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 2024-12-06.

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	10% 5% 1 3 2 2 2 2 4 5% 64%	105424
	Night	13% 5% 1 3 2 2 3 2 4 5% 59%	103016
Transparency	Day	24% 4 7% 17% 24% 24%	105174
	Night	28% 4 7% 17% 22% 21%	102855
Seeing	Day	12% 9% 26% 33% 15% 4	15223
	Night	14% 3 15% 36% 28% 5%	24562

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	24% 11% 2 4 3 2 3 2 3 4 41%	2539
	11th to 20th	Day	24% 9% 3 7% 4 4 3 3 4 4 35%	2732
	21st to end	Day	26% 10% 2 6% 3 3 4 3 4 6% 33%	3146
February	1st to 10th	Day	22% 9% 2 3 2 2 3 2 4 5% 46%	1710
	11th to 20th	Day	17% 11% 2 4 3 3 3 3 5% 5% 45%	1798
	21st to end	Day	23% 11% 2 3 3 2 2 2 3 5% 44%	1570
March	1st to 10th	Day	22% 18% 3 7% 4 4 3 3 5% 5% 28%	3195
	11th to 20th	Day	16% 7% 1 4 2 2 2 2 3 5% 56%	3490
	21st to end	Day	21% 12% 3 6% 4 4 4 4 5% 7% 30%	4000
April	1st to 10th	Day	16% 8% 3 4 4 3 3 4 5% 7% 44%	2280
	11th to 20th	Day	14% 5% 1 4 3 3 3 4 5% 6% 51%	2249
	21st to end	Day	12% 4 1 3 3 3 3 3 5% 7% 57%	2280
May	1st to 10th	Day	8% 3 1 3 2 2 2 2 3 5% 69%	3921
	11th to 20th	Day	4 2 1 2 1 2 2 2 3 5% 77%	4377
	21st to end	Day	5% 3 1 2 2 2 3 2 4 5% 71%	4813
June	1st to 10th	Day	7% 4 1 3 3 3 3 2 5% 7% 62%	2660
	11th to 20th	Day	2 2 1 2 1 2 2 2 3 5% 79%	2660
	21st to end	Day	3 2 1 1 1 1 1 1 3 3 83%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 93%	4420
	11th to 20th	Day	2 1 1 1 1 1 2 2 3 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 2 2 2 87%	4529
August	1st to 10th	Day	2 1 1 2 2 2 2 2 3 5% 78%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	3760
	21st to end	Day	2 1 1 1 1 1 1 1 2 90%	4180
September	1st to 10th	Day	2 1 1 1 1 1 2 2 3 87%	2280
	11th to 20th	Day	4 3 1 2 2 1 2 2 3 4 75%	2280
	21st to end	Day	4 2 1 2 2 1 2 2 2 5% 77%	2060
October	1st to 10th	Day	4 2 1 3 2 2 3 3 4 5% 73%	3180
	11th to 20th	Day	7% 2 1 2 2 2 2 2 4 5% 72%	3161
	21st to end	Day	6% 3 1 4 3 3 4 3 4 6% 63%	3448
November	1st to 10th	Day	14% 5% 2 3 4 3 2 2 4 8% 53%	1778
	11th to 20th	Day	15% 4 1 3 4 3 4 3 5% 7% 51%	1800
	21st to end	Day	15% 5% 1 4 3 3 4 3 5% 5% 53%	1675
December	1st to 10th	Day	17% 6% 2 4 3 3 4 3 4 6% 48%	1560
	11th to 20th	Day	29% 10% 3 5% 3 3 3 3 5% 5% 32%	1928
	21st to end	Day	15% 7% 2 6% 4 4 5% 4 6% 8% 37%	2794
January	1st to 10th	Night	19% 10% 2 5% 3 3 3 3 5% 6% 40%	4406
	11th to 20th	Night	24% 8% 2 5% 4 4 4 3 5% 5% 36%	4230
	21st to end	Night	26% 9% 2 6% 5% 4 4 3 4 5% 33%	4521
February	1st to 10th	Night	20% 7% 1 4 3 3 3 2 4 5% 48%	2470
	11th to 20th	Night	18% 7% 1 5% 3 3 4 3 5% 6% 46%	2268
	21st to end	Night	20% 9% 2 3 2 3 3 3 4 6% 46%	1884
March	1st to 10th	Night	25% 18% 2 4 3 3 3 2 4 4 31%	3674
