Davis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Davis clear sky chart from 2005-11-06 to 2025-04-11.

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	7% 4 1 4 3 3 3 3 4 6% 64%	117521
	Night	8% 4 1 4 3 3 3 3 4 6% 62%	119626
Transparency	Day	22% 4 8% 19% 24% 23%	117275
	Night	25% 4 9% 19% 23% 21%	119461
Seeing	Day	8% 5% 17% 27% 28% 14%	15746
	Night	9% 4 18% 41% 25% 3	25178

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	17% 6% 2 5% 4 5% 5% 4 6% 6% 41%	3682
	11th to 20th	Day	14% 8% 3 6% 3 3 3 2 3 4 51%	4100
	21st to end	Day	18% 7% 2 6% 3 4 4 3 4 6% 41%	4510
February	1st to 10th	Day	18% 6% 3 6% 4 4 4 2 3 5% 46%	1800
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 46%	1851
	21st to end	Day	13% 5% 2 4 3 4 3 3 4 7% 51%	1650
March	1st to 10th	Day	10% 8% 3 8% 5% 4 5% 4 6% 6% 40%	4775
	11th to 20th	Day	12% 5% 2 5% 4 3 3 4 6% 7% 49%	5000
	21st to end	Day	11% 5% 2 6% 4 5% 5% 5% 6% 8% 42%	5617
April	1st to 10th	Day	11% 7% 2 5% 4 3 4 4 5% 8% 48%	2400
	11th to 20th	Day	8% 4 1 4 3 3 4 3 5% 6% 58%	2251
	21st to end	Day	3 3 1 2 3 3 4 3 4 9% 64%	2280
May	1st to 10th	Day	5% 2 1 2 2 2 2 2 3 5% 76%	3922
	11th to 20th	Day	2 2 2 1 2 2 2 3 5% 80%	4301
	21st to end	Day	2 1 1 1 1 2 2 4 6% 79%	4813
June	1st to 10th	Day	4 3 1 2 2 2 3 3 4 7% 69%	2660
	11th to 20th	Day	1 1 1 1 2 2 2 4 5% 82%	2660
	21st to end	Day	2 1 1 1 1 2 1 3 3 85%	2660
July	1st to 10th	Day	1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 87%	4307
	21st to end	Day	1 1 1 1 1 2 3 89%	4455
August	1st to 10th	Day	1 1 1 2 1 2 2 2 5% 5% 78%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	3821
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 3 88%	2280
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 6% 76%	2280
	21st to end	Day	3 2 1 2 1 2 1 1 3 4 81%	2030
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 77%	3180
	11th to 20th	Day	3 2 1 3 2 2 3 2 4 6% 73%	3161
	21st to end	Day	4 2 1 4 3 3 3 3 4 6% 67%	3448
November	1st to 10th	Day	8% 4 1 3 3 3 4 3 4 8% 60%	1854
	11th to 20th	Day	10% 6% 2 6% 4 4 3 4 5% 6% 50%	1800
	21st to end	Day	11% 4 1 5% 3 3 4 4 5% 7% 53%	1630
December	1st to 10th	Day	8% 4 1 4 3 4 3 4 5% 11% 51%	2620
	11th to 20th	Day	24% 9% 4 11% 6% 4 4 2 4 5% 25%	3028
	21st to end	Day	23% 9% 4 8% 4 4 4 3 5% 7% 28%	4004
January	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	6262
	11th to 20th	Night	14% 6% 2 6% 3 3 3 2 4 5% 52%	5880
	21st to end	Night	15% 7% 3 6% 3 3 4 3 4 5% 47%	6468
February	1st to 10th	Night	17% 5% 2 4 4 3 3 3 4 5% 50%	2600
	11th to 20th	Night	12% 4 2 4 4 3 4 4 5% 7% 51%	2434
	21st to end	Night	12% 4 2 4 2 3 3 3 4 7% 56%	1980
March	1st to 10th	Night	11% 6% 3 7% 5% 4 4 4 5% 7% 45%	5117
	11th to 20th	Night	14% 4 2 4 3 3 4 3 5% 7% 53%	5000
