Hewlett Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hewlett Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	18% 11% 4 9% 5% 4 4 3 5% 6% 30%	108677
	Night	19% 14% 5% 9% 5% 4 4 3 4 5% 27%	109881
Transparency	Day	53% 6% 12% 20% 8% 1	112190
	Night	58% 5% 11% 17% 8% 1	114532
Seeing	Day	27% 15% 29% 19% 8% 2	14827
	Night	27% 14% 28% 23% 8% 1	24070

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	16% 22% 7% 15% 6% 4 4 3 3 5% 15%	3597
	11th to 20th	Day	12% 19% 9% 21% 8% 4 3 2 4 4 13%	3570
	21st to end	Day	14% 19% 8% 14% 8% 7% 5% 3 4 3 16%	4126
February	1st to 10th	Day	18% 17% 6% 14% 6% 5% 4 3 4 4 20%	1800
	11th to 20th	Day	16% 17% 6% 12% 7% 5% 4 3 4 5% 22%	1802
	21st to end	Day	19% 17% 5% 11% 7% 5% 3 3 4 4 21%	1570
March	1st to 10th	Day	22% 14% 4 10% 5% 4 3 3 4 5% 27%	3116
	11th to 20th	Day	15% 20% 5% 9% 5% 4 4 3 3 5% 27%	3437
	21st to end	Day	15% 10% 4 10% 5% 4 4 3 4 6% 36%	4060
April	1st to 10th	Day	22% 14% 3 7% 4 4 4 2 4 5% 32%	2280
	11th to 20th	Day	23% 10% 3 5% 4 4 3 3 3 5% 36%	2254
	21st to end	Day	25% 10% 3 7% 4 3 4 3 5% 6% 32%	2358
May	1st to 10th	Day	22% 10% 3 7% 5% 4 4 4 5% 6% 30%	4426
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 3 5% 7% 32%	4398
	21st to end	Day	16% 6% 3 5% 5% 5% 4 4 7% 9% 36%	4837
June	1st to 10th	Day	23% 8% 2 6% 4 5% 5% 3 6% 7% 30%	2660
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 4 7% 8% 35%	2660
	21st to end	Day	22% 7% 2 6% 5% 4 4 4 6% 6% 33%	2660
July	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 8% 9% 38%	4440
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 38%	4440
	21st to end	Day	13% 6% 2 6% 4 4 4 5% 7% 9% 40%	4814
August	1st to 10th	Day	17% 8% 3 7% 4 5% 5% 3 6% 9% 34%	4365
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 4 6% 8% 36%	3921
	21st to end	Day	18% 6% 2 6% 3 4 4 4 6% 9% 39%	4180
September	1st to 10th	Day	16% 6% 2 6% 5% 3 4 4 5% 7% 42%	2280
	11th to 20th	Day	15% 6% 2 5% 4 4 3 4 4 7% 46%	2257
	21st to end	Day	19% 6% 3 6% 4 4 5% 4 5% 6% 38%	2066
October	1st to 10th	Day	22% 8% 2 7% 4 5% 4 3 5% 6% 32%	1900
	11th to 20th	Day	21% 13% 3 8% 4 4 5% 3 5% 6% 28%	1900
	21st to end	Day	28% 11% 3 7% 4 5% 3 3 4 6% 26%	1926
November	1st to 10th	Day	18% 9% 3 8% 6% 5% 4 4 6% 6% 30%	1761
	11th to 20th	Day	17% 13% 3 8% 6% 6% 4 3 5% 4 29%	1670
	21st to end	Day	21% 18% 5% 10% 6% 4 4 3 4 5% 20%	1600
December	1st to 10th	Day	22% 23% 6% 12% 6% 3 4 3 4 4 12%	2600
	11th to 20th	Day	19% 20% 7% 11% 5% 3 4 2 4 4 20%	2986
	21st to end	Day	21% 18% 4 12% 5% 4 3 3 3 5% 21%	3960
January	1st to 10th	Night	14% 24% 9% 15% 6% 4 3 2 3 3 17%	6349
	11th to 20th	Night	12% 22% 9% 18% 8% 5% 5% 3 3 3 12%	6315
	21st to end	Night	17% 19% 9% 14% 7% 7% 4 3 3 3 14%	6812
February	1st to 10th	Night	18% 19% 6% 12% 7% 4 4 3 5% 4 18%	2600
	11th to 20th	Night	15% 16% 6% 11% 7% 3 5% 3 4 5% 23%	2483
