An unusually humid start to summer is likely to stay until mid next week. Humidity, the amount of moisture in the air, makes the temperature feel a degree or two warmer than it really is. This is because humans cannot cool themselves by sweating as effectively in these conditions, since water doesn't evaporate as readily. High humidity is particularly noticeable at night-time, where the moisture traps the heat, and so temperatures don't fall as dramatically. Night-time temperatures in Sydney haven't dropped below 20 degrees since the start of the new year, the first time this has occurred so early in summer since 2000. In particular, last night (6th January) was the city's warmest since December 2013. This humidity is caused by a moist airmass filtering from the tropics and lingering over New South Wales. Usually, a fresh southerly change and cool air mass from the South dries out the NSW coast. However, a very moist trough deepening over the nation along with a tropical low over the Kimberley progressing southeastward is largely preventing any cool change. Models are indicating a southerly change to come around mid next week. Until then, it is advisable to drink plenty of fluids and leave that doona in the cupboard!