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 2026-06-16.

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	18% 11% 4 9% 5% 5% 4 3 5% 6% 30%	138041
	Night	19% 13% 5% 10% 5% 4 4 3 4 5% 27%	133960
Transparency	Day	54% 5% 11% 20% 9% 1	141508
	Night	58% 5% 11% 17% 8% 1	138582
Seeing	Day	27% 16% 29% 18% 8% 2	16554
	Night	27% 14% 28% 23% 8% 1	25682

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	15% 22% 7% 16% 6% 5% 4 3 4 5% 14%	4661
	11th to 20th	Day	11% 20% 9% 20% 8% 5% 4 2 3 4 12%	4650
	21st to end	Day	11% 17% 8% 15% 10% 9% 6% 2 4 3 15%	5346
February	1st to 10th	Day	18% 16% 6% 14% 6% 5% 4 3 4 4 20%	1890
	11th to 20th	Day	16% 16% 6% 11% 7% 5% 4 3 4 5% 22%	1895
	21st to end	Day	19% 18% 6% 12% 7% 5% 3 3 4 4 19%	1730
March	1st to 10th	Day	20% 15% 5% 11% 5% 3 4 3 3 5% 25%	4576
	11th to 20th	Day	12% 16% 4 9% 6% 6% 5% 3 5% 6% 29%	5031
	21st to end	Day	20% 11% 4 10% 5% 4 4 2 3 4 32%	5951
April	1st to 10th	Day	22% 14% 4 7% 4 4 4 2 4 5% 31%	2520
	11th to 20th	Day	23% 11% 3 6% 4 5% 3 3 3 5% 35%	2494
	21st to end	Day	24% 10% 3 7% 4 3 4 3 4 5% 33%	2606
May	1st to 10th	Day	25% 11% 3 7% 4 4 4 3 5% 6% 26%	8237
	11th to 20th	Day	19% 9% 3 8% 5% 4 5% 3 5% 6% 32%	8221
	21st to end	Day	23% 8% 3 6% 4 4 4 3 5% 7% 31%	9060
June	1st to 10th	Day	22% 8% 2 7% 4 5% 5% 3 6% 7% 31%	2940
	11th to 20th	Day	17% 6% 2 6% 5% 5% 4 4 7% 8% 35%	2870
	21st to end	Day	21% 7% 2 6% 5% 4 5% 4 6% 6% 34%	2800
July	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	6360
	21st to end	Day	13% 5% 2 5% 4 4 4 4 7% 9% 43%	6926
August	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 6% 9% 35%	4503
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 4 6% 8% 37%	4047
	21st to end	Day	18% 6% 2 6% 3 4 4 4 6% 9% 39%	4312
September	1st to 10th	Day	16% 5% 2 5% 4 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	14% 6% 2 5% 4 4 3 4 5% 7% 47%	2376
	21st to end	Day	19% 6% 3 6% 4 4 4 4 5% 6% 38%	2174
October	1st to 10th	Day	22% 8% 2 7% 4 5% 4 3 5% 6% 35%	2000
	11th to 20th	Day	20% 13% 3 8% 4 4 5% 3 5% 6% 28%	2000
	21st to end	Day	27% 10% 3 7% 4 5% 3 3 4 6% 26%	2027
November	1st to 10th	Day	19% 10% 4 8% 6% 5% 4 4 5% 6% 29%	1851
	11th to 20th	Day	17% 14% 4 9% 6% 6% 4 3 5% 4 28%	1753
	21st to end	Day	21% 18% 5% 10% 6% 4 4 3 4 5% 19%	1680
December	1st to 10th	Day	21% 23% 6% 13% 6% 4 4 3 4 4 12%	2680
	11th to 20th	Day	19% 20% 7% 11% 5% 3 4 3 4 4 20%	3066
	21st to end	Day	21% 18% 4 13% 5% 4 3 3 3 5% 20%	4048
January	1st to 10th	Night	14% 24% 8% 15% 6% 4 3 2 3 4 17%	8234
	11th to 20th	Night	15% 22% 9% 18% 8% 5% 5% 3 3 3 11%	8235
	21st to end	Night	14% 16% 8% 14% 8% 8% 5% 3 4 4 16%	8817
February	1st to 10th	Night	17% 18% 6% 12% 7% 4 4 3 5% 4 18%	2730
	11th to 20th	Night	15% 17% 7% 11% 7% 3 5% 3 4 5% 24%	2610
	21st to end	Night	18% 16% 6% 10% 6% 4 4 3 4 5% 24%	2076
