How would you describe Tucson's climate?  Hot and dry?  You're basically conveying information about temperature and precipitation. Here we'll mainly concern ourselves with temperature.  With just two numbers, the annual mean or annual average temperature and the annual range of temperature, you can give someone a pretty complete idea of the temperature in Tucson (or some other location) and how it changes during the year.  

Before we look at the factors that determine annual mean and range of temperature, here are some average temperature and precipitation data for Tucson.


1.   Monthly average temperatures are plotted here.  To determine the annual mean temperature add the twelve monthly average temperatures and divide by 12 (68.5 F for the data shown here).  You can get a pretty good estimate of the annual mean temperature by adding the highest and lowest average monthly temperature values and dividing by 2 (69 F).

Average Monthly Temperature and Precipitation data for Tucson

Average
Monthly Temp
 Month 
Average
Max Temp
 Average
Min Temp
 Difference
 Precipitation
52
 Jan
 64
 39
 25
 0.99
55
 Feb
 68
 42
 26
 0.88
59
 Mar
 73
 45
 28
 0.81
66
 Apr
 82
 50
 32
 0.28 (see Note 2)
74
 May
 90
 59
 31
 0.24
84
 Jun
 100
 68
 32
 0.24
86
 Jul
 100
 73
 27
 2.07 (see Note 3)
85
 Aug
 97
 72
 25
 2.30
81
 Sep
 94
 68
 26
 1.45
70
 Oct
 84
 57
 27
 1.21
59
 Nov
 72
 45
 27
 0.67
52
 Dec
 65
 39
 26
 1.07 (see Note 4)

Notes
2.   April, May, and June is generally the driest time of year in Tucson.  This is reflected in the low montly average precipitation values.  It is fairly common to go a month or more without rain at this time of year.

      Because the air is dry and the skies are cloud free, there is generally a large difference between daytime high and nighttime low temperatures. 

3.   The summer thunderstorm season usually begins in early July when the daily average dew point temperature remains 54 F or above for three days in a row.  July, Aug, and Sept. are usually the wettest months of the year in Tucson.  Tucson gets nearly half its annual rainfall during the summer thunderstorm season. 

      Note how the difference between daytime high and nighttime low temperatures decreases once the summer thunderstorm season gets underway.  This is due to the increase in humidity and cloud cover.  Clouds will lower the daytime high temperature and raise the nighttime minimum temperature.

4.   During the winter, middle latitude storms will occasionally drop far enough south to bring precipitation to southern Arizona.  Sometimes these storms will pull up moisture from the tropics and rainfall amounts can be significant.

There are three or four main factors that determine a region's annual mean and annual temperature range.


Latitude affects both the annual mean and the annual range of temperature.  The polar regions have colder annual average temperatures than any other location on earth.  The South Pole is in the middle of a large land mass (Antarctica), the North Pole is ocean.  Much of Antarctica is found at high altitude.  These factors work together to make Antarctica and the South Pole colder than the North Pole.  The hottest regions on earth are found near 30 latitude, not at the Equator.  This is, if you remember, where the optimal combination of sun elevation angle and length of day delivers the greatest amount of sunlight energy to the ground.

The annual range of temperature increases with increasing latitude.  There is little or no seasonal change at the Equator.

A region surrounded by land will have a much larger annual range of temperature than a region surrounded by or near a large body of water.  Oceans are slow to warm during the summer and slow to cool during the winter.  This is partly because water has a higher specific heat than soil.  Some other factors come into play.  The figure below tries to explain why soil and water warm at different rates during the summer


Water has a higher specific heat.  Some of the incoming energy is used to evaporate rather than warm water.  Incoming sunlight penetrates into a body of water and is used to try to warm a larger mass of water.  These three factors mean that water will warm more slowly and won't get as hot during the summer as land.  There is also a larger diurnal temperature range over land than over ocean.  If you've ever been to the beach in the summer you probably remember  that the sand on the beach gets much hotter during the day than the ocean water.  We'll see this effect in action in a later lecture on satellite photographs.

The table below summarizes the three controls of temperature that we have covered so far.  One of them affects both the annual mean and annual range, one affects just the mean, and the other just the annual range.


