Atmospheric Moisture

• Takes the form of water vapor, liquid water, and ice, moisture is transferred via moving air.

Rules for Atmospheric Moisture §

• More humid air = More water in the air.

• Humid air has a higher dew point temperature than less humid air. Low dew point = low humidity.

  • Topographical features that block moisture and wind also reduce the rate of change of the dew point.
  • Receiving little insolation also lowers dew point significantly.
  • High insolation = higher energy = more humidity = higher dew point.

• A belt of high specific humidity straddles the equator, coinciding with the tropics. This is due to insolation.

• Ocean currents influence specific humidity as they move warm and cold water which brings more / less humidity respectively.

• Land influences humidity due to topography that influences wind currents.

• Dry Air is associated with low humidity. Thus, deserts have low humidity.

• Cold seasons have lower specific humidity.

• As air rises, the pressure decreases. In an adiabatic situation, the decrease in pressure means increase in volume and decrease in temperature.

• Each lapse rate tells something different about the air parcel

  • Unstable: If the environmental lapse rate is higher than the (un)saturated adiabatic lapse rate, then the rising air parcel will remain warmer than its surroundings and it will continue rising
  • Conditionally Unstable: If the environmental lapse rate is between the saturated and unsaturated adiabatic lapse rates, then the addition of moisture will increase the likelihood for rising air. Hence, its motion will be dependent on moisture.
  • Stable - if the environmental lapse rate is lower than thee (un)saturated adiabatic lapse rate, then the air will sink and we will have a temperature inversion. In this case, the air parcel remains cooler than its surroundings and so will sink.

• Convergent winds will move upward until saturated or cooled.

• Cold air sinks and gradually becomes unsaturated due to heating and loss of moisture

• Warm air rises and gradually becomes saturated due to cooling. When it is fully saturated, clouds form.

Clouds and Precipitation §

• Clouds form either due to the cooling of air or humidity increase

• The presence of a fog means that surface level temperature has reached the dew point

• The occurrence of fog is dependent on climate, temperature and typical humidity levels. It may also depend on topography that can influence the wind directions.

  • Fog is frequent along the windward sides of mountain ranges
  • Most common along coastlines, over large bodies of water and in certain low areas due to the water providing additional moisture.
  • Radiation fogs are common in inland areas where there is a significant drop in temperature.
  • Advection fogs are common where cold ocean currents extend into otherwise warm coastal areas.
  • Evaporation fogs are common on the open ocean.

• The state of the precipitate depends on the temperature of the cloud.

  • Warm clouds = Lower altitude = more liquid precipitate.
  • Cold clouds = higher altitude = more solid precipitate.

• Precipitation is concentrated in equatorial, oceanic areas.

• Precipitation generally decreases poleward (due to being too cold), inland (due to less moisture), and downwind from major mountain ranges (due to the terrain)

Humidity §

• Humidity - the amount of water vapor in the atmosphere, which can be expressed in many ways.

  • Absolute Humidity - the mass per volume of the water in the air. Since this changes when air expands or compresses, it is not useful in understanding the atmosphere.
  • Vapor Pressure - denoted the amount of pressure contributed by the water vapor.
  • Water Vapor Capacity - denoted , it is the maximum amount of water vapor the air can hold. It varies by temperature, increasing exponentially.
    • Unsaturated air - occurs when the weather is fair. It is when more water vapor can be introduced in the air.
    • Saturated air - the air has as much water vapor as possible. This is indicated with the formation of precipitation and drops of water (i.e., clouds).
    • Supersaturated - the air has more water vapor than its water vapor capacity.
  • Relative Humidity - he ratio of how much water vapor is in the air relative to the water vapor capacity, indicating how close the air is to saturation. It varies with temperature.
  • Specific Humidity - denoted . It is expressed as the ratio of the mass of water vapor in some body of air to the total mass of that air. Not dependent on temperature, pressure or volume.
  • Dew Point - the temperature to which a volume of air must be cooled to become saturated with water vapor, which then condenses into dew on solid.
    • It is cooling which causes dew formation since cooling makes the water vapor capacity lower.
    • Dew Point Depression - the difference between the current air temperature and the dew point temperature.
    • It is a good indicator of humidity compared to relative humidity since it does not vary due to the temperature at the time of day.
    • The dewpoint is an indicator of the arrival of the seasons and of winds.

• Air at low temperatures can hold much less water vapor. Thus, in cooling the air we change its ability to evaporate moisture. We achieve more relative humidity.

• Specific humidity varies as a function of latitude. Higher specific humidity near the equator.

Rising Air §

Lapse Rate §

• An adiabatic process is one where energy is not exchanged between the parcel and its surroundings. Otherwise it is diabatic.