March	1st to 10th	Night	21% 14% 5% 10% 6% 4 3 2 3 4 28%	5484
	11th to 20th	Night	18% 15% 4 7% 4 4 4 3 5% 7% 30%	5145
	21st to end	Night	21% 15% 4 9% 5% 3 3 3 3 4 29%	5222
April	1st to 10th	Night	23% 11% 3 8% 5% 3 4 3 4 5% 30%	2100
	11th to 20th	Night	24% 10% 3 6% 5% 4 3 2 4 5% 34%	2075
	21st to end	Night	23% 11% 3 6% 4 3 3 3 3 5% 34%	2031
May	1st to 10th	Night	26% 11% 3 8% 5% 4 4 4 6% 7% 22%	4660
	11th to 20th	Night	21% 9% 3 8% 4 4 4 4 5% 7% 31%	4639
	21st to end	Night	22% 7% 3 6% 4 5% 4 3 6% 7% 32%	5114
June	1st to 10th	Night	20% 6% 2 7% 5% 4 5% 5% 5% 8% 32%	1680
	11th to 20th	Night	17% 7% 2 6% 6% 3 4 3 6% 7% 39%	1640
	21st to end	Night	19% 9% 3 6% 5% 4 5% 4 5% 8% 34%	1600
July	1st to 10th	Night	15% 6% 3 6% 5% 5% 5% 4 7% 8% 37%	3600
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	3600
	21st to end	Night	11% 6% 3 6% 4 4 5% 3 6% 8% 43%	3926
August	1st to 10th	Night	16% 8% 2 8% 4 4 4 4 5% 7% 37%	2643
	11th to 20th	Night	17% 6% 3 8% 4 4 4 3 5% 7% 39%	2969
	21st to end	Night	11% 6% 3 7% 5% 4 5% 4 6% 8% 41%	3586
September	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 5% 7% 44%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 7% 50%	2023
	21st to end	Night	17% 7% 3 6% 4 4 4 3 6% 7% 38%	2224
October	1st to 10th	Night	21% 8% 2 5% 5% 4 4 4 5% 6% 36%	2400
	11th to 20th	Night	20% 10% 3 7% 5% 5% 4 3 5% 6% 33%	2400
	21st to end	Night	27% 10% 3 8% 5% 5% 4 3 4 5% 26%	2811
November	1st to 10th	Night	20% 13% 3 6% 4 3 4 3 5% 6% 32%	2667
	11th to 20th	Night	19% 13% 4 8% 4 4 4 3 5% 6% 30%	2861
	21st to end	Night	22% 17% 5% 9% 5% 5% 4 3 4 4 22%	2940
December	1st to 10th	Night	20% 24% 7% 14% 6% 4 4 3 3 4 11%	4720
	11th to 20th	Night	24% 17% 5% 10% 5% 4 3 2 4 4 22%	5348
	21st to end	Night	25% 17% 5% 12% 5% 4 3 2 3 5% 17%	7150

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	73% 6% 8% 11% 2	4662
	11th to 20th	Day	77% 3 7% 6% 6% 1	4680
	21st to end	Day	74% 4 8% 7% 6% 1	5346
February	1st to 10th	Day	68% 4 9% 15% 3 1	1890
	11th to 20th	Day	66% 4 11% 13% 7%	1904
	21st to end	Day	69% 4 8% 12% 6% 1	1730
March	1st to 10th	Day	63% 3 10% 13% 8% 2	4579
	11th to 20th	Day	56% 6% 14% 14% 9% 1	5031
	21st to end	Day	55% 4 7% 17% 15% 2	5951
April	1st to 10th	Day	58% 3 7% 19% 11% 2	2520
	11th to 20th	Day	54% 3 7% 19% 14% 2	2494
	21st to end	Day	53% 4 10% 16% 14% 3	2606
May	1st to 10th	Day	58% 5% 9% 19% 9% 1	8249
	11th to 20th	Day	52% 6% 12% 21% 9% 1	8225
	21st to end	Day	52% 6% 9% 24% 9% 1	9061
June	1st to 10th	Day	50% 6% 12% 19% 12% 1	2940
	11th to 20th	Day	45% 7% 12% 25% 10% 2	2870
	21st to end	Day	50% 7% 11% 21% 11% 1	2800
July	1st to 10th	Day	42% 6% 14% 27% 11%	6290
	11th to 20th	Day	39% 7% 15% 27% 13%	6285
	21st to end	Day	36% 6% 13% 37% 8%	6728
August	1st to 10th	Day	46% 6% 15% 23% 9%	4489