One final factor:

Cities on the west coast and east coast of the US can have very different climates even if they are at the same latitude and altitude.  A cold southward flowing ocean current is found along the West Coast.  The warm Gulf Stream current flows northward along the East Coast.  Winds at middle latitudes generally blow from west to east.  The city on the West Coast will feel the full moderating effect of the ocean.  The city on the East Coast will be affected by the Gulf Stream current and also by winds blowing across the interior of the US.

A graphical summary.  You find cold locations over land at high latitudes (Northern Canada, Siberia).  Antarctica is the coldest region because it is found at high latitude, is a land mass, and much of Antarctica is high altitude.  The hottest regions on earth are found at low altitude in the middle of land masses near 30 latitude.

What kind of climate would you expect to find at Point X in the figure above.  I.e. at a point near the Equator in the middle of the Pacific Ocean?   The answer to the question includes a short story that features such things as carved wooden pigs, tropical island beverages, and something called betelnut.


The photograph above, taken on Kapingamarangi Atoll, shows a group of people that were participating in a very large international project called the Tropical Ocean Global Atmosphere/Coupled Ocean Atmosphere Response Experiment.  The instructor of this course and one of the other people in the photograph had just installed and tested the tall white lightning detector seen at the edge of the photograph.  They were about to leave Kapingamarangi and travel to Papua New Guinea to install detectors at two additional sites.





It's not particularly easy to travel to Kapingamarangi, it is very remote.  At the time of our field experiment, we had to first fly to Pohnpei (an island in the Federated States of Micronesia).  We planned then to travel onboard a cargo ship that normally sails to Kapingamarangi once a month.  In our case we discovered, after reaching Pohnpei, that the ship departure had been delayed.  We would eventually wait 3 weeks on the island of Pohnpei before departing for Kapingamarangi.  That gave us plenty of time to visit and learn about the island of Pohnpei.



Pohnpei is a fairly large island and, together with some of the other Micronesian islands, is a popular snorkeling and scuba diving destination.   Pohnpei also has a weather station that is operated by the US National Atmospheric and Oceanic Administration (NOAA). 

Pohnpei is located at low latitude in the middle of the Pacific Ocean.  Both of those factors will reduce the annual range of temperature.  The annual range is less than one degree (it is about 34 F in Tucson)!

The average monthly temperatures in Pohnpei range from a high of 80.8 F in February and March to a low of 80.0 F in July.  The all-time record high temperature is 96 F, it has never dropped below 66 F on Pohnpei.The controls of temperature that we have learned about can have quite an effect.

The following precipitation data for Kolonia, the largest town on the island, show that Pohnpei is also one of the rainiest locations on earth

Close to 400 inches of rain may fall in the interior of Pohnpei.  The rainiest location on earth is in Hawaii with about 460 inches of rain per year.

We learned quite a bit more about life in the tropics during our 3 weeks on Pohnpei.  For example we saw a lot of pigs and learned about their importance in the local culture and economy.




The Micro Glory (shown below) is the ship that carried us from Pohnpei to Kapingamarangi and back.  The ship carries supplies to the people on Kapingamarangi and some other small islands along the way.  The islanders pay for the supplies with pigs (the pigs are later sold on Pohnpei).  We shared deck space on the Micro Glory on the trip back to Pohnpei with 20 to 30 pigs (they were hoisted aboard in nets).


Most of the lower deck in the photo above (under the hoists) was occupied by pigs on the return trip.  One of the pigs died on the return trip - that was a very serious matter.

We also had a chance to sample some of the local beverages.


Drinking kava or sakau (as it is called on Pohnpei) turns your mouth and throat numb.  It is supposed to relax you, make you sleep more fully, and doesn't seem to have any after effects.  Until fairly recently you could buy kava in pill form at local supermarkets.  However, because of reports that it can cause serious liver problems, that is no longer the case.  There are no reports of liver problems when drinking kava that has been prepared in the traditional way.  Here is a link to a Wikipedia article on kava.

We never tried betelnut.  Areca nuts are wrapped in betel leaves and chewed together with lime (lime is pretty caustic, that is one of the reasons I didn't try betelnut).  The resulting mixture is a mild stimulant (some people add tobacco to the mix).  The most interesting aspect, however, is that chewing betelnut colors your mouth and teeth bright red.  You don't swallow betelnut, you spit it out.  You see the bright red stains on sidewalks and the ground wherever you go.  Most hotels will also have a large sign near the entrance reminding guests not to chew betelnut inside the hotel.  You can read more about betelnut here.