Phillips Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Phillips Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	6% 10% 3 7% 4 4 4 3 5% 6% 48%	105458
	Night	7% 10% 4 8% 4 4 4 3 5% 6% 45%	102815
Transparency	Day	35% 6% 10% 20% 19% 11%	105211
	Night	37% 7% 11% 19% 17% 9%	102650
Seeing	Day	13% 17% 29% 26% 13% 3	15304
	Night	12% 9% 21% 37% 18% 2	25005

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	6% 21% 9% 13% 6% 5% 4 3 3 5% 25%	2539
	11th to 20th	Day	7% 23% 7% 13% 5% 3 3 3 4 4 27%	2704
	21st to end	Day	11% 26% 5% 9% 5% 4 4 2 4 5% 25%	3146
February	1st to 10th	Day	10% 19% 7% 13% 5% 4 4 3 4 5% 27%	1710
	11th to 20th	Day	9% 20% 5% 11% 6% 4 4 3 5% 7% 28%	1704
	21st to end	Day	8% 19% 8% 13% 5% 5% 4 4 4 5% 26%	1570
March	1st to 10th	Day	7% 21% 6% 11% 4 5% 3 2 4 5% 31%	3307
	11th to 20th	Day	6% 13% 5% 7% 4 3 3 3 5% 5% 47%	3490
	21st to end	Day	10% 21% 6% 11% 6% 6% 5% 4 5% 7% 19%	3825
April	1st to 10th	Day	11% 14% 4 9% 4 5% 5% 4 5% 6% 32%	2141
	11th to 20th	Day	9% 12% 4 7% 6% 4 5% 4 5% 6% 38%	2437
	21st to end	Day	9% 12% 4 7% 5% 4 4 4 6% 7% 38%	2470
May	1st to 10th	Day	8% 15% 3 6% 5% 4 5% 4 5% 6% 37%	4110
	11th to 20th	Day	4 5% 2 5% 4 4 4 4 6% 8% 55%	4071
	21st to end	Day	8% 10% 3 7% 4 4 4 3 4 6% 46%	4655
June	1st to 10th	Day	8% 6% 2 5% 4 4 5% 4 6% 8% 48%	2732
	11th to 20th	Day	8% 6% 2 5% 3 4 4 4 6% 7% 52%	2850
	21st to end	Day	3 3 1 3 2 3 3 3 5% 6% 69%	2850
July	1st to 10th	Day	2 1 1 2 1 2 2 2 4 5% 79%	4532
	11th to 20th	Day	2 3 1 4 3 3 3 2 5% 7% 67%	4329
	21st to end	Day	3 4 1 3 3 4 4 3 6% 6% 63%	4454
August	1st to 10th	Day	4 3 1 4 4 4 4 4 5% 7% 60%	4092
	11th to 20th	Day	3 2 1 3 3 4 3 3 5% 6% 67%	4028
	21st to end	Day	2 2 1 3 2 3 3 2 4 5% 72%	4521
September	1st to 10th	Day	4 3 1 4 3 3 4 3 4 6% 66%	2181
	11th to 20th	Day	5% 4 1 4 3 3 4 3 5% 7% 61%	2090
	21st to end	Day	7% 4 2 3 3 3 3 2 4 5% 63%	2090
October	1st to 10th	Day	7% 5% 1 3 2 3 3 3 3 5% 65%	3335
	11th to 20th	Day	5% 7% 2 6% 3 3 4 2 4 5% 59%	3150
	21st to end	Day	6% 12% 3 6% 4 3 4 4 6% 8% 45%	3296
November	1st to 10th	Day	12% 13% 4 7% 4 4 4 3 4 6% 39%	1764
	11th to 20th	Day	11% 12% 5% 7% 4 3 4 3 6% 7% 39%	1733
	21st to end	Day	7% 13% 4 9% 6% 4 3 3 5% 6% 39%	1596
December	1st to 10th	Day	11% 17% 6% 10% 7% 4 4 3 4 5% 30%	1520
	11th to 20th	Day	10% 16% 4 13% 7% 5% 6% 4 7% 6% 22%	1926
	21st to end	Day	8% 11% 6% 12% 5% 6% 5% 5% 5% 7% 30%	2510
January	1st to 10th	Night	7% 16% 8% 17% 5% 4 3 3 5% 6% 25%	4406
	11th to 20th	Night	9% 21% 7% 15% 6% 4 3 2 4 6% 23%	4256
	21st to end	Night	9% 24% 7% 11% 5% 4 4 3 4 5% 23%	4521
February	1st to 10th	Night	10% 17% 5% 11% 6% 4 4 4 6% 6% 27%	2470
	11th to 20th	Night	8% 16% 5% 9% 5% 4 4 2 6% 6% 36%	2360
	21st to end	Night	6% 17% 6% 12% 5% 5% 4 4 5% 6% 30%	1884
March	1st to 10th	Night	6% 15% 5% 9% 5% 5% 5% 4 6% 7% 31%	3566
