The Hydrologic Cycle

The hydrologic cycle represents an unending cycle of water as it circulates between the atmosphere and the Earth. There are several component to the cycle: evaporation, transpiration, evapotranspiration, condensation, precipitation, infiltration, and runoff.

Make sure you know the definition of each of these components. See Key Points #1 for definitions of the first three items.


Expressions of Atmospheric Moisture

When we discuss humidity and water vapor content, we are only speaking about the invisible water vapor (the gas), and not the visible clouds.

Specific Humidity (q) = mass of water vapor / mass of all of the air. Units = g/kg

Specific humidity is a "good"/direct measure of moisture content. If you have two parcels of air, sample A has 3 g/kg and sample B has 5 g/kg, which parcel has more water vapor? Answer.

We have two values for q. The actual value, q (value at a particular time) and the saturation value, qs (air's capacity which depends on temp - we look this up in a table or read it off a graph, see below). We can use the RH formula (shown below) in the following way to calculate RH...

RH = q/qs * 100



From the above graph, we can easily read off the saturation specific humidity by using the air temperature. Two examples are given to show the relationship between air temperature and saturation values. Sample A has an air temperature of 40°F. We find 40°F and read upward to the curve (red line) and over to the left to read the saturation specific humidity. The value is roughly 8 g/kg. Sample B is warmer, the temperature is 80°F and we read a value for saturation specific humidity of ~21 g/kg. Notice the relationship, the warmer the air, the more water vapor it can "hold". There are graphs such as these for absolute humidity (not discussed in this lecture) and vapor pressure. They are all read and interpreted the same way. Warmer air can "hold" more water vapor than colder air.


Vapor Pressure (e) = pressure that the water vapor exerts. Units = millibars (mb)

Vapor pressure is a "good"/direct measure of moisture content. If you have two parcels of air, sample A has a vapor pressure of 8.2 mb and sample B has a vapor pressure of 4.6 mb, which parcel has more water vapor? Answer.

We also have two values for e. The actual value, e (value at a particular) and the saturation value, es (air's capacity depending on temp. - we look this up in a table or read it off a graph). We can use the RH formula in the following way to calculate RH given vapor pressures...

RH = e/es * 100


Relative humidity (RH) = ratio of water vapor content to maximum amount of water vapor possible at a given temperature (capacity). Units = %

The main equation for relative humidity is:

RH = content (actual moisture value) / capacity (saturation value) * 100 = ____%


The interpretation of RH is...

50% RH indicates that the air is holding half of the water that it is capable of holding. How much is that? Well, it depends on how much it is capable of holding and that depends on the temperature.

Air is saturated when RH = 100%. Relative humidity has an inverse relationship with temperature such that when temperature is the highest (mid to late pm), RH is the lowest and when temperature is at a minimum, RH is highest. This assumes that actual vapor content is not changing. Because RH is temperature dependent, the value changes when the temp. changes so it is misleading and NOT a "good" measure of actual moisture content (unless you know the temperature also).

Relative humidity then varies over the course of a day since temperature is changing. There is an inverse relationship between the two variables. If we hold vapor content fixed...


* RH is lowest when the temperature is the warmest; and highest when the temperature is the coldest (see diagram below).



Dew Point Temperature (Td) = temperature that the air must be cooled to in order for condensation to occur. Units = °C or °F

Dew point temperature is a "good"/direct measure of moisture content. The higher the dew point, the more moisture there is in the atmosphere.

Determining Saturation from our Moisture Indicators...

The air is saturated (contains all the water that it can possibly hold) if any of the following are true...

If one of these is true, then all of them are true and condensation CAN occur. There is no guarantee that condensation WILL occur, so I've used the word CAN rather than WILL in this case.

Condensation and Its Forms...

As air temperature reaches the dew point temperature, the air reaches saturation. Further cooling resulting in condensation. In order for water to condense, it needs a solid surface to condense onto. In the atmosphere, condensation requires the presence of condensation nuclei (tiny airborne particles) which provide the required surface. What about dew and frost?