	21st to end	Night	18% 16% 6% 10% 7% 4 3 3 4 5% 25%	1885
March	1st to 10th	Night	21% 12% 4 8% 5% 4 3 3 3 5% 32%	3724
	11th to 20th	Night	23% 19% 4 8% 4 4 3 2 3 5% 25%	3520
	21st to end	Night	18% 14% 4 8% 5% 3 3 3 3 4 35%	3571
April	1st to 10th	Night	23% 11% 3 8% 5% 3 4 3 4 5% 31%	1900
	11th to 20th	Night	23% 9% 3 6% 5% 4 4 3 4 5% 34%	1875
	21st to end	Night	23% 11% 3 6% 4 3 3 3 4 5% 33%	1837
May	1st to 10th	Night	24% 10% 3 8% 5% 4 4 4 5% 7% 26%	2514
	11th to 20th	Night	22% 9% 3 7% 4 4 4 4 4 6% 33%	2495
	21st to end	Night	17% 6% 2 5% 4 5% 4 3 6% 8% 40%	2738
June	1st to 10th	Night	20% 6% 2 7% 5% 4 5% 5% 5% 8% 31%	1520
	11th to 20th	Night	17% 6% 2 6% 5% 3 4 4 6% 7% 40%	1520
	21st to end	Night	19% 9% 3 7% 4 4 5% 4 5% 8% 33%	1520
July	1st to 10th	Night	16% 7% 3 7% 5% 5% 4 4 6% 6% 38%	2520
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 8% 41%	2520
	21st to end	Night	12% 7% 3 7% 4 4 5% 4 7% 7% 40%	2739
August	1st to 10th	Night	16% 8% 2 8% 5% 4 5% 4 5% 7% 37%	2561
	11th to 20th	Night	17% 6% 3 8% 4 4 4 4 5% 7% 38%	2876
	21st to end	Night	11% 6% 3 7% 5% 4 5% 4 6% 8% 41%	3476
September	1st to 10th	Night	16% 7% 2 6% 4 3 4 3 5% 7% 44%	1900
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 7% 49%	1922
	21st to end	Night	17% 7% 3 6% 4 4 4 3 6% 7% 38%	2113
October	1st to 10th	Night	22% 9% 2 6% 5% 4 4 4 5% 6% 34%	2280
	11th to 20th	Night	21% 10% 3 7% 5% 5% 4 3 5% 6% 33%	2280
	21st to end	Night	27% 10% 3 8% 5% 5% 4 3 4 5% 26%	2670
November	1st to 10th	Night	19% 12% 3 7% 5% 3 4 3 5% 6% 33%	2537
	11th to 20th	Night	19% 12% 4 8% 4 4 4 4 5% 6% 30%	2725
	21st to end	Night	22% 17% 5% 9% 5% 5% 4 3 4 4 22%	2800
December	1st to 10th	Night	21% 24% 7% 14% 6% 4 4 3 3 3 11%	4580
	11th to 20th	Night	24% 17% 4 10% 5% 4 3 2 3 4 23%	5208
	21st to end	Night	25% 17% 5% 12% 5% 5% 4 2 4 5% 17%	6996

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	71% 7% 8% 12% 2	3598
	11th to 20th	Day	75% 4 8% 6% 6%	3600
	21st to end	Day	72% 5% 9% 7% 6% 1	4126
February	1st to 10th	Day	68% 4 9% 15% 4 1	1800
	11th to 20th	Day	66% 4 11% 12% 7%	1811
	21st to end	Day	68% 4 8% 13% 6% 1	1570
March	1st to 10th	Day	61% 5% 10% 17% 7% 1	3119
	11th to 20th	Day	60% 6% 12% 13% 8% 1	3437
	21st to end	Day	48% 5% 9% 21% 14% 2	4060
April	1st to 10th	Day	57% 3 7% 19% 11% 2	2280
	11th to 20th	Day	52% 4 7% 19% 14% 2	2254
	21st to end	Day	54% 4 10% 15% 14% 3	2358
May	1st to 10th	Day	54% 6% 11% 19% 9% 1	4440
	11th to 20th	Day	50% 8% 12% 20% 8% 1	4402
	21st to end	Day	42% 8% 12% 24% 12% 1	4837
June	1st to 10th	Day	51% 7% 12% 18% 11% 1	2660
	11th to 20th	Day	45% 6% 13% 24% 11% 2	2660
	21st to end	Day	50% 7% 11% 20% 11% 1	2660
July	1st to 10th	Day	40% 6% 14% 28% 11%	4370
	11th to 20th	Day	40% 7% 16% 30% 7%	4365
	21st to end	Day	39% 8% 14% 30% 9%	4661
August	1st to 10th	Day	47% 6% 15% 22% 9%	4351
