Hearst
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hearst clear sky chart from 2005-11-06 to 2025-02-25.

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	16% 13% 4 10% 6% 5% 5% 4 5% 6% 27%	108372
	Night	14% 14% 5% 11% 7% 6% 5% 4 5% 5% 23%	110491
Transparency	Day	57% 6% 12% 18% 7%	111816
	Night	61% 6% 11% 16% 6%	115210
Seeing	Day	23% 15% 32% 21% 7% 2	15832
	Night	22% 12% 31% 25% 8% 1	25220

By Month

Percentage Cloud Cover NE (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	8% 19% 8% 22% 9% 8% 6% 3 5% 4 10%	3158
	11th to 20th	Day	8% 13% 7% 21% 9% 6% 5% 5% 10% 5% 12%	3333
	21st to end	Day	9% 16% 9% 14% 7% 7% 8% 6% 7% 5% 14%	3982
February	1st to 10th	Day	8% 14% 4 10% 7% 8% 11% 5% 7% 5% 21%	1645
	11th to 20th	Day	8% 17% 5% 12% 8% 6% 7% 5% 6% 6% 19%	1753
	21st to end	Day	8% 16% 6% 13% 7% 6% 8% 6% 6% 5% 19%	1599
March	1st to 10th	Day	11% 15% 4 10% 5% 7% 6% 4 5% 5% 28%	3118
	11th to 20th	Day	10% 19% 6% 14% 10% 5% 4 3 4 4 22%	3470
	21st to end	Day	14% 15% 5% 14% 5% 6% 5% 3 4 5% 23%	3863
April	1st to 10th	Day	17% 16% 5% 11% 5% 4 4 4 3 5% 25%	2326
	11th to 20th	Day	19% 14% 4 9% 5% 4 4 3 4 5% 28%	2442
	21st to end	Day	16% 13% 4 8% 5% 4 5% 3 5% 6% 32%	2470
May	1st to 10th	Day	16% 13% 4 9% 5% 4 4 3 5% 5% 31%	4135
	11th to 20th	Day	16% 13% 3 9% 5% 5% 5% 4 5% 7% 29%	4711
	21st to end	Day	22% 12% 3 6% 4 4 4 3 5% 6% 31%	5185
June	1st to 10th	Day	24% 9% 3 6% 4 4 4 4 5% 7% 30%	2850
	11th to 20th	Day	18% 7% 3 6% 4 5% 5% 4 6% 8% 36%	2850
	21st to end	Day	20% 7% 2 6% 5% 4 5% 4 5% 7% 32%	2850
July	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 7% 35%	4760
	11th to 20th	Day	13% 7% 3 7% 4 4 4 4 6% 8% 39%	4760
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 9% 37%	4950
August	1st to 10th	Day	18% 7% 2 7% 4 5% 5% 4 6% 9% 34%	4373
	11th to 20th	Day	17% 6% 2 5% 4 3 4 3 5% 7% 46%	4044
	21st to end	Day	14% 9% 3 8% 5% 5% 4 3 5% 7% 37%	4532
September	1st to 10th	Day	20% 8% 3 7% 4 4 4 3 4 6% 36%	2313
	11th to 20th	Day	23% 11% 3 7% 5% 4 4 3 5% 7% 29%	2090
	21st to end	Day	22% 12% 3 7% 5% 4 3 2 4 5% 31%	2090
October	1st to 10th	Day	19% 18% 3 7% 4 4 4 3 4 6% 28%	1990
	11th to 20th	Day	25% 21% 5% 7% 5% 3 4 3 4 5% 18%	1796
	21st to end	Day	23% 24% 6% 9% 4 4 4 2 4 4 16%	1881
November	1st to 10th	Day	20% 21% 6% 12% 5% 4 3 3 5% 5% 16%	1671
	11th to 20th	Day	18% 24% 7% 15% 7% 4 3 3 4 4 12%	1600
	21st to end	Day	18% 22% 8% 17% 7% 6% 4 3 3 4 8%	1405
December	1st to 10th	Day	11% 23% 8% 20% 15% 5% 3 2 2 3 7%	2280
	11th to 20th	Day	10% 17% 8% 13% 8% 11% 6% 7% 6% 5% 10%	2621
	21st to end	Day	16% 21% 8% 13% 7% 6% 5% 3 5% 5% 11%	3476
January	1st to 10th	Night	6% 16% 9% 18% 10% 9% 7% 4 5% 4 11%	6808
	11th to 20th	Night	7% 17% 8% 18% 10% 7% 6% 7% 9% 4 7%	6571
	21st to end	Night	8% 16% 8% 14% 7% 7% 8% 5% 6% 5% 16%	6996
February	1st to 10th	Night	7% 13% 5% 13% 8% 8% 9% 7% 5% 6% 19%	2755