• A parcel of air denotes a discrete mass of air. We use this to study the motion of air.

• The lapse rate denotes how much a rising air parcel cools given elevation. (so its units are ). This assumes an adiabatic process.

  • Convention: Positive = cools with height. Negative = warms with height.
  • If the parcel rises adiabatically and does not become saturated, the lapse rate is constant. It is referred to as the unsaturated adiabatic lapse rate.
  • Otherwise, if the parcel was saturated, moisture comes out of the vapor since the water vapor capacity decreased/.
    • Such a parcel does not cool as much as unsaturated air. Saturated air is warmed by latent heat.
    • The lapse rate of saturated air is less than that of unsaturated.
    • The rate at which it cools is the saturated adiabatic lapse rate. It is dependent on temperature since higher temperature = more water vapor.

• Air adjacent to a rising parcel of air can have a totally different lapse rate than the rising air, and these rates can vary significantly from day to day and place to place, depending on the local weather, climatic setting, and other factors

  • This lapse rate is known as the environmental lapse rate.
  • The environmental lapse rate can be negative which indicates a temperature inversion.

• The surface affects the rising of the air since each material on the surface responds differently to insolation, which then affects the temperature and humidity of the air.

  • The loss of plant cover (i.e., due to deforestation) increases surface temperature
  • Urbanization creates heat islands which increases surface temperatures.
  • Changes to the environment affect the environmental lapse rate. ¹Deforestation and Urbanization both contribute to high surface temperatures especially at night
  • Water has more thermal inertia which means it reacts more slowly to insolation.
    • In warm ocean currents, near-surface air temperature is warm but higher up it becomes cooler, resulting in the formation of clouds.
    • In cold ocean currents, near surface air temperature is cool and the temperature higher up is no different. This means environmental lapse rate is gentler and the atmosphere is more stable (no precipitation)
    • In snowy and icy areas, since the albedo is high, insolation is trapped and the surrounding air remains cold, sinking back to the ground (i.e., temperature inversion). Warming these areas makes the air less stable as the lapse rate changes.

Convection §

• Air can rise for a variety of reasons

  • Free Convection - air rises due to unstable atmospheric conditions
  • Forced Convection - air is forced to rise due to external factors even if the atmosphere is stable.

• Convergence and Divergence affect the rising of air.

• Convergence occurs when wind systems collide with each other. This causes the air to rise upward. This is forced.

  • When the air reaches the dew point, it condenses and precipitates.
  • Low level convergence is important in regions such as the ITCZ.
  • It occurs when:
    • Wind slows down (i.e., in a sea breeze). The slowed air collides with the faster air behind it.
    • Wind collides with a landmass. This produces an orographic effect where the air rises, then cools above, forming a cloud above the mountain ²
      • At the level when air becomes saturated, clouds form. This is the lifting condensation level. Then it cools.
      • Air that is leeward is warmer than the air at the same elevation on the windward side.

• Rossby waves form as the upper level air circulates around bends in the polar front jet stream.

  • When the air changes direction, the air converges, sinks and warms adiabatically, causing high pressure near the surface,
  • On the opposite side of the trough, the air speeds up, causing air to diverge and spread apart. Air from below flows into the divergence zone and *causes low pressure *

• Warm air fronts are forced over cold air fronts . This is called frontal lifting

Clouds §

Clouds §

• Clouds are the accumulations of liquid water and ice suspended in the air. They can give information about the moisture at a particular point in time.

• Clouds are classified in three ways

  • Cloud Form
    • Cumuiform - taller than they are wide or at least look lumpy.
    • Stratiform - have the appearance of strata or layers
    • Cirriform - feathery wispy clouds
  • Cloud Altitudes
    • Low Clouds - low on the troposphere
    • Alto / Mid-level Clouds - clouds in between low and high.
    • High Clouds - clouds that are at the top of the troposphere
  • Precipitation - clouds hold precipitate which may drop when the cloud can no longer hold their weight.
    • Nimbus clouds - clouds that are experiencing precipitation