	21st to end	Night	12% 6% 2 5% 4 3 4 3 6% 7% 47%	5344
April	1st to 10th	Night	7% 5% 2 4 3 4 4 3 5% 8% 56%	2000
	11th to 20th	Night	7% 4 1 3 4 2 3 3 5% 6% 62%	1883
	21st to end	Night	4 2 1 2 2 2 2 4 4 7% 70%	1900
May	1st to 10th	Night	2 1 1 1 1 1 2 2 4 6% 79%	3037
	11th to 20th	Night	2 1 1 1 1 2 1 2 5% 83%	2590
	21st to end	Night	2 1 1 1 1 2 2 3 5% 83%	2762
June	1st to 10th	Night	4 1 3 2 3 3 2 3 7% 71%	1520
	11th to 20th	Night	1 1 1 2 2 2 1 2 4 84%	1520
	21st to end	Night	2 1 1 1 1 1 1 1 2 3 86%	1520
July	1st to 10th	Night	1 1 1 1 1 94%	2540
	11th to 20th	Night	1 1 1 2 1 1 2 1 2 3 86%	2653
	21st to end	Night	1 1 1 1 1 1 1 2 5% 87%	3109
August	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 77%	2848
	11th to 20th	Night	1 1 1 1 1 2 3 89%	3022
	21st to end	Night	2 1 1 1 1 1 1 2 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 3 88%	1900
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 5% 78%	1900
	21st to end	Night	1 1 1 2 1 1 1 2 3 5% 82%	2145
October	1st to 10th	Night	2 1 1 2 2 2 2 2 4 5% 78%	3800
	11th to 20th	Night	2 1 1 2 1 2 2 2 4 5% 78%	3777
	21st to end	Night	5% 1 1 3 3 3 4 3 4 6% 67%	4121
November	1st to 10th	Night	7% 4 1 3 3 3 3 3 5% 8% 59%	2443
	11th to 20th	Night	9% 4 2 4 3 2 3 3 5% 7% 58%	2600
	21st to end	Night	10% 4 2 4 3 3 3 3 4 6% 58%	2765
December	1st to 10th	Night	9% 4 1 4 3 3 4 3 5% 7% 59%	4560
	11th to 20th	Night	20% 8% 3 8% 6% 5% 5% 4 6% 8% 27%	5176
	21st to end	Night	22% 8% 3 7% 5% 4 5% 4 5% 6% 31%	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	41% 9% 14% 20% 12% 4	3682
	11th to 20th	Day	38% 4 7% 18% 19% 14%	4100
	21st to end	Day	42% 6% 8% 13% 19% 11%	4510
February	1st to 10th	Day	42% 4 8% 21% 18% 8%	1800
	11th to 20th	Day	36% 6% 11% 21% 18% 7%	1860
	21st to end	Day	34% 5% 9% 25% 20% 7%	1650
March	1st to 10th	Day	42% 7% 9% 21% 18% 3	4775
	11th to 20th	Day	32% 6% 12% 27% 14% 8%	5000
	21st to end	Day	36% 7% 15% 27% 12% 2	5617
April	1st to 10th	Day	34% 5% 12% 23% 19% 7%	2400
	11th to 20th	Day	25% 6% 13% 24% 23% 9%	2254
	21st to end	Day	19% 5% 12% 29% 27% 8%	2280
May	1st to 10th	Day	13% 3 8% 28% 25% 23%	3922
	11th to 20th	Day	9% 3 5% 27% 39% 16%	4304
	21st to end	Day	9% 4 10% 34% 30% 14%	4814
June	1st to 10th	Day	17% 3 10% 27% 29% 14%	2660
	11th to 20th	Day	7% 2 8% 21% 34% 27%	2660
	21st to end	Day	7% 2 3 12% 38% 39%	2660
July	1st to 10th	Day	3 1 2 6% 15% 74%	4351
	11th to 20th	Day	7% 1 3 8% 16% 66%	4232
	21st to end	Day	3 2 2 7% 25% 61%	4315
August	1st to 10th	Day	9% 2 3 9% 26% 51%	4076
	11th to 20th	Day	3 2 2 10% 42% 42%	3859
	21st to end	Day	6% 1 2 11% 39% 41%	4179
September	1st to 10th	Day	5% 2 2 8% 30% 53%	2280
	11th to 20th	Day	13% 2 5% 22% 40% 18%	2280
	21st to end	Day	11% 2 4 19% 35% 30%	2030
October	1st to 10th	Day	12% 2 4 12% 27% 43%	3180