	11th to 20th	Night	3 10% 4 6% 4 4 4 3 6% 8% 47%	3490
	21st to end	Night	8% 12% 5% 10% 5% 5% 6% 5% 6% 8% 31%	3828
April	1st to 10th	Night	8% 13% 4 6% 5% 4 4 4 6% 6% 41%	2035
	11th to 20th	Night	9% 9% 4 8% 5% 5% 4 3 6% 6% 41%	1691
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 5% 50%	1710
May	1st to 10th	Night	7% 13% 3 8% 4 4 5% 4 5% 6% 42%	2860
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 6% 7% 55%	2834
	21st to end	Night	8% 6% 2 5% 4 4 5% 4 5% 7% 49%	2929
June	1st to 10th	Night	6% 6% 2 5% 5% 4 4 5% 6% 8% 50%	1442
	11th to 20th	Night	9% 4 1 5% 5% 3 3 4 4 7% 56%	1330
	21st to end	Night	3 3 1 4 3 2 4 2 4 5% 68%	1330
July	1st to 10th	Night	2 2 1 3 2 3 2 2 5% 5% 72%	2409
	11th to 20th	Night	2 1 4 3 3 3 4 5% 7% 68%	2631
	21st to end	Night	2 2 1 4 3 3 3 3 5% 7% 66%	3109
August	1st to 10th	Night	2 2 1 4 3 3 5% 4 7% 9% 59%	2847
	11th to 20th	Night	3 3 1 5% 4 4 4 3 4 6% 63%	2811
	21st to end	Night	3 3 1 2 2 2 2 2 4 5% 74%	3146
September	1st to 10th	Night	4 3 1 4 2 3 3 3 4 4 70%	1996
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 6% 62%	2090
	21st to end	Night	8% 3 1 4 3 2 3 2 4 5% 65%	2090
October	1st to 10th	Night	5% 4 1 3 2 2 2 2 4 6% 69%	3640
	11th to 20th	Night	10% 6% 2 5% 3 4 3 2 4 5% 54%	3778
	21st to end	Night	6% 10% 2 6% 3 3 4 3 5% 8% 48%	4261
November	1st to 10th	Night	10% 12% 3 7% 4 4 5% 3 5% 7% 42%	2535
	11th to 20th	Night	9% 11% 4 7% 5% 4 4 4 6% 7% 40%	2645
	21st to end	Night	6% 12% 4 9% 6% 5% 4 3 5% 6% 39%	2800
December	1st to 10th	Night	10% 16% 5% 10% 5% 4 4 4 6% 6% 31%	2660
	11th to 20th	Night	11% 15% 5% 11% 7% 5% 5% 4 6% 6% 26%	3271
	21st to end	Night	8% 13% 6% 12% 5% 5% 5% 4 6% 7% 29%	5154

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	62% 6% 12% 16% 3	2539
	11th to 20th	Day	60% 6% 13% 13% 7% 1	2704
	21st to end	Day	62% 7% 12% 13% 6% 1	3146
February	1st to 10th	Day	59% 8% 11% 16% 6% 1	1710
	11th to 20th	Day	55% 8% 14% 15% 7% 2	1713
	21st to end	Day	59% 7% 12% 14% 7% 1	1570
March	1st to 10th	Day	54% 7% 9% 20% 10% 1	3307
	11th to 20th	Day	37% 7% 11% 31% 14% 1	3490
	21st to end	Day	59% 12% 13% 13% 4	3825
April	1st to 10th	Day	50% 7% 13% 16% 11% 3	2141
	11th to 20th	Day	44% 7% 10% 23% 13% 2	2440
	21st to end	Day	42% 8% 11% 25% 13%	2470
May	1st to 10th	Day	41% 9% 9% 24% 15% 2	4110
	11th to 20th	Day	24% 7% 10% 35% 21% 4	4074
	21st to end	Day	36% 8% 9% 32% 14% 1	4655
June	1st to 10th	Day	32% 7% 14% 29% 17% 2	2732
	11th to 20th	Day	29% 6% 10% 22% 23% 9%	2850
	21st to end	Day	15% 4 6% 21% 38% 16%	2850
July	1st to 10th	Day	10% 3 3 11% 23% 50%	4463
	11th to 20th	Day	16% 3 4 10% 25% 41%	4254
	21st to end	Day	19% 4 6% 11% 32% 28%	4314
August	1st to 10th	Day	22% 4 6% 13% 32% 24%	4078
	11th to 20th	Day	17% 4 6% 17% 36% 21%	4067
	21st to end	Day	14% 3 5% 18% 31% 28%	4520
September	1st to 10th	Day	19% 4 7% 17% 31% 21%	2181
