How Does Specific Humidity Vary Globally and Seasonally?
HUMIDITY OF THE AIR VARIES greatly from region to region and between different altitudes in the atmosphere. It also varies from one time to another, such as between different seasons. To compare different regions, altitudes, and seasons, we generally use specific humidity, which expresses the amount of water vapor in the air, independent of variations in temperature. From what you know about the Earth, which regions do you think have the highest specific humidity and which ones have the lowest? Do you think these will vary between seasons?
How Does Specific Humidity Vary Globally?
Specific humidity offers some advantages for comparing moisture of various places with different temperatures. The globes below depict specific humidity averaged over an entire year. On all the globes and maps on these next two pages, blues and greens represent higher values of specific humidity, whereas tan represents low values. Observe the globes below, identifying areas that are unusually high or low, and consider possible explanations for these humid and dry places, respectively.
1. The main pattern that emerges from inspection of these globes is that specific humidity varies primarily as a function of latitude — areas near the equator have much higher specific humidity than areas farther north and south (toward the poles).
2. A belt of high specific humidity straddles the equator, coinciding with the tropics. This region receives, on average, the most insolation, which in turn causes a relatively high amount of evaporation and higher content of water vapor in the air. Also, warmer air has a relatively high capacity for water vapor (recall the blue curves from the previous pages).
3. The lowest values are near the poles, where cold temperatures and a cover of ice over the surface limit the amount of evaporation. Also, cold air has a lower capacity for water vapor. For all these reasons, there is less water vapor in polar regions than at lower latitudes.
4. The patterns of specific humidity are less complicated over the oceans than over land. The patterns over the oceans do not exactly follow latitude, mostly because of the influence of ocean currents that move warm and cold water north and south along the edges of continents. In the Atlantic Ocean, the warm Gulf Stream Current brings warm water northward along the East Coast of North America. The northward flow of warm water is accompanied by a northward expansion of moderate humidities toward Europe.
5. Patterns of specific humidity are more complicated on land, mostly reflecting the influence of topography, especially large mountain belts that interfere with prevailing winds, such as the Andes of South America.
6. Very low specific humidity also characterizes land areas along the subtropics, including the northern part of Africa. This region is the site of the Sahara Desert, the world's largest desert. We commonly associate deserts with dry air, and this is indeed reflected in the low values of specific humidity. The low humidity continues eastward across deserts of the Arabian Peninsula and onto southern Asia.
7. In the ocean west of southern Africa is another bend in the patterns of humidity. Lower specific humidities extend to the north offshore of the west coast of southern Africa and then bend westward into the South Atlantic Ocean. Think about what might cause this before reading on.
8. As you probably surmised, this pattern off southern Africa reflects another ocean current, but this time a cold current that brings cold water north, accounting for the lower humidity.
9. The annual average of specific humidity represented on these globes does not tell the whole story. In some regions, there are huge seasonal variations in humidity as the prevailing wind directions shift, such as in association with a monsoon. Before we explore these seasonal variations on the next page, think about how the large patterns on both these globes might change from January (the northern winter and southern summer) to July (northern summer, southern winter).
How Does Specific Humidity Vary from Season to Season?
Specific humidity varies from one season to the next, as represented by the two maps below, which show average specific humidities for the months of January and July. On both maps, note the values for where you live, and think about how this relates to the typical weather you experience at these two times of year.
1. In winter, specific humidity is very small at the poles, because there is so little energy available to evaporate surface water, most of which is frozen anyway.
2. In the mid-latitudes, specific humidity values are fairly low in winter, largely because the moderately cool temperatures limit the amount of evaporation.
3. Where water is significantly warmer than the air above it, such as in the British Isles and over the mid-latitude oceans, the warm, moist air adjacent to the water will rise more because it is less dense. This increased air turbulence allows for more water to evaporate, thereby increasing the humidity.
4. The most humid regions are tropical and equatorial surfaces.
5. Large topographic features, like the Andes Mountains of South America, influence regional patterns as the southeast trade winds push moist air up against the east side of the Andes.
6. The Southern Hemisphere is mostly water, so specific humidity is generally high, particularly in January, the heart of the Southern Hemisphere summer.
7. The overall pattern is fairly similar for July, even though winter and summer have switched between the Northern and Southern Hemispheres. As in January, the highest values of humidity are mostly in the tropics and the lowest values are near the poles.
8. A striking difference from January is the appearance of a region of high humidity in southeastern Asia. This is related to the Asian monsoon, where a seasonal change in prevailing wind direction brings warm, moist air from the adjacent oceans northward into Asia, spreading humidity across India, China, Korea, Japan, and large areas of southeastern Asia.
9. In July, during the Northern Hemisphere summer, humid air reaches farther northward in response to the increased summer temperatures and the resulting increase in evaporation.
10. At this time, during the Southern Hemisphere winter, desert regions of the Southern Hemisphere, such as central Australia and southern Africa, have much lower humidity than in January. This is due to the sinking motion from subtropical high-pressure areas, which brings drier, subsiding air from aloft down to the surface.
11. Because the Southern Hemisphere has much less land than the Northern Hemisphere, it appears more similar between January and July than does the Northern Hemisphere.