If you like temperatures that are quite warm and humid with breezes,
you've picked a good time to go.
First of all, you can find a chart showing the climate in Buenos Aires
Alvear Palace Hotel
As you can see, the typical high temperature in December is 80 degrees
F, dropping to about 60 at night. Of course, some days will be hotter,
others cooler. December is when the weather is heating up, so
temperatures will likely be higher later in the month.
The chart also shows a typical rainfall of about three inches during
the month, or an average of 0.1 inches per day. It is one of the
rainier months of the year, so you can assume it will be humid (a fact
which is verified elsewhere on the page).
The figures given here are similar:
Here are some other sources indicating that you can expect warm and
humid weather in the Argentine summer:
RipioTurismo (follow menus to find the climate)
Buenos Aires Fast Facts
As for Rio de Janeiro, the weather is much the same. In fact, the
EuroWeather site listed above gives the same average December
temperature for Rio (22 degrees C or 72 degrees F; remember that's
average of day and night), although with slightly more precipitation
in Rio, so it'll be a bit more humid. Temperatures can reach 100
degrees F, although that's more likely in January or February during
the peak of the summer.
Here are some other sites that discuss summer weather in Rio:
Tips for Travelers
"Coastal cities such as Rio de Janeiro and Salvador have tropical
climates with warm temperatures offset by the constant sea breeze; the
average temperature is in the eighties."
"Most of Brazil from Rio de Janeiro to the north is blessed with a
tropical climate where it is difficult to tell when spring becomes
summer and summer, autumn. Rio's average temperature, for example, is
around 27ºC (80º F) which climbs to 40ºC, the low hundreds, during the
summer months that stretch from December through to March. ... North
of Rio, the weather becomes noticeable more tropical...."
Rio de Janeiro for Beginners
"Summer is probably Rio's most popular season. It starts to get hotter
around October, and summer vacations start in mid-December. On sunny
days, digital thermometers in Ipanema and Copacabana may read 40° C,
about 104 F. If you do not turn on the air conditioner, when you leave
the shower start wondering if you forgot to towel off, or if you're
already breaking a new sweat! The tropical summer sun is very hot
after 10 a.m. If you have fair skin (and even if you don't), take it
easy and do not push your luck."
If you'd like to see what the weather was like in Rio on any given day
in the past, you can go to this page and enter the date near where it
The above sites indicate that weather in the cities north of Rio is
similar to what it is in Rio, with climate gradually becoming more
tropical as you head north. If you're interested in seeing the weather
for a particular city in Brazil, you can find that out here:
I hope you find this information useful. Have a great trip!
Google search term: "buenos aires" climate "average temperature"
Google search term: "rio de janeiro" climate "average temperature"
I also searched at http://www.wunderground.com using the country