On calm, clear nights, the ground and lower atmosphere cool by radiation. When the dew point is reached, water vapor condenses into liquid water droplets on the cold surfaces at the ground. This is dew.


The frost point is the name given to the dew point when it is below freezing. On calm, cool nights when the air cools to the frost point, frost will appear. In this case, water vapor turns directly into ice. What is the name of this process? Answer. Note: frost is NOT frozen dew.


Fog is just a cloud located near the ground. The most important three types are...



Air rises into the atmosphere, cooling as it ascends. At some height, the air cools to the dew point and condensation occurs upon the condensation nuclei. These nuclei can consist of dust, pollen grains, soil particles, soot, ash, salt, etc.

The cloud classification system we use was created by Luke Howard in 1803. His system is based on Latin words to describe clouds as they appear to a ground observer.

Clouds are also divided into categories based on height of the cloud base: high, middle, low, and clouds of vertical development

High Clouds

Types: Cirrus, Cirrostratus, Cirrocumulus








Middle Clouds

Types: Altostratus, Altocumulus






Low Clouds

Types: Stratus, Nimbostratus, Stratocumulus








Clouds of Vertical Development

Types: Cumulus (fair weather), Cumulus Congestus, Cumulonimbus


Cumulus Congestus






Unusual Cloud Types

Lenticular - form when moist air crosses over a mountain barrier - lens shaped clouds

Cumulus Mammatus - only cloud that forms where air is sinking. Associated with severe weather, but often seen afterwards

Contrails (Condensation Trails) - come from the water vapor added tot he air from the jet engine exhaust










The size and shape of the clouds that we just looked at in the previous section are controlled by atmospheric stability

We need to define a "parcel" - think of it as a body of air with specific temperature and humidity characteristics - like a "balloon of air" without the balloon. It can expand and contract and there is no mixing of parcel air with outside air. This is defined as an adiabatic process.

*Remember: a lapse rate is a change in temperature with height.

*As air rises, the parcel expands and temperature drops. As air sinks, the parcel compresses and the temperature increases. These temperature changes in the parcel are simply due to expansion and contraction/compression and are therefore, adiabatic.


Lapse Rates

Environmental Lapse Rate (ELR): actual change in temperature with height

Measured with a weather balloon (radiosonde). The Radiosonde is a package of instruments launched by a weather balloon into the atmosphere to measure various quantities in the vertical. Temperature, moisture, pressure, wind speed and direction can all be determined and be sent back to the surface via radio transmitter. Balloon launches occur twice per day all over the world. The time of launch at any particular location is that corresponding to 0000 and 1200 Greenwich Mean Time (GMT) - the time at Greenwich, England. Here in Arizona, our launches occur at 5 a.m. and 5 p.m. since we are 7 hours earlier than GMT.

Dry Adiabatic Lapse Rate (DALR): applies to the parcel. Rate is 10°C / 1000 m or 5.5°F / 1000 ft

DALR is used when the air parcel is NOT SATURATED. The rate of cooling (rising air) or warming (sinking air) is constant and shown above.

Wet Adiabatic Lapse Rate (WALR): applies to the parcel. Rate average is 6°C / 1000 m or 3.3°F / 1000 ft

WALR is used when the air parcel IS SATURATED. The rate of cooling is slower than the DALR since condensation is occurring and latent heat is being released. The rate varies over time and from place to place. The average WALR is shown above.





**So starting at the ground, if the air is unsaturated, rising air cools at the DALR. Cooling will continue until the dew point is reached (Lifting Condensation Level). At this point the air is saturated and further cooling results in condensation (clouds form). From this point on up, the air cools at the WALR. In the diagram (above left), notice the slope of the line for the dry adiabatic lapse rate relative to the saturated or wet adiabatic lapse rate. The more towards vertical the line is, the slower the rate of cooling as we go up. The picture on the right shows the cloud bases clearly - the location of the cloud base is the LCL.

The Mountain Problem...

The side of the mountain that faces the wind is the windward side and the other side is the leeward side. Air rises up the windward side, it cools at the DALR until the dew point is reached. This is the point of saturation, RH = 100%, and the cloud base is reached. After this point until the top of the mountain is reached, air cools at the WALR. As the air sinks down the leeward side of the mountain, it warms at the DALR.