	11th to 20th	Night	8% 16% 5% 14% 9% 6% 6% 5% 6% 5% 19%	2533
	21st to end	Night	7% 16% 6% 12% 8% 6% 7% 5% 4 6% 21%	1949
March	1st to 10th	Night	17% 16% 4 9% 5% 5% 6% 4 5% 6% 24%	3722
	11th to 20th	Night	10% 15% 7% 15% 8% 4 4 2 3 4 27%	3489
	21st to end	Night	13% 11% 5% 13% 9% 7% 6% 3 4 4 25%	3798
April	1st to 10th	Night	16% 16% 4 9% 5% 5% 3 3 4 4 30%	1853
	11th to 20th	Night	18% 12% 3 9% 4 5% 5% 4 5% 6% 30%	1687
	21st to end	Night	17% 12% 4 8% 5% 3 3 3 3 5% 36%	1710
May	1st to 10th	Night	16% 13% 4 7% 4 3 4 3 5% 6% 35%	2804
	11th to 20th	Night	17% 11% 3 8% 4 3 4 3 6% 5% 35%	2193
	21st to end	Night	18% 11% 3 7% 4 4 4 4 6% 6% 35%	2402
June	1st to 10th	Night	19% 9% 2 6% 3 3 4 3 6% 8% 36%	1330
	11th to 20th	Night	17% 8% 3 5% 4 5% 6% 4 5% 7% 38%	1330
	21st to end	Night	20% 6% 2 5% 4 4 5% 4 6% 8% 37%	1330
July	1st to 10th	Night	15% 6% 2 5% 5% 5% 4 5% 7% 8% 38%	2210
	11th to 20th	Night	14% 6% 3 6% 5% 4 4 5% 5% 8% 42%	2210
	21st to end	Night	12% 6% 3 5% 4 4 5% 4 5% 8% 44%	2612
August	1st to 10th	Night	12% 7% 2 6% 5% 4 4 4 5% 8% 43%	2564
	11th to 20th	Night	15% 7% 2 5% 3 4 4 4 6% 8% 44%	2785
	21st to end	Night	12% 6% 2 5% 4 4 4 3 4 7% 48%	3124
September	1st to 10th	Night	18% 7% 3 7% 4 4 5% 3 5% 7% 38%	1844
	11th to 20th	Night	22% 9% 2 6% 3 3 4 3 4 6% 36%	2090
	21st to end	Night	23% 11% 2 6% 4 3 3 3 5% 6% 35%	2090
October	1st to 10th	Night	21% 13% 3 6% 4 4 3 2 5% 6% 32%	2189
	11th to 20th	Night	25% 17% 5% 9% 4 4 4 3 5% 5% 20%	2382
	21st to end	Night	21% 22% 6% 10% 5% 4 3 3 5% 5% 16%	2717
November	1st to 10th	Night	21% 18% 6% 12% 5% 6% 4 3 4 5% 16%	2626
	11th to 20th	Night	20% 21% 6% 13% 8% 5% 4 3 4 3 13%	2800
	21st to end	Night	19% 21% 8% 16% 7% 5% 3 2 3 4 11%	2990
December	1st to 10th	Night	9% 24% 10% 20% 11% 7% 4 2 2 3 8%	4910
	11th to 20th	Night	9% 17% 6% 15% 10% 10% 6% 5% 5% 5% 13%	5586
	21st to end	Night	17% 22% 7% 13% 7% 7% 5% 5% 5% 5% 9%	7502

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	78% 5% 7% 6% 4	3159
	11th to 20th	Day	69% 4 10% 15% 2	3362
	21st to end	Day	70% 4 6% 13% 6% 1	3982
February	1st to 10th	Day	62% 5% 10% 17% 6%	1645
	11th to 20th	Day	64% 5% 12% 13% 6%	1762
	21st to end	Day	64% 5% 10% 13% 7% 1	1599
March	1st to 10th	Day	57% 7% 9% 15% 11% 1	3120
	11th to 20th	Day	64% 7% 9% 15% 5%	3470
	21st to end	Day	62% 7% 7% 13% 11% 1	3863
April	1st to 10th	Day	61% 6% 7% 17% 8% 1	2326
	11th to 20th	Day	58% 5% 9% 18% 10% 1	2442
	21st to end	Day	53% 6% 9% 16% 14% 2	2470
May	1st to 10th	Day	51% 9% 11% 17% 12% 1	4147
	11th to 20th	Day	50% 10% 14% 16% 9% 1	4715
	21st to end	Day	51% 8% 11% 20% 9% 1	5185
June	1st to 10th	Day	52% 8% 10% 17% 11% 1	2850
	11th to 20th	Day	45% 8% 12% 22% 12%	2850
	21st to end	Day	49% 8% 13% 21% 9% 1	2850
July	1st to 10th	Day	47% 7% 12% 27% 7%	4686
	11th to 20th	Day	41% 8% 19% 23% 9%	4679
