to end	Day	11% 9% 23% 32% 20% 5%	495
November	1st to 10th	Day	18% 5% 19% 28% 25% 5%	284
	11th to 20th	Day	11% 6% 20% 33% 24% 6%	270
	21st to end	Day	12% 9% 21% 33% 20% 5%	214
December	1st to 10th	Day	23% 8% 18% 28% 15% 8%	180
	11th to 20th	Day	21% 11% 20% 24% 17% 8%	180
	21st to end	Day	25% 9% 23% 26% 13% 4	198
January	1st to 10th	Night	27% 3 17% 29% 18% 6%	760
	11th to 20th	Night	19% 3 20% 39% 16% 3	740
	21st to end	Night	23% 2 21% 28% 21% 4	792
February	1st to 10th	Night	22% 4 17% 35% 19% 2	720
	11th to 20th	Night	20% 6% 17% 33% 20% 4	704
	21st to end	Night	24% 5% 19% 36% 13% 2	592
March	1st to 10th	Night	28% 6% 16% 30% 17% 3	720
	11th to 20th	Night	23% 5% 15% 29% 24% 3	720
	21st to end	Night	23% 6% 17% 33% 18% 2	792
April	1st to 10th	Night	18% 4 18% 38% 20% 2	720
	11th to 20th	Night	16% 5% 18% 35% 22% 4	711
	21st to end	Night	11% 5% 16% 38% 25% 6%	720
May	1st to 10th	Night	10% 4 21% 37% 24% 4	658
	11th to 20th	Night	9% 2 12% 38% 35% 4	354
	21st to end	Night	9% 2 12% 34% 40% 4	389
June	1st to 10th	Night	8% 2 13% 35% 37% 6%	360
	11th to 20th	Night	3 2 12% 43% 36% 4	360
	21st to end	Night	2 1 8% 39% 44% 6%	360
July	1st to 10th	Night	1 4 39% 53% 4	350
	11th to 20th	Night	1 3 42% 47% 7%	348
	21st to end	Night	1 4 31% 52% 11%	538
August	1st to 10th	Night	4 1 5% 37% 46% 8%	537
	11th to 20th	Night	1 1 5% 42% 41% 9%	687
	21st to end	Night	1 5% 37% 52% 5%	792
September	1st to 10th	Night	2 1 6% 38% 48% 6%	720
	11th to 20th	Night	4 1 9% 35% 40% 11%	720
	21st to end	Night	4 2 13% 39% 33% 10%	720
October	1st to 10th	Night	7% 3 11% 41% 31% 7%	720
	11th to 20th	Night	9% 3 11% 37% 34% 6%	720
	21st to end	Night	10% 4 14% 33% 30% 10%	792
November	1st to 10th	Night	18% 2 12% 34% 26% 8%	740
	11th to 20th	Night	13% 2 18% 38% 24% 5%	760
	21st to end	Night	15% 4 17% 39% 19% 6%	760
December	1st to 10th	Night	21% 4 17% 36% 16% 5%	760
	11th to 20th	Night	29% 3 17% 31% 17% 3	760
	21st to end	Night	27% 4 20% 32% 15% 2	836

Page generated at 2024-01-12 22:38:14. Version ac07

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