We may have just finished what reportedly ranks as the fifth-warmest January on record, but by the end of this week, we’ll be experiencing our coldest weather in seven years when an Arctic blast dips into the United States and delivers temperatures that will start to plummet during the day on Friday and extend through Sunday morning. The weatherman is telling us that the temp will be at six below zero in the Boston area when we wake up Saturday morning and will reach no higher than 15 throughout the entire day. In addition, the Arctic blast will be riding a northwest wind of 20-30 miles per hour, bringing the dreaded wind chill factor into the range of -20 or lower. For those of us who live along the coastline, we will be treated to the eerie spectre of “sea smoke,” the water vapor that forms when really cold air moves over the relatively warmer water (which presently is at a temperature of about 40 degrees in Boston Harbor). Fortunately, this cold snap will be gone in a snap — less than 48 hours — with the temperatures getting back to more-seasonable levels by Sunday when the wind direction shifts from the northwest to the southwest. Needless to say, outdoor activity of just about any kind will be impossible during the day on Saturday. While that may be a minor inconvenience for most of us, for those who are homeless, the extreme cold will be deadly, particularly for the recently-homeless who have not experienced weather this cold. We trust that local officials and charitable organizations will do their best to get the homeless off the streets and into warming shelters during this life-threatening period. As for the rest of us (who can turn our TV onto some golf tournament in California or Hawaii for a vicarious experience), we should make sure that all windows and storm windows are shut tight throughout our homes to ensure that cold air does not infiltrate, with the potential to cause freezing pipes. It also is smart to make sure that our thermostats are kept high enough to ensure that outside walls that have water or heating lines (and that may not be fully-insulated) are kept warm enough to prevent freeze-ups. We realize that this will increase our heating bills — but that will be far cheaper than repairing the damage from a frozen pipe that bursts. Ol’ Man Winter has been kind to us up to now — but this weekend he’ll be letting us know that we should not take him for granted