	11th to 20th	Day	13% 4 7% 25% 31% 20%	3161
	21st to end	Day	19% 3 8% 22% 33% 15%	3448
November	1st to 10th	Day	24% 5% 10% 27% 24% 10%	1854
	11th to 20th	Day	35% 6% 11% 24% 18% 6%	1800
	21st to end	Day	30% 7% 10% 22% 21% 9%	1630
December	1st to 10th	Day	27% 6% 11% 30% 19% 6%	2620
	11th to 20th	Day	62% 4 7% 13% 11% 2	3028
	21st to end	Day	54% 6% 13% 17% 8% 2	4004
January	1st to 10th	Night	34% 8% 19% 23% 12% 4	6262
	11th to 20th	Night	36% 4 9% 17% 22% 12%	5880
	21st to end	Night	39% 5% 9% 13% 22% 13%	6468
February	1st to 10th	Night	38% 4 8% 21% 20% 10%	2600
	11th to 20th	Night	32% 5% 10% 23% 21% 8%	2436
	21st to end	Night	29% 4 9% 25% 24% 8%	1980
March	1st to 10th	Night	37% 6% 13% 20% 20% 5%	5117
	11th to 20th	Night	31% 5% 14% 23% 17% 10%	5000
	21st to end	Night	35% 6% 14% 26% 15% 3	5344
April	1st to 10th	Night	28% 5% 14% 22% 23% 8%	2000
	11th to 20th	Night	23% 4 12% 28% 21% 11%	1883
	21st to end	Night	15% 5% 13% 30% 29% 8%	1900
May	1st to 10th	Night	9% 4 10% 25% 31% 20%	3037
	11th to 20th	Night	7% 2 6% 25% 40% 19%	2590
	21st to end	Night	7% 2 9% 39% 29% 14%	2762
June	1st to 10th	Night	15% 4 10% 26% 31% 14%	1520
	11th to 20th	Night	7% 2 7% 21% 34% 29%	1520
	21st to end	Night	7% 1 3 11% 33% 43%	1520
July	1st to 10th	Night	3 1 1 5% 15% 75%	2500
	11th to 20th	Night	7% 1 2 6% 14% 69%	2605
	21st to end	Night	4 1 1 6% 25% 63%	3011
August	1st to 10th	Night	10% 3 2 7% 24% 54%	2839
	11th to 20th	Night	3 1 2 7% 37% 50%	3050
	21st to end	Night	7% 1 2 8% 33% 49%	3498
September	1st to 10th	Night	5% 2 2 7% 30% 55%	1900
	11th to 20th	Night	11% 3 5% 19% 38% 24%	1900
	21st to end	Night	8% 1 5% 19% 32% 35%	2145
October	1st to 10th	Night	11% 2 4 11% 19% 52%	3800
	11th to 20th	Night	10% 3 6% 29% 29% 23%	3777
	21st to end	Night	18% 4 5% 20% 32% 21%	4121
November	1st to 10th	Night	23% 5% 9% 25% 22% 16%	2443
	11th to 20th	Night	26% 4 12% 27% 23% 9%	2600
	21st to end	Night	28% 5% 8% 21% 26% 12%	2765
December	1st to 10th	Night	25% 5% 8% 28% 23% 11%	4560
	11th to 20th	Night	53% 7% 10% 16% 12% 2	5176
	21st to end	Night	53% 6% 12% 16% 9% 3	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	32% 6% 18% 23% 18% 3	217
	11th to 20th	Day	17% 7% 19% 21% 26% 11%	400
	21st to end	Day	15% 6% 18% 23% 23% 14%	440
February	1st to 10th	Day	21% 8% 16% 21% 23% 12%	400
	11th to 20th	Day	17% 11% 21% 25% 17% 9%	444
	21st to end	Day	12% 10% 22% 29% 19% 7%	414
March	1st to 10th	Day	15% 10% 25% 28% 15% 6%	500
	11th to 20th	Day	18% 9% 22% 22% 18% 11%	500
	21st to end	Day	15% 12% 23% 26% 19% 6%	550
April	1st to 10th	Day	13% 9% 23% 26% 18% 11%	500
	11th to 20th	Day	10% 9% 25% 26% 16% 14%	453
	21st to end	Day	4 7% 22% 27% 25% 14%	460
May	1st to 10th	Day	6% 6% 25% 31% 22% 10%	463