	11th to 20th	Night	19% 5% 1 3 2 2 2 2 3 5% 56%	3490
	21st to end	Night	24% 11% 2 5% 4 3 4 3 6% 6% 33%	3653
April	1st to 10th	Night	15% 9% 2 6% 3 3 3 2 4 5% 47%	1900
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 4 6% 52%	1879
	21st to end	Night	11% 4 1 3 3 3 2 3 4 6% 61%	1900
May	1st to 10th	Night	6% 6% 1 2 2 2 3 2 3 5% 67%	3048
	11th to 20th	Night	5% 2 1 1 1 1 2 2 3 6% 77%	2516
	21st to end	Night	5% 2 1 2 2 2 2 2 3 5% 75%	2762
June	1st to 10th	Night	7% 4 1 3 2 3 3 3 4 5% 65%	1520
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 79%	1520
	21st to end	Night	2 2 2 2 1 2 2 3 3 81%	1520
July	1st to 10th	Night	1 1 1 1 1 2 93%	2540
	11th to 20th	Night	2 2 1 2 1 2 2 1 3 4 80%	2540
	21st to end	Night	1 1 2 1 1 2 2 3 3 85%	3014
August	1st to 10th	Night	2 2 1 3 2 2 2 2 4 4 75%	2848
	11th to 20th	Night	1 1 1 1 1 1 2 2 89%	3088
	21st to end	Night	3 1 1 1 1 1 1 1 1 2 87%	3498
September	1st to 10th	Night	2 1 1 1 1 1 1 2 3 87%	1900
	11th to 20th	Night	4 2 1 2 1 1 2 2 3 4 77%	1900
	21st to end	Night	3 2 2 2 2 2 2 3 3 78%	2114
October	1st to 10th	Night	4 2 1 3 2 2 2 2 3 3 77%	3800
	11th to 20th	Night	4 2 2 1 2 2 2 2 4 78%	3777
	21st to end	Night	10% 3 1 3 2 3 3 2 4 6% 63%	4121
November	1st to 10th	Night	16% 5% 1 3 2 2 3 3 4 6% 55%	2521
	11th to 20th	Night	12% 4 2 4 3 2 3 3 5% 7% 55%	2600
	21st to end	Night	14% 5% 2 4 3 3 3 2 4 6% 54%	2720
December	1st to 10th	Night	21% 5% 1 3 2 2 3 3 4 6% 51%	2736
	11th to 20th	Night	32% 8% 2 3 3 2 3 3 4 6% 33%	3276
	21st to end	Night	21% 8% 3 6% 4 5% 4 4 5% 6% 34%	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 7% 22% 16% 3	2539
	11th to 20th	Day	52% 6% 8% 14% 12% 8%	2732
	21st to end	Day	52% 5% 9% 17% 12% 5%	3146
February	1st to 10th	Day	42% 3 9% 18% 18% 10%	1710
	11th to 20th	Day	41% 4 9% 17% 21% 7%	1808
	21st to end	Day	45% 3 8% 21% 15% 7%	1570
March	1st to 10th	Day	55% 9% 9% 12% 11% 4	3195
	11th to 20th	Day	32% 5% 8% 32% 16% 8%	3490
	21st to end	Day	48% 11% 13% 17% 9% 2	4000
April	1st to 10th	Day	39% 7% 12% 22% 16% 4	2280
	11th to 20th	Day	32% 7% 13% 25% 18% 5%	2252
	21st to end	Day	26% 6% 13% 29% 23% 4	2280
May	1st to 10th	Day	18% 5% 11% 29% 20% 16%	3921
	11th to 20th	Day	13% 3 7% 28% 39% 10%	4380
	21st to end	Day	16% 5% 12% 35% 28% 5%	4814
June	1st to 10th	Day	22% 6% 10% 23% 29% 10%	2660
	11th to 20th	Day	10% 3 7% 21% 37% 21%	2660
	21st to end	Day	9% 2 3 14% 36% 36%	2660
July	1st to 10th	Day	3 1 2 5% 15% 74%	4351
	11th to 20th	Day	7% 2 3 7% 17% 64%	4345
	21st to end	Day	6% 1 2 5% 25% 61%	4386
August	1st to 10th	Day	10% 2 2 8% 21% 57%	4076
	11th to 20th	Day	4 1 2 6% 39% 48%	3796
	21st to end	Day	6% 1 2 9% 42% 41%	4179