	11th to 20th	Day	45% 6% 16% 25% 7%	3960
	21st to end	Day	42% 5% 17% 26% 10%	4180
September	1st to 10th	Day	41% 6% 15% 26% 12% 1	2280
	11th to 20th	Day	38% 5% 12% 30% 14% 1	2257
	21st to end	Day	45% 7% 13% 25% 10%	2066
October	1st to 10th	Day	49% 5% 12% 26% 9%	3160
	11th to 20th	Day	50% 5% 15% 23% 6%	3140
	21st to end	Day	48% 4 18% 24% 6%	3151
November	1st to 10th	Day	53% 6% 14% 20% 6% 1	1761
	11th to 20th	Day	58% 5% 11% 16% 8% 2	1670
	21st to end	Day	67% 7% 12% 10% 4	1600
December	1st to 10th	Day	77% 5% 10% 7% 1	2600
	11th to 20th	Day	69% 5% 8% 12% 5%	2986
	21st to end	Day	68% 4 8% 15% 4	3960
January	1st to 10th	Night	73% 6% 9% 9% 2	6349
	11th to 20th	Night	77% 5% 8% 7% 3	6360
	21st to end	Night	75% 4 7% 7% 6% 1	6812
February	1st to 10th	Night	69% 5% 10% 11% 4	2600
	11th to 20th	Night	64% 4 10% 16% 5%	2485
	21st to end	Night	62% 5% 10% 15% 8%	1885
March	1st to 10th	Night	56% 5% 9% 21% 8% 1	3734
	11th to 20th	Night	61% 6% 14% 12% 6% 1	3521
	21st to end	Night	52% 5% 6% 22% 13% 2	3571
April	1st to 10th	Night	56% 4 8% 20% 11% 1	1900
	11th to 20th	Night	52% 5% 9% 19% 13% 1	1875
	21st to end	Night	54% 4 9% 17% 13% 3	1837
May	1st to 10th	Night	55% 8% 12% 15% 10% 2	2520
	11th to 20th	Night	50% 7% 13% 19% 9% 1	2496
	21st to end	Night	41% 7% 13% 20% 17% 2	2739
June	1st to 10th	Night	49% 8% 14% 18% 11% 1	1520
	11th to 20th	Night	42% 6% 14% 20% 15% 1	1520
	21st to end	Night	48% 8% 13% 19% 12%	1520
July	1st to 10th	Night	45% 5% 12% 23% 14% 1	2480
	11th to 20th	Night	40% 6% 15% 31% 8%	2472
	21st to end	Night	41% 8% 14% 25% 12% 1	2651
August	1st to 10th	Night	46% 7% 15% 23% 9%	2551
	11th to 20th	Night	45% 6% 15% 25% 9% 1	2903
	21st to end	Night	39% 6% 15% 28% 10% 1	3476
September	1st to 10th	Night	41% 6% 13% 28% 12% 1	1900
	11th to 20th	Night	35% 6% 15% 30% 14% 1	1922
	21st to end	Night	44% 5% 12% 26% 12% 1	2113
October	1st to 10th	Night	47% 5% 13% 25% 10% 1	3800
	11th to 20th	Night	46% 4 16% 24% 10%	3779
	21st to end	Night	52% 4 17% 20% 6%	4395
November	1st to 10th	Night	52% 6% 14% 18% 8% 1	2537
	11th to 20th	Night	55% 6% 13% 17% 7% 2	2725
	21st to end	Night	66% 5% 13% 12% 4	2800
December	1st to 10th	Night	79% 5% 8% 7% 1	4580
	11th to 20th	Night	67% 5% 8% 13% 6%	5208
	21st to end	Night	70% 5% 8% 10% 7%	6996

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	27% 23% 33% 13% 6%	200
	11th to 20th	Day	29% 21% 34% 12% 4 1	200
	21st to end	Day	27% 24% 30% 17% 2	257
February	1st to 10th	Day	40% 24% 27% 9% 1	300
	11th to 20th	Day	32% 30% 30% 8% 1	349
	21st to end	Day	36% 22% 29% 12% 2	382
March	1st to 10th	Day	31% 19% 31% 15% 4	460
	11th to 20th	Day	33% 19% 31% 13% 2 1	460
	21st to end	Day	35% 19% 26% 14% 6% 1	506
April	1st to 10th	Day	36% 16% 30% 13% 4	460
	11th to 20th	Day	36% 18% 21% 16% 6% 3	452
	21st to end	Day	37% 15% 27% 12% 7% 2	460