	11th to 20th	Day	6% 2 23% 27% 27% 14%	488
	21st to end	Day	3 5% 18% 27% 30% 17%	529
June	1st to 10th	Day	4 5% 20% 28% 30% 13%	490
	11th to 20th	Day	1 5% 17% 32% 37% 8%	490
	21st to end	Day	1 1 16% 33% 36% 13%	490
July	1st to 10th	Day	1 1 8% 39% 37% 14%	486
	11th to 20th	Day	1 8% 33% 41% 17%	484
	21st to end	Day	1 10% 33% 39% 17%	531
August	1st to 10th	Day	2 1 11% 33% 38% 14%	487
	11th to 20th	Day	1 11% 30% 36% 20%	477
	21st to end	Day	1 9% 27% 40% 23%	539
September	1st to 10th	Day	1 1 7% 29% 41% 21%	490
	11th to 20th	Day	3 3 12% 29% 33% 19%	490
	21st to end	Day	2 3 14% 24% 34% 22%	490
October	1st to 10th	Day	2 6% 14% 26% 27% 24%	490
	11th to 20th	Day	5% 5% 14% 22% 28% 25%	490
	21st to end	Day	8% 5% 17% 26% 27% 18%	539
November	1st to 10th	Day	8% 6% 20% 20% 26% 20%	390
	11th to 20th	Day	11% 5% 18% 23% 31% 13%	300
	21st to end	Day	13% 3 15% 33% 24% 11%	215
December	1st to 10th	Day	17% 7% 19% 19% 26% 14%	200
	11th to 20th	Day	18% 6% 19% 27% 21% 11%	200
	21st to end	Day	21% 6% 22% 28% 18% 5%	220
January	1st to 10th	Night	17% 5% 19% 34% 22% 4	800
	11th to 20th	Night	16% 5% 16% 36% 24% 3	800
	21st to end	Night	17% 4 16% 34% 26% 4	880
February	1st to 10th	Night	19% 5% 17% 35% 21% 2	800
	11th to 20th	Night	14% 5% 22% 36% 21% 3	784
	21st to end	Night	13% 6% 23% 38% 19% 2	660
March	1st to 10th	Night	16% 7% 20% 35% 20% 2	800
	11th to 20th	Night	16% 7% 18% 34% 23% 1	800
	21st to end	Night	14% 6% 24% 37% 18% 1	880
April	1st to 10th	Night	9% 5% 24% 41% 20% 1	800
	11th to 20th	Night	8% 6% 25% 37% 20% 3	753
	21st to end	Night	5% 4 24% 43% 22% 2	760
May	1st to 10th	Night	4 4 28% 40% 22% 1	682
	11th to 20th	Night	3 2 21% 51% 21% 1	422
	21st to end	Night	4 2 26% 47% 21%	411
June	1st to 10th	Night	4 2 24% 49% 21% 1	380
	11th to 20th	Night	1 2 27% 50% 20%	380
	21st to end	Night	1 1 23% 53% 22% 1	380
July	1st to 10th	Night	1 14% 62% 23%	370
	11th to 20th	Night	1 14% 59% 24% 1	429
	21st to end	Night	1 1 10% 57% 29% 2	624
August	1st to 10th	Night	2 1 16% 54% 26% 1	567
	11th to 20th	Night	1 15% 52% 30% 2	684
	21st to end	Night	1 1 12% 48% 37% 2	836
September	1st to 10th	Night	1 2 10% 48% 38% 2	760
	11th to 20th	Night	2 2 17% 44% 32% 4	760
	21st to end	Night	1 2 16% 39% 36% 6%	760
October	1st to 10th	Night	3 3 15% 39% 34% 6%	760
	11th to 20th	Night	4 3 17% 36% 34% 5%	760
	21st to end	Night	6% 3 16% 36% 31% 9%	836
November	1st to 10th	Night	8% 5% 16% 33% 32% 5%	780
	11th to 20th	Night	9% 3 18% 40% 27% 4	800
	21st to end	Night	11% 5% 16% 38% 25% 4	800
December	1st to 10th	Night	16% 5% 17% 34% 23% 6%	800
	11th to 20th	Night	23% 5% 14% 33% 23% 3	800
	21st to end	Night	20% 8% 20% 32% 18% 1	880

Page generated at 2025-04-08 06:17:26. Version ac07

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