September	1st to 10th	Day	5% 2 2 8% 29% 54%	2280
	11th to 20th	Day	14% 3 5% 20% 36% 22%	2280
	21st to end	Day	13% 2 5% 18% 32% 29%	2060
October	1st to 10th	Day	15% 3 4 11% 26% 42%	3180
	11th to 20th	Day	16% 3 5% 17% 39% 20%	3161
	21st to end	Day	22% 3 7% 20% 32% 16%	3448
November	1st to 10th	Day	31% 4 9% 19% 23% 14%	1778
	11th to 20th	Day	33% 5% 10% 19% 23% 10%	1800
	21st to end	Day	33% 4 8% 17% 24% 13%	1675
December	1st to 10th	Day	38% 5% 7% 19% 20% 11%	1560
	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% 6% 12% 23% 13% 4	4406
	11th to 20th	Night	50% 6% 10% 12% 13% 9%	4230
	21st to end	Night	53% 6% 9% 13% 12% 7%	4521
February	1st to 10th	Night	40% 3 6% 19% 19% 13%	2470
	11th to 20th	Night	39% 5% 7% 20% 21% 8%	2269
	21st to end	Night	39% 4 9% 23% 18% 6%	1884
March	1st to 10th	Night	56% 7% 8% 14% 11% 4	3674
	11th to 20th	Night	33% 4 7% 23% 25% 7%	3490
	21st to end	Night	49% 10% 12% 19% 8% 3	3653
April	1st to 10th	Night	40% 5% 12% 19% 18% 6%	1900
	11th to 20th	Night	33% 6% 13% 24% 16% 8%	1879
	21st to end	Night	25% 6% 11% 33% 20% 5%	1900
May	1st to 10th	Night	19% 7% 13% 25% 24% 13%	3048
	11th to 20th	Night	12% 2 8% 26% 37% 16%	2516
	21st to end	Night	13% 4 13% 39% 25% 5%	2762
June	1st to 10th	Night	21% 7% 11% 26% 24% 12%	1520
	11th to 20th	Night	10% 3 6% 22% 35% 24%	1520
	21st to end	Night	10% 3 4 12% 32% 39%	1520
July	1st to 10th	Night	3 1 2 6% 16% 73%	2500
	11th to 20th	Night	10% 2 2 7% 15% 63%	2492
	21st to end	Night	6% 2 2 5% 22% 63%	2919
August	1st to 10th	Night	13% 2 2 9% 23% 52%	2839
	11th to 20th	Night	4 1 2 8% 36% 48%	3118
	21st to end	Night	8% 1 2 12% 33% 44%	3498
September	1st to 10th	Night	5% 1 3 8% 32% 51%	1900
	11th to 20th	Night	12% 3 6% 18% 40% 21%	1900
	21st to end	Night	12% 2 4 19% 32% 30%	2114
October	1st to 10th	Night	15% 2 4 10% 23% 46%	3800
	11th to 20th	Night	12% 2 5% 24% 36% 21%	3777
	21st to end	Night	23% 3 4 18% 32% 19%	4121
November	1st to 10th	Night	29% 5% 9% 20% 20% 16%	2521
	11th to 20th	Night	29% 4 11% 20% 25% 11%	2600
	21st to end	Night	33% 3 7% 19% 23% 15%	2720
December	1st to 10th	Night	36% 5% 8% 19% 18% 14%	2736
	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% 7% 20% 33% 11% 3	294
	21st to end	Day	26% 8% 22% 28% 13% 4	407
February	1st to 10th	Day	28% 12% 23% 25% 11% 2	370
	11th to 20th	Day	25% 15% 28% 22% 10% 1	430
	21st to end	Day	24% 15% 27% 25% 8% 1	382
March	1st to 10th	Day	32% 17% 27% 17% 6% 1	460
	11th to 20th	Day	26% 19% 23% 18% 12% 3	460
	21st to end	Day	25% 17% 31% 20% 7%	506
April	1st to 10th	Day	20% 16% 30% 23% 10% 1	460
	11th to 20th	Day	18% 17% 28% 26% 9% 1	452
	21st to end	Day	11% 14% 28% 29% 15% 3	460