May	1st to 10th	Day	28% 17% 27% 20% 7% 1	463
	11th to 20th	Day	22% 15% 31% 20% 9% 2	487
	21st to end	Day	20% 15% 30% 24% 10% 1	529
June	1st to 10th	Day	29% 14% 28% 21% 7% 1	490
	11th to 20th	Day	18% 14% 35% 22% 9% 3	490
	21st to end	Day	28% 13% 30% 17% 10% 1	490
July	1st to 10th	Day	17% 9% 31% 28% 13% 2	485
	11th to 20th	Day	16% 12% 29% 29% 11% 3	484
	21st to end	Day	18% 7% 33% 31% 9% 2	532
August	1st to 10th	Day	19% 8% 25% 29% 15% 3	486
	11th to 20th	Day	16% 8% 27% 28% 15% 6%	479
	21st to end	Day	18% 11% 32% 23% 10% 6%	539
September	1st to 10th	Day	19% 11% 27% 24% 13% 7%	490
	11th to 20th	Day	15% 7% 26% 23% 18% 12%	490
	21st to end	Day	20% 13% 26% 24% 14% 4	490
October	1st to 10th	Day	28% 16% 23% 16% 12% 4	490
	11th to 20th	Day	29% 18% 28% 17% 6% 2	490
	21st to end	Day	36% 14% 26% 18% 5%	452
November	1st to 10th	Day	29% 18% 29% 16% 7% 1	395
	11th to 20th	Day	31% 17% 32% 12% 6% 1	270
	21st to end	Day	37% 19% 32% 8% 5%	200
December	1st to 10th	Day	40% 14% 28% 16% 4 1	200
	11th to 20th	Day	35% 24% 29% 10% 3	200
	21st to end	Day	38% 15% 27% 16% 2 1	220
January	1st to 10th	Night	32% 18% 30% 16% 3	800
	11th to 20th	Night	33% 21% 25% 16% 4	800
	21st to end	Night	24% 20% 31% 20% 5%	880
February	1st to 10th	Night	30% 22% 30% 16% 2	800
	11th to 20th	Night	26% 20% 36% 16% 3	783
	21st to end	Night	28% 19% 32% 18% 3	629
March	1st to 10th	Night	29% 18% 28% 22% 3	760
	11th to 20th	Night	30% 20% 29% 16% 4 1	760
	21st to end	Night	30% 13% 30% 19% 6%	836
April	1st to 10th	Night	30% 15% 31% 21% 3	760
	11th to 20th	Night	31% 17% 26% 20% 6%	751
	21st to end	Night	32% 12% 29% 20% 7%	697
May	1st to 10th	Night	28% 8% 30% 25% 9% 1	380
	11th to 20th	Night	18% 11% 36% 29% 6% 1	374
	21st to end	Night	16% 11% 32% 32% 9%	411
June	1st to 10th	Night	23% 8% 27% 37% 6%	380
	11th to 20th	Night	17% 9% 30% 31% 12% 1	380
	21st to end	Night	21% 9% 30% 30% 11%	380
July	1st to 10th	Night	12% 5% 31% 43% 8% 1	370
	11th to 20th	Night	14% 6% 28% 35% 16% 1	368
	21st to end	Night	15% 6% 30% 38% 9% 1	416
August	1st to 10th	Night	16% 6% 22% 39% 16%	407
	11th to 20th	Night	20% 4 20% 35% 20% 2	616
	21st to end	Night	14% 7% 29% 33% 15% 1	836
September	1st to 10th	Night	19% 8% 28% 30% 16% 1	760
	11th to 20th	Night	18% 7% 20% 32% 19% 5%	760
	21st to end	Night	19% 11% 24% 27% 17% 2	760
October	1st to 10th	Night	27% 12% 27% 20% 13% 1	760
	11th to 20th	Night	27% 13% 26% 25% 8% 1	760
	21st to end	Night	35% 13% 28% 21% 4	836
November	1st to 10th	Night	29% 15% 29% 20% 7% 1	780
	11th to 20th	Night	28% 17% 28% 17% 10% 1	800
	21st to end	Night	34% 19% 25% 16% 6%	800
December	1st to 10th	Night	38% 18% 25% 16% 4	800
	11th to 20th	Night	32% 20% 30% 14% 3 1	800
	21st to end	Night	37% 16% 27% 17% 4	880

Page generated at 2025-02-18 03:26:29. Version ac07

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