	11th to 20th	Day	45% 6% 16% 25% 8%	4086
	21st to end	Day	42% 5% 17% 26% 10%	4312
September	1st to 10th	Day	40% 6% 14% 27% 12% 1	2400
	11th to 20th	Day	37% 5% 12% 31% 15% 1	2376
	21st to end	Day	45% 6% 12% 25% 10%	2174
October	1st to 10th	Day	47% 5% 11% 27% 10%	3260
	11th to 20th	Day	50% 5% 15% 23% 7% 1	3240
	21st to end	Day	48% 4 18% 23% 6%	3252
November	1st to 10th	Day	55% 6% 13% 19% 6%	1851
	11th to 20th	Day	59% 5% 12% 15% 7% 2	1753
	21st to end	Day	68% 7% 12% 10% 4	1680
December	1st to 10th	Day	76% 5% 10% 8% 1	2680
	11th to 20th	Day	69% 5% 8% 13% 5%	3066
	21st to end	Day	68% 4 8% 15% 4	4048
January	1st to 10th	Night	73% 5% 9% 9% 3	8234
	11th to 20th	Night	79% 4 7% 7% 4	8280
	21st to end	Night	73% 4 7% 9% 7%	8817
February	1st to 10th	Night	69% 5% 10% 11% 5%	2730
	11th to 20th	Night	64% 4 10% 17% 5%	2612
	21st to end	Night	63% 5% 10% 15% 7%	2076
March	1st to 10th	Night	61% 3 7% 16% 10% 2	5494
	11th to 20th	Night	53% 6% 18% 14% 7% 2	5146
	21st to end	Night	59% 4 5% 20% 10% 2	5222
April	1st to 10th	Night	57% 4 8% 19% 11% 1	2100
	11th to 20th	Night	54% 5% 9% 18% 13% 1	2075
	21st to end	Night	54% 4 8% 18% 14% 3	2031
May	1st to 10th	Night	59% 6% 11% 15% 8% 1	4666
	11th to 20th	Night	52% 5% 13% 19% 10% 1	4640
	21st to end	Night	49% 6% 11% 20% 13% 1	5115
June	1st to 10th	Night	48% 7% 13% 19% 12% 1	1680
	11th to 20th	Night	43% 6% 14% 20% 15% 1	1640
	21st to end	Night	48% 8% 13% 19% 12%	1600
July	1st to 10th	Night	43% 6% 16% 22% 13% 1	3560
	11th to 20th	Night	41% 5% 14% 28% 11% 1	3552
	21st to end	Night	38% 7% 12% 30% 13%	3809
August	1st to 10th	Night	45% 7% 15% 23% 9%	2633
	11th to 20th	Night	45% 6% 14% 25% 9% 1	2996
	21st to end	Night	40% 6% 14% 28% 10% 1	3586
September	1st to 10th	Night	40% 6% 12% 29% 12% 1	2000
	11th to 20th	Night	34% 6% 14% 31% 14% 1	2023
	21st to end	Night	44% 5% 12% 25% 12% 1	2224
October	1st to 10th	Night	46% 5% 13% 25% 11% 1	3920
	11th to 20th	Night	46% 4 16% 24% 10% 1	3899
	21st to end	Night	52% 4 16% 21% 6%	4536
November	1st to 10th	Night	53% 5% 14% 18% 8% 1	2667
	11th to 20th	Night	55% 5% 13% 17% 7% 2	2861
	21st to end	Night	66% 5% 13% 12% 4	2940
December	1st to 10th	Night	78% 5% 8% 7% 1	4720
	11th to 20th	Night	67% 5% 8% 13% 6%	5348
	21st to end	Night	70% 5% 8% 10% 7%	7150

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% 22% 33% 13% 5%	220
	11th to 20th	Day	31% 23% 31% 11% 3 1	220
	21st to end	Day	27% 23% 30% 17% 3	283
February	1st to 10th	Day	37% 22% 29% 11% 1	330
	11th to 20th	Day	31% 29% 31% 9% 1	382
	21st to end	Day	35% 22% 31% 10% 1	446
March	1st to 10th	Day	31% 22% 29% 14% 4	540
	11th to 20th	Day	37% 18% 29% 13% 2 1	540
	21st to end	Day	36% 19% 26% 13% 6% 1	594
April	1st to 10th	Day	37% 17% 30% 12% 4	540
	11th to 20th	Day	37% 18% 22% 15% 5% 2	532
	21st to end	Day	36% 17% 27% 12% 6% 1	540