May	1st to 10th	Day	10% 16% 33% 26% 10% 4	463
	11th to 20th	Day	10% 14% 25% 31% 16% 5%	488
	21st to end	Day	10% 10% 25% 32% 17% 7%	529
June	1st to 10th	Day	7% 10% 25% 38% 13% 7%	490
	11th to 20th	Day	4 13% 29% 36% 14% 4	490
	21st to end	Day	3 4 29% 40% 16% 8%	490
July	1st to 10th	Day	2 26% 48% 18% 6%	486
	11th to 20th	Day	1 24% 48% 20% 7%	484
	21st to end	Day	1 1 27% 45% 19% 7%	531
August	1st to 10th	Day	3 3 24% 45% 18% 6%	487
	11th to 20th	Day	3 3 29% 40% 18% 7%	477
	21st to end	Day	2 3 19% 52% 21% 4	539
September	1st to 10th	Day	2 3 28% 45% 19% 3	490
	11th to 20th	Day	8% 6% 28% 37% 19% 3	490
	21st to end	Day	3 8% 28% 41% 18% 2	490
October	1st to 10th	Day	4 10% 29% 32% 20% 5%	490
	11th to 20th	Day	8% 8% 24% 35% 19% 6%	490
	21st to end	Day	11% 10% 24% 31% 19% 5%	539
November	1st to 10th	Day	17% 6% 21% 27% 23% 5%	314
	11th to 20th	Day	13% 8% 20% 32% 22% 6%	300
	21st to end	Day	16% 9% 19% 33% 19% 4	237
December	1st to 10th	Day	22% 8% 17% 28% 15% 9%	190
	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	20% 3 20% 39% 16% 3	760
	21st to end	Night	23% 3 21% 28% 20% 4	836
February	1st to 10th	Night	23% 5% 18% 34% 18% 2	760
	11th to 20th	Night	21% 6% 18% 33% 19% 3	744
	21st to end	Night	23% 5% 20% 36% 13% 2	628
March	1st to 10th	Night	29% 6% 17% 29% 16% 3	760
	11th to 20th	Night	23% 6% 16% 29% 24% 3	760
	21st to end	Night	24% 6% 18% 33% 17% 2	836
April	1st to 10th	Night	17% 5% 18% 38% 20% 2	760
	11th to 20th	Night	16% 6% 17% 35% 23% 4	751
	21st to end	Night	11% 5% 17% 38% 25% 5%	760
May	1st to 10th	Night	10% 4 22% 37% 23% 4	693
	11th to 20th	Night	9% 2 11% 39% 36% 4	374
	21st to end	Night	9% 2 12% 36% 38% 4	411
June	1st to 10th	Night	7% 2 14% 36% 36% 6%	380
	11th to 20th	Night	3 2 13% 43% 35% 4	380
	21st to end	Night	2 1 9% 40% 42% 5%	380
July	1st to 10th	Night	1 4 40% 51% 4	370
	11th to 20th	Night	1 1 4 43% 45% 6%	368
	21st to end	Night	1 4 32% 52% 11%	568
August	1st to 10th	Night	4 1 5% 39% 45% 7%	567
	11th to 20th	Night	1 1 6% 44% 39% 9%	726
	21st to end	Night	1 1 5% 37% 51% 4	836
September	1st to 10th	Night	2 1 8% 39% 45% 6%	760
	11th to 20th	Night	4 1 10% 37% 38% 10%	760
	21st to end	Night	3 2 13% 40% 32% 10%	760
October	1st to 10th	Night	7% 3 12% 42% 30% 6%	760
	11th to 20th	Night	8% 3 12% 38% 34% 5%	760
	21st to end	Night	10% 4 15% 34% 28% 9%	836
November	1st to 10th	Night	17% 2 14% 34% 25% 8%	780
	11th to 20th	Night	14% 3 19% 36% 22% 5%	800
	21st to end	Night	16% 5% 17% 37% 19% 6%	800
December	1st to 10th	Night	21% 4 17% 36% 17% 5%	782
	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 2024-12-03 03:45:15. Version ac07

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