	21st to end	Day	41% 8% 18% 27% 6%	4793
August	1st to 10th	Day	45% 8% 16% 26% 5%	4358
	11th to 20th	Day	39% 6% 17% 33% 5%	4083
	21st to end	Day	46% 6% 14% 25% 8% 1	4531
September	1st to 10th	Day	49% 7% 14% 22% 7% 1	2313
	11th to 20th	Day	55% 8% 18% 16% 4	2090
	21st to end	Day	56% 6% 11% 19% 7%	2090
October	1st to 10th	Day	55% 6% 18% 15% 6%	3302
	11th to 20th	Day	62% 5% 9% 19% 5%	2960
	21st to end	Day	72% 5% 11% 7% 5%	3081
November	1st to 10th	Day	69% 6% 11% 11% 2	1671
	11th to 20th	Day	77% 5% 9% 8% 2	1600
	21st to end	Day	82% 5% 6% 6% 2	1405
December	1st to 10th	Day	85% 3 6% 5% 1	2280
	11th to 20th	Day	72% 7% 13% 6% 2	2621
	21st to end	Day	75% 4 7% 13% 1	3476
January	1st to 10th	Night	75% 5% 9% 5% 4 1	6808
	11th to 20th	Night	74% 6% 9% 9% 3	6617
	21st to end	Night	67% 5% 9% 15% 4	6996
February	1st to 10th	Night	63% 7% 8% 14% 7%	2755
	11th to 20th	Night	64% 5% 10% 15% 6%	2535
	21st to end	Night	62% 6% 11% 14% 6% 1	1949
March	1st to 10th	Night	59% 6% 12% 15% 8%	3733
	11th to 20th	Night	60% 5% 9% 18% 7%	3490
	21st to end	Night	60% 6% 8% 12% 12% 1	3798
April	1st to 10th	Night	57% 6% 7% 17% 11% 2	1853
	11th to 20th	Night	53% 6% 9% 17% 13% 1	1687
	21st to end	Night	51% 5% 9% 16% 17% 3	1710
May	1st to 10th	Night	47% 9% 11% 21% 12% 1	2811
	11th to 20th	Night	47% 8% 13% 22% 9% 1	2194
	21st to end	Night	47% 7% 12% 20% 13% 1	2403
June	1st to 10th	Night	46% 7% 11% 22% 12% 2	1330
	11th to 20th	Night	44% 7% 11% 22% 15% 1	1330
	21st to end	Night	43% 9% 13% 23% 13%	1330
July	1st to 10th	Night	40% 9% 14% 29% 8%	2175
	11th to 20th	Night	39% 7% 17% 25% 12%	2168
	21st to end	Night	38% 6% 16% 29% 11%	2529
August	1st to 10th	Night	39% 8% 17% 30% 5%	2555
	11th to 20th	Night	38% 7% 17% 31% 7%	2812
	21st to end	Night	35% 7% 12% 36% 9%	3124
September	1st to 10th	Night	46% 6% 15% 24% 8% 1	1844
	11th to 20th	Night	48% 8% 20% 18% 5%	2090
	21st to end	Night	50% 6% 15% 21% 8%	2090
October	1st to 10th	Night	52% 5% 12% 22% 8%	3657
	11th to 20th	Night	57% 6% 15% 18% 4	3957
	21st to end	Night	67% 6% 15% 10% 2	4467
November	1st to 10th	Night	70% 7% 10% 11% 2 1	2626
	11th to 20th	Night	77% 6% 8% 8% 1	2800
	21st to end	Night	79% 5% 7% 6% 2	2990
December	1st to 10th	Night	84% 4 5% 5% 2	4910
	11th to 20th	Night	72% 6% 9% 10% 3	5585
	21st to end	Night	76% 5% 6% 10% 2	7502

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	18% 15% 41% 20% 5% 2	200
	11th to 20th	Day	16% 14% 36% 26% 7% 1	243
	21st to end	Day	18% 16% 33% 25% 6% 2	330
February	1st to 10th	Day	17% 24% 36% 18% 5% 1	300
	11th to 20th	Day	18% 18% 41% 19% 4 1	300
	21st to end	Day	24% 22% 40% 12% 2	237
March	1st to 10th	Day	17% 16% 44% 20% 2 1	270
	11th to 20th	Day	24% 15% 32% 22% 5% 2	450
	21st to end	Day	23% 14% 34% 22% 5% 2	506
April	1st to 10th	Day	20% 21% 33% 18% 6% 2	460
	11th to 20th	Day	27% 20% 27% 18% 6% 2	452