Rising air leads to cloudiness and precipitation making the windward side the wetter and cooler side. The leeward side with air warming as it descends, leads to cloudless skies and dry conditions. This side is also known as the "rainshadow". Lack of precipitation leads to rather sparse vegetation on the lee side. This side can be very dry creating a semi-arid region or a desert (rainshadow desert).


Determining Stability

Stability determines the likelihood of clouds, precipitation, and thunderstorms. There are four categories of stability, three of which are common. Stable air (absolute stability), unstable air (absolute instability) and conditionally unstable air. The fourth category is neutral stability.

Stable conditions: Rising air that is cooler than its surroundings will sink back to earth since it has a higher density. Air that returns back to its point of origin is called stable. During stable conditions, clouds will tend to have flat tops and bases: the stratiform group of clouds. Stable air is most common in the morning / early part of the day.

Unstable conditions: Rising air that is warmer than its surroundings will continue to rise - accelerating away from its point of origin. This is called unstable. During unstable conditions, clouds will show vertical development: the clouds of vertical development group (cumuloform). Unstable air is most common in the afternoon when surface air temperatures are the warmest.

Conditionally unstable conditions: In this case, the condition that determines stability is whether or not the air is saturated. Air tends to be stable if the air is unsaturated and unstable if the air is saturated.

Neutral stability conditions: If the air has neither the tendency to rise or sink, it is considered neutral.

There are two methods to assess stability, the parcel method which is mentioned above, and the lapse rate method. We'll start with the lapse rate method.

Lapse Rate Method of Determining Stability

We compare the ELR to the DALR and WALR numerically. If...

ELR < WALR = absolutely stable example: ELR=4°C / km < WALR=6°C / km

ELR > DALR = absolutely unstable example: ELR=13°C / km > DALR=10°C / km

DALR > ELR > WALR = conditionally unstable example: 10°C / km > 7°C / km > 6°C / km

ELR = DALR or ELR = WALR both situations would be neutral stability

Parcel Method of Determining Stability

Compare the temperature of the parcel to the surrounding environmental temperature.


Mechanisms for Lifting Air and Cloud Development

There are four mechanisms that are responsible for the development of clouds.

1. Convectional lifting: warm air rises

2. Orographic lifting: air forced over a topographic barrier. Windward vs. leeward, rainshadow effect terms come in here.

3. Convergent or cyclonic lifting: lifting associated with low pressure and general air convergence

4. Frontal lifting: lifting at frontal boundaries


Precipitation Processes and Forms of Precipitation

Water vapor condenses on condensation nuclei (the solid surface required for condensation) in the atmosphere to produce clouds. Cloud droplets are very small and are easily suspended by the upward moving air where the cloud now exists. In order for precipitation to occur, the tiny cloud droplets must grow much larger. The two processes that cause cloud droplets to grow big enough to fall as precipitation are the collision-coalescence process and the Bergeron ice-crystal process.

Collision-Coalescence Process

Bergeron Ice Crystal Process



Precipitation Types

Rain: drops with a diameter greater than or equal to 0.5 mm (you don't need to know that).

Drizzle: smaller in diameter than rain drops.

Virga: rain that evaporates before reaching the ground..

Snow: solid form of precipitation composed of ice crystals in a complex six-sided form.

Blizzard: combination of low temperatures, large accumulation of snow and strong winds

Sleet: frozen ice pellets that result from snow that melts within a warm atmospheric layer and then refreezes before reaching the ground.

Freezing rain: supercooled water that reaches the ground and freezes on contact with the frozen surface. Similar to sleet except that the sub-freezing layer near the ground is too thin for the water droplets to have time to refreeze.

Hail: produced within cumulonimbus clouds as frozen droplets are carried around within the cloud. Hailstones grow larger the more times they make a "round trip" through the cloud. Once the hailstone is too heavy to be supported by the updraft of the thunderstorm, the stone falls to earth.



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