According to the website www.weather-and-climate.com, average December temperatures in Bethlehem range between a daily high of 14 degrees C and an overnight low of 7. Also, its climate is influenced by the Mediterranean effect, which produces summer drought and winter rain. As a consequence, one fifth of its annual rainfall (111 mm) falls during December, resulting in an average of six rainy days for that month.
Latitude is not the only thing which determines climate. Other factors are also at play, including altitude. Bethlehem is 775 metres (2,543 feet) above sea level - something which needs to be considered when comparing it with other places of similar latitude. Reflecting its higher altitude, some of Bethlehem's precipitation falls as snow, and on average the month of December sees one day of snowfall.
While some may describe an overnight low of 7 Deg.C (44.6 Deg.F) as being "mild", it is hardly conducive to sleeping out under the stars - particularly if there is also a good chance of precipitation, whether in the form of rain or snow.
Furthermore, these "average" air temperatures are measured under standard conditions - inside an enclosure known as a "Stevenson Screen", which is mounted four feet above the ground. This temperature can be quite different from the ground - or "grass" - temperature, which is more applicable to anybody camping out in the open (as the shepherds of Jesus's time were described as doing).
In terms of both latitude and altitude, Bethlehem is similar to Australia's New England Plateau region. Around the towns of Glen Innes and Inverell, one does not willingly camp out in the open during the southern hemisphere's equivalent of December!
Although extremely distrustful of anything the JWs / WTS have ever said, on this particular matter, they way well have a point!