What part of the year is the air above the Pacific Northwest typically the driest (least amount of water vapor)?
What part of the year is the above the Pacific Northwest typically the coldest?
The answers to these two questions are the same: right around now! But why?
Let's start with a measure of the total water vapor in a vertical column (called precipitable water) for Quillayute, WA (UIL), on the NW Washington coast (see below). The black line is the average and the red line is the extreme high values. Lowest values (driest air) in late March/early April. Interestingly, the driest air is not when our precipitation is the least (late July/early August). The amount of moist is highest during our driest period. Very strange.
Next, let's go to a specific level (850 hPa--about 5000 ft). The mixing ratio (the amount of water vapor-in grams- per kilogram of dry air) shows a similar pattern, with driest conditions in early spring.
How do we explain this weird situation? The answer is found with temperature.
The amount of water vapor that can be held (or contained) in a volume of air depends on temperature, with warm air able to hold more water vapor. Here is the temperature at 850 hPa (again about 5000 ft). Ah ha! It looks like the moisture! The warmest temperatures aloft are in early August and the coolest average temperatures are in late winter (March).
But that leaves another question. Why are air temperatures aloft coolest in March? Why aren't the temperatures aloft coolest when the sun is weakest (late December)?
The reason is that there is substantial thermal inertia of the atmosphere--it takes a while to warm it up. Like a giant flywheel. And remember that the atmosphere cools as along as long as the amount of energy leaving the atmosphere (by radiation and other processes) exceeds what is coming in (solar radiation, heat provided by the condensation of water vapor)
The surface of the earth has less thermal inertia and tend to respond more rapidly to heating/cooling at the earth's surface then the atmosphere above. Thus, the lowest surface air temperatures tend to be in middle to late January in our region, not the spring.
There is an interesting meteorological implication of the different rate of heating between air near the surface and aloft: the atmosphere over our region tends to be most unstable (tendency to convect) during spring compared to any other time of the year.
The atmosphere becomes less stable when the rate of cooling with height (called the lapse rate) is greatest. If in late winter or early spring, the ground warms up faster than the air aloft, then the lapse rate becomes larger during that period. This is particularly true in spring when the sun (and thus heating near the ground) is getting large.
This figure shows the lapse rate in the lowest 10,000 ft above Quilluyate across the year (black is average, red line is extreme max, blue line extreme min). The lapse rate is largest in April.
Large lapse rates promote instability, which produces cumulus and cumulonimbus clouds, which in turn bring the famous convective rain showers of NW spring. So spring is a showery period in our region because of the surface warms more quickly than the atmosphere above.