	21st to end	Day	23% 18% 29% 18% 9% 2	460
May	1st to 10th	Day	25% 15% 35% 17% 7% 2	477
	11th to 20th	Day	23% 17% 31% 21% 6% 3	675
	21st to end	Day	26% 13% 32% 22% 6% 1	734
June	1st to 10th	Day	28% 13% 32% 19% 6% 2	680
	11th to 20th	Day	22% 14% 34% 20% 8% 1	680
	21st to end	Day	23% 12% 30% 22% 12% 2	680
July	1st to 10th	Day	19% 12% 32% 25% 10% 3	671
	11th to 20th	Day	18% 12% 30% 26% 12% 2	668
	21st to end	Day	17% 9% 32% 28% 10% 4	739
August	1st to 10th	Day	19% 10% 32% 26% 11% 2	652
	11th to 20th	Day	17% 10% 26% 25% 16% 5%	479
	21st to end	Day	21% 13% 28% 22% 12% 4	539
September	1st to 10th	Day	22% 15% 31% 21% 9% 2	490
	11th to 20th	Day	29% 14% 31% 15% 9% 3	490
	21st to end	Day	29% 10% 30% 20% 10% 1	490
October	1st to 10th	Day	30% 16% 26% 21% 6% 1	490
	11th to 20th	Day	33% 18% 32% 14% 3	386
	21st to end	Day	35% 19% 27% 16% 2	330
November	1st to 10th	Day	33% 21% 30% 12% 4	254
	11th to 20th	Day	38% 22% 27% 11% 3	200
	21st to end	Day	31% 25% 33% 10% 3	200
December	1st to 10th	Day	25% 17% 30% 21% 7% 2	200
	11th to 20th	Day	20% 19% 40% 17% 4 2	200
	21st to end	Day	27% 15% 32% 19% 6% 1	220
January	1st to 10th	Night	20% 14% 36% 21% 8% 1	1000
	11th to 20th	Night	20% 13% 33% 26% 6% 2	1000
	21st to end	Night	16% 11% 33% 28% 10% 2	1100
February	1st to 10th	Night	14% 14% 33% 29% 9% 1	955
	11th to 20th	Night	15% 15% 34% 27% 8% 1	783
	21st to end	Night	16% 15% 34% 27% 7% 1	647
March	1st to 10th	Night	16% 14% 35% 27% 7% 1	760
	11th to 20th	Night	19% 16% 35% 23% 7% 1	760
	21st to end	Night	17% 13% 35% 25% 8% 1	829
April	1st to 10th	Night	23% 14% 31% 23% 7% 1	570
	11th to 20th	Night	24% 12% 29% 25% 9% 1	563
	21st to end	Night	22% 12% 31% 25% 9% 1	570
May	1st to 10th	Night	21% 10% 32% 29% 8%	570
	11th to 20th	Night	22% 10% 33% 27% 6% 1	378
	21st to end	Night	22% 12% 32% 28% 7%	411
June	1st to 10th	Night	24% 12% 28% 24% 11% 1	380
	11th to 20th	Night	21% 11% 26% 33% 8%	380
	21st to end	Night	19% 9% 29% 32% 10% 1	380
July	1st to 10th	Night	18% 8% 29% 32% 11% 1	370
	11th to 20th	Night	16% 8% 33% 33% 10% 1	368
	21st to end	Night	17% 6% 29% 32% 15%	538
August	1st to 10th	Night	15% 5% 32% 32% 14% 2	567
	11th to 20th	Night	17% 6% 25% 34% 16% 2	564
	21st to end	Night	18% 8% 28% 30% 14% 2	627
September	1st to 10th	Night	21% 9% 30% 26% 11% 2	604
	11th to 20th	Night	27% 10% 32% 22% 9%	760
	21st to end	Night	28% 9% 27% 26% 9% 1	760
October	1st to 10th	Night	28% 14% 28% 21% 8% 1	760
	11th to 20th	Night	36% 10% 29% 19% 5% 1	760
	21st to end	Night	33% 12% 26% 23% 6%	836
November	1st to 10th	Night	30% 17% 29% 18% 5% 1	780
	11th to 20th	Night	32% 14% 30% 18% 6%	800
	21st to end	Night	30% 16% 32% 16% 6%	990
December	1st to 10th	Night	21% 15% 34% 22% 8% 1	1000
	11th to 20th	Night	20% 13% 36% 23% 8% 1	1000
	21st to end	Night	25% 14% 29% 20% 9% 2	1100

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