• Cloud types

Type	Description
Cumulus	Puffy. Produced by air rising due to free convection. Air is potentially unstable
Altocumulus	Puffy, wispy clouds that contain both liquid and ice particles.
Cirrocumulus	Partly lumpy and partly wispy. They are composed almost entirely of ice due to the altitude
Stratocumulus	Cumulus clouds that occur in layers. Typically low clouds that are composed entirely of tiny drops of liquid water
Cumulonimbus	Unstable, tall cumulus clouds. They contain water at the base and ice at the top. Usually lightning strikes due to them. Well developed clouds form an anvil shape as the stronger upper level winds push the moisture sideways
Cirrus	Wispy clouds high in the atmosphere. These clouds are composed of ice crystals that are easily smeared by strong winds. The wispiness comes from the ice refracting the light. Indicates stormy weather is coming in the next few days
Contrail	Condensation trails formed by jet engines for example . Form when water vapor encounters the colder air outside, causing water to condense or ice to form. They can then be dispersed and sheared by the wind.
Stratus	Have a layered aspect to them. Clouds that are observed when the sky is overcast. Formed due to forced convection. Oriented laterally implying vertical motion is resisted by atmosphere.
Cirrostratus	High level clouds that are made of ice.
Altostratus	Mid level stratiform clouds that contain both ice and liquid water. Indicates high overcast.
Nimbostratus	Stratiform that is precipitating. Typical in regional storms
Fog	A cloud that hugs the ground closely.

Fog §

• Fogs form due to the simultaneous addition of moisture and chilling of air.

  • Evaporation fog - Formed when cold air flows over warm water. As the unstable air rises, it incorporates water molecules on the surface. Evaporation fogs rise in vertical columns.

  • Valley Fog - formed when rugged terrain traps atmospheric moisture in a valley and additional moisture is added from surrounding bodies of water .

  • Precipitation fog - formed alongside precipitation that occurs along a warm front’s boundary with a cold front. This happens when the cold air is dry and the raindrops evaporate due to the warm air, increasing its humidity.

  • Radiation fog - occurs when the ground is significantly cooled due to heat loss, and especially when the surface level winds are humid. If the surface is cooled sufficiently, the fog will persist.

  • Advection fog - fogs produced due to advection — when warm air flows over a cold surface and is cooled to the dew point.

  • Upslope fog - fogs produced by air travelling uphill that is cooled due to low atmospheric pressure to the dew point where it settles.

Precipitation §

• Precipitation - process whereby liquid droplets of water (raindrops), solid bits of ice (snowflakes and hail), or some combination of these fall from the sky due to a cloud being too heavy for uplift winds to carry.

• Precipitate can form in a variety of ways:

  • Collision-coalescence - when a larger droplet becomes larger by colliding and incorporating smaller droplets
  • The small droplets can also slide past the larger droplets, leaving no net change in size.
  • Alternatively, wind resistance can reshape the falling drop until it splits into two. This shrinking also happens due to evaporation.

• Saturation is reached with slightly less water vapor when air is in contact with ice in the cloud than when it is in contact with liquid water, even at the same temperature. Less water vapor can exist when there is ice

  • Air becomes unsaturated next to a liquid drop, causing water to evaporate and relative humidity to increase
  • Liberated water vapor molecules diffuse and aggregate into a nearby snowflake.
  • Bergeron process - the ice in the cloud might melt which causes ice to become liquid and liquid to become vapor which becomes more ice (by aggregation) until it falls down.

• Cloud Seeding - a process wherein the formation of precipitate is made more efficient by either (artificially) cooling the cloud or adding solid particles to facilitate ice crystal formation.

Sleet §

• Sleet - pertains to a mixture of snow and rain, usually involving small frozen ice pellets.

• Freezing Rain - precipitation that reaches the ground as raindrops but immediately freezes upon contact with cold objects on the surface.

• Both form via temperature inversions that causes the precipitate to freeze, unfreeze, and then refreeze. However, sleet has freezing temperatures reaching higher up, whereas freezing rain does not.

• For example, this forms when a warm front meets a cold front and it rises up enough to form another cold region.

• In such a circumstance, the height of the warm front influences the precipitate.

• From lowest to highest altitude. -> Rain, Freezing Rain, Sleet, Snow.

• Terrain can also influence the formation of sleet and freezing rain. This happens in cold air dams where a warm air mass is wedged above a cold air mass.

Extremes §

• Drought - an unusually extended period when losses of surface water to evaporation and transpiration from vegetation in the area are uncharacteristically higher than the addition of water to the surface by precipitation.

• Droughts can be caused by atypical weather patterns

  • Persistent high pressure with sinking air
  • An unusual change in wind direction that brings in dry air.
  • A temporary change in the direction or strength of an ocean current that induces temporary climate changes,

• The return period is the average length of time expected to see between events that equal or exceed a given magnitude. Calculated as follows

• The primary impact of a drought involves the dying of crops. The secondary impact involves indirect effects such as economic losses.

  Links §

• Reynolds Ch. 4

Footnotes §

1. To be exact, it steepens ↩

2. Hence why thunderstorms form in mountain peaks more often than adjacent valleys. Another reason is because wind moves upslope (see here) and can therefore collide at the apex. ↩