	11th to 20th	Day	22% 5% 9% 23% 28% 14%	2090
	21st to end	Day	24% 5% 7% 25% 32% 8%	2090
October	1st to 10th	Day	23% 3 6% 18% 34% 15%	3335
	11th to 20th	Day	28% 4 7% 34% 18% 8%	3150
	21st to end	Day	35% 6% 16% 26% 16% 1	3296
November	1st to 10th	Day	45% 8% 14% 20% 11% 2	1764
	11th to 20th	Day	42% 8% 18% 22% 8% 2	1733
	21st to end	Day	45% 7% 15% 20% 11% 2	1596
December	1st to 10th	Day	56% 8% 12% 16% 5% 4	1520
	11th to 20th	Day	58% 10% 13% 16% 2	1924
	21st to end	Day	48% 11% 15% 15% 10%	2510
January	1st to 10th	Night	57% 8% 12% 17% 6%	4406
	11th to 20th	Night	63% 7% 10% 14% 4 1	4256
	21st to end	Night	63% 6% 13% 11% 6% 1	4521
February	1st to 10th	Night	55% 8% 14% 17% 6%	2470
	11th to 20th	Night	49% 7% 14% 20% 10% 1	2361
	21st to end	Night	52% 7% 13% 19% 6% 2	1884
March	1st to 10th	Night	48% 9% 11% 20% 11% 1	3566
	11th to 20th	Night	30% 10% 16% 27% 17% 1	3490
	21st to end	Night	44% 13% 16% 22% 4	3828
April	1st to 10th	Night	42% 8% 14% 19% 14% 4	2035
	11th to 20th	Night	40% 7% 14% 23% 14% 2	1691
	21st to end	Night	34% 7% 14% 25% 19%	1710
May	1st to 10th	Night	39% 9% 9% 22% 18% 2	2860
	11th to 20th	Night	23% 7% 12% 28% 24% 6%	2835
	21st to end	Night	30% 10% 12% 27% 20% 2	2929
June	1st to 10th	Night	29% 8% 13% 24% 23% 3	1442
	11th to 20th	Night	27% 6% 10% 22% 23% 12%	1330
	21st to end	Night	17% 4 4 18% 31% 26%	1330
July	1st to 10th	Night	14% 3 3 10% 19% 51%	2369
	11th to 20th	Night	14% 3 4 10% 24% 45%	2583
	21st to end	Night	17% 4 4 10% 34% 31%	3011
August	1st to 10th	Night	18% 5% 6% 12% 23% 35%	2838
	11th to 20th	Night	22% 4 4 13% 35% 22%	2838
	21st to end	Night	14% 3 5% 18% 32% 29%	3146
September	1st to 10th	Night	18% 3 8% 12% 29% 30%	1996
	11th to 20th	Night	22% 6% 6% 22% 27% 18%	2090
	21st to end	Night	22% 4 6% 24% 33% 10%	2090
October	1st to 10th	Night	18% 3 6% 19% 37% 16%	3640
	11th to 20th	Night	33% 3 7% 30% 18% 8%	3778
	21st to end	Night	33% 6% 11% 30% 18% 2	4261
November	1st to 10th	Night	41% 8% 12% 22% 16% 2	2535
	11th to 20th	Night	41% 7% 16% 24% 11% 1	2645
	21st to end	Night	44% 8% 13% 22% 10% 2	2800
December	1st to 10th	Night	51% 8% 16% 15% 7% 3	2660
	11th to 20th	Night	56% 8% 14% 16% 6%	3272
	21st to end	Night	49% 10% 13% 18% 10%	5154

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	25% 20% 22% 23% 8% 1	180
	11th to 20th	Day	18% 13% 33% 23% 11% 2	227
	21st to end	Day	15% 17% 32% 24% 11%	297
February	1st to 10th	Day	21% 25% 29% 23% 3	270
	11th to 20th	Day	20% 22% 32% 18% 6% 2	309
	21st to end	Day	18% 26% 34% 17% 5% 1	382
March	1st to 10th	Day	19% 30% 28% 20% 3	460
	11th to 20th	Day	14% 23% 33% 23% 7% 1	460
	21st to end	Day	23% 29% 28% 16% 3	506
April	1st to 10th	Day	24% 23% 26% 21% 5% 1	460
	11th to 20th	Day	21% 30% 29% 13% 7%	452
	21st to end	Day	19% 25% 25% 22% 7% 2	460
May	1st to 10th	Day	13% 23% 36% 20% 6% 1	463
	11th to 20th	Day	15% 18% 31% 25% 10% 2	488
	21st to end	Day	16% 11% 28% 25% 16% 5%	629
June	1st to 10th	Day	9% 14% 25% 29% 19% 4	680
	11th to 20th	Day	13% 16% 28% 30% 12% 1	680
	21st to end	Day	4 10% 28% 33% 21% 4	680
July	1st to 10th	Day	3 8% 29% 36% 21% 3	559
	11th to 20th	Day	3 7% 37% 38% 14% 1	484
	21st to end	Day	3 5% 27% 38% 22% 4	531
August	1st to 10th	Day	7% 8% 32% 32% 18% 3	487
	11th to 20th	Day	3 9% 32% 33% 18% 5%	477
	21st to end	Day	4 13% 26% 32% 22% 4	539
September	1st to 10th	Day	4 11% 30% 29% 20% 6%	490
	11th to 20th	Day	7% 17% 28% 23% 21% 5%	490
	21st to end	Day	8% 13% 26% 30% 17% 6%	490
October	1st to 10th	Day	13% 15% 25% 24% 19% 5%	400
	11th to 20th	Day	10% 22% 16% 25% 20% 6%	300
	21st to end	Day	16% 17% 30% 26% 12%	330
November	1st to 10th	Day	18% 19% 24% 25% 9% 5%	300
	11th to 20th	Day	13% 19% 33% 24% 9% 2	300
	21st to end	Day	16% 15% 30% 30% 8% 1	299
December	1st to 10th	Day	18% 12% 31% 27% 10% 1	270
	11th to 20th	Day	17% 17% 37% 24% 6%	270
	21st to end	Day	24% 20% 30% 19% 6% 1	205
January	1st to 10th	Night	18% 8% 24% 34% 15% 1	760
	11th to 20th	Night	19% 10% 23% 32% 14% 2	760
	21st to end	Night	18% 10% 22% 35% 14% 2	836
February	1st to 10th	Night	17% 14% 26% 33% 10% 1	760
	11th to 20th	Night	13% 13% 28% 34% 11% 2	744
	21st to end	Night	12% 14% 29% 32% 11% 1	628
March	1st to 10th	Night	12% 11% 30% 36% 9% 1	760
	11th to 20th	Night	10% 10% 26% 37% 15% 1	760
	21st to end	Night	16% 12% 26% 35% 11% 1	836
April	1st to 10th	Night	14% 13% 23% 35% 13% 2	732
	11th to 20th	Night	13% 13% 29% 33% 11% 1	563
	21st to end	Night	12% 11% 24% 33% 18% 1	570
May	1st to 10th	Night	12% 10% 24% 35% 17% 2	570
	11th to 20th	Night	11% 7% 17% 37% 25% 3	561
	21st to end	Night	15% 7% 18% 35% 22% 4	617
June	1st to 10th	Night	8% 6% 20% 36% 27% 2	492
	11th to 20th	Night	9% 6% 21% 43% 19% 1	380
	21st to end	Night	5% 3 14% 45% 27% 6%	380
July	1st to 10th	Night	2 1 12% 49% 31% 5%	370
	11th to 20th	Night	3 2 14% 54% 26% 1	420
	21st to end	Night	3 1 9% 48% 35% 3	624
August	1st to 10th	Night	5% 1 12% 49% 31% 1	567
	11th to 20th	Night	2 2 15% 45% 33% 3	564
	21st to end	Night	5% 4 17% 42% 29% 3	627
September	1st to 10th	Night	5% 4 15% 43% 29% 4	690
	11th to 20th	Night	7% 5% 16% 41% 26% 6%	760
	21st to end	Night	10% 5% 14% 40% 25% 6%	760
October	1st to 10th	Night	12% 6% 16% 38% 25% 3	760
	11th to 20th	Night	12% 7% 15% 38% 26% 3	760
	21st to end	Night	11% 10% 18% 41% 17% 3	836
November	1st to 10th	Night	15% 9% 19% 39% 16% 2	780
	11th to 20th	Night	13% 10% 28% 36% 11% 1	845
	21st to end	Night	11% 13% 23% 36% 14% 4	1000
December	1st to 10th	Night	18% 12% 24% 36% 10%	950
	11th to 20th	Night	19% 12% 28% 30% 10% 1	950
	21st to end	Night	19% 16% 27% 28% 8% 1	1033

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