Wednesday, June 6, 2018

Warmest Day of the Year

"Just wait until the dog days of August" - Anonymous

June is upon us and many locations have already experienced triple-digit heat. Some locations in the Upper Midwest topped the century mark in May! Will these be the warmest temperatures of the year? Of course no one has a crystal ball, but climatology allows us to peek into the record books and figure out what time of the year is the hottest.

The thing about climatology is that no two places are exactly alike. It surprises no one that the climate of Texas is different than the climate of California. What most people do not realize is that patterns of hot/cold and wet/dry are wildly variable. For example, summer is the wet time of year in the central U.S., but summer is the dry time of year along the West Coast. Temperatures share a similar characteristic of varying across relatively short distances.

Back to the hottest day of the year. When I was growing up in eastern Texas, everyone knew that the hottest time of the year was August. The map below shows the average date (actually the median date) for the occurrence of the single hottest day of the year. It turns out that this aphorism is accurate for only a small slice of the U.S. centered on eastern Texas and adjacent areas.

People that live in western Texas have a much different view of the summer. Residents of El Paso and Midland, Texas, know that late June is the hottest time of the year. Why such variability? The energy output from the sun does not change along lines of latitude. The reasons for the variability of the annual maximum temperature dates are complex. 

In eastern Texas, July and August are the two driest months of the year. Without much soil moisture, the sun's energy is fully devoted to heating the ground – not evaporating water. In western Texas and southern Arizona/New Mexico, June and early July is the sunniest and driest time of the year. Later in July, the Southwest Monsoon sets in. To a large degree, cloud and precipitation climatology drive the variations in the map. More clouds and precipitation mean cooler temperatures during the warm time of the year. Fewer clouds and precipitation mean higher temperatures.

How often is June the month where the highest temperature of the year is observed? What about July? August? The three maps below show the relative frequency for the the hottest day of the year to occur in each of the three core summer months (June, July, and August). Only a small area in the southwestern U.S. is likely to see the hottest temperature of the year occur in June. The same is true for August (more so than June though). The big "winner" is July. For 78% of the U.S. and Canada, July is the most common month to observe the highest temperature of the year.

Perception of weather and climate does not usually match what the data tells us. In the case of the hottest time of year, the Venn Diagram of perception and reality has significant overlap, but far less than most people realize.

If we revisit this article at the end of July and ask if the hottest temperature of the year is behind us, for the vast majority of you, the answer will be yes.

Bonus maps:

© Brian Brettschneider

Sunday, December 17, 2017

Christmas Climatology

The following is a list or random Christmas Climatology facts and figures. All data come from the GHCN-D database from NCEI.

A) No station outside of Hawaii has ever had a 10" rain event on Christmas Day. The Continental U.S. record is 9.84" at Dothan, AL, in 1964. LINK

B) Hawaii has seen four 10" rainfalls on Christmas Day. Three in 1927 on the Big Island. LINK

C) The wettest Christmas *may be* 1975 and the driest *may be* 1999. LINK

D) A closer look at the ESRL 20th Century Reanalysis data set (1851-2014) shows that Christmas 1889 was the warmest during that time period (also exceeding 2015). Note: this data set models temps using SLP, SST, and sea ice. LINK

E) The coldest Christmas is easily 1983. Christmas 1983 is not only the coldest Christmas Day on record. In the R1 Reanalysis database, Dec 25, 1983, is the second coldest of any day between Jan 1, 1948, and present [12/22/1989]. LINK1 LINK2

F) The coldest Christmas in Alaska was 1961.

G) The warmest Christmas in Alaska is a near tie between 1971 and 1985.

H) The lowest Christmas Day temperature in the Lower 48 was -53°F at Riverside, OR, in 1924. LINK 

I) La Pryor, TX, hit 93°F on Christmas Day 1955. That is the U.S. Christmas record. 

J) The lowest Christmas high temperature was -25°F at Wolf Point, MT, in 1983. LINK 

K) The lowest max (coldest high temperature) on an Alaska Christmas was -56°F. Allakaket (1917) and Eagle (1961) share the honor. LINK

L) The lowest low temperature on Christmas Day in Alaska was a chilly -66°F at Allakaket in 1954. LINK 

M) The warmest Christmas Day temperature in Alaska was 57°F at Copper Center School in 1962. LINK 

N) The greatest Christmas Day snow is 44.0" at the Mount Rainer Paradise Ranger Station in 2015. Note: This is possibly a three-day total. The next highest total is 40.2" at Portola, California, in 1971. LINK 

O) The deepest snow depth on Christmas Day is 160". Both Mt. Rainer Paradise Ranger Station (1996) and Mt. Baker Lodge (1948) achieved this value. LINK 

P) There are 4,469 stations with at least 50 years of Christmas Day temperature data. All but 21 (not incl Hawaii) have had at least one Christmas freeze. 7 in Florida, 13 in California, 1 in Louisiana. LINK

Q) Utqiaġvik (Barrow), Bettles, Northway, Tok, Kotzebue, Ft. Yukon, and Tanana, Alaska, have never had a Christmas Day temperature above freezing. [min 50 years]. LINK 

R) In Alaska, Tok (5°F), Northway (9°F), and Ft. Yukon (9°F) have never had a Christmas low temp warmer than 10°F. [min 50 years] LINK

S) In 1919, only 2.6% of stations in the Lower 48 had measurable precipitation on Christmas Day. In 1940, 48.0% of stations had measurable precipitation. LINK

T) There are seven station that have not recorded measurable precipitation on Christmas Day [min 50 years]. LINK

U) The Otis 2 NE Cooperative station in Oregon has recorded measurable precipitation on 53 of 65 Christmas Days. That is the highest percentage outside of Hawaii. LINK. They also have the longest current streak of Christmas' with measurable precipitation. 23 in a row and counting. Outside the Pacific NW, only Bradford, PA, has had 20 consecutive Christmas' with measurable precip (1961-1980). LINK

V) The Houghton Lake AP, Michigan, station once received measurable snow on 17 consecutive Christmas Days (1964-1980). LINK 

W) Map showing the record lowest Christmas Day temperature for stations with 40+ years of data. LINK

X) Map showing the warmest Christmas Day temperature on record using stations with at least 40 years of data. LINK

Y) Mould Bay, Canada, has Christmas Day data for 64 years. They have never recorded a Christmas temperature above 0°F.

Z) U.S. Stations with 10+ consecutive White Christmases overlaid on historical probability. Flagstaff, AZ, is a near certainty to end their run of 11 straight White Christmases. Doesn't look good for Wolf Canyon, NM, either. Every other dot looks safe. LINK 

AA) Coldest Christmas on record based on Reanalysis data: 1851-present. Note: pre-1948 methodology is less robust. LINK

AB) Historical chance of measurable snow on Christmas Day. LINK 

AC) Stations that have ever had a White Christmas. LINK

Thursday, September 28, 2017

Map Collection

A) Temperature maps (percentiles, medians, and averages).



C) Daylight & Twilight

D) Record Consecutive Temp & Precip Days

E) Wettest / Driest  Months & Seasons

F) Rick's Wet Season Index

G) Holiday Climatology

H) Eclipse Maps

I) Precipitation Percentiles

J) Precipitation Recurrence Intervals

K) Defining Seasons

L) ESRL Rankings Website Output

M) Days per Year Threshold Count

N) First/Last Temperature Date

O) Single Year/Season Analysis

P) Snow Depth

Q) Seasonal Midpoints

R) Change Across Time Periods

S) All-Time Extreme

T) Severe Weather

U) Random Stuff