May	1st to 10th	Day	31% 18% 27% 17% 7% 1	543
	11th to 20th	Day	23% 16% 32% 19% 8% 2	567
	21st to end	Day	23% 16% 30% 22% 9% 1	617
June	1st to 10th	Day	28% 14% 29% 22% 7% 1	570
	11th to 20th	Day	17% 17% 35% 20% 8% 3	550
	21st to end	Day	27% 14% 32% 17% 10% 1	530
July	1st to 10th	Day	17% 10% 33% 26% 12% 2	525
	11th to 20th	Day	16% 12% 28% 30% 10% 2	524
	21st to end	Day	17% 7% 35% 30% 9% 1	576
August	1st to 10th	Day	18% 7% 25% 31% 15% 3	526
	11th to 20th	Day	16% 8% 27% 27% 16% 6%	519
	21st to end	Day	17% 11% 32% 23% 11% 6%	583
September	1st to 10th	Day	19% 11% 26% 24% 14% 6%	530
	11th to 20th	Day	14% 7% 26% 24% 19% 11%	530
	21st to end	Day	20% 13% 27% 23% 13% 4	530
October	1st to 10th	Day	26% 15% 25% 16% 12% 5%	530
	11th to 20th	Day	28% 19% 28% 18% 6% 2	530
	21st to end	Day	36% 14% 26% 18% 5%	487
November	1st to 10th	Day	30% 19% 29% 15% 7%	425
	11th to 20th	Day	31% 17% 32% 11% 7% 2	293
	21st to end	Day	37% 19% 32% 7% 5%	220
December	1st to 10th	Day	38% 15% 29% 15% 3	220
	11th to 20th	Day	35% 23% 31% 9% 2	220
	21st to end	Day	39% 16% 27% 15% 2 1	242
January	1st to 10th	Night	32% 19% 29% 17% 3	840
	11th to 20th	Night	34% 21% 25% 16% 4	840
	21st to end	Night	24% 20% 31% 19% 5%	924
February	1st to 10th	Night	29% 21% 30% 18% 2	840
	11th to 20th	Night	26% 20% 36% 16% 3	823
	21st to end	Night	28% 18% 33% 18% 2	692
March	1st to 10th	Night	30% 18% 29% 20% 3	840
	11th to 20th	Night	31% 20% 29% 17% 3 1	840
	21st to end	Night	31% 14% 30% 19% 6%	924
April	1st to 10th	Night	31% 16% 31% 20% 3	840
	11th to 20th	Night	32% 17% 25% 19% 6%	831
	21st to end	Night	31% 13% 29% 20% 6%	771
May	1st to 10th	Night	29% 9% 30% 24% 8%	420
	11th to 20th	Night	19% 11% 35% 29% 6%	414
	21st to end	Night	17% 11% 33% 31% 8%	455
June	1st to 10th	Night	23% 8% 27% 37% 6%	420
	11th to 20th	Night	18% 10% 30% 30% 11% 1	410
	21st to end	Night	21% 9% 30% 30% 11%	400
July	1st to 10th	Night	12% 5% 31% 42% 9% 1	390
	11th to 20th	Night	14% 6% 28% 35% 15% 1	388
	21st to end	Night	15% 7% 30% 38% 9% 1	438
August	1st to 10th	Night	15% 6% 22% 41% 16%	429
	11th to 20th	Night	20% 4 20% 34% 20% 2	649
	21st to end	Night	15% 7% 29% 33% 15% 1	880
September	1st to 10th	Night	19% 8% 28% 30% 16% 1	800
	11th to 20th	Night	17% 6% 20% 33% 19% 5%	800
	21st to end	Night	19% 11% 24% 27% 17% 2	800
October	1st to 10th	Night	26% 12% 27% 20% 14% 1	800
	11th to 20th	Night	26% 13% 27% 25% 8% 1	800
	21st to end	Night	34% 13% 28% 21% 5%	880
November	1st to 10th	Night	30% 15% 28% 20% 6% 1	820
	11th to 20th	Night	28% 17% 28% 17% 9% 1	840
	21st to end	Night	34% 19% 25% 16% 5%	840
December	1st to 10th	Night	37% 19% 25% 15% 4	840
	11th to 20th	Night	32% 21% 29% 14% 3 1	840
	21st to end	Night	37% 16% 27% 17% 3	924

Page generated at 2026-06-13 02:31:40. Version ac07

Copyright © 2026